nc_growler

My '91 GT has developed a bit of a stutter under load.

Two weeks ago it was up at VIR doing parade laps (pretty hot ones actually) and did great. And then drove home with no issues what so ever.

Here's what I'm experiencing.

Heavy acceleration (not WOT) causes the car to miss or stutter.

But if I accelerate more evenly and with a lighter foot, the car will run to full throttle without missing and with apparent power.

But if at any point in the RPM range I "stuff it" the car stutters.

No dash indications.

It's been a really wet May/June here. But I have not had the car out in the rain and the car is garaged.

I just finished reading this thread on power/load, but I don't think it helped:

https://rennlist.com/forums/928-foru...power+throttle

Thoughts?

Fuel pressure? Electrics? Coil wires?

I believe it has been 3 years (7K miles since new plugs, rotors, caps). Pretty sure wires are original.

SteveG

How much fuel in tank? Fuel pump or injector? WOT switch.

Speedtoys

Whats the condition of the coils, and coil grounds?

Age of wires is a contender here too.

nc_growler

SteveG: Tank is nearly full. Fuel pump and filter replaced after failure on way back from SITM 2011. Injectors were all replaced 9K miles ago. WOT switch is an interesting question. I'll hook up my 928DIAG tool and test the switch sensor tomorrow.

Speedtoys: Yeah, these were my first thoughts too. I'm going to pull the coil wires and check for corrosion tomorrow as well.

It's strange to me that if I let it run up, the car runs up to 6,000RPM just fine... Maybe it's leaning out on fuel when I push...

Mrmerlin

does the car have an intank pump-?
Replace the ignition wires with the Beru set Roger sells,
its plug and play and comes with new ignition coil wires 4 of them,
for the different coil connections

Whitesands

Could be the King leads.

Hilton

This is my first question too. Disconnect the main pump, jump the fuel pump relay and go see if the in-tank pump is running (you need to disconnect the external pump so you can hear the in-tank one).

I'd then check the throttle switch is closing at "WOT" (around 2/3 throttle). This can be tested at the LH plug with a multimeter.

After that I'd suspect the MAF's voltage/response - how old is it?

nc_growler

Thanks for questions about the in-tank pump.

I replaced the intank pump with a strainer when I replaced the fuel pump and filter. Been there, done that

928DIAG tool has a WOT Switch test that I will use to verify and update thread with.

MAF is original (car has 76K miles). I may swap with a friend before replacing that.

MrMerlin - Yeah, I had been thinking about replacing those anyway... Had them on my "Roger List". Do you know if the original wires from Porsche actually had little tabs with the cylinder numbers on them? The current set little white clip-on's with the numbers printed on them.

Bill Ball

Yes, I had exactly this problem and found this.

nc_growler

Long story here.

Ran 928Diag tool and found no issues. No ignition failures detected. Ran out of ideas on what might be wrong.

Ended up transporting car to Brad at Digital Chassis in Durham, NC.

After looking for vacuum leaks and obvious issues, we began to suspect MAF/LH.

Sent the MAF and LH out for inspection (LH was fine. MAF was 40 millivolts out of spec). Had MAF rebuilt while I was away on vacation in Norway. Rich at Electronik Repair, Inc. did a great job turning that around for us but I had the car transported back home while we waited.

Got back from Norway to find MAF waiting. Installed new Hall Generator/Sensor as I was getting Hall Sensor failures. Turns out top mounting screw had fallen out and was probably causing the fault, but I had the sensor so I installed the new one as well as rebuilt the connector. Thanks to Neilh for stopping by and providing moral support.

Installed rebuilt MAF. Same issue. Car starts... Runs for 10-12 seconds, then chokes and dies. Same old problem!!!

Transported car back to Brad at Digital Chassis and he spent some additional time on diagnosis. Brad knows these cars. In an moment of inspiration he jumpered the Relay Ignition Circuit Monitor out and the car started running perfectly!

Talk about your unique failures. The monitor that supposed to tell you if there is an ignition issue fails and causes a very difficult diagnosis!

Luckily Brad has a used part handy and I'm rolling again! And several other things are now mysteriously fixed.

Used to be that car would not consistently stop fuel flow when decelerating (foot completely off gas). The car now consistently stops fuel flow and I have engine compression braking...

Brad says that when this component fails it appears that after the start program expires, the monitor shuts down 4 cyclinders, but it was not shutting down the fuel injection. We were smelling a lot of extra fuel. I guess it was a good thing the car wouldn't run for more than 20 seconds in this condition.

I'm grinning and driving again!

nc_growler

So... Was this a common problem? Or was it as unique as I think it was?

Mrmerlin

uneek problem

nc_growler

Yeah. Thought so. A real curve ball. Didn't see that one coming.

Hoping that's the last strange one I get to diagnose for awhile.

M. Requin

One for the files!