Bit of history:
Standard VR6, runs well, no miss fires or other issues. No faults logged on VAG-COM.
Last year the MOT tester said it only just got through the emissions test, he had to clean up the AFM connection and then he only just siad he got it through after lots of attempts (i'm not convinced the AFM connection was a problem).
I promptly forgot about this for this year, of course the MOT tester this time wasn't prepared to fiddle around and try lots of times to get it through.
So results:
[code]
Fast Idle
RPM: 2598 Pass
CO: 0.571 Fail (max 0.3)
HC: 39 Pass
Lambda: 0.988 Pass
2nd Fast Idle Test
RPM: 2616 Pass
CO: 0.504 Fail (max 0.3)
HC: 32 Pass
Lambda: 0.989 Pass
Natural Idle test
RPM: 704 Pass
CO: 0.429 Pass (max 0.5)
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I watched as he tested, and the CO fluctuated in and out of the PASS zone each time a test was run at constant RPM.
On VAG-COM the Lamba value fluctuates between 0.9 and 1.1 at constant RPM. The idle speed also fluctuates slightly, I thought this was normal.
First question - should Lambda flutuate at a constant RPM that much?
Second question - How do I fix it? :lol:
The MOT tester was ex-VW and seemed to think a change of oil and filter would make a difference, otherwise who knows because theres so much that will imapct it, so he wasn't much use (last oil change 2k miles ago).
Edited by Pugugly on 16/01/2009 at 18:24
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No guarantee, but a dose of VW's own injector cleaner fuel additive (part number G 001 700 03) may help, plus a good Italian tune-up several times before the test and once more on the way to the test. No CAT on the car, presumably? What fuel are you using? Shell V-Power or equivalent may help too, given its reputed cleaning properties.
I had a 1996 Golf VR6 Highline which always had emissions values so low that the testers often remarked on them. It was always fed Shell Optimax and the VW additive. There is one exception to that history: when my other half used it for too many short trips, which resulted in higher emissions.
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No CAT on the car presumably?
Yes theres a Cat. 119k mile original by the look of it and history.
What fuel are you using?
I try to use Shell VPower, but currently it's on a tank full of ASDA's convenient cheapest.
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Here's some information about why the output from a car's lambda sensor fluctuates, and why it's normal.
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Petrol engines give their maximum power when running slightly rich, and their maximum economy when running slightly weak when compared to stoichometric.
The value reported during MOT testing is the calculated lambda value, and this represents the excess air ratio. So, if lambda is greater than 1, the mixture is weak. In engine fuelling calculations, there's also a commonly used greek letter, phi, which denotes the excess fuel ratio. Lambda = 1 / phi
This value is not related to the output of the car's lambda sensor which is sometimes called an oxygen sensor. The car's lambda sensor cannot calculate a lmbda value, it switches near the stoichometric point, and simply tells the engine management system rich or weak. This means that the sensor is quite non-linear.
During warm up, the lambda sensor is too cold to switch, and the car relies on its open loop mapping (the open loop mapping is the starting point for all the fuelling calculations). As the exhaust heats up, the lambda sensor switches for the first time - this signal is detected by the engine management computer which then switches into closed loop mode.
When running in closed loop mode, the cat needs to be held close to stoichometric to allow both oxidation and reduction reactions to occur. As the lambda sensor is downstream of the combustion, the signal it reads is delayed. This delay, combined with the non-linear response of the sensor means that the control cannot be smooth, like idle speed control for example, but oscillates between rich and weak. This oscillation is about once per second at tickover, but, becomes faster at higher engine speed as the delay becomes shorter.
This non-linear response is also why the lambda sensor doesn't have full authority in closed loop mode. For example during a transient, like acceleration, the lambda sensor can only "say" weak, it can't give any indication about how weak the mixture is. So, there's no quick way for the ECU to reach the right fuelling point quickly using that approach. Instead, the open loop mappings plus stored adaptive corrections are used to get close to the operating point quickly, and then use the lambda sensor to prod the system back towards stoichometric.
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You should see the voltage output from the lambda sensor fluctuate smartly at about 1 cycle per second at tick over, speeding up when you rev the engine.
In your case the reported lambda value from the MOT machine is below 1 - the mixture is too rich.
In VAG Com, does it tell you the trim, block learn, or integrator readings for the closed loop fuelling? If so, are these also indicating a rich mixture?
If so, you need to go through all the possibilities tha might result in a rich mixture - for example, is the coolant temperature sensor correctly indicating the engine temperature (or has some muppet soldered in a resistor?), is the fuel pressure correct? is the vacuum connection to the fuel presure regulator correct? is the air flow meter reading correctly (if it's electrically OK, possibly only a substitution would reveal this conclusively).
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In your case the reported lambda value from the MOT machine is below 1 - the mixture is too rich.
The lambda in Vag-Com fluctuates between 0.9 and 1.1. Fuel economy has always been very good (early-mid 30's) for the driving I do so I never considered it to be rich. I take it the MOT reading is an average?
In VAG Com does it tell you the trim block learn or integrator readings for the closed loop fuelling? If so are these also indicating a rich mixture?
I think it might do. I will run the diagnostic again - not sure I could interpret those values!
Coolant temp reading mirrors the dash temp display so i assume correct as they are seperate sensors.
Not sure on fuel pressure, I may rig up a gauge. Vac to regulator is connected to inlet manifold.
AFM value changes in vag com with throttle, as you say only substitutuion would reveal if its the correct reading I suppose.
Thanke for the help!
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The thing to be careful of here - which I hinted at in my (rather long) post is that the output of the car's lambda sensor and the lambda reading of the MOT station are entirely different things;
The lambda sensor output simply reports whether the mixture is rich or weak relative to stoichometric - the lambda sensor doesn't provide any (meaningful) information about how far rich or weak the engine is operating.
The MOT lambda value is a calculated by comparing the relative amounts of the gases measured by the exhaust gas analyser - some of the averaging is done by the time delay caused by the gas moving through the cat (in fact this is one of the sizing considerations for a cat - to ensure the gas is in there for a sufficient period of time).
- not sure I could interpret those values!
I'm not a VW expert, but, these are usually given in the form of a number in a range. Zero to 255 with a nominal value of 128, i.e., somewhere in the middle, is typical. If you find a value that's way off centre, it's an indicator of a long term fuel mixture problem
If everything electrical checks out, the other remaining task is to check that the cat is actually working - this is best checked by reading the temperature rise across the cat.
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1st things 1st though?
What condition is the air filter in? replace it anyway?
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The lambda probe in the corrado (is the engine ABV?) is upstream of the catayltic converter. It could be that the system is working fine, hence no issues on vag-com, but the CO part of your 3 way catalyst is knackered hence no amount of tuning will make it pass the MOT. You will have to replace the cat
Edited by enda1 on 16/01/2009 at 15:18
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It may be the cat but I dont see that as automatic because of the HC readings.
Would expect them to be way high too (not just the co).
As it stands I would be guessing at air flow (filter) or sensor. But its just a guess at the moment!
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Not saying its automatically the cat but since it is not part of the closed loop fueling system, which includes the lambda probe, then the engine has no way of altering its mixture dependent on the output from the cat and hence may be operting properly. You could spend may hours and pounds trying to fix a problem in the engine which doesn't exist.
If your HC is passing it would suggest (though I don't know what previous results were) that the fueling is fine as you don't excess hydrocarbon (unburnt fuel) in the exhaust. This would suggest that the airflow/fuel ratio is fine.
A new air filter and cleaner for the injectors is fairly cheap and you colud try that and then get it retested but if that doesn't work then its either an electronic type problem or the cat.
You say that you just about passed last year but not this year. The 3 way cat covers unburnt HC, NOx, and CO. It may be that with age it is diminishing in effectiveness particularly with respect to CO
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You say that you just about passed last year but not this year.
I'll dig out the emissions printout if I have it from last year and see if it was just the CO that scraped through or not.
Yes the mechanic I used last year said it just scraped through last year. Said he had to hold it at high revs for ages to get it really hot then quickly do the test.
The tester this year just did it from idle etc and didnt try to get it hot.
IIRC CATs work better the hotter they are and engines are more efficient etc at optimum temperature.
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>>This would suggest that the airflow/fuel ratio is fine.
Not really. HCs stay low until a cylinder begins to miss or there's lots of oil being burnt - in pre-cat days, engines ran a bit smoother if you richened them up a bit, giving low HC figures.
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few possibilities, i would try a different fuel!.I have had fiats struggle on MOT's with supermarket fuel & go straight through with a tank of quality.
I would check to make sure you don't have a small vacum leak, this will make the rev's drift slightly up & down when holding steady & co a fraction too high as the air leak is being compensated. Are there any small leaks in the exhaust. A tiny leak at the manifold can cause this. The only time you may hear a leak is when cold & may tick under load.
HC's not bad but cat may be lazy.Oxy sensors may be getting lazy. have you looked at live data of sensors to see if they are switching quickly.
If the fuel trim was more than 20% out a long term fuel trim fault would of flagged as a fault code.
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30 mpg and better on a VR6. That is exceptionally good fora car like that. Did I read that the test was done cold? Hmm, just an idea but sounds like it is not thrashed. Maybe needs an oil change (nice clean oil with no fuel contaminant), a good thrashing to clear its throat, and insist the test is done hot. The figures do not look that bad and could be brought back with just a service and "Italian" tune up.
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I reseated all vacuum hoses on the inlet side, checked the exhaust over pre-cat, fitted new dizzy cap & rotor, new air filter, new oil and filter, half a tank of V power and a damn good thrashing of the engine must have had some effect as it passed this morning.
Not sure all that was necessary but GSF are doing free delivery for online orders and I had some bits on the shelf so did it all.
30 mpg and better on a VR6. That is exceptionally good fora car like that.
Thats the on board computer reading on premium fuel. On a long 70mph motorway run I can get it up to 35mpg for a trip. This may of course be wildly inaccurate but it checked out last time I kept track of miles per fill up. On supermarket fuel it drops a bit. On track days it takes a nose dive :)
Edited by was8v on 21/01/2009 at 10:16
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Over ten years of ownership from new, my Golf VR6 did just under 30mpg; that excludes the time when it was mostly driven by my then partner, whose short trips and other driving characteristics took it well under 25mpg. I often saw 33+ mpg on steady long trips. The best I ever saw on the trip meter was over 37mpg.
Edited by Roger Jones on 21/01/2009 at 11:22
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Congratulations on the pass. You must be pleased. I just wondered whether your fuel consumption indicated quite gentle driving - hence needing a good thrashing to clear it out. I also had a Golf VR6 for a while ( most unreliable car I have ever had) and although the fuel consumption was never as bad as I anticipated - 30+ MPG to me seemed very good. Maybe the oil change and good thrashing cleared it out and gave the cat more clear surface area to work. Unburned fuel in the oil does fume off and is burned when the engine is running, of course causing higher emissions. You'll know what to do next year.
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VR6: "most unreliable car I have ever had"
Sorry to see that. Mine? Totally reliable.
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Out of interest when was it registered in 1992? If it was before 1st August then it needs to undergo the less stringent pre-cat emissions test.
Regards
832
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I would say this has to be a sensor wear.
My Passat VR6 was wrecked by a leaking fuel pressure regulator which nobody could diagnose for 18 months (until I saw fuel spray from the perforated diaphragm.) However it still passed two MOTs whilst running roughly with a lake of fuel in the inlet manifold, because the Motronic system could always pull the emissions back within limits.
You will probably face the same difficulties I had = no fault code than nobody knows what is wrong. However what lead me to eventually find the diaphragm failure was a long flow chart to diagnose a non specific engine management/fuel system fault that I downloaded from the USA (where more such information has to be published).
This flow chart started with a full perssure and flow check on the fuel system and then worked through every possible sensor fault. It was very good but would take about two days to go through the lot and therfore no garage (or customer) was happy to follow it through.
You need to find a specialist who is prepared to follow such a methodical procedure and not jump to conclusions to be sure that you get to the fault without wasting money on hunches.
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as youve thrashed the living daylights out of the car and got the cat up to its full operating temprature then it will pass the mot test, seems to me you will have the same problem next year, would have been easier just to put a new cat on it. not cheeper though.
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