For a start I have a 1984 Porsche 944 NA. I've been chasing many issues around the car, I found one of my old underlying issues today, its very odd and hard to explain. I think I might have a short somewhere but really have no clue.

Issue in simple terms:
I crank the car on 4 injectors, doesn't start. Unplug 1 injector then crank the car and it idles for 5-6 seconds and dies (I normally unplug cylinder 1's injector because its easy to get to). Plug the unplugged injector back in and crank on 4 and doesn't start, but after cranking on 4 injectors you can start it for 5-6 seconds on 3 injectors.

Origin/Full Story of my issues:
https://rennlist.com/forums/924-931-...-no-start.html

 

Yeah, that tracks. I bet that if you were to pull up the boots on the injector connectors, that you'd find there's all kinds of degraded insulation.

There are companies (Lindsey Racing?) that sell new harnesses, that you might want to look into.

Good luck

 

Would it be the injector harness or could it go back even further in the harness? Is there a clear reason the car would only want to run on 3 for 5-6 seconds? This car has taken me for a ride.

I'm local to Lindsey Racing and I've looked into their harnesses.

 

On my car, I didn't look too far back as the damage that I saw was right at the bend, where the wiring entered the plug. I wasn't getting weird running issues but there were definitely breaks on the power and ground wires and I figured it wouldn't be long until I did start seeing impacts from that.

ETA: On my car, the wiring was pretty OK once I traced things back into the bulk of the wire bundle, it was really just the sharp bends at the plugs. For the part of the harness that went into the car, that stuff was in great shape: unfaded colors, pliable, even the PVC sheathing was flexible(ish). You can get connector kits that will let you just replace the plug by splicing into the harness a little, which can be way more cost effective than a new harness.

The injectors batch fire when they're grounded by the DME, so a problem at one injector might translate itself to a problem at all of them (maybe? My EE knowledge is less than fantastic). If you check your plugs, are they wet? I'm wondering if it's possible you're running really, really rich and that eventually kills the engine. Maybe it's just lucky that you pulled the plug where the short is but, if you pull the plug on cylinder 2, can you replicate the crank on 4 cylinders, unplug one injector, engine fires on 3 cylinders behavior?

The other thing you can check is the grounds on the bellhousing and block, as that is the path for the injector ground. If those aren't clean and tight, a little bit of vibration once the engine gets going could be dislodging whatever minimal grounding is happening when the engine is off.

Cheers

 

I'm not very knowledgeable with electrics however I understand a little. The plugs are wet and I have pulled my rail off and done an injector test with all 4 spraying fuel in equal amounts. I will have to try and look for damage, the grey outside of the injector harness on my car is open to the elements in 1 place. I also can't find the harness grounds for the life of me, I'm not sure if I'm not seeing them because they are covered in dirt and oil or what.

Thanks for the advice

I'll go try a different injector connecter to pull, and try to find my grounds. < lol

 

Check this out, pretty good guide:
Cheers

 

Grounds are coated in a thick layer of dirt and oil grime, might need to clean those. All 4 injector plugs look good too.

This is the only part of the injector harness damage I can see.



Edit: I absolutely love NC944ER. He is amazing.

 

You won't believe this, I unplugged 2 injectors because why not, the car straight up started and idled better than the 3 injectors, it didn't idle long but it idled better. The car previously only fired on 2 injectors before I replaced them. Now the actual crazy part, I unplugged 3 injectors, so the car was on 1, and it started as soon as I turned the key, sounded good like the 2 injectors and idled just as long. Could a bad ground get remotely close to causing this? I also check all injector pins and they all have continuity to each other, I'm unsure if they should or if they are shorted since its batch fire.

 

Nothing about that makes any sense to me. Unless you're somehow just simply flooding the car when all 4 are hooked up. But that doesn't even make sense. Each cylinder has its own runner. They don't share a fuel supply past the injectors. I would say you have an issue where there is not enough signal (power) to fire them all when all are hooked up. But they squirt fine when doing the 4-cup test. So that doesn't seem to be it either. Just like with Millermatic's saga, I'm invested in your story. I'm on the edge of my seat and dying to know what you find as root cause.

I didn't even know these WOULD start and run on one cylinder.

 

To be quite honest, I'm on the same page as you, this car has taken me for a ride without even driving it. 2 weeks after I got the car I drove it around the block twice and I found out later it had two injectors not firing, It was driving great on two cylinders and now it doesn't want to stay running on two. Something about the car changed but I'm not sure what changed.

I've seen Millermatic on the forum, is it the thread titled with door chime?

 

Remove the fuel rail and insert each injector into a container of equal volume. Crank the engine for several seconds. Evaluate results. Then electrically unplug one injector, repeat test. Evaluate results. Repeat for all scenarios you tried with engine.What did you discover?

Verify injectors are for this engine. Check for procedure on Clark's Garage. Injectors are low impedance. Also Stan Weiss has a tabulation of injector part # vs flow rate and impedance/resistance.

A high impedance injector requires more current to open. Unplugging an injector provides an opportunity for more current to the other injectors. There is only one +12v supply to all 4 injectors but each pair had a separate ground. The low impedance injector opens and "latches" open while the DME controls how long the injector stays open thru the ground.

 

This thread will not receive anymore updates from me, the issue has been resolved and the update will be posted to the link below.

Link to origin story: https://rennlist.com/forums/924-931-...-no-start.html