Nissan Micra 1.4SE Sep 2000 - Excess mpg - Air Temp reads constant 42 C

Honest John Back Room » Technical matters
What is life like with your car? Let us know and win £500 in John Lewis vouchers | No thanks
View Threaded
Sun 23 Jan 2011 15:31
Nissan Micra 1.4SE Sep 2000 - Excess mpg - Air Temp reads constant 42 C - Paul_L

Hi - the car is using way too much fuel - about 20 mpg. Supposedly recent MOT (bought the car from a dealer - MOT is one month ago.)

I have a simple diagnostic tool (ELM327) to read the diagnostics. There is a warning that the cat converter is out of limits but I doubt that has anything to do with it. Other than that everything looks normal except the air temperature sensor is reading a constant 42 deg C when the outside temperature is more like 5 or 6 deg C. Could this be an issue? It does not seem to have caused the ECU to log it as a fault.

Reply | Report as offensive | Link
Sun 23 Jan 2011 15:54
Nissan Micra 1.4SE Sep 2000 - Excess mpg - Air Temp reads constant 42 C - Peter D

What does the temp gauge read in the dash. The engine believe it is cold and is overfuelling that's why the cat limits are out. Regards Peter

Reply | Report as offensive | Link
Sun 23 Jan 2011 15:55
Nissan Micra 1.4SE Sep 2000 - Excess mpg - Air Temp reads constant 42 C - Peter D

I think I mis-read the post. The 'air' temp reads 42 are you sure that is not 42F not 42C. Regards Peter

Reply | Report as offensive | Link
Sun 23 Jan 2011 16:36
Nissan Micra 1.4SE Sep 2000 - Excess mpg - Air Temp reads constant 42 C - Paul_L

It is 42 deg C. The temp dial on the dashboard looks normal. Its a little a bit above half way up.

Reply | Report as offensive | Link
Sun 23 Jan 2011 21:51
Nissan Micra 1.4SE Sep 2000 - Excess mpg - Air Temp reads constant 42 C - unthrottled
Best bet is to ask a garage to run an emissions test. Look at the CO reading. If it's more than about 1% from a warm engine, then it's running rich. Efficiency falls off pretty quickly as the AFR gets leaner than stoichiometric, since the fuel is chucked out of the exhaust as carbon monoxide-effectively wasted petrol.
Reply | Report as offensive | Link
Mon 24 Jan 2011 14:05
Nissan Micra 1.4SE Sep 2000 - Excess mpg - Air Temp reads constant 42 C - Paul_L

Good idea. I have an old Gunsons CO meter at the back of my garage somewhere. I'll give it a whirl.

Reply | Report as offensive | Link
Mon 24 Jan 2011 20:04
Nissan Micra 1.4SE Sep 2000 - Excess mpg - Air Temp reads constant 42 C - unthrottled
The air intake temperature reading is interesting. The effect it would have on economy isn't predictable. Hot air is les dense than cold air, so if the ECU thinks the air is hotter than it really is, it will underfuel the engine, which will tend to increase economy. However, at 42C, the ECU might be worried about knock and retard the spark timing. This will tend to decrease economy.
Usually the O2 sensor in th exhaust will detect if the mixture is wrong, and the ECU will adjust the fuelling accordingly. However, the O2 sensor only has limited authority, if a small correction to the fuelling doesn't give the right O2 readings, the ECU will assume the O2 sensor to be faulty, trigger an emissions fault, and ignore the output.

Unfortunately, A CO reading on its own isn't a great deal of use. The problem is that the exhaust gas reading is an average of all the cylinders. Say for example, 2 cylinders were 10% rich, and 2 were 10% lean. The catalytic converter could then sort out the emissions to give you a near 'normal' exhaust composition. But the lost work from the rich cylinders isn't regained by the lean ones. This all sounds very complicated, but it isn't that bad. Pull the plugs and have a look at the electrodes. They should all have a very similar colour. If some are whiteish and others dark with soot deposits, then you have maldistribution of fuel/air between cylinders. If not, then we look elsewhere!
Reply | Report as offensive | Link
Tue 25 Jan 2011 09:54
Nissan Micra 1.4SE Sep 2000 - Excess mpg - Air Temp reads constant 42 C - Paul_L

Just for information the emissions results from December 7th MOT are:

Fast idle

CO 0.031 % vol (Max limit 0.2)

HC 2 ppm vol (Max limit 200)

Lambda 1.008 (Min limit 0.970, max limit 1.030)

Natural idle:

CO 0.077 %vol (Max limit 0.300)

Reply | Report as offensive | Link
Tue 25 Jan 2011 21:18
Nissan Micra 1.4SE Sep 2000 - Excess mpg - Air Temp reads constant 42 C - unthrottled
Nothing appears wrong there at all. Of course this is no guarentee that idle reflects loaded operation, but those results are really very good. Have you checked silly things like sticking brakes or handbrakes? Any unusual drivetrain noise? Are the emissions results from before or after the mileage nosedived?
Reply | Report as offensive | Link
Thu 27 Jan 2011 00:12
Nissan Micra 1.4SE Sep 2000 - Excess mpg - Air Temp reads constant 42 C - unthrottled
Just had a thought. Have you checked for fuel leaks? either in the tank, or the fuel line, or a badly fitting filler cap. This would explain horrendous consumption, but with no running difficulties. Try running the engine with the car in the garage, and walk around with your hydrocarbon sensor (nose!) and see if you can smell anything.
Reply | Report as offensive | Link
Thu 27 Jan 2011 19:05
Nissan Micra 1.4SE Sep 2000 - Excess mpg - Air Temp reads constant 42 C - Paul_L

A bit of egg on my face here. Confession - I must have misread the 42 deg C being constant. Apologies to the person who responded early on and suggested this to me. However - after being a lot more careful over what I was doing this time - the long term fuel trim reads +16.4% and is obviously wrong. I cannot see any reason for this. The only way to reset the value seems to be to pay a Nissan dealer a rip off £90 for 5 minutes work. Any other ideas?

Reply | Report as offensive | Link
Thu 27 Jan 2011 19:06
Nissan Micra 1.4SE Sep 2000 - Excess mpg - Air Temp reads constant 42 C - Paul_L

There is a bit of smell of petrol but more what I would expect from a rcih mixture.

Reply | Report as offensive | Link
Thu 27 Jan 2011 20:12
Nissan Micra 1.4SE Sep 2000 - Excess mpg - Air Temp reads constant 42 C - unthrottled
With fuel injection and catalytic converters a rich mixture doesn't necessarily smell of fuel. Unburned fuel is a sign of poor mixture preparation rather than the actual equivalence ratio. Rich mixtures waste fuel via incomplete oxidation to carbon monoxide-which is odourless. Have you looked at the plugs for signs of soot? I still think you've got a fuel leak somewhere. It can be a swine to find because petrol will evaporate and not leave puddles. I'd jack it up, run the engine to get the fuel flowing through the lines and crawl underneath armed with a sensitive nose.

The thing is, a 10% rich mixture will only lead to about an 8% drop in economy. To really hit your economy, the mixture would have to be seriously rich-to the point where it would start causing driveability problems. What does your CO meter give you? A 10% rich equivalence ratio gives about 2.5% CO.

If you don't trust the readings, try the 'extra fuel test'. With the engine running, spray some propane (cigarette lighter refill) into the intake and see how the engine initially responds. If revs momentarily rise, you have a lean mixture, if the revs fall slightly, the mxture is about stoichiometric, if the engine stumbles, the mixture is rich. I don't think this is the problem though.
Reply | Report as offensive | Link
Fri 28 Jan 2011 01:33
Nissan Micra 1.4SE Sep 2000 - Excess mpg - Air Temp reads constant 42 C - rrvfireblade

and walk around with your hydrocarbon sensor (nose!) and see if you can smell anything...........well ive heard the nose called many things in my time UNTHROTTLED but never a hydrocarbon sensor lol,,that was a cracker mate,tickled me that did..

Reply | Report as offensive | Link
Sat 29 Jan 2011 14:01
Nissan Micra 1.4SE Sep 2000 - Excess mpg - Air Temp reads constant 42 C - Paul_L

Oh no. Please don't PUNish me any more! (Shows the state I am getting in to.)

Any way. Did the CO test this morning with my ageing Gunson. To all intents and purposes I would say the CO was zero or crtainly very low. the Gunson is not the most accurate instrument but there is no doubt the reading drops loads as soon as the sensor is placed in the exhaust pipe.

There is no real smell of petrol - certainly not enough for a leak. I have my daughter doing a carefully controlled MPG check now but it will be a few days before I get the result of that. I am going to try to reset the ECU by disconnecting the battery for a few hours and then shorting the terminals (not the battery!) together for 15 mins. I am not sure if this works in the late 2000 on Micra. Does anybody know?

Reply | Report as offensive | Link
Sat 29 Jan 2011 14:46
Nissan Micra 1.4SE Sep 2000 - Excess mpg - Air Temp reads constant 42 C - unthrottled

It's good enough-it's not an MOT test. Like I said, a10% rich mixture would give about 8% drop in economy at most-and the CO reading of ~3% or 30,000ppm. I'm a really stumped now. If it's not leaking/evaporating, and it's not being thrown out of the exhaust, where is it going?? Misfiring would give driveability problems, incorrect spark timing (very unlikely) would also give driveability problems. A significant loss of compression would give lumpy idling and noticeable oil consumption.

Getting a definitive fuel consumption figure is hard-and slow. Trying to use the petrol gauge is a waste of time-you'll get a wildly different figure every time.

Reply | Report as offensive | Link
Sat 29 Jan 2011 15:43
Nissan Micra 1.4SE Sep 2000 - Excess mpg - Air Temp reads constant 42 C - Paul_L

Yipee! Managed to rest the long term fuel trim! This is exactly what I did but is much along the lines suggested in various sources on the Internet.

Disconnect battery negative lead. Connected a 4.7k 1/2W resistor between the battery positive and the disconnected battery negative lead for 5 minutes. This was to ensure a controlled discharge of any capacitors. Checked the voltage across the resistor which was then less than 10mv and decreasing. I then tied the disconnected battery negative lead on to the positive lead and left it like that for 15 minutes. I then freed the battery negative lead and checked the voltage between it and the positive lead which was then 2.5mv and not changing. I reconnected the battery negative lead to the battery terminal and checked the long term fuel trim again and it was zero!

The short term fuel trim goes plus and minus around zero when the engine is warm so I am hopeful the long term fuel trim will stabilise at a low number. Time will tell. We will restart the mpg test today and I will report back in a few days.

Thanks for the comments re CO numbers. As I reported early on we have not had the car long. I have quizzed my daughter on her driving technique and I know that for the first two days she was driving in sport mode and I know that is disastrous for fuel consumption. The tyre pressures were a bit low and are now correct which one person on the Internet reported had a big effect on fuel consumption although I have never noticed this on my own cars. I can believe the remaining problem is down to the fuel trim but I have no idea why it was such a high value in the first place.

Reply | Report as offensive | Link
Sat 29 Jan 2011 17:11
Nissan Micra 1.4SE Sep 2000 - Excess mpg - Air Temp reads constant 42 C - unthrottled
Moderately low tyre pressures will reduce fuel consumption by 1 or 2 per cent per tyre-it's not that significant, that's why you haven't noticed it in the past. Hypermilers often run 40+ psi in a bid for a few extra mpg. I tried it-the ride was awful!

Have you been basing this high consumption inquiry just upon the digital read out on the dash, and not cross checking with actual brim to brim calculations...??
Reply | Report as offensive | Link

10 warranty myths exposed

Here’s how to sort the fact from the fiction when it comes to car warranties.

Cheap Car Warranties