Poor running at low RPM diagnostic (with fuel trim graph)

[alex22077]

Hi All!

Have had a running issue with my 2001 Toyota Celica (2zz 1.8 engine) for the past few years and would really appreciate any help or advice.

On idle (750RPM) or low RPMs (around 1000RPM) my fuel trims drop to around -11 indicating a rich condition. However the engine seems to run rough with this adjustment and fairly easy to stall, no power. RPMs also seem unstable such as when disengaging the clutch, they drop fast then bounce back up to idle quickly. Seems more apparent when the engines at operating temperature.

As the RPM increases the trims go to around 0 and drives normal. I have changed most relevant sensors such as MAF, O2, plugs, VVTI oil filter and cleaned intake. No change.

Images below are from a typical short trip with throttle position, MAF and fuel trim reading. Hope someone can understand them. I suspect a leak on the exhaust side before the O2 and have found the exhaust system has not been support properly and wonder if a crack has been caused????


[url]



Thanks for looking =)

[E-bmw]

Your suggestions all are possibilities (can't see the attachment due to work servers, so will have another look later) if it is running rich the fuel trims are low to put less fuel in, as you obviously know.

Does that engine have a throttle control valve?

You mention the exhaust, but have you checked it for any leaks/cracks

[alex22077]

Thanks for your reply!

Its a driveby wire which is working fine. Idle air
control valve has also been cleaned and tested ok.

Couldn't see any exhaust cracks with my eyes bur found some missing manifold headshild which makes me wonder if I'm close as someone's removed it.

Just wondering if anyone could interpret the graph's as I'm by no means an expert.

[GreenV8S]

alex22077 said:

On idle (750RPM) or low RPMs (around 1000RPM) my fuel trims drop to around -11 indicating a rich condition.

Do you have the actual lambda readings? I see a couple of 'volts' lines but not sure whether they are trying to show lambda - they look pretty flat.

If you have a lot of exhaust gas reversal that could make it run rich + badly. Do you have a wild cam? Plenum or multi throttle bodies? To see whether it is anything like that you could try retarding the spark in the problematic region so that you have less depression. It will make it run hotter so maybe not something you would want as a solution but could give a clue about the nature of the problem.

ETA you mention of an IACV suggests it is a plenum setup in which case a wild cam could cause exhaust gas reversal and distribution problems.

Edited by GreenV8S on Monday 25th February 20:24

[melhookv12]

Might be worth putting some decent injector cleaner through it.

Could the fuel trims be showing a rich mixture due to poor injector spray patterns ?

[stevieturbo]

So you say it has DBW....and also an idle air valve ? That seems a tad unusual.

You say it has had an issue for a few years......so has it ever actually ran correctly ?

For some reason your pics arent working for me.

You say it's running rich....exactly how rich ? gas analyser ? Presumably it has been MOT'd at some point ? emissions test results ?

Some posters above are starting to talk about cams and other stuff.....you give no indication this is anything other than a totally standard engine ?

You say you've swapped various parts and no change....often a bad idea. Not directly related here perhaps...but Scanner Danner offers great teaching videos, and this very recent one deals with a garage who build in problems by changing parts for no reason.

https://www.youtube.com/watch?v=F2gi5hRV3kg

[alex22077]

GreenV8S said:

alex22077 said:

On idle (750RPM) or low RPMs (around 1000RPM) my fuel trims drop to around -11 indicating a rich condition.

Do you have the actual lambda readings? I see a couple of 'volts' lines but not sure whether they are trying to show lambda - they look pretty flat.

If you have a lot of exhaust gas reversal that could make it run rich + badly. Do you have a wild cam? Plenum or multi throttle bodies? To see whether it is anything like that you could try retarding the spark in the problematic region so that you have less depression. It will make it run hotter so maybe not something you would want as a solution but could give a clue about the nature of the problem.

ETA you mention of an IACV suggests it is a plenum setup in which case a wild cam could cause exhaust gas reversal and distribution problems.

Edited by GreenV8S on Monday 25th February 20:24

This was my first attempt at using a graphing application and forgot to scale up the lambo results. I will do another run with this software and try again with better sample rates to.



Regarding mods, this is a stock engine.

I am also not sure how I can retard the spark as this is a stock ECU and tunes itself.

[Dr Doofenshmirtz]

Let it run rough at idle for 5 minutes or so, then pull the spark plugs and check their colour.
Could be a dodgy coil pack? If they're all an even colour, then it's not that.

[buggalugs]

Dodgy coil pack would show lean mixture due to excess oxygen surely

Leaky injector maybe?

I bet it’s only one cylinder, looking at the plugs is a good shout

[alex22077]

melhookv12 said:

Might be worth putting some decent injector cleaner through it.

Could the fuel trims be showing a rich mixture due to poor injector spray patterns ?

Changed the injectors as once the engines warmed up it can sometimes be rough to start (sometimes an extra second cranking but noticeable) which is often sign of leaky injectors. No difference with new injectors.

Edited by alex22077 on Wednesday 27th February 07:57

Edited by alex22077 on Wednesday 27th February 07:58

[alex22077]

stevieturbo said:

So you say it has DBW....and also an idle air valve ? That seems a tad unusual.

You say it has had an issue for a few years......so has it ever actually ran correctly ?

For some reason your pics arent working for me.

You say it's running rich....exactly how rich ? gas analyser ? Presumably it has been MOT'd at some point ? emissions test results ?

Some posters above are starting to talk about cams and other stuff.....you give no indication this is anything other than a totally standard engine ?

You say you've swapped various parts and no change....often a bad idea. Not directly related here perhaps...but Scanner Danner offers great teaching videos, and this very recent one deals with a garage who build in problems by changing parts for no reason.

https://www.youtube.com/watch?v=F2gi5hRV3kg

They moved to electric throttle in later years.

Never idles or ran properly in low RPMs, bought the car with this issue as found the previous owner hasn't secured the intake filter properly so was running without filter for who knows how long... this lead to me clean all intake, throttle, IAC, MAF and then start changing sensors once they made no difference. Don't like changing parts either but thought it would be an easy fix due to dirt build up.

Compression test showed engine is 190PSI on all cylinders so dirt doesn't seem to of damaged anything.

[alex22077]

Got better reading from my daily driving. Enlarged the O2 reading in-case it makes sense to anyone:





Also a reading from some idling once warmed up a little:



Fuel trims always return to around -11 at idle to low rpms.

[alex22077]

Dr Doofenshmirtz said:

Let it run rough at idle for 5 minutes or so, then pull the spark plugs and check their colour.
Could be a dodgy coil pack? If they're all an even colour, then it's not that.

Worth a shot, will check them over the weekend. Spark plugs have been in over around 9K miles so worth looking at them all.

[buggalugs]

With you mentioning the intake not being secured or put together correctly, I wonder if there’s an issue with the evap system purging, or if it’s been bypassed or somehow messed with leading to it sucking fuel vapours all the time.

[alex22077]

buggalugs said:

With you mentioning the intake not being secured or put together correctly, I wonder if there’s an issue with the evap system purging, or if it’s been bypassed or somehow messed with leading to it sucking fuel vapours all the time.

Wonder if I disconnect the hose to the intake and block off where it connect (to stop vacuum leak) the fuel trims will stabilize.

Would leaving the EVP hose disconnected cause any problems while driving?

[buggalugs]

I wouldn't think so, seems like a quick thing to check and elimitate anyway.

I am the kind of saddo that watches videos of people fixing cars and I saw one rich at idle the other day. It was a VAG with a mecanically driven fuel pump and the seal had gone, dumping fuel into the oil, then it was running rich at idle via the PCV. I think that fuel pump arrangement is a VAG thing though not seen one of those on a toyota.