1990 740 #4 cylinder not firing

hyp2bsqr · Post by **hyp2bsqr** » 24 Sep 2013, 22:05

Ok so I went out for a test drive, wasn't pushing it and the engine stumbled, coughed and then I lost #4 cylinder. I did a compression check and it is 130# and the rest around 150#. Swapped injectors and no change. I don't have an NOID to test the ECU out on the injector wiring.

Here is what is even more weird. If I start it on 3 cylinders and turn the headlights on the RPMs drop significantly and it runs rougher. Additionally if I pull the starter solenoid green switch wire the RPMs change. To me this smacks of some strange electrical issue.

This car has been screwed with by the PO who is very poor with electrics and I am at a loss as where to start with this thing. Previously the car ran on all 4 cylinders just fine.

If anyone has any suggestions as to an approach I would appreciate it. This thing has been sitting for 3 months now because I have about given up on it and am close to just tearing the entire harness out since it has been compromised in so many places with those gawd awful 3M clip-on splicing connectors. 3M should go to federal prison for that invention.

Anyway HELP please:(

Pete

hyp2bsqr · Post by **hyp2bsqr** » 25 Sep 2013, 08:02

UPDATE:

I have checked all the obvious stuff like plugs and wired, I should have listed that. Keep in mind it was running fine until it stumbled and I started moving stuff under the manifold.

I left an important factor out. This car has been modded significantly. The intake and rail are custom fab to accommodate huge injectors and big hairdryer.

Back to the electrics. If the headlights were shorted they wouldn't work - they do. To me it seems like a ground issue but as I said I have tried everything.

Does anyone know what of the harness I can unplug before I lose my engine control? I want to eliminate as much as I can.

UPDATE:

Since the injector all fire from the same power source I swapped injectors to eliminate a bad injector. Then I swapped #3 injector cable over to #4 and vice versa. #3 fired using #4 cable but #4 using #3 cable is still dead.

If the piston were bad I'd expect no compression. However I have 125-130LBS after 2-3 revolutions. The other cylinders measure 150LBS.

Could the core of the issue be a stuck valve?

Any help would be appreciated.

Pete

hyp2bsqr · Post by **hyp2bsqr** » 25 Sep 2013, 10:03

Porblem solved and I feel like a schmuck. Bottom line - partially stuck valve (suspected but not entirely proven) AND more importantly an intermittently bad #4 plug wire. More specifically an intermittent short to the radio suppression shield that only stopped shorting under heat. Chalk that up to another weird one.

I still have some bizarre voltage drop issue based on the behavior with the headlights and starter lead but I can at least run the car.

Thanks everyone!

lindy8_man · Post by **lindy8_man** » 27 Sep 2013, 12:14

Thanks for the trouble shooting description above! Every car has their little quirks. But there is nothing new under the sun!

As for the voltage drops and even loss of RPMs. I would check your coil packs (or replace, they are not that $) Also I would get your alternator checked. Headlights and starters are huge Amperage draws. When there is a heavy draw, the alternator adds physical resistance to the belt and therefore a drop in hp. You are not blowing fuses, so I wouldn't suspect a short somewhere. But you might check the headlight circuit to see if you have any 3M parasites drawing current away...