Carpokes Porsche Forum

johnb wrote: Tue Oct 07, 2025 6:38 pm I'm chasing an odd issue with my 944 Turbo and I could use some input from the brain trust!

Lately when the engine warms up it'll start bucking/misfiring under load at part throttle. It's fine under very light throttle. I can kind of ease my way through it and get into the higher rpms. So it seems to be load related. There doesn't seem to be any rpm or road speed limit as long as I get there with minimal load.

If I literally floor it, there's no stumbling, it's perfectly smooth. But it won't accelerate at all.

In light throttle when I'm not having the issue, AFRs seem normal. When it happens, my gauge swings wildly from rich to lean and back again. I'm not sure if wideband gauges can tell you anything reliable under hard misfires though?

I should mention I only have the wideband for diagnostics, the car is 100% stock.

I just put a new coil in in it and a new coil to distributor cap wire. I haven't changed the ignition parts yet but I have them on hand so I will try those.

But here's where it gets odd! The other day after getting it hot during a test run, I let it sit for a few minutes, and it would not start again. Within a few seconds of cranking I could smell fuel, really strongly. After a few failed attempts to start I pulled the plugs and the engine was totally flooded.

So I let it sit with the plugs loosened, and later it started up after a lot of cranking. But when it did run, it was crazy rich, like buried in the 10s and it only came up to 11-12 when it warmed up. I have never seen that before. I drove it again in that condition and the stumbling issue was exactly the same as before. So I don't think it's a lack of fuel that's causing the issue, though I could be wrong.

Next day when it was nice and cool I tested the fuel pressure. Everything was bang on - the pressure was the same as it has always been, 2.5 bar with the engine off, and ~2 running. The regulator and damper both hold vacuum and there's no sign of fuel leaking into the vacuum lines from either one.

Tonight this no-start, fuel smell thing happened again. I did a few test drives after changing the coil and now it won't start. I can smell the fuel.

I am getting tachomter bounce while cranking during these no-start situations.

Other stuff I have checked:

* the stumbling issue is present with 2 DMEs. I have a spare on hooked up at the moment for other reasons and the issue is the same.
* the DME temp sensor in the block gives normal readings, and in fact I actually tested the voltage inside the DME because I happen to have it opened up at the moment. The reading indicated that it's correctly sensing a hot engine. So that covers the harness too.
* the AFM air temp sensor reading looked normal too.
* it's not the TPS, I unplugged it and there was no change in the stumbling issue. Also I can tell from my AFR gauge that the idle switch is working
* there's no obvious loose hose clamps or anything like that.

I know I should do a full vaccum and boost leak test and a bunch of other stuff, and I will but I'm just looking for any suggestions in case someone can put me on the right track quickly!

I know it's possible I have 2 totally unrelated issues with the stumbling thing and the super rich, no start thing.

What makes a 944 run rich if it's not the fuel pressure? Or was it just showing rich because I flooded it previously due to some issue unrelated to fuel?

Thanks for reading if you got this far.

Tom wrote: Tue Oct 07, 2025 6:47 pm

johnb wrote: Tue Oct 07, 2025 6:38 pm I'm chasing an odd issue with my 944 Turbo and I could use some input from the brain trust!

Lately when the engine warms up it'll start bucking/misfiring under load at part throttle. It's fine under very light throttle. I can kind of ease my way through it and get into the higher rpms. So it seems to be load related. There doesn't seem to be any rpm or road speed limit as long as I get there with minimal load.

If I literally floor it, there's no stumbling, it's perfectly smooth. But it won't accelerate at all.

In light throttle when I'm not having the issue, AFRs seem normal. When it happens, my gauge swings wildly from rich to lean and back again. I'm not sure if wideband gauges can tell you anything reliable under hard misfires though?

I should mention I only have the wideband for diagnostics, the car is 100% stock.

I just put a new coil in in it and a new coil to distributor cap wire. I haven't changed the ignition parts yet but I have them on hand so I will try those.

But here's where it gets odd! The other day after getting it hot during a test run, I let it sit for a few minutes, and it would not start again. Within a few seconds of cranking I could smell fuel, really strongly. After a few failed attempts to start I pulled the plugs and the engine was totally flooded.

So I let it sit with the plugs loosened, and later it started up after a lot of cranking. But when it did run, it was crazy rich, like buried in the 10s and it only came up to 11-12 when it warmed up. I have never seen that before. I drove it again in that condition and the stumbling issue was exactly the same as before. So I don't think it's a lack of fuel that's causing the issue, though I could be wrong.

Next day when it was nice and cool I tested the fuel pressure. Everything was bang on - the pressure was the same as it has always been, 2.5 bar with the engine off, and ~2 running. The regulator and damper both hold vacuum and there's no sign of fuel leaking into the vacuum lines from either one.

Tonight this no-start, fuel smell thing happened again. I did a few test drives after changing the coil and now it won't start. I can smell the fuel.

I am getting tachomter bounce while cranking during these no-start situations.

Other stuff I have checked:

* the stumbling issue is present with 2 DMEs. I have a spare on hooked up at the moment for other reasons and the issue is the same.
* the DME temp sensor in the block gives normal readings, and in fact I actually tested the voltage inside the DME because I happen to have it opened up at the moment. The reading indicated that it's correctly sensing a hot engine. So that covers the harness too.
* the AFM air temp sensor reading looked normal too.
* it's not the TPS, I unplugged it and there was no change in the stumbling issue. Also I can tell from my AFR gauge that the idle switch is working
* there's no obvious loose hose clamps or anything like that.

I know I should do a full vaccum and boost leak test and a bunch of other stuff, and I will but I'm just looking for any suggestions in case someone can put me on the right track quickly!

I know it's possible I have 2 totally unrelated issues with the stumbling thing and the super rich, no start thing.

What makes a 944 run rich if it's not the fuel pressure? Or was it just showing rich because I flooded it previously due to some issue unrelated to fuel?

Thanks for reading if you got this far.

My knee-jerk reaction would be weak ignition components -- wires often get overlooked, cap, rotor, plugs. Stumbling under load and running fine without load is a pretty classic weak ignition symptom. The rich exhaust could be explained by the fuel just not igniting. If you have new wires, plugs, cap, and rotor anyway, I'd definitely put those in and cross your fingers. In a dark garage, if the wires are really bad, you can sometimes seem them arcing...

johnb wrote: Tue Oct 07, 2025 6:38 pm I'm chasing an odd issue with my 944 Turbo and I could use some input from the brain trust!

Lately when the engine warms up it'll start bucking/misfiring under load at part throttle. It's fine under very light throttle. I can kind of ease my way through it and get into the higher rpms. So it seems to be load related. There doesn't seem to be any rpm or road speed limit as long as I get there with minimal load.

If I literally floor it, there's no stumbling, it's perfectly smooth. But it won't accelerate at all.

In light throttle when I'm not having the issue, AFRs seem normal. When it happens, my gauge swings wildly from rich to lean and back again. I'm not sure if wideband gauges can tell you anything reliable under hard misfires though?

I should mention I only have the wideband for diagnostics, the car is 100% stock.

I just put a new coil in in it and a new coil to distributor cap wire. I haven't changed the ignition parts yet but I have them on hand so I will try those.

But here's where it gets odd! The other day after getting it hot during a test run, I let it sit for a few minutes, and it would not start again. Within a few seconds of cranking I could smell fuel, really strongly. After a few failed attempts to start I pulled the plugs and the engine was totally flooded.

So I let it sit with the plugs loosened, and later it started up after a lot of cranking. But when it did run, it was crazy rich, like buried in the 10s and it only came up to 11-12 when it warmed up. I have never seen that before. I drove it again in that condition and the stumbling issue was exactly the same as before. So I don't think it's a lack of fuel that's causing the issue, though I could be wrong.

Next day when it was nice and cool I tested the fuel pressure. Everything was bang on - the pressure was the same as it has always been, 2.5 bar with the engine off, and ~2 running. The regulator and damper both hold vacuum and there's no sign of fuel leaking into the vacuum lines from either one.

Tonight this no-start, fuel smell thing happened again. I did a few test drives after changing the coil and now it won't start. I can smell the fuel.

I am getting tachomter bounce while cranking during these no-start situations.

Other stuff I have checked:

* the stumbling issue is present with 2 DMEs. I have a spare on hooked up at the moment for other reasons and the issue is the same.
* the DME temp sensor in the block gives normal readings, and in fact I actually tested the voltage inside the DME because I happen to have it opened up at the moment. The reading indicated that it's correctly sensing a hot engine. So that covers the harness too.
* the AFM air temp sensor reading looked normal too.
* it's not the TPS, I unplugged it and there was no change in the stumbling issue. Also I can tell from my AFR gauge that the idle switch is working
* there's no obvious loose hose clamps or anything like that.

I know I should do a full vaccum and boost leak test and a bunch of other stuff, and I will but I'm just looking for any suggestions in case someone can put me on the right track quickly!

I know it's possible I have 2 totally unrelated issues with the stumbling thing and the super rich, no start thing.

What makes a 944 run rich if it's not the fuel pressure? Or was it just showing rich because I flooded it previously due to some issue unrelated to fuel?

Thanks for reading if you got this far.

johnb wrote: Tue Oct 07, 2025 7:10 pm

Tom wrote: Tue Oct 07, 2025 6:47 pm

johnb wrote: Tue Oct 07, 2025 6:38 pm I'm chasing an odd issue with my 944 Turbo and I could use some input from the brain trust!

Lately when the engine warms up it'll start bucking/misfiring under load at part throttle. It's fine under very light throttle. I can kind of ease my way through it and get into the higher rpms. So it seems to be load related. There doesn't seem to be any rpm or road speed limit as long as I get there with minimal load.

If I literally floor it, there's no stumbling, it's perfectly smooth. But it won't accelerate at all.

In light throttle when I'm not having the issue, AFRs seem normal. When it happens, my gauge swings wildly from rich to lean and back again. I'm not sure if wideband gauges can tell you anything reliable under hard misfires though?

I should mention I only have the wideband for diagnostics, the car is 100% stock.

I just put a new coil in in it and a new coil to distributor cap wire. I haven't changed the ignition parts yet but I have them on hand so I will try those.

But here's where it gets odd! The other day after getting it hot during a test run, I let it sit for a few minutes, and it would not start again. Within a few seconds of cranking I could smell fuel, really strongly. After a few failed attempts to start I pulled the plugs and the engine was totally flooded.

So I let it sit with the plugs loosened, and later it started up after a lot of cranking. But when it did run, it was crazy rich, like buried in the 10s and it only came up to 11-12 when it warmed up. I have never seen that before. I drove it again in that condition and the stumbling issue was exactly the same as before. So I don't think it's a lack of fuel that's causing the issue, though I could be wrong.

Next day when it was nice and cool I tested the fuel pressure. Everything was bang on - the pressure was the same as it has always been, 2.5 bar with the engine off, and ~2 running. The regulator and damper both hold vacuum and there's no sign of fuel leaking into the vacuum lines from either one.

Tonight this no-start, fuel smell thing happened again. I did a few test drives after changing the coil and now it won't start. I can smell the fuel.

I am getting tachomter bounce while cranking during these no-start situations.

Other stuff I have checked:

* the stumbling issue is present with 2 DMEs. I have a spare on hooked up at the moment for other reasons and the issue is the same.
* the DME temp sensor in the block gives normal readings, and in fact I actually tested the voltage inside the DME because I happen to have it opened up at the moment. The reading indicated that it's correctly sensing a hot engine. So that covers the harness too.
* the AFM air temp sensor reading looked normal too.
* it's not the TPS, I unplugged it and there was no change in the stumbling issue. Also I can tell from my AFR gauge that the idle switch is working
* there's no obvious loose hose clamps or anything like that.

I know I should do a full vaccum and boost leak test and a bunch of other stuff, and I will but I'm just looking for any suggestions in case someone can put me on the right track quickly!

I know it's possible I have 2 totally unrelated issues with the stumbling thing and the super rich, no start thing.

What makes a 944 run rich if it's not the fuel pressure? Or was it just showing rich because I flooded it previously due to some issue unrelated to fuel?

Thanks for reading if you got this far.

My knee-jerk reaction would be weak ignition components -- wires often get overlooked, cap, rotor, plugs. Stumbling under load and running fine without load is a pretty classic weak ignition symptom. The rich exhaust could be explained by the fuel just not igniting. If you have new wires, plugs, cap, and rotor anyway, I'd definitely put those in and cross your fingers. In a dark garage, if the wires are really bad, you can sometimes seem them arcing...

Thanks Tom. That was my thought too - I definitely can't see arcing but I remember it very clearly with the old wires that I replaced in 2012. Even so, those and the cap and rotor are definitely prime candidates.

I was wondering what might make it not happen at WOT (though it still has no power). The extra fuel might make it easier for the spark, just enough to stop a hard misfire. Likewise for the warmup period.

I just got it to start again a few minutes ago and the AFR did clear up to normal levels after a while. So I think maybe it was showing rich the last time because the sensor itself was doused in raw fuel.

Anyway I did manage to capture a good dwell image for the other thread...

Tom wrote: Tue Oct 07, 2025 7:15 pm

johnb wrote: Tue Oct 07, 2025 7:10 pm

Tom wrote: Tue Oct 07, 2025 6:47 pm

My knee-jerk reaction would be weak ignition components -- wires often get overlooked, cap, rotor, plugs. Stumbling under load and running fine without load is a pretty classic weak ignition symptom. The rich exhaust could be explained by the fuel just not igniting. If you have new wires, plugs, cap, and rotor anyway, I'd definitely put those in and cross your fingers. In a dark garage, if the wires are really bad, you can sometimes seem them arcing...

Thanks Tom. That was my thought too - I definitely can't see arcing but I remember it very clearly with the old wires that I replaced in 2012. Even so, those and the cap and rotor are definitely prime candidates.

I was wondering what might make it not happen at WOT (though it still has no power). The extra fuel might make it easier for the spark, just enough to stop a hard misfire. Likewise for the warmup period.

I just got it to start again a few minutes ago and the AFR did clear up to normal levels after a while. So I think maybe it was showing rich the last time because the sensor itself was doused in raw fuel.

Anyway I did manage to capture a good dwell image for the other thread...

Maybe I missed the WOT part? What happens at WOT? I was thinking no power meant it was bogging down, which would still be consistent with weak ignition... ?

johnb wrote: Tue Oct 07, 2025 7:18 pm

Tom wrote: Tue Oct 07, 2025 7:15 pm

johnb wrote: Tue Oct 07, 2025 7:10 pm

Thanks Tom. That was my thought too - I definitely can't see arcing but I remember it very clearly with the old wires that I replaced in 2012. Even so, those and the cap and rotor are definitely prime candidates.

I was wondering what might make it not happen at WOT (though it still has no power). The extra fuel might make it easier for the spark, just enough to stop a hard misfire. Likewise for the warmup period.

I just got it to start again a few minutes ago and the AFR did clear up to normal levels after a while. So I think maybe it was showing rich the last time because the sensor itself was doused in raw fuel.

Anyway I did manage to capture a good dwell image for the other thread...

Maybe I missed the WOT part? What happens at WOT? I was thinking no power meant it was bogging down, which would still be consistent with weak ignition... ?

If I literally floor it, it doesn't stumble, it's smooth. It just won't accelerate.

Tom wrote: Wed Oct 08, 2025 2:18 pm Darn it, filter-eating rodents was going to be my next guess!

It's official, now I've seen everything. Sorry that happened. Cue @dr bob and his four-dimensional rodent regulation routine....

I'd pull the i/c pipes and j-boot and see that you find. You can also get a visual on the turbo impeller that way. I recently used a cheap endoscope on my exhaust and, if I saw that in my car, might be temped to snake one into the intake and cylinders to see if anything is in there.

https://carpokes.com/viewtopic.php?p=48296#p48257