Can't hold rpm steady at 1500 to 2000 rpm - is this normal?

[Oxytorch]

Hmmm

In my 06 GS, with the car stationary and in N, I've noticed that I can't hold rpm steady at 1500 to 2000 rpm or thereabouts. Periodically, every say 5 seconds, rpm will drop by as 200 to 500 rpm for a few seconds and then it bounces back up again. The higher the rpm, the steadier it gets. Over about 2500 rpm it's pretty stable. There is no obvious rpm fluctuation at idle, although I can often feel a mild brief roughness occurring periodically.

Just wondering if anyone else has noticed this. Maybe it's just an effect of O2 sensors feeding back and adjusting fuel trims?

Thanks.
John

 

[Tubber273]

I had this same problem, except mine threw CEL codes P0300,301,302,303,305 (misfires), and a few others on top of that!
It rarely happens now after being fixed and sometimes a battery reset will fix it up.
FYI...the fix was over 1.5 years and included new O2 sensors, new cats, new coils, and a new ECU. (yes a small fortune)
I've even driven the codes away.

Best advice - try a reset.
Keep an eye on the CEL and record any codes if they come up.

 

[allandwf]

It's not air con kicking in?

 

[conaero]

Suggests air getting in somewhere, check for split pipe to the plenum after the MAF.

 

[voicey]

Does it happen when cold or hot or both?

What are the fuel trims for both banks?

 

[Oxytorch]

Hey voicey,
I've been a little too busy to check forums lately.
I've only checked for this when warm but I will check when cold.
Long term fuel trims are -16% bank 1 and -18% bank 2 indicating a bit rich both banks, so probably not unmetered air, which should lead to a lean situation.
I have not been able to find a cause for the rich trim figures, which have pretty much stayed the same since I bought the car 2 years ago. I have tried new MAFs (one was a dud that made the car really run like ****), replaced both forward wide band O2 sensors, and had injectors removed, cleaned and tested.
I experience very occasional brief hesitations when driving under low load conditions. It also tends to have a few hesitations straight after fuel fill up. I have bought a fuel rail pressure tester but haven't got around to use it yet. I need to have the leaking radiator replaced soon, so I will get a new coolant temp sensor fitted at the same time. When driving off cold, the car is always a bit lurchy for the first 200 - 300 metres, which I guess is just related to oil distribution - I'm not in the habit of letting it idle for any time before driving off.
No CEL codes, not even pending CELs.

 

[Oxytorch]

Wow. I didn't realise C RAP would get asterisked out! I will soap my mouth immediately!

 

[Contigo]

Not like an Ozzy to swear!

 

[voicey]

Those trims are huge - when they hit 20% (either way) you'll be getting a CEL. The trims are pretty much the same on both sides so it must be something that influences both banks. I would want to scope the MAF signal as my first port of call. At idle it should be a lovely clean and smooth signal. When you rev the engine it should also give a clean trace on the scope.

Have you checked the operation of the secondary air injection system?

 

[stefanocirillo]

Apologies if this sounds rude or patronizing (it isn't supposed to be). But why does it matter if you cant hold constant rpm in neutral? These cars don't like running from cold, they will hesitate and lurch on the first few pulls but once up to temp it runs fine. I find if i let it idle for a minute or two before pulling away on cold it remedies everything. If this is the case with yours, maybe just ignore? I think we all get overly concerned with our cars looking for faults because they italian and highly tuned. Case studies on the forum of worst case scenarios certainly don't help. Regarding running rich, i thought all GS/4200s did this to factor for differing fuel qualities etc. The amount of glorious bangs from the exhausts suggests a lot of unburnt fuel being dumped. I for one enjoy this characteristic

I could be talking rubbish but just my 2 cents.

 

[voicey]

Apologies if this sounds rude or patronizing (it isn't supposed to be). But why does it matter if you cant hold constant rpm in neutral? These cars don't like running from cold, they will hesitate and lurch on the first few pulls but once up to temp it runs fine. I find if i let it idle for a minute or two before pulling away on cold it remedies everything. If this is the case with yours, maybe just ignore?

In my opinion, it matters because a normally operating engine would be able to hold a steady rpm in neutral. Coupled with the excessive fuel trims, it would say to me that there is an issue. Everyone is free to ignore my advice (and I'm sure many do) but if it were my car or one I looked after then I would want it running properly.

Regarding running rich, i thought all GS/4200s did this to factor for differing fuel qualities etc. The amount of glorious bangs from the exhausts suggests a lot of unburnt fuel being dumped. I for one enjoy this characteristic

Modern cars, including Maseratis, try to run with as clean a combustion as possible. They do this by sampling the exhaust gases and use that info to tweak the length of the fuel injector pulse. These are the fuel trims - the car in this example wont be running rich (or lean) but the ECU is having to reduce the time the injector is open by 16-18% in order to get it to run at the correct AFR. Without this correction the car would run very rich and you could say goodbye to the cats.

If whatever is causing the "rich" condition gets worse then the ECU will reach the limit of how much it can correct the fuelling - the least of the problems at this stage is a CEL. The worst is that it will really start to run rich (as there is no more correction available).

There's nothing to suggest stop using the car at present but I would want to know what underlying issue there is, in case it gets worse and leads to expensive problems.

 

[Oxytorch]

Thank you for your input voicey.
I don't have an oscilloscope but I am able to log data with my scan tool.
I have two recorded two graphs.
The 1st shows rpm, absolute throttle position and MAF, starting at idle and then trying to hold rpm at 1500. You can see some wonder at idle but worse fluctuations when a little throttle applied. There's a bit of fluctuation in both MAF and absolute throttle position (accelerator pedal held completely steady), but not sure if this degree of fluctuation is abnormal or not.
2nd graph shows rpm, MAF and STFT bank 1. At idle, rpm wondering a little, fairly steady MAF, but occasional dips in STFT approaching -10%. This time when trying to hold 1500 rpm, much wider swings in rpm and very erratic STFT but fairly stable MAF. Pushing up to 2100 rpm, rpm becomes very steady accompanied by initial positive deflection in STFT (and much more linear STFT) then drifting toward 0 and again fairly stable MAF. At the end, I briefly revved to ~3400 rpm.

Your suggestion about the secondary air injection system could be on the right track. But how does one check for correct function or faults? Would it cause CEL codes?
Cheers.

 

[Oxytorch]

NB with respect to my descriptions, the graphs uploaded in the wrong order.

 

[Contigo]

Unplug your MAF sensor and go for a drive to rule out MAF failure.

 

[voicey]

Ideally you'd want to scope the MAF signal as the sample rate of the scan tool is low. However, the variation of airflow (kg/s) at idle and the STFT at idle would be enough for me to suspect the MAF. When was this last changed and for what unit (ie: did you get a genuine Bosch)?

Also, the chart of throttle position when you are trying to hold the rpm at 1,500 is all over the place. You state that you were actually holding the pedal steady in one place - so there is a problem with the throttle signal. Try observing the signal with the engine off for different pedal positions.

Unplug your MAF sensor and go for a drive to rule out MAF failure.

Good plan - when the MAF is unplugged the ECU resorts to a map based on throttle position. If the pedal position signal is a problem the issue will still be present.

 

[Oxytorch]

Ok. Unplugged MAF and here's how it looks:
Still get fluctuating rpm. There does appear to be a correlation between unsteady rpm and some fluctuation in absolute throttle position. When TPS is stable, rpm steadier and much less fluctuation of STFT. However, rpm still up and down toward the end when TPS seemed quite stable.


With MAF plugged back in, at idle rpm quite steady, STFT fairly tight although tending to slightly negative. With a little accelerator pedal, rpm up and down along with wobbly TPS and erratic STFT. This persisted with slight gradual depressing of the accelerator pedal as can be seen. Then at the end accelerated to about 2000 rpm - nice and smooth with smooth TPS reading and much tighter but again slightly negative STFT. With MAF plugged in, TPS signal seemed a little more unsteady.



So unplugging MAF doesn't solve the problem. To me there seems to be some correlation between wobble in TPS and fluctuating rpm but the even more striking correlation is with the erratic STFT, which jumps from abnormally rich to abnormally lean. I don't believe this should be happening with our wideband O2 sensors. Chicken or egg? Is the erratic STFT a sign of some air fuel problem or is it a false reading which is then causing rpm to bounce around due to it rapidly altering the injector pulse. Could front O2 sensors be to blame? If so, why only when rpm between 1000-2000? Faulty TPS? I dunno. When taken to the Maserati dealer, no faults found but no explanation provided either.

 

[Oxytorch]

Didn't mean to attach last graph, which shows only TPS and rpm with MAF unplugged. Initial fluctuation is just from trying to get rpm up. rpm steadied out toward the end with steady TPS.

 

[Contigo]

It's probably one of five things

1) MAF
2) Crank Position sensor
3) O2 sensors
4) Air Leak
5) Throttle Body

 

[voicey]

In your first chart (MAF unplugged) was the wandering TPS as result of you moving the accelerator pedal or was the pedal steady?

I still think the MAF could be bad and would want to scope the signal to be sure. When was it last replaced and for what type of unit?

 

[ecarca]

Did you sort this out? It looks a lot like my problem and I am still looking for a solution. I am quite persistent and I hope I'll be driving with my MAF plugged one of these days.
I think I will order the accelerator pedal potentiometer soon to try this out.