Punto 1.2 injection-fuel trim diagnosis

[Lowtechguy]

Hi Guy,

Just to remind you I have a 1.2 fire engine in a punto mk2 using OBD2.

I had a slight problem with the mot last week being very over rich on the fuel at idle, I had suspected the icv and replaced this and everything now seems ok (according to my laptop water temp, lambdas etc all working fine).

My question is the Short term and long term fuel trims (what they mean?)...

With the dodgy idle control valve they were both set at +99% however since fitting the new one the;

short term is set at -100%
and
long term +99%

Both at idle, when you rev they both adapt as you would expect both getting nearer to 0%.

Do you think this is because 1 injector has a problem and when you rev the car this is less noticeable as the injector is working harder anyway/fuel drips are less a problem at higher revs?

Or

Do you think this is simply a case of the car adapting the modifications it has had since the last mapping session (new cam, 3.7kg flywheel and larger throttlebody).

I'm confused..

Many thanks for reading this in advance.

Cristian.

[Guy Croft]

When operating hot in closed loop the ecu reads the oxygen sensor and decides if the exhaust is correct.
If the fuelling is lean it will adjust the fuelling incrementally eg: by 10% at a time until it's right using the 'short term trim' function, and then reprogramme the fuel map/memory with a new setting called the 'long term trim' which will go up to 10% - the value tested & proved correct with short term trim, when that's done it will reset the % adjustment on the short term trim for that load position to zero.

If you raise the engine speed and they both go to zero, you can assume I guess that the exhaust is right and no adjustments are in process. You'd need to drive and examine the trim in motion to get a clearer idea of how the ecu is coping.

These figures - well, I'm driving a bit blind on this, they would tend to suggest that:

short term is set at -100% It's trying hard to lean out but I've never heard of such a high short term trim value, normally there are limits around, dunno 20% on short term trim.

long term +99% It's trying to add a huge amount of extra fuel to the ecu memory fuel presets.

That doesn't make sense to me at all. Did you enter those -100% +99% presets? Are they the maximum presets or actual running values at idle?
How are you reading that data? Laptop?

It takes a few seconds for the long term to stabilise and thus I would expect, on a hot engine with fully functional Lambda at steady idle that the short term maybe might show small % for a period and then drop to zero, with a commensurate increase in the long term value at that speed.

Any faulty part in the fuel or ignition system be it plug, lead, injector, sensor can upset the fuelling and the trim systems will try to compensate. However if the short term value is at a very high limit (ie: at max all the time) the ecu cannot cope with reprogramming at all, because it depends on some response available from the short term to test alterations.

I haven't ever actually programmed this, just trying to help here, so if anyone else who has can add or amend anything please do so. Unfortunately I don't either have an OE diagnostic manual for the Punto 1.2 MPI.

GC

[Lowtechguy]

Guy as always thank you so much.

I am plugged in via OBD and a laptop, the values I posted are correct according to the laptop at idle and as you say its so odd I couldnt get my head around it. when I rev the car the values are changing up and down but are more central to zero (they are never exactly zero).

I took the car for a drive (the first time in 5 months) last night so tonight I will re-evaluate the data, I am hopeing after reading your post that the short term would shift the long term values, if not I will get a friend to drive while I read the laptop again.

(Water temp, lambda, ignition advance timing and idle speed were all fine, hence my thinking about the injectors (batch injection))

Many thanks,

Cristian.

[NickRP]

Christian,

are you SURE your long term correction limits are programmed right?

Check with the company that did the mapping for you - maybe they were trying to overcome something by limiting long term adjustment to 99% (i.e. that it cannot go LOWER than 99%). Or they've simply made an error.

I think that the issue here is that for some reason your long term trim won't come down, and then the only thing the unit can do is to try to adjust by short time trim.

BTW, it is perfectly normal to have those values slightly different from 0.

Best regards,
Nikola

[Guy Croft]

Nik, hi

thanks for joining this thread and contributing your very expert advice.

GC

[NickRP]

It was my pleasure.

I forgot to add that long term correction limits can be different for different RPM ranges, which can explain why the problems were present only at idle.

Best regards,
Nikola

[Lowtechguy]

Blimey, Thats interesting...

I thought that the ecu was trying to overfuel a lean cylinder because of a poor injector or something.

That makes alot of sence now.

The mapping guy will have even more questions this time round.

Many thanks to both of you.

Cristian.