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I think many failures are due to silly long life service intervals.
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Inreresting to see "John" say that Diesels sometimes blow their turbos due to bearings goingbecause people don't idle the cars after a run.
Myth.
Not a myth with big diesels on trucks and boats. I used to work with a company that sold turbo oilers - a big canister that stored up oil when the engine was running, and then released it into the turbo bearings when the engine stopped (and the oil pressure dropped).
Like this - only much bigger - www.racerpartswholesale.com/product/4964/Accusump2
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Watercooled bearing housings are certainly included in many turbo's designed for small diesels in the 4 to 6 litre range; this is to avoid carbonisation of oil and resultant stuck piston ring seals if the engine is shut down hot.
Without watercooled housings it is advisable to idle for a minute or so after high speed running, i.e. not turn off immediately after pulling into a motorway service area.
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I recall reading the handbook on a Metro Turbo I once owned. It clearly stated that it was very advisable to let the engine idle for a (min or two?) after a fast run to let the turbo cool down without wrecking the thing due to heat soak. ok. , a long time ago but the principle surely remains the same.
in any case it costs next to nothing to do it and it's usually covered by the final mile or so home (or destination) running at 30 mph or less
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I have been working on turbo cars since about 1979, well remember swapping out turbos on the original audi quattro. Not seen many diesel turbos where problems can be said to be due to heat.
On petrol engine turbo cars its advisable to let the turbo spin down and oil refresh itself for a minute or two, although this normally happens naturally unless you are coming straight off an autobahn into a rest area.
Diesel turbos run cooler and in fact all modern turbos are much more durable than they used to be.
When a diesel turbo is blasting down the motorway the turbo is not actually working very hard. High temperatures are only really experienced when say towing a caravan up a long incline in the summer with 35 deg heat and the air con running full blast. Then you can get up to some pretty hight temperatures, but then they are designed to deal with it.
After changing literally hundreds of turbos over 30 years I would say diesel turbo faults are due to the following (in order),
1 VNT vane problems due to sticking - often on sedately driven cars where driver changes up early and the vanes never get any excercise.
2. Actuator problems, especially on the garrett turbos a la Ford where the actuator just packs up. Pugs are common for this too.
3. Lube problems due to carbon fouling up the oil ways in the turbo. The carbon comes from the engine, NOT from the turbo. Some engines run dirty and so there is a lot of soot circulating in the oil, this tends to precipitate out in the turbo and can block the gauze filter in the oil feed.
To avoid turbo lube problems you need to keep the oil clean. This means a fully synth oil and change it regularly. the oil itself can last a long time, but it becomes laden with soot, carbon, after a while and the additives that keep the carbon in suspension become less effective after a time. A good synth oil will resist high temps without carbonising in the turbo. The inexperienced see carbon in a turbo and blocked gauze and assume it is due to heat in the turbo, it isn't it comes from the engine itself.
If I had been working a diesel turbo car hard (like the towing example) then I would give it a good idle to let everything even out before shutting off. In normal day to day driving this is not necessary at all.
In high performance petrol engine turbo cars I always let them idel for a minute after a fast run.
Edited by WorkshopTech on 12/02/2010 at 20:28
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Thanks WT, useful info. A lot of common sense in that post.
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1 VNT vane problems due to sticking - often on sedately driven cars where driver changes up early and the vanes never get any excercise.
Have you tried this stuff WT?
www.innotecworld.com/c-1064-turbo-clean-set.aspx
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Have you tried this stuff WT? www.innotecworld.com/c-1064-turbo-clean-set.aspx
Yes, a lot of people have tried it with mixed results.
In my experience once the VNT starts playing up then best to replace if you want long term relibaility.
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Very informative post Workshop Tech Cheers
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I don't think they water cool turbo's on diesels because they run at a much lower temperature. The exhaust temperature is lower. I've never understood the EGR principle of diverting exhaust back into the engine to cool the combustion chambers because diesel runs cooler anyway although I know it's 'supposed' to reduce emissions. Diesel turbo's used to last a lot longer than petrol turbo's due to the lower temperatures involved but with modern engines they are failing much more quickly and regularly.
I did a chemistry degree, but it was *a while ago* so there may be others out there that correct any errors in my explanation.
Basically, EGR is all about nitrogen oxides (NOx) control. They are produced during high temperature combustion of fuel (road fuels, gas, coal etc) in air (which is 78% Nitrogen, 21% Oxygen). It's more of a problem in diesel than petrol (I think) because the compression ration in diesel engines is higher, so combustion temperature is higher. During the power stroke, the fragments of fuel and oxygen flying about leads to a side reaction with nitrogen from the air that forms the NOx. Reducing the combustion temperature reduces the amount of NOx produced.
So where does EGR feature? Once you've burnt fuel in the air, some of the oxygen has been replaced by CO2 in the exhaust. EGR diverts some of this back into the inlet - diluting the oxygen in the fresh air with exhaust air that has less oxygen in it.
By diluting the oxygen with CO2, the combustion in the cylinder happens more slowly - releasing about the same amount of energy over a longer time. This means the temperature of combustion in the cylinder is lower, and less NOx gets formed. This means less acid rain, and less kids/grannies getting asthma attacks from poor urban air quality etc.
Hope that helps.
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Pre-cats I wonder if petrol engines produced much more NOx than diseasels as the 1000's of electric arcs produced per minute at the sparkplugs create a nice soup of NOx.
Since the cat virtually eradicates NOx, diesel has perhaps overtaken petrol for this particular pollutant.
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I think NOX prodcution is basically temp dependent. Cats on petrol engines (three way cats0 reduce NOx a vast amount. Diesels have always produced a lot of Nox. Mind you, I'm not a university educated expert, so I could be wrong..?
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Thanks R2-CMax for that explanation. My old 80 Tdi had a blocked EGR valve. When it was removed and inspected, the black gunk was completely solid like coal, the only way to get it out would be to physically attack it. I just got a new one, although if I'd kept the car, I would have ended up with an EGR replacement pipe from Allard.
That car put me off diesels, I now run a beautiful, smooth, reliable, normally aspirated straight six petrol, and it will be a sad day if I have to give it up.
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I checked the car manual on the 1.5l turbo diesel Rio and it states that in normal driving the car does not need idling before turning off (I do not believe that though and let the car idle for 20 - 30 seconds).
The manual says that the only time to let the car idle (to cool the turbo) is after either towing or climbing a hill.
Maybe diesels do run the turbo's at lower exhaust temperatures but I would have thought the turbo's work harder and run at higher pressures as I don't think diesel turbo's have waste gate's to keep a certain pressure.
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I checked the car manual on the 1.5l turbo diesel Rio and it states that in normal driving the car does not need idling before turning off The manual says that the only time to let the car idle (to cool the turbo) is after either towing or climbing a hill.
Yes, that is what I stated a bit further up this thread.
I don't think diesel turbo's have waste gate's to keep a certain pressure.
They are mostly VNT design, which is more sophisticated than a wastegate.
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WT,
is there any maintenance regime or servicing one can give to the variable vane mechanism of a modern turbo?
apart from risking the wrath of the BR by suggesting (wasting money according to some) keeping good quality oil servicing up of course..;)
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Not really. There are no servicable aspects to the VNT design.
We have found that the cars that mostly give trouble with VNT are often sedately driven examples where the driver keeps the revs down and changes up as early as possible. So the VNT rarely gets to move through its full range and gets sticky. The best thing is not to change up too early and regularly give the engine some revs up toward the red line to get the turbo spinning and work the VNT mechanism.
A lot of diesel drivers are economy fanatics and always change up at 2000rpm.
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Ref WT's comment about dependency of NOx and temperature, when I was working on a gas turbine project a few years back, I was informed that the ratio is a fourth power [ double the temperature gives 16 times the NOx.] In practice, we found that reducing turbine inlet temperature by 20 degrees dropped NOx by almost two thirds.
Back approximately to thread, a lot of the Japanese SUV diesels have run-on timers which keep the engine ticking over for a time even after key removal and door locking. Note that we get Japanese spec units, and may be different from European spec.
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Not really. There are no servicable aspects to the VNT design.
Much obliged WT.
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So the VNT rarely gets to move through its full range and gets sticky. The best thing is not to change up too early and regularly give the engine some revs up toward the red line to get the turbo spinning and work the VNT mechanism. A lot of diesel drivers are economy fanatics and always change up at 2000rpm.
If the engine makes it's max torque below 2000rpm, as is common for TDs, then the variable vane will be fully doing its stuff when under load despite being below 2000 rpm, not that I am suggesting to never exceed 2000rpm BTW.
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If the engine makes it's max torque below 2000rpm, as is common for TDs, then the variable vane will be fully doing its stuff when under load despite being below 2000rpm
I think the idea is that you rev it hard to allow the soot build up on the vanes to be blasted away. So you are working the VNT mechanism and keeping it clear at the same time.
Edited by corax on 13/02/2010 at 20:42
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If the engine makes it's max torque below 2000rpm as is common for TDs then the variable vane will be fully doing its stuff when under load despite being below 2000 rpm not that I am suggesting to never exceed 2000rpm BTW.
It doesnt work like that. Max torque doesnt mean vanes are fully open.
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It doesnt work like that. >>
It does work like that, a modern TD under load (uphill, trailer etc) will deploy the VV to gain max boost at well below 2000rpm.
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It does work like that, a modern TD under load (uphill, trailer etc) will deploy the VV to gain max boost at well below 2000rpm.
No, the vanes are like a camera aperture. At low revs they are closed to stop any air passing through the turbine, creating higher boost for more torque.
When the engine revs are higher, the vanes open to allow some air to pass through the turbine, as it is past it's torque peak .
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>>www.honeywell.com/sites/portal?smap=turbo&page=tur...i
ts_VNTworks&theme=P1
Thanks Old Navy. Thats what I was trying to explain, though not very well! But the point is that you need to rev the engine properly to exercise the vanes, as WT says. I'll remember that if I ever buy a diesel again.
Edited by corax on 14/02/2010 at 13:55
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It does work like that a modern TD under load (uphill trailer etc) will deploy the VV to gain max boost at well below 2000rpm.
Why do you keep spouting off about stuff you know nothing about? I have been working on VNT turbos since they first introduced. Been on the Garrett courses. I do know how one works!
You misunderstand how these work. I wont explain it here, but look on the web, Im sure there are plenty of explanations.
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>>No, the vanes are like a camera aperture ......................... >>
Depends on how you look at it, from the perspective of a big turbo for high rpm and the resulting lag or from the small turbo offering low rpm benefits though strangling the higher rpm performance.
Why do you keep spouting off about stuff you know nothing about? I have been working on VNT turbos since they first introduced. Been on the Garrett courses. >>
So you should know how it works!
(I'll ignore the insult!)
>>I wont explain it here>>
I'll help you then.
A non VV turbo able to sustain boost at high RPM need to be of a size that would make it unresponsive at low rpm. A VV t/c features vaccuum or electronically controlled vanes that enable responsiveness at lower rpm as well as sustained boost at higher rpm.
Of course what constitutes high and low rpm is different for diesel and petrol engines.
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Cheddar,
Think less about apertures and large or small turbos, because that's not what variable nozzles are about.
Think more about the nozzles being used to alter the *angle* that the exhaust gas leaves the turbine, and how that needs to vary as either the turbine speed varies or how the mass flow through the turbine changes.
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N_C
You might be able to clear something up for me on this.
Variable Nozzle Turbines (VNTs) are associated with an adjustable vane system.
What is the relevance of the term "nozzle"? I always associate a nozzle with an outlet - something like the tapered device on the exit of a jet engine, or even the add on attachments for a hairdryer. In the context of a VNT, does the nozzle part relate to the function the vanes perform in altering the flow of gases, or is there something else in conjunction with the vanes which moves around?
I always thought the function of the vane movement was to alter the "angle of attack" of the vanes, and therefore adjust their responsiveness to the airflow to cater for different loads and operating conditions, but I'm thinking maybe not after reading this.
Cheers
DP
Edited by DP on 14/02/2010 at 13:32
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DP
VNT are also known as variable geometry turbos (VGT) which is prob a better term.
Basically as the vanes are moved the aspect ratio of the turbo is altered because the area between the vane tips alters. When the vanes are at a shallow angle (low engine rpm) the angle of attack of the gas onto the blades is close to 90 degrees so this helps get the turbine speed up (more efficient). So its BOTH angle of attack and area (aspect ratio) that is altered.
Generlly low rpm = lower gas velocity and vanes at shallow angle. High rpm = high gas velocity and mass flow rate so vanes fully open.
Edited by WorkshopTech on 14/02/2010 at 13:43
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WT,
Thanks for a very clear answer.
Cheers
DP
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Whilst on VV turbo's.
I assume it's the inlet turbines that can vary angle, or do the exhaust side vanes vary too...i would have though the exhaust side a extremely hostile environment for anything to continue working well.
edit...already answered by WT as he speaks of gas entering, still impressed anything moving can survive there long term.
Edited by gordonbennet on 14/02/2010 at 13:47
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edit...already answered by WT as he speaks of gas entering still impressed anything moving can survive there long term.
Thats why there have been very few petrol VNT designs - they run too hot and wont survice. Diesels run cooler and so VNT has become common on them. Diesel turbos usualyy run 700-800 deg but petrol turbos run upwards of 1000deg.
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Thats why there have been very few petrol VNT designs - they run too hot and wont survice. Diesels run cooler and so VNT has become common on them. Diesel turbos usualyy run 700-800 deg but petrol turbos run upwards of 1000deg.
Petrol VNTs have untill now been confined to mega-bucks supercars because the need to be made from exotic high end materials, but VAG are using one on their 1.5TSi Evo engine. Its the first Vnt in a mass produced car. That combined with the miller cycle giving a short compression stroke and a long expansion stroke is said to take petrol efficiency into new territory.
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Sorry DP, I just realised I had written about gas leaving the turbine - of course, I menat gas entering the turbine.
You're absolutely right to think about angle of attack - the main thing the variable nozzles do is to match the airflow conditions of the air entering nearly tangentially making the radial inflow of gas compatible with the speed of the turbine.
Misaligning the nozzles will cause turbo output to fall - which under some conditions is also useful.
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Thank you N_C, and again to WT.
It was the nozzle terminology that confused me.
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Cheddar
I am talking about variable vane NC, as per my previous explanation.
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When you are in a hole, stop digging!
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When you are in a hole stop digging!
WT, why be so objectional rather than offer rational argumentation? I mean you have answered DP quite politely though when somewhere dares to question you you get rather objectionable, do you treat your customers in this way?
Have a look at the following it supports my previous explanation perfectly, the opening paragraph:
"the problem with the turbocharger that we?ve all come to know and love is that big turbos do not work well at slow engine speeds, while small turbos are fast to spool but run out of steam pretty quick"
paultan.org/2006/08/16/how-does-variable-turbine-g.../
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>> When you are in a hole stop digging! >> WT why be so objectional rather than offer rational argumentation? I mean you have answered DP quite politely though when somewhere dares to question you you get rather objectionable do you treat your customers in this way?
I am not normally objectionalble at all. But you do seem to consistently post incorrect information/advice. The way a VNT operates is a matter of fact, its not something that needs to be argued about or questioned. I have had numerous turbos in bits on the bench, I know how they work. THen you post up something telling me that is incorrect when there is plenty of info out there if you care to search for it and educate yourself.
Sorry if it appears rude, I just dont tolerate fools gladly as the saying goes.
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But you do seem to consistently post incorrect information/advice.
You really can be most unpleasant!
I know perfectly well how a VV t/c works and have explained as such, if you disagree perhaps explain why rather than continue to be objectionable.
I think one issue with this is the premise from which you look at it - whether you see VV enabling a small turbo to sustain boost at high rpm or a big turbo to respond at low rpm. Your comments seems to err towards the former though I reckon the latter is perhaps a better rationale.
Lastly you say VNT which seems to be uniquely Garrett terminology as far as I can see where as variable vane encompasses the genre.
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www.technologie-entwicklung.de/Gasturbines/VNT15-T...l
Worth a read and a nice simple demonstration of how the VNT system works at the very bottom of the page
;o)
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Worth a read and a nice simple demonstration of how the VNT system works at the very bottom of the page
Thanks Dox, good find i know a bit more now.
I'm still impressed that those vanes can operate well bearing in mind the environment they have to survive in.
As WT noted above, very little to be done there in the form of servcing unless prepared to strip out, clean and replace any bearings...hardly a task for the unskilled home mechanic on the kitchen table..;)
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I read the term VVT to mean the turbine blades (thought these were the vanes being talked about...) altered pitch. Now i know better!
Naff name, misleading! :-)
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Just to add a little to your post R2-CMax. The higher pressure in a Diesel at the point of ignition is a big factor as well as temperature. Diesels run unthrotled so fill their cylinders completely even under low loads, when you press the accelerator you are just adding more fuel, so there is a lot of extra Oxygen present. Petrol cars need to run with an accurately controlled mixture (stoichiometric) with little excess Oxygen available to form NOx, The amount of air has to exactly match the amount of fuel, It was only with the development of lean burn engines that much NOx was produced but still lower than diesel.
It this feature of always filling its cylinders and compressing it more that makes diesels more efficient and allows more of the heat to be turned into mechanical energy with less heat chucked out the exhaust and its also why petrol exhaust is hotter. Many modern petrol engines use EGR not so much to reduce NOx but to increase eficiency, make them closer to diesel but filling their cylinders more without adding more Oxygen.
Edited by Stanb Sevento on 10/06/2017 at 11:34
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I don't think they water cool turbo's on diesels because they run at a much lower temperature. The exhaust temperature is lower. I've never understood the EGR principle of diverting exhaust back into the engine to cool the combustion chambers because diesel runs cooler anyway
You're right about the turbo cooling on pertols, as petrol EGT can get the turbo up to near red heat if you're not carefull.
However thats not the reason why exhaust is diverted back to the cyls.
It is done to reduce emisions primarily NoX. In a lab it was found that bleeding an inert gas such as argon into the engine gave reduced emissions. Obviously we are not going to carry a tank of argon around that is expensive and would need refilling etc. So an alternative solution was to use exhaust gases which works reasonably well, and may even help in the respect that unburnt fuel in the exhaust gets a second chance to burn.
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Why are EGR valves so problematic?. Is it because the soot from the exhaust builds up in the narrowest part of the valve and eventually blocks it? Are they still failing now or are they improving?
If you continually work the engine hard when it's up to temperature will this reduce the likelihood of EGR problems?
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An EGR reduces Nox and unburnt hydrocarbons though is detrimental to CO and CO2.
A Cat reduces Nox to N and O, CO to CO2 and unburnt hydrocarbons to CO2 and H2O.
So why do have vehicles fitted with EGRs and Cats and use CO2 to measure emmisions?
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CO2 is not an atmospheric pollutant - there are EU directives that prescribe the maximum levels of NOx/HC/CO from engines - hence the need for Cats/EGRs.
CO2 is primarily an indication of fuel consumption, and has nothing to do with the 'cleanliness' of combustion (other than some devices that reduce NOx/HC/CO cause increased inefficiency and thus increased CO2 emissions).
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My '93 Landcruiser has a water cooled turbo.
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Why are EGR valves so problematic?. Is it because the soot from the exhaust builds up in the narrowest part of the valve and eventually blocks it? Are they still failing now or are they improving? If you continually work the engine hard when it's up to temperature will this reduce the likelihood of EGR problems?
One of the reasons for the build up is that diesels work at very high pressure and end up with high crankcase pressures. The gas from the crankcase is fed int the exhaust but it contains a lot of oil mist and this sooty oily gas deposits in the EGR. Newer engines have more sofisticated oil separators to much reduce this.
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Why are EGR valves so problematic?. Is it because the soot from the exhaust builds up in the narrowest part of the valve and eventually blocks it? Are they still failing now or are they improving? If you continually work the engine hard when it's up to temperature will this reduce the likelihood of EGR problems?
One of the reasons for the build up is that diesels work at very high pressure and end up with high crankcase pressures. The gas from the crankcase is fed int the exhaust but it contains a lot of oil mist and this sooty oily gas deposits in the EGR. Newer engines have more sofisticated oil separators to much reduce this.
Thanks Stanb. It's only been 7 years since I asked the question but it was worth the wait :-)
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Oops I did not even notice corax. It is Big Johns fault I blame him, he must be a bad person.LOL The wife says I live in the past anyway.
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Oops I did not even notice corax. It is Big Johns fault I blame him, he must be a bad person.LOL The wife says I live in the past anyway.
OK I'll have to change to "Big bad John" LOL
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