CrashTest

I'm really frustrated with this issues, I can't seem to diagnose what is going wrong. I had an issue where my 98 2.5 wouldn't hold an idle when hot. It was waver up and down and eventually stall. Found the ICV was cracked so I replaced it with a new gasket. The idle problem was fixed.

Now the car is having intermittent issues with stumbling or hesitation. I thought it was old gas, but have run two tanks through since then as well as injector cleaner. It tends to happen right after acceleration at nearly open/wide open throttle above 5500rpm. The car will pull fine, but after I let off, will stumble for a bit. If you short-shift for a bit it will calm down and drive normally. If I drive always under 4000rpm I never see any issues. I can commute fine but am always worried it'll start acting up. The idle is right about 600-700rpm, but is a little unsteady. I don't remember it moving around at all before all these issues.

I cleaned the MAF (looked really clean already), cleaned the TB and ICV passages. I tried to check crankcase vacuum in case it's the AOS, haven't been able to adapt my vac. gauge to the dipstick but if I open the oil filler, the idle rises and gets rough. There is def. some vacuum but I can easily lift the cap on and off. Audible hissing when open, but the car doesn't stall.

I check codes (no CEL) and there's nothing stored or pending. Pulled some data at hot idle after driving home:

RPM between 780-815
MAF between 3.8 and 4.3 (I believe g/s, meaning 13.7-15.5 kg/hr)
Short trim 1 between -3.0 and 3.9, mostly -3 to -0.8
long trim 1 is 3.1
short trim 2 between -2.3 and 2.3
long trim 2 is 0.8


Any ideas? I've got a fuel filter to install, mainly because it's got 113k and might have never been changed.

Macster

If the fuel filter is original I think I'd start with that. The symptom (sort of) reads like a fuel supply problem.

The engine's behavior when the filler tube cap is removed is normal. Removing the cap represents a big air leak and the engine will run a bit rough but the DME should adapt to the point the engine's running pretty good but probably not as good as it would with the cap in place. Of course when the cap is installed then the engine can react again and then the DME has to unlearn what it learned after the cap was removed.

I'm always a bit concerned when I see short and long term fuel trim differences between banks. However, I'm not sure what's normal that is how much difference there can be and not be of any real concern.

One thought is as long as there is no error code that the two can differ up to that point but still it just feels wrong to me to see much difference between the two banks.

Based on the long term fuel trims it does look like bank #1 requires more fuel than bank #2.

However, with the Bank #1 short term fuel trims mostly in the -3.0 to -0.8 range this looks like the DME is moving towards less fuel for that bank.

So, even though the oil filler tube cap removal "test" did not find the cap stuck to the filler tube due to excessive low pressure I'm going to have to go with AOS.

Now this does not mean that you should run right out and replace the AOS. There's that fuel filter I mentioned.

However, a fuel filter is a common failure to both banks and that both banks differ in their fueling suggests the problem is not a common failure.

Now you might think the AOS is a common failure too, but based on the connection of the AOS to the intake manifold it can affect one bank more than another at least at first. Once the failure progresses then its negative effect more fully involves both banks.

Since you have the fuel filter go ahead and install it.

You might have to let the AOS mature or progress further to the point you can make the call. At 113K miles if the AOS is original it is due to fail, past due.

I'm not a big fan of replacing parts without some firm diagnosis the parts in question need replacing, but with the AOS and its age and miles, with the behavior of the engine I would not consider it at all premature to replace the AOS.

'course it is your call. And you can always run this by a trusted tech. He would of course be at the car and can check for other possible explanations.

CrashTest

Thanks. I'll swap the fuel filter this weekend.

Another thought is I have taken the AOS hose on and off a few times to swap the ICV and clean the TB. Those o-ring seals don't always snap on very well. I might clean and check those to make sure it's not a vacuum leak there.

-David

CrashTest

Replaced the fuel filter but idle is still a little high and uneven. Not hesitation, but that was also sporadic.

Should I clear the DME with a battery disconnect to re-set it after the filter change?

I'm also going to bypass the AOS temporarily with a plug near the TB and see if that changes idle behavior.

Macster

It would have been better to clear the DME before the retest but 1) I do not like the battery disconnect technique as this I think presents too much of a risk to zapping the car's electrical system and 2) I don't think in this case it would have made any difference.

If you are sure you can bypass/eliminate a possible bad AOS from the equation to see if then the engine's idle settles down, and you are sure this bypass action does not present any danger to the engine, I guess that is a good next step.

You have to be sure the bypass is done right, and what you do makes sense, so if the behavior is still present you know that it is *not* the AOS.

An incorrectly performed test, or an invalid test, sets you on a course that is based on knowing something that is not correct.

Allow me to get a little Zen-ish: It is not what you don't know that hurts you, it is what you know that is not so that hurts you.

GBoxS

@CrashTest

I know this is a shot in the dark given the time that has past, but did you ever figure this out?

I have a 01 S that experiencing the same.

Byprodriver

2000 & newer T/B are not as finicky, Get some brake cleaner spray & clean it in place 1st.
Thanks for searching!

GBoxS

Okay will do. Thank you!