odonnell

Having an issue with the injector circuit drawing too much current, and the DME kills the motor within a split second of it catching upon startup. Fuel pressure is perfect, replaced FPR less than a month ago and it runs like a boss whenever the DME lets the injectors pulse.

It was running perfectly and starting first time every time until today, I warm-started it and it was up to the old game again. Unhooking an injector harness didn't affect it when I tried to start it on 3 cyls the first time, but on the second attempt it caught and ran. I plugged the injector back in and it started the very next try, on all cylinders, and was fine for the drive back to my place.

I think they're the factory early injectors, although they were rebuilt and balanced by Witchunter about 7-8 years ago and have never been a problem before. Im running a 1987 DME as of a couple months ago, and have heard the injector impedences are different between early and late but not enough to matter. There is a little bit of oxidation on them, but has never been enough for me to worry or anything.

Anyone have any wisdom? Does this sound like a problem with the injectors, or somewhere else?

william_b_noble

sounds like a problem elsewhere - the DME doesn't have a way of monitoring current - are you sure there isn't a bad connection to the crank sensors?

thomasmryan

If you do need to replace them....I have had good luck with injectors from the guys at five-o motor sports in land rovers and mercedes vehicles.

See if they have something that matches the parameters of your motor if you need to replace them.

odonnell

I was reading that the injector drivers will auto-shutdown if there is too high of a load on them. Is that incorrect? My crank sensors are relatively new Bosch parts, and the connectors are in good shape. I pulled the boots back and they look great. Full continuity through harness. When it is running, jiggling the harness there will not induce a stall. It starts and catches, but then dies, so I wasn't suspecting them.

Thanks for the tip, the next question was going to be "so what injectors are you guys running." Saw a bunch of stuff on fleabay that would work, but unsure of quality.

Arominus

Even small differences in impedance can have an effect on the driver circuit if its degrading. Lower impedence is more work for the circuit. Sounds like your dme is having a hard time with it. Have access to another dme? Or another set of 87-88 injectors? I'd look at that long with your wiring.

Arominus

Editing posts from my phone sucks. Anyways, start with the wiring. Get a buddy and a multi meter and loom at the injector leads back to the dme connector. Your looking for massive differences

odonnell

Thanks. Just to clarify - I'm looking for internal resistance in the harness?

I have a good early dme and AFM I will swap in, if the injectors turn out to be fine.

V2Rocket

it shouldnt be the injectors, the problem is somewhere else.
why do you think the injectors are getting too much current? what kind of test did you do to determine that?

odonnell

The car was insta-dying and I got it to run on 3 cylinders by unplugging one injector. The lower current draw allowed the DME injector drivers to pulse without going into self-preservation mode... or at least that's what my troubleshooting led me to. Better details in first post.

Arominus

Yes, if you have a wire going south it can make a lot more work for the injector driver. Clean out the connectors with de-oxit or something similar as well. The dme/afm swap is a good idea too, even with the NA-tune your DME's injector driver could be getting old too.

Rogue_Ant

The DME does monitor current - it simply does it all in hardware.
The injector driver circuit adjusts itself based off of injector current.

Good thinking. A wiring issue is the most likely culprit.
I would also measure the resistance on each injector, just to make sure none of them have an internal short.

T&T Racing

When the DME senses a predetermined high level of current draw on the fuel injector circuit, ie fuel injection wiring harness, the DME will turn off the engine.
Since you have replaced the fuel pressure regulator, I would say you have high resistance in the fuel injector wiring harness.

Is the harness the original supplied with the car? It is diffiult to find the weak or wiring lead that is causing the high resistance.

If so, purchase a new one from Lindsey Racing, a complete harness including new injector connections. Just need to find the wires numbers from the DME harness, 2 wires, connect the wires and good to go for another 25 years.

odonnell

Thanks for the advice guys. I played electrician and found the harness and injectors to be fine, from a resistance standpoint. Stock harness AFAIK. I installed a new Bosch fuel pump, because maybe the old pump was under-performing at certain temps and injector demand, and it's been running great so far. I'm secretly saving up money for standalone because the old Bosch FI stuff is holding this car back from being a reliable daily...

V2Rocket

Tell me more about this overheating injector circuit.
High fuel pressure could make the injectors harder to open, drawing more current, right?

odonnell

For posterity sake, this issue was either due to a bad fuel pressure damper or my harness being ratty. The problems with warm starts never went away until I gutted all the Motronic parts. I had replaced the damper just before that, which helped (old one was leaking through the diaphragm) but it still was never right. Runs and starts perfect these days and is being daily driven again.