944 will only run with 3 injectors plugged in?

prior944 · 05-07-2015, 07:02 AM

So last summer my 86' 944 N/A died all of the sudden while driving on the highway, I pulled over tried to start again and it turns over no problem but will not fire. Had the car towed home and have been trouble shooting on and off since. I have spark and was able to get it to run a few seconds with a shot of quick start. I then tired pulling an injector connection and it starts and runs no problem, but the second i plug that injector in it dies immediately. This lead me to believe the FPR had gone bad so i replaced it, but it didn't make a difference. I have pulled all the injectors, tested and cleaned them, tried jumping the DME, and replacing the ECU under the passengers floor and still no change. I tested the fuel pressure its good; 36 psi with the fuel pump jumpered; 30 psi while the car is running (with one injector unplugged), and if i pull off the vacuum line while its running i get about a 7psi jump. I have replaced the plugs, wires, cap, and rotor and the fuel pump and fuel filter. Also it doesn't matter which injector i unplug i pick a different one each time. I'm all out of ideas... if anyone has any suggestions they would be greatly appreciated.

Jfrahm · 05-07-2015, 07:24 AM

Fuel damper pissing gas into the vacuum system? It could also be the DME temp sensor, if it falses cold it will run pig rich to compensate.

kdjones2000 · kdjones2000's Garage

Weak fuel pump as well can cause this.

kevingross · 05-08-2015, 08:07 AM

Make sure that the 4th injector is not electrically shorting things. A 944 will idle on three cylinders but not two. The eight-valve cars share DME output drivers as a pair: one drives two injectors, another drives the other two. So if by plugging into a shorted injector you disable it and its partner, the car will die.

Just a theory.

Tom944 · Tom944's Garage

I would ohm out all the injector wiring. Sounds like with them all plugged in the resistance is too high to fire any of the injectors. There is likely a fault in the common side. Not sure if that is the ground or positive without looking at the schematic.

prior944 · 05-10-2015, 06:33 PM

Thanks for all the suggestions!! I will try them all out this week and post how it goes!

prior944 · 05-21-2015, 05:32 AM

well i have tried checking the resistance of all the injectors, I've checked the wiring, the mass air flow sensor and the throttle position sensor and everything has been up to spec. One thing i noticed is as long as the car is cold I can usually get it to start and run for about 10 seconds... it runs really rough and only idles at about 600 rpm, but it checked the fuel pressure when it was idleing like this and it was too high.. about 35psi, then as soon as i unplugged a fuel injector the fuel pressure dropped back down to 29 psi and the rpm came back up to 1000 and ran much better.... I am confused why the fuel pressure would drop when I unplug an injector?