Hesitations after refuelling (& rich fuel trims) 2006 gransport

[Oxytorch]

I've posted on here before about the rich fuel trims (generally -15 to -20% using an ELM27 scan tool) and intermittent hesitations. No engine codes except for when I installed a replacement MAF sensor that was faulty. I have had the car in recently to the local Maserati dealership - using SD3 no problems found except for a fan resistor error code, so the resistor was replaced. Has anyone else used a scan tool to check their fuel trims because although its not bad enough to throw a code (needs to go over the +/- 25% threshold) I think it should ideally be +/- 10%. However, mechanics seem to ignore this clue unless it actually throws a error code.

Now, something I've noticed lately is that whenever I refuel, the hesitations occur reliably when driving away from the gas station. I feel a brief hesitation that lasts 1-2 seconds that repeats about every 5 seconds. However, after driving less than half a kilometre, the hesitations are gone. Googling this symptom, in other makes of cars, it seems to indicate a stuck open EGR valve. Is this something that could occur in the gransport? It could also explain the rich fuel trims maybe. I don't ink the charcoal canister has ever been changed but not sure what effect that could have.

Looking forward to getting your advice.

John

 

[hodroyd]

Not come across that one John, but somebody else might have..!!

 

[conaero]

Carbon canisters need replacing.

 

[Oxytorch]

Carbon canisters need replacing.

Ok. So how easy is that to do. Having just paid $3k for the recent service at the Maserati dealership, I don't want to spend too much more this year. For that sum, they did the sd3 test, supplied and replaced fan resistor, replaced gearbox/diff oil, replaced coolant and refitted the decats which apparently had a leak at the join.

 

[voicey]

I would be concerned about those fuel trims - ideally you want +/- 5%. You probably know this, but since they are negative numbers the ECU is shortening the injector pulse as the lambda is sensing too much fuel in the exhaust gas. I suspect that this will get worse and when the ECU hits the threshold of +/- 25% it will stop correcting and throw a CEL. At this point if the problem continues to get worse you'll have unburnt fuel in the exhaust which is not good for the cats.

I'm not familiar with how Maserati configure the 4.2 engine - do you have separate MAF for each bank?

What are the long term fuel trims on each bank - are they equal?

PS: You need a new mechanic if they are ignoring this issue!

 

[Oxytorch]

I would be concerned about those fuel trims - ideally you want +/- 5%. You probably know this, but since they are negative numbers the ECU is shortening the injector pulse as the lambda is sensing too much fuel in the exhaust gas. I suspect that this will get worse and when the ECU hits the threshold of +/- 25% it will stop correcting and throw a CEL. At this point if the problem continues to get worse you'll have unburnt fuel in the exhaust which is not good for the cats.

I'm not familiar with how Maserati configure the 4.2 engine - do you have separate MAF for each bank?

What are the long term fuel trims on each bank - are they equal?

PS: You need a new mechanic if they are ignoring this issue!

Thanks voicey. Both banks are showing similar long term fuel trims of -15 to -20%. I think bank 1 usually 1-2 % more negative than the other. There is only 1 MAF, which I have replaced with a new one. Also installed two new air fuel sensors and had the fuel injectors professionally cleaned and spray tested.

Unfortunately it's slim pickings when it comes to Maserati mechanics here in Brisbane. There is just one indie "automation" who I was dissatisfied with. A Ferrari indie Enzotech was suggested but seemed uninterested in my problem. That's why I went to the official Maserati stealership, which was the only option left.

 

[voicey]

Right, I should preface this post with the warning that my expertise lies with the Ferrari marque (I'm on this forum to learn about Maseratis).

I've been through my Maserati technical material and have ascertained the following (please correct me if I'm wrong):

1) There is only one ECU for the engine (doesn't matter in this case but it is different to Ferrari V8s which have two)
2) There is only one MAF (as you note and different to Ferrari)
3) There are separate fuel trims for each bank (as you note)
4) There is separate Lamdba monitoring for each bank

The ECU is reducing the fuel as it thinks the engine is running rich when sampling the exhaust gases. There are a few main causes of this:

a) Incorrect airflow measurement
b) Incorrect exhaust gas sampling
c) Leak allowing air into the exhaust (usually where the manifold meets the head)
d) Fuel pressure too low

Since your issue is on both banks to a similar degree I would guess the issue is upstream of the engine - this leaves me with a) and d).

I'd be looking at....

- The MAF (replaced - but I'd still scope the output)
- Inlet Air temperatures (check with your OBD scanner)
- Coolant temperature sender
- Fuel pressure

I'll give it some more thought and would appreciate it if someone more experienced with Maseratis would chime in....

 

[CatmanV2]

Hi Voicey. I was hoping you'd chip in

Wondering why a or d would make the exhaust seem rich? I'd have thought both those conditions would have made the engine run leaner?

As ever, just curious.
C

 

[voicey]

CORRECTION:

d) Fuel pressure too high

That's what you get for not paying proper attention (I'm watching a movie with the wife)! Obviously, if the fuel pressure is too high then there'll be too much injected into the cylinders for any given pulse width - the ECU would then trim back the time the injector is open for.

Even if the fuel pressure is normal, you may have leaking injectors which let fuel into the cylinders when they are supposed to be closed. However this would normally start manifesting itself on one or two injectors first so the chances are the LTFT would go wild on one bank only in this scenario.

With regard to the air flow measurement, a MAF can over or under estimate the amount of air passing it. In this case the MAF has been renewed but it is still worth putting an oscilloscope on the output to make sure the signal is what one would expect. Another important factor is that the ECU also takes into account the temperature of the air and the coolant when deciding how long to open the injectors for, hence my suggestions.

One final thought - how is the battery? The ECU also varies the injector pulse width according to the voltage detected. I keep on banging on about it, but a healthy and well maintained battery is the cornerstone to worry free Italian supercar motoring.

 

[CatmanV2]

CORRECTION:

d) Fuel pressure too high

Glad I'm not losing the total plot. So what about air leaking into the exhaust manifold? Wouldn't that make the ECU richen the mixture?

C

 

[chrisbinsb]

I don't know that this addresses all the issues you are having but regarding the valve: I had an issue with the car not starting strongly after refueling. Once the engine caught it would run weakly briefly before returning to normal. It turned out the valve was stuck open so we replaced it and the problem was eliminated. The valve was cheap and easily replaced - it's located just under the plastic engine shroud on the left side if the car.

 

[Oxytorch]

Hi Voicey
Thank you for your thoughts.
The coolent temp and air intake temp seem normal when checked with the scan tool.
If it's a problem beyond the air intake, the possibilities, as you state, are leaky injectors, high fuel pressure, or faulty air fuel sensor falsely sensing rich.
I do remember reading somewhere about the possibility of exhaust manifold leak (before the air fuel sensors) but not sure of the mechanism.
Because it's showing rich on both banks and at an almost identical level, it would mean that the problem would have to be affecting both sides. Therefore a both fuel pumps malfunctioning, leaking injector both banks, exhaust manifold leak both sides or faulty air fuel sensors both sides. This all seems unlikely. Nevertheless, I did have the fuel injectors removed, cleaned ultrasonically and bench tested, and replaced both air fuel sensors with new ones. None of this made any impact on the long term fuel trims.

Before then engine, the only possibility is a faulty MAF sensor but replacing it with a new one also had no effect on the long term fuel trims. That's why the hesitations after refuelling started to make me think about a problem with the evaporative system, which would affect both banks.

The comments about the battery are interesting. The battery is near new as a result of mysterious intermittent start problem that afflicted my car recently. http://www.sportsmaserati.com/showthread.php/14412-Gransport-starter-issue-where-are-all-the-ground-connections The cause of this remains a mystery to me but it was fixed by an auto electrician by wiring additional permanent ground connections to the starter motor and start button. At about this time, the alternator was also failing due to corroded main power cable connection. The alternator was refurbished and a new power cable installed. I guess there could still be a lingering electrical issue but everything electrical is working normally now, so it would be hard to find.

Mysterious.

J

 

[voicey]

Glad I'm not losing the total plot. So what about air leaking into the exhaust manifold? Wouldn't that make the ECU richen the mixture?

C

You are, of course, correct again. That'll teach me to post whilst drinking beer and being distracted.

Hi Voicey
Thank you for your thoughts.
The coolent temp and air intake temp seem normal when checked with the scan tool.
If it's a problem beyond the air intake, the possibilities, as you state, are leaky injectors, high fuel pressure, or faulty air fuel sensor falsely sensing rich.
I do remember reading somewhere about the possibility of exhaust manifold leak (before the air fuel sensors) but not sure of the mechanism.
Because it's showing rich on both banks and at an almost identical level, it would mean that the problem would have to be affecting both sides. Therefore a both fuel pumps malfunctioning, leaking injector both banks, exhaust manifold leak both sides or faulty air fuel sensors both sides. This all seems unlikely. Nevertheless, I did have the fuel injectors removed, cleaned ultrasonically and bench tested, and replaced both air fuel sensors with new ones. None of this made any impact on the long term fuel trims.

Before then engine, the only possibility is a faulty MAF sensor but replacing it with a new one also had no effect on the long term fuel trims. That's why the hesitations after refuelling started to make me think about a problem with the evaporative system, which would affect both banks.

The comments about the battery are interesting. The battery is near new as a result of mysterious intermittent start problem that afflicted my car recently. http://www.sportsmaserati.com/showthread.php/14412-Gransport-starter-issue-where-are-all-the-ground-connections The cause of this remains a mystery to me but it was fixed by an auto electrician by wiring additional permanent ground connections to the starter motor and start button. At about this time, the alternator was also failing due to corroded main power cable connection. The alternator was refurbished and a new power cable installed. I guess there could still be a lingering electrical issue but everything electrical is working normally now, so it would be hard to find.

Mysterious.

J

I concur with your analysis. There's not much else in the system.





On the evap side - since it recycles fuel vapour back into the inlet manifold it may be putting too much in. If the valve chrisbinsb reports stuck open for him is the evap valve then you may want to look at it.

 

[CatmanV2]

You are, of course, correct again. That'll teach me to post whilst drinking beer and being distracted.

I tend to have more success that way

C