Bill Ball

A coupe of weeks ago my car developed a very strong stutter on acceleration. Since I have a wideband installed as well as the stock O2 sensor in my X-pipe, I noticed it went lean, maybe AFR of 16 or so at this point. Of course, it should go rich if anything when I accelerate, and I usually see 13s when hammered. The stutter is severe, more so than the AFR would seem to explain, but the AFR going lean on acceleration is new. I buy it as indicating a fuel issue, but I did check over the ignition system. The stutter is not always there, but once it develops it seems to get worse, eventually affecting partial throttle and then the idle as well. It is worse under low-RPM heavy load (not necessarily WOT) and sometimes seems to clear at higher RPM, but that is not consistent.

Here's what I know.

- Seems sometimes to be OK then appear after the car is completely warmed up.
- There are no stored error codes in the LH or EZK.
- The throttle switch shows proper function in the diagnostic test.
- Swapping in another LH did not change the behavior.
- Swapping another MAF was inclusive as the spare MAF I had seemed way off and the car ran more poorly with it even not accelerating. [EDIT: I just repeated this and gave the car a chance to adjust to the other MAF. Eventually I got the same stutter during a drive.]
- Fuel pressure is fine - I checked it - even though the pump is original. Fuel filter is not very old.
- I swapped the O2 sensor and that seemed to fix it, which I sure didn't expect. But 5 days later, it started doing the same thing again. And it seemed completely fine, accelerating like a beast for those 5 days, including a nice long drive during which I beat the crap out it as I am inclined to do. Not one problem during that time...until today.
- So, I swapped in new injectors since the current injectors are original and have never been serviced. After that it appeared things were good again, but after 10 minutes of driving, when I whomped on it, it started stuttering badly again. It's was not the injectors.

I think I ruled out the MAF. I am going to borrow a Sharktuner so I can log the MAF signal and other variables.

What else could I be missing?

PorKen

Could be ignition too?

Lean AFR can be caused by unburned fuel = too much oxygen.

Leon Speed

Sounds like ignition to me. Ground straps and/or coils. But then again I don't know much

Herman K

My MB had a stutter for a long time all hints/advise was directing me to loose connections (resolder) in the fuel relay.

After resolder and new relays the problem was still there until I found out that on of the female prongs in the holder on the MB (on the 928 this would be inside the CE panel) receiving the relay was bad and had burned it self 1/2 way of some times making contact and some times not.

Don't know if this helps but it crossed my mind that you could have some thing similar bugging you.

the flyin' scotsman

how old is the fuel pump? could it be failing under heavy load?

Bill Ball

The fuel pump is original. When the stuttering is bad it affects the idle and throttle response without a load. The fuel pressure was fine while it still stuttered under those conditions. Sure, I suppose it could still be the fuel pump. Other than replacing it presumptively, I would need to get a remote fuel pressure gauge to check this.

Bill Ball

OK, I guess I follow that. It sure feels like an ignition stutter. I checked over the coil wires, which at times had gotten corroded in the past. They are clean. Nothing is loose and the car seems to run fine at times. All the other parts of the system are fairly fresh except the coils are original. There's no ignition miss at other times and it cruises well. Not sure how to proceed with diagnosing the ignition system.

I just swapped the other MAF back in and gave the car a chance to adjust to it. Eventually it did the same stuttering with lean AFR. It also seems the car does not downshift properly during these episodes, as if the load signal is not right.

the flyin' scotsman

when my pump failed the car stopped like a light switch but re-started after being flat bedded home with no symptoms of anything being wrong including great pressure. It didnt make it half way down the 1/2 km driveway before it quit again.

Bill Ball

I will look at this.

The only other thing that I should note is that I REINSTALLED THE KEYLESS ENTRY, modifying the wiring to it a bit from what I had before, right before this problem started. I don't think I could have gotten anywhere near any of the ignition or LH wires, but who knows? It's always worthwhile to look at the last thing you worked on.

Bill Ball

I did notice that refueling seemed to relieve the stuttering once for a short while, so that's another possible indicator for the fuel pump perhaps struggling. Also, the other day, when the tank got to about 1/4th full I did hear a whirring noise that went away on refueling. It didn't stutter at any point during that day, but I have never heard any fuel pump noise before unless the tank was essentially bone dry, exposing the in-tank pump. The in-tank pump was replaced at 150K miles even though it was fine and the rubber was not split. I suppose it could be acting up as well, but I've got a lot of gas to drain in that case.

dprantl

Do hear a hiss when you remove the fuel cap? And how old is the TempII sensor?

Dan
'91 928GT S/C 475hp/460lb.ft

Bill Ball

Never noticed a hiss. I can check Temp II with Theo's diagnostic, but I would expect failure to go rich and if it grounded I would expect it just doesn't do anything. It can't lean the mixture beyond the usual hot readings AFAIK.

Lizard928

voltage to the fuel pump is dropping. As the RPMs increase the Alt generates more voltage helping to (partially) overcome the problem.

Although, I wouldnt rule out an ign problem too.

Bill Ball

I will fuss with and potentially replace the fuel pump. It is old and original, but so's just about everything else on my car. I may have access to a remote (electric) fuel pressure gauge tomorrow.

WICruiser

You may want to check the tank venting system. You can have a great pump but if the tank can not get air in you can not get fuel out. The tank vent system typically is tied into the intake system to provide a purge of the canister when the engine is running, off idle, that would create significant vacuum that would diminish with increasing load. An easy (not sure if this is really easy) would be to disconnect the tank vent line from the canister so the tank can breath and see if it makes any difference.