Jman2678

Having a bit of a strange issue on my early 85 N/A 944. She's been one of those cars for about the last 3 years that just finds a way to defy expected behavior. This specific issue is with the fuel injectors. I replaced the eBay injectors that were installed by the previous owner after discovering that they are high impedance injectors(specifically measuring them at ~14.8Ohms) and subsequently learning the damage they can cause. Upon switching out the injectors with ~2.8Ohm injectors, I'm now having an issue where I believe the DME is not powering them when all 4 are connected. From what I've read if the injectors are pulling too high amperage or if they have too much resistance the DME will cut power to them. I switched back to high impedance injectors and it fired right up. This makes no sense to me because if the DME was going to cut power due to high resistance from injectors it would do it then. So I switched back to low impedance injectors and I can only get it to fire by unplugging one injector. It doesn't matter which one, I've tried every one. I'm getting about 36PSI at the rail, close to 38(this may not be 100% right, I didn't feel like double checking since my gauge is a pain in the *** to setup on the pump and I'm going off memory of this morning) at the pump. I have spark on all cylinders with or without all injectors being plugged in. Clarks pointed me to FPR however I'm doubting that due to it working just fine with high impedance injectors. I'm somewhat at a loss of where to go now. I've always gone by you need spark, fuel, compression and timing for a running car but that's not working here lol. When I get back this evening I'll be checking to see if I can feel all 3 injectors firing, then unplug a different injector, plug in whatever injector was unplugged before and repeating to test all injectors are clicking. Any advice on what steps are next would be much appreciated. If I left out any info I'll update my original post.

EDIT: I did want to mention, my current next step is to test continuity from DME connector to injectors. Only slight issue is that I can't find a single DME Pinout diagram ANYWHERE for an early model N/A, only turbo or late model. I'm very nervous about running with one of those and throwing parts at a bad schematic. My current thought process is to test continuity between all DME pins and the injectors, if I find one that has continuity then at least I can rule out a short along the way either way.

orig944

Check to make sure a previous owner did not somehow put a late DME in your car. It would be strange, but stranger things have happened. You should have 944 618 111 01.

orig944

I should have read your whole post. Since pulling any injector gets them to work. I would suspect an issue on the positive supply to the injectors, or a ground problem for the DME. In between the injectors and DME are two separate circuits with two injectors each. Your issue is where there is a common circuit for all four injectors. Your fuel pump/dme relay is part of the positive supply, I suggest jumpering it out with the famous 3 wire jumper. Clark's has the details.

Jman2678

Can confirm DME is early. I had the same thought about the fuel pump/dme relay, but if the fuel pump is working wouldn’t that be ruled out? Also, making up a new jumper now, glove box jumper snuck away again. I’ll let you know what happens.

Jfrahm

I would at least check for fuel leaking from the FPR or damper, which is common. Fooling with the injectors might be masking the problem.

Jman2678

Well DME relay seems good, with 3 wire jumper still no start with 4 injectors. Just checked and have no fuel smell in FPR or damper lines.

Jfrahm

OK, they might still leak under vacuum but that's something. Also worth checking the DME temp sensor to see if it's misreading and causing the DME to run super rich.

Jman2678

The issue isn't a rich condition causing no start, the injectors are not firing due to presumably either pulling too much current or the DME reading excess resistance or something along those lines.

Zirconocene

Have you checked the grounds on the engine and bellhousing? Clean them up with some sandpaper, DeOxit them, clean the attachment points on the driveline parts, and see if that makes any difference. You might also want to (carefully) inspect the wiring harness and ground wires. As was mentioned, the DME grounds the circuit to fire the injectors, so it's best to make sure that those pathways are as good as you can make them.

Cheers

Jfrahm

Maybe so, or horses not zebras and it's just pig rich for more typical reasons.

Jman2678

Yep, while sorting out an issue with the cooling fans I thoroughly cleaned all grounding points based off of a chart I found at that time. Included in those was the bellhousing and engine as well as about 5 more points. I inspected the ground wires this evening and everything seemed okay, no exposed wires anywhere along the path that I could see. Do you or anyone intelligent enough to read a wiring schematic know where exactly the injectors are grounded(or I guess where the DME relay is grounded?). While looking for DME pinout diagrams I found that the ground provided to the injectors through the relay is from two of the pins on the DME connector. One for each set of 2 injectors. Knowing where that grounded to physically on the car would be helpful so I can visibly(and mulit-meter) verify I have good ground. Also, just as an aside, I wanted to verify that it is actually that the injectors aren't being powered at all, I pulled them and had them sitting on cups to measure the gas output. With all 4 connected no gas whatsoever, with only 3 I get good spray. Can't I go under the wire sleeve on the injectors and check for power and ground while cranking? See if I have continuity with known good ground?

Jman2678

Jfrahm my last comment should help clarify what I mean.

Zirconocene

On a 1988 (this is the earliest year I have a wiring diagram for) the injector grounds run to pins 14 and 15 on the DME, and to ground points MP IX and VIII. MP IX looks like it's on the engine, while MP VIII is not so clear to me, perhaps that's the one on the bellhousing. MP X is also right in the vicinity of MP VIII on the diagram of grounds, so I'm not completely sure.

Apologies in advance if this doesn't apply to your car; I always get stuff wrong with non-16V models.

Cheers

Jman2678

I'll go hunting and see, thanks for a next step!

orig944

The DME ground is up under the dash near the fuse/relay panel. Also, check the red/black wire at the positive battery terminal for corrosion.