Carpokes Porsche Forum

I have been having an issue for some time now with my relatively-new-to-me 1989 944 S2 with ~215k miles. The car was undriveable didn't run at 208k when I acquired the car, so a lot has been refreshed, and it runs great on dry days now. When it's wet out, I have the following symptoms, and I'd love the community help troubleshooting.

* The car will lose power suddenly when driving. It won't happen right away, but it will happen during a rainy drive within 20-25 minutes. It will also linger with the car the next day for a bit, even if it's sunny and warm.
* .It can stall out, but I can usually save it if I clutch in and slam on the accelerator to rev the motor. It will also generally always start right up, and it will idle fine. The power only really cuts out when the engine is under load. Sometimes that happens immediately if you haven't revved the engine high enough before lifting the clutch pedal. When I've had to get home and not be stuck, I've come home with a burnt clutch smell because clutch drops are reliable ways to get going again.
* It will be less likely to happen when the the tachometer is kept higher, like dropping a gear to keep the tach over 4-5k, but that is not a given.
* I have the Focus9 DME relay with the diagnostic LEDs and the pre-prime fuel pump timer. That might explain why it always starts up right away because fuel is always available at startup.
* I have witnessed the Focus9 DME relay lights when this is happening. The lights for fuel pump power and fuel pump control signal from the ECU blink when the engine hesitates.
* I have replaced the DME with a jumper to force the fuel pump to stay on. This keeps the car from stalling out, but it will still hesitate. occasionally. If I just keep my foot on the gas, it will continue again.

Things I'm confident of...
* I have reliable power at 14V everywhere I can test.
* I have cleaned every ground I can find, including the ones behind the headlights, the ones all around the engine, the ones back by the fuel pump, the ones near the fuse box (above and below), near the battery, etc.
* The cam position sensor is new, as well as the connector that disintegrated when I first touched it.
* All ignition wires, distributor, the ignition coil, and the "other ignition coil" that is behind the left headlight are new/good.
* All fuel lines and filters (outside and inside the tank) and the pump are recently replaced.
* The battery is good and reasonably new.
* All the ignition components under the hood are staying dry, even when I drive without the engine trays installed. The same for the cam position sensor, engine reference sensor, and the fuel injector harnesses.
* I idled the car in my driveway for 30-40 minutes and just sprayed everything under the hood with a hose for that hole time. It never hesitated or skipped or anything. Every connector and ground connection got a straight blast of hose water jetted at it with no impact, but it wouldn't drive well after that. Whatever water had gotten _somewhere_ needed to be gone before the car drove well enough again.
* The dashboard under the driver's side is staying dry. There's no water getting into the wiring under the fuse box, and the driver's side floor is staying dry (except for whatever comes from my shoes). The floor and ECU under the passenger side is also staying dry.
* The distributor is staying dry. The S2 doesn't have that extra seal behind it that could allow water in.

Top Theories:
* Water is getting _somewhere_. It sounds obvious, but that explains why it doesn't happen right away, and it would explain why the issue may continue the next day with nice weather for a little while.
* When it happens, the ECU is interrupted. That would explain the ECU signal for the fuel pump cutting out. That would explain the engine still hesitating when the fuel pump jumper is forcing the fuel pump on permanently.
* The tray under the windshield over the HVAC fans has a few cracks in it. Maybe water is getting under that tray and grounding something out?
* Either the engine reference sensor or the cam position sensor (or the signal from them) is getting interrupted. That would cause the ECU (probably?) to cut the fuel pump signal and ignition.

I'm running out of ideas. Has anyone else ever run into anything like this?

That's a really comprehensive list of things you've checked already.

The lights on your focus9 DME relay seem the most informative. If I understand correctly, it sounds like the ECU is stumbling in its signal to keep the fuel pump energized? Though when you have the DME relay bypass installed, the car still hesitates? That's fascinating.

It definitely sounds electrical in nature, especially based on those factors.

However, is it possible water is getting into the fuel system? Like a tiny leak into the fuel tank? (Water sinks in fuel, right?) Maybe the engine can handle a little bit of water, but feed it enough and it stalls?

Maybe the lights blinking on the Focus9 DME relay are a _result_ of the car stalling (and the ECU cutting the fuel supply deliberately) and not pointing to the cause of the stumble.

In that case, maybe it's fuel delivery then. Clogged fuel filter? Gunk in the tank? Failing fuel pressure regulator/damper/vac lines?

But that doesn't explain why it only happens when wet.

This whole post I'm thinking out loud. I'm just as stumped as you.

Sound like a tricky one, sorry about that. If it idles and revs fine with no load, but stumbles under load, my first thought is a weak spark. Moisture can cause all sorts of ignition problems, with spark jumping from plug to head, from wire to chassis, from cap post to post, resistance in wires, coil wires to ground, etc. The relay lights might suggest otherwise, but hard to say. The bad news is that there are an endless number of possibilities. The good news is the problem is repeatable with a garden hose. I would be inclined to start collecting test info. Monitor power at the DME/coil/fuel pump, etc. before and after the garden hose (scope is best, or good multimeter, or even a 12v LED); if that gives no clues, pull a spark plug wire and check the strength of the spark before and after watering it down; then look at the O2 signal to see if the car is stuck lean or rich when stumbling -- essentially trying to see what's going wrong when it stumbles (weak spark, fuel supply, erratic spark, bad AFR caused by a bad sensor input, etc.) so that you can start to see why that's happening. My guess would be a harness issue, but impossible to say. You might check the battery voltage itself when stumbling too. A bad 12v power cable that is partially grounding out due to moisture can cause all sorts of issues.

I found my oil pressure sender was disconnected from both wires. One wire broke from it's ring terminal, and the other one just pulled itself off the spade connector. I'm not sure if those were supposed to be two different connection mechanisms originally or if that's been repaired before. Either way, both wires are connected again. I really doubt that this is the electrical ground issue I'm looking for because I don't see how either of these could have been causing issues only when wet, but it's fixed now.

I also found the oxygen sensor wire wasn't properly supported. The metal clip wasn't attached to the stud over the torque tube, but it is now. Again, that isn't necessarily a source of the problem, but it does suggest I should take a closer inspection at the cable to the O2 sensor when I get a chance. It's clearly been replaced before, and it's got a lot of wiring splices along its path wrapped in just electrical tape. Again, no obvious culprits, but it certainly doesn't look great.

I have another theory to investigate too. I have the alarm control unit near the DME (928.637.104.05). I don't have specific reasons to suspect this, but I'm pretty sure interrupting the voltage signal to the DME relay is the way this implements it's immobilizer. Again, this unit has stayed dry when this problem is occurring, but I don't really know all the things it's connected to. I wonder if something is convincing the alarm unit to cut power to the DME relay.

neilschelly wrote: Tue Oct 24, 2023 6:08 am I found my oil pressure sender was disconnected from both wires. One wire broke from it's ring terminal, and the other one just pulled itself off the spade connector. I'm not sure if those were supposed to be two different connection mechanisms originally or if that's been repaired before. Either way, both wires are connected again. I really doubt that this is the electrical ground issue I'm looking for because I don't see how either of these could have been causing issues only when wet, but it's fixed now.

I also found the oxygen sensor wire wasn't properly supported. The metal clip wasn't attached to the stud over the torque tube, but it is now. Again, that isn't necessarily a source of the problem, but it does suggest I should take a closer inspection at the cable to the O2 sensor when I get a chance. It's clearly been replaced before, and it's got a lot of wiring splices along its path wrapped in just electrical tape. Again, no obvious culprits, but it certainly doesn't look great.

I have another theory to investigate too. I have the alarm control unit near the DME (928.637.104.05). I don't have specific reasons to suspect this, but I'm pretty sure interrupting the voltage signal to the DME relay is the way this implements it's immobilizer. Again, this unit has stayed dry when this problem is occurring, but I don't really know all the things it's connected to. I wonder if something is convincing the alarm unit to cut power to the DME relay.

Could be the dodgy o2 sensor wiring. Tough problem cause it is not instantly reproducable.
What you could do is try driving the car as usual with the o2 sensor unpluged. The DME should run on default maps which aren't the best, but at least you can rule out the o2 sensor if the problem still remains.

In the back left of the engine bay there is a plug, it carries the power wire for the coil from the ignition switch. Check that the plug isn't totally disintegrated, that was a phantom electrical issue on a 90S2 cab we have.

Seagrams wrote: Thu Nov 02, 2023 3:50 pm Could be the dodgy o2 sensor wiring. Tough problem cause it is not instantly reproducable.
What you could do is try driving the car as usual with the o2 sensor unplugs. The DME should run on default maps which aren't the best, but at least you can rule out the o2 sensor if the problem still remains.

That's my best running theory. I'm planning to dig into the O2 sensor tonight. After looking over the wiring diagrams, it looks like a short in the O2 sensor heater circuit could actually ground out the ECU signal for the fuel pump. I need to look closer tonight at that. I'm crossing my fingers that it's as easy as repairing/replacing the O2 sensor.
-N

stitch2k1 wrote: Thu Nov 02, 2023 5:07 pm In the back left of the engine bay there is a plug, it carries the power wire for the coil from the ignition switch. Check that the plug isn't totally disintegrated, that was a phantom electrical issue on a 90S2 cab we have.

Can you take a picture of the plug by chance? Driver's side (fuse box) or passenger (battery) side?
-N

I pulled the O2 sensor, and I decided it was worth replacing after seeing it too. There was definitely some peeling electrical tape from at least two spots on the wiring. Now, I'm just waiting for some rain to really test it.

I _did_ go for a drive-thru car wash, which has definitely triggered this before. There were no issues. There's no rain in the forecast for at least the next week, and I've already turned off the hoses outside for winter, so I was impatient. I'm hesitant to call victory yet, but I'm cautiously optimistic.
-N

neilschelly wrote: Sun Nov 05, 2023 4:51 pm I pulled the O2 sensor, and I decided it was worth replacing after seeing it too. There was definitely some peeling electrical tape from at least two spots on the wiring. Now, I'm just waiting for some rain to really test it.

I _did_ go for a drive-thru car wash, which has definitely triggered this before. There were no issues. There's no rain in the forecast for at least the next week, and I've already turned off the hoses outside for winter, so I was impatient. I'm hesitant to call victory yet, but I'm cautiously optimistic.
-N

I pulling for ya!

"Cautiously optimistic" is definitely better that "I'm out of options and have reached the end of the rope".

Let us know if all still good after the next big deluge!