Nissan Micra 2000 1.4 SE+ - Long Term Fuel Trim

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Sun 17 Apr 2011 22:01
Nissan Micra 2000 1.4 SE+ - Long Term Fuel Trim - Futurism

Have had this car for a little while now and have an ongoing problem with lumpy idle and the long term fuel trim showing very rich. Car has done just shy of 51000 miles. It had a fault code of P0171 for a while and the front oxygen sensor was changed by a garage. After the fault came back, they decided it was the MAF in the Bosch throttle body and they replaced this. The fault kept recurring and they changed the MAF a further two times (one of them had a faulty air temperature sensor on the side). After the problem did not improve, I had another garage take a look at the car and it was discovered that the previous garage had fitted a universal 4-wire back oxygen sensor to the front! The fuel trim at this point was hanging between +20-23%.

Having had the front changed for a legitmate Bosch sensor (part number: 0258005263) and the wiring harness reconnected where it had been cut by the last garage the car seemed to be running a lot better and the fuel trim started reducing (they said it should freefall after a few hundred miles back down towards 0%) and wouldn't need anything else doing to it. It got down to 9% last week when I last checked so I thought it was indeed nearly fixed. However, tonight I noticed the idle was very rough when I pulled up after a 25 mile journey so I plugged in the ELM327 tool I'd been using to monitor the trims and it has gone back up to +17%!

I'm kind of out of ideas now unless the aftermarket MAF that was put in is faulty or the throttle body is on it's way out - note, this is the later Bosch system that began in Sep 2000 when this car was first bought (hence the odd connector for the oxygen sensor which was used for 4 months by Nissan). Any help or ideas appreciated please? :)

Edited by Futurism on 17/04/2011 at 22:02

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Sun 17 Apr 2011 23:25
Nissan Micra 2000 1.4 SE+ - Long Term Fuel Trim - unthrottled

The injectors are not metering the fuel as the ECU expects from it's base map. 'Trim' refers to the process of adding/subtracting injection duration over what the ECU predicts is necessary for the air flow. The prediction is cross checked with the lambda sensor feedback to fine tune the fuelling. I would suspect the fuel pressure regulator is malfunctioning. The pressure regulator maintains the pressure in the fuel rail at a constant value above manifold pressure which will vary from ~0.2-1.0bar absolute. The other option (unlikely) is that some or all of your injectors have deposits on the nozzles restricting fuel flow rate.

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Mon 18 Apr 2011 16:36
Nissan Micra 2000 1.4 SE+ - Long Term Fuel Trim - Falkirk Bairn

Put some BP Ultimate, Shell V-power and give it a Italian Tune-up over 10miles or so.

" The car might be constipated and needing a blow through"

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Mon 18 Apr 2011 16:47
Nissan Micra 2000 1.4 SE+ - Long Term Fuel Trim - Futurism

Aah, fuel pressure and injectors is one thing I've not thought of. By 'Italian Tune-Up' are you referring to sustained high revs as I've tried the whole 3rd/4th gear on the motorway before and it never did much. Should have added to my original post that STFT is consistently going to +10% which is probably what is pushing up the LTFT over time?

MPG return are not what I'd call abysmal, but they're not the best - mid 30s as roughly as can be guaged from brim to brim with mixed driving the same routes the last few weeks.

Edited by Futurism on 18/04/2011 at 16:49

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Mon 18 Apr 2011 17:15
Nissan Micra 2000 1.4 SE+ - Long Term Fuel Trim - unthrottled

'Put some high octane fuel in an engine that doesn't require it and give it a good thrashing'

What a useless suggestion.

My car has lived on a diet of the cheapest supermarket swill for 15 years and the injectors haven't given any problems.

OP: Why are you measuring fuel trim? The important question is whether the air to fuel ratio is correct for all cylinders and fuel trim data isn't going to provide that. I'm not a gasoline injection expert by any means but I'd expect the fuel trim to be quite small. I'd expect the lambda sensor to only have the 'authority' to change the fuelling by +/-10%, to prevent a faulty lambda sensor from dictating a nonsense fuel curve that might damage the engine.

The lumpy idle is probably due to a lean mixture at idle. Does the exhaust spit or pop slightly at irregular intervals. Lean mixturs tend to cause this. Rich mixtures tend to give a softer exhaust note than lean ones. How does the engine cope with transients? Does it stumble when you snap the throttle open?

Have a look at your spark plugs. Are they all the same colour, preferably light brown? If one of them is a different colour, then the problem probably lies wit hthat injector rather than the rail pressure.

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Mon 18 Apr 2011 17:35
Nissan Micra 2000 1.4 SE+ - Long Term Fuel Trim - Futurism

I have been measuring fuel trim as that was what was causing the DTC P0171 to trip in the first place by going over +25%. The exhaust doesn't spit or pop that I have noticed, the revs dip down to about 500 at idle once warm and the steering wheel starts vibrating noticeably. Sometimes the idle hunts between 500 and 750 for a few moments before settling at the lower value. The spark plugs were changed a few months back for Bosch FR8DCX as the Denso/NGKs have been known to cause problems with the coil pack engines. They were quite dark when replaced, nearly black, I will have to take another look at these ones.

The engine seems to respond well, it is quite willing to rev and accelerate, even if you put your foot right down through a gear. There was a slight hesitation before the oxygen sensor was changed recently but this seems to have gone.

In regards fuel, I have been only filling with Shell regular petrol.

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Mon 18 Apr 2011 19:51
Nissan Micra 2000 1.4 SE+ - Long Term Fuel Trim - unthrottled

Quiet exhaust, black plugs, good throttle response-sounds like you're well on the rich side! If it is rich, I don't understand why the ECU is adding more fuel...

It's worthwhile noting, that the ECU doesn't 'know' what each cylinder is doing-the ECU assumes that each injector will flow fuel at the same rate and will set tyhe same pulse width for every cylinder. Due to manufacturing tolerences this is not exactly true, and the ECU will use the lambda sensor to fine tune the fuelling.

If the lambda sensor keeps 'seeing oxygen' in the exhaust, from say one lean cylinder, the ECU will keep adding fuel to ALL the cylinders until the oxygen in the exhaust goes away-this would result in the lean cylinder becoming stoichiometric-and all the other cylinders becomming rich. This is why I asked if the plugs were all the same colour.

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