944S cutting out after warmup

[3liter914-6]

Resurrected an '88 944S that had been sitting for about 6 or 7 years and largely been a pretty smooth process, but I've just started driving it and a problem has emerged that I'm not sure what's causing it. Just throwing it out there in case this is a common issue that has a known fix. I did some googling and nothing jumped out at me, but perhaps someone's encountered this before.

Replaced so far and other possibly useful info: fuel lines, pump (Bosch), filter and in tank screen. URO DME relay after problem started with no change in behavior. Replaced DME coolant temp sensor due to cold start issues (ICV was removed by PO which seems to be crux of that issue). New O2 sensor (OE Bosch). Cam sensor connector crumbled and I soldered on a new one. Car seems to have chip as I found an EEPROM that appears to be the original in the collection of parts that came with it.

It seems like after the car has warmed up (about 15 mins of driving) it starts bogging as if fuel or ignition is cutting out. Cycling the ignition key while driving makes the issue go away immediately, but will typically return pretty quickly once it starts happening. Car restarts fine if it stalls.

Last night ordered new cam sensor, crank sensor. Holding off on FPR as I can't see why a bad or dying FPR would be heat related, open to suggestions that might be a cause.

[Tom]

Resurrected an '88 944S that had been sitting for about 6 or 7 years and largely been a pretty smooth process, but I've just started driving it and a problem has emerged that I'm not sure what's causing it. Just throwing it out there in case this is a common issue that has a known fix. I did some googling and nothing jumped out at me, but perhaps someone's encountered this before.

Replaced so far and other possibly useful info: fuel lines, pump (Bosch), filter and in tank screen. URO DME relay after problem started with no change in behavior. Replaced DME coolant temp sensor due to cold start issues (ICV was removed by PO which seems to be crux of that issue). New O2 sensor (OE Bosch). Cam sensor connector crumbled and I soldered on a new one. Car seems to have chip as I found an EEPROM that appears to be the original in the collection of parts that came with it.

It seems like after the car has warmed up (about 15 mins of driving) it starts bogging as if fuel or ignition is cutting out. Cycling the ignition key while driving makes the issue go away immediately, but will typically return pretty quickly once it starts happening. Car restarts fine if it stalls.

Last night ordered new cam sensor, crank sensor. Holding off on FPR as I can't see why a bad or dying FPR would be heat related, open to suggestions that might be a cause.

Those are always tough ones. If you can warm it up in the garage and wait for it to stumble, you might have a better chance of diagnosing it by checking things while it is acting up -- spark strength, fuel pressure, pulling individual spark plug leads and fuel injector clips, etc. My wild guess is that it's electrical of some kind. I wouldn't think that cycling the key would fix a fuel blockage or bad FPR, etc. Beyond that, it's tough to even guess. But I will anyway. Have you tried a different coil? It's not necessarily a 944 thing, but sometimes coils will fail that way. They are ok when cold, but after running a while they warm up and expand just enough to cause issues internally. Spark plug wires can do the same (pop the hood in a dark location and look for arcing. Or just change them, along with the plugs, cap & rotor, if they've all been sitting 6 or 7 years. Cracked solder joints are a similar issue -- they can be fine until the DME gets warm and a crack expands just enough to make a joint fail. Speed and reference sensor connectors are on the perennial top-5 things to check when these cars have odd problems, along with the DME grounds and fuel injector connectors. Just spit-balling a bit to throw out things to consider...

Read all about the speed and ref sensor/fuel injector connectors, and how they fail, here:
https://www.carpokes.com/viewtopic.php?t=565&hilit=JPT

[3liter914-6]

I appreciate the brain storm. I did replace the plug wires, plugs, rotor (don't think I did cap) when I did the t-belt, forgot to mention that.

Hopefully not DME, but the fact a reset cures it (temporarily) had me suspicious. It feels to me like fuel, but I haven't had the opportunity to give it a test, I guess that's next. I will probably throw a coil at it, was also curious about the ignition module, but with all this stuff I feel like an ECU reset wouldn't change much that would allow it to run fine again. So I'm guessing the issue must be with the ECU. Maybe the cam or crank sensor are giving irregular readings and at some # of cycles of bad data the ECU gets confused and starts timing ignition/injection incorrectly. No idea if that's a plausible idea with the motronic. I'll see if I can get a better idea of whether it's spark or fuel this weekend, just wasn't sure if this was something others have encountered before with a known culprit. We'll see if the speed and cam sensor change things when they show up.

[barnwerks]

How about the ignition switch?
There is an electrical portion of the key switch assembly, fastened (2 screws) to the end of the keyway housing. This failure can be easily diagnosed with a spare switch plugged into the dash harness.

[walfreyydo]

+1 ignition switch would be high on my list given that cycling the ignition key makes it go away.

If that doesnt resolve the issue you can start looking at testing the AFM and re-tracking the arms/contact strip

https://www.clarks-garage.com/shop-manual/elect-25.htm
https://www.clarks-garage.com/shop-manual/elect-17.htm
https://www.clarks-garage.com/shop-manual/elect-22.htm

And also recommend you test components before replacing, good info on troubleshooting and diagnosis here
https://www.clarks-garage.com/shop-manual/ts-01.htm

[3liter914-6]

I will probably add the ignition switch to my list of things to "just replace", it's something I considered in the diagnosis, but I feel like I would see other signs for the switch -- I still have full power to radio, headlights, hvac, etc during this. I suppose it could be just enough to reduce the voltage to the ECU and cause the injectors or ignition to drop out.

I appreciate the AFM suggestion. It was high on my list of suspicious items as the car came with a "spare" AFM with paint pen on it looking like it came from LKQ back in 2017 around when the car stopped being driven. I've had a few E30s so it's not a completely unfamiliar beast although I've always tested on car, the 9V approach is a new twist.

I promise I'm not just throwing parts at the car, when I did the t-belt the cam sensor connector was held together with prayers and capton tape and crumbled to dust when I unwrapped the connectors. I crimped on a new 3 pin power timer, but put it on my list of things to replace prior to megasquirting. The crank sensor connector had not fully disintegrated, but was well on it's way and I have it wrapped in duct tape for now and it's been in my shopping basket for a few weeks waiting to collect an order large enough to justify shipping. So these are both things that need to be replaced sooner or later. The goal is solid daily driver so I'm not too upset about replacing other things that could be contributing that in some way I consider maintenance on 40 year old car -- coil, ICM, etc. Diagnosis is challenging though as it's just me and when this happens the car doesn't really stay running, and the problem disappears on restart so difficult to tell whether it's a fuel or spark issue and do much in the way of troubleshooting while the problem is active.

[Tom]

I will probably add the ignition switch to my list of things to "just replace", it's something I considered in the diagnosis, but I feel like I would see other signs for the switch -- I still have full power to radio, headlights, hvac, etc during this. I suppose it could be just enough to reduce the voltage to the ECU and cause the injectors or ignition to drop out.

I appreciate the AFM suggestion. It was high on my list of suspicious items as the car came with a "spare" AFM with paint pen on it looking like it came from LKQ back in 2017 around when the car stopped being driven. I've had a few E30s so it's not a completely unfamiliar beast although I've always tested on car, the 9V approach is a new twist.

I promise I'm not just throwing parts at the car, when I did the t-belt the cam sensor connector was held together with prayers and capton tape and crumbled to dust when I unwrapped the connectors. I crimped on a new 3 pin power timer, but put it on my list of things to replace prior to megasquirting. The crank sensor connector had not fully disintegrated, but was well on it's way and I have it wrapped in duct tape for now and it's been in my shopping basket for a few weeks waiting to collect an order large enough to justify shipping. So these are both things that need to be replaced sooner or later. The goal is solid daily driver so I'm not too upset about replacing other things that could be contributing that in some way I consider maintenance on 40 year old car -- coil, ICM, etc. Diagnosis is challenging though as it's just me and when this happens the car doesn't really stay running, and the problem disappears on restart so difficult to tell whether it's a fuel or spark issue and do much in the way of troubleshooting while the problem is active.

Clarks is awesome, but see our AFM test guide linked below as well. It allows you to fully test without taking anything apart... For a car that's been sitting 7 years, I think you get a lot more latitude to swap in new parts -- things degrade just sitting for years. Speed and ref sensors wrapped in duct tape have been known to cause all sorts of problems just like you are experiencing, so that one is a no-brainer...

https://www.carpokes.com/viewtopic.php?t=561

[3liter914-6]

Haven't had much time to tinker with the car the last few months, but been doing a little thinking here and there and the symptoms high fuel pressure caught my eye:

The high differential pressure across the injectors causes them to draw excessive current. The excessive current is seen by the injector drivers which subsequently shutdown. If you have a condition where the car will not start or starts and almost immediately dies, try disconnecting the wire for one (1) injector.

Not my problem but interesting.

I'd also been having a problem with long cranking on startup and the fuel pump and check valve are brand new, so that was another symptom that pushed me towards wanting to check. I looked on Amazon and found that the kits are listed as having a 12x1.50 adapter so I figured I was good and pulled the trigger. Unfortunately it didn't exactly work. Tightened it down and got a reading a bit lower than was expected, but was accompanied by a steady drip, drip, drip from the fittings. I tried everything I could think of to get it to seal -- o-rings, cut up silicone tubing to serve as a washer, various sealants and still with the drip. Some people have had success with hose clamping some fuel line over the fitting, but a trip to AutoZone was uneventful as all they had was 1/4". I'd resigned myself to ordering up the parts to custom fabricate an adapter and waiting a week or so to get it done when I decided I had one more trick up my sleeve. The fitting really wants an internal flare on the female adapter so I decided to print one.

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I figured I'd give PLA a shot first as proof of concept before I cracked open a fresh spool of TPU, but there was no need to go that far as after a test fit and a quick redesign it snugged up and was finally drip free. Unfortunately it really wasn't bleeding off any pressure and PSI at idle was still 30. I'd already pulled the FPR to see if I can figure out which one it was, definitely an aftermarket regulator (looks like WVE based on the markings) and already suspected someone had replaced with an 8V unit, which seems to be confirmed.

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I have a set of 4-hole Volvo 33lb injectors squirreled away for my planned microsquirt conversion that I could probably swap in and be ok (high z, so theoretically safe but not optimal). But, in the end I decided to just replace the FPR , so a correct pressure Delphi FPR should be on it's way tomorrow morning, so we'll see if that helps with my issue, but it seems like it can't hurt. Especially as the pressure bleeds down instantly after you shut the pump off, so if nothing else it'll hopefully help with my extended cranking on every start. Pretty happy I was able to find an $18 + pennies in PLA solution to checking my fuel pressure too.

[3liter914-6]

PLA flare insert still holding up without leaks after a couple of days with E10 exposure. The new FPR showed up today and reads right about where it should be (48 +/- 3) at idle and holds pressure after shutdown. Test drive will have to wait, but it's definitely something that wasn't in spec.

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[Tom]

I'd be tempted to put a piece of PLA in a pickle jar full of gas, and see how if it dissolves at all. I'd just be a little worried about sending dissolved pla through the injectors....?