fuel injection problem

Nick Wulf · 08-29-2007, 02:13 PM

I have an early 85 944. I just replaced 2 valves and the timing belt. as soon as i got the engine back together correctly, i tried to start the engine. No luck. I traced the problem to the fuel injection, none of the injectors are working. The injector plugs have 12 volts power when grounded to the block, but none when completing a curcuit back to the DME. I figuired that if i get power through each pin when grounded to the block, then is the relay still good? or the computer? does it mean anything?

idontknow · 08-29-2007, 04:15 PM

Does the tach bounce while cranking? They're tied (somewhat) to the fuel injectors and that'd tell you if the reference sensors are working.

bholmes · bholmes's Garage

If I remember corectly the injectors are a broken ground switch, meaning that they have power to them all the time and what trigger them is the completing of the ground circuit, that explains why when it's grounded to the motor it reads 12v. Check for fuel pressure per clarks garage and let us know how it goes!

bholmes · bholmes's Garage

Sorry, i didn't notice this was your first post. Welcome to the board! and the clarks garage thing is clarks-garage.com good luck!

Nick Wulf · 08-29-2007, 07:34 PM

I have fuel pressure, a little over 1 bar. The tach reads nothing when cranking, but i'm posotive the flywheel sensors are hooked up, and hooked up correctly. So if i have power to the pins, then it means nothing about the DME or the relay, because the DME only interupts the flow of power.

bholmes · bholmes's Garage

that is my understanding, as long as i'm not mistaken. Look up a schematic for wiring on the DME and then test it according to that. Or If it is a broken ground type switch you could set your meter to measure ohms of resistance and then ground one probe to the engine or chassis and the other probe put in the ground wire for the injector. If your meter reads zero ohms when the injectors are supposed to be firing (you'd probobly have to crank the engine over) you have continuity, however if you read a bogus reading or something more than zero ohms you could have a problem. That may help. I've never had to do it before so i'm far from an expert.

Slam · 08-30-2007, 11:15 AM

If the tach isn't bouncing I'd suspect the sensors (as stated) and/or the DME relay. Did you have the sensors unplugged at any time? Check the connectors at the back of the engine, right by the firewall (one's marked 'DG' - IIRC - can't remember the other). Your ignition switch could be another culprit. Have you got spark?

fast924S · fast924S's Garage

Did you gap your speed and reference sensors correctly???

Nick Wulf · 08-30-2007, 05:39 PM

it has spark, and i didn't take off the flywheel sensors. The meter reads 1ohm when cranking with 1 probe on the engine, and 1 probe in the injector ground wire

idontknow · 08-30-2007, 06:11 PM

From Clarks Garage
"Another way to check the operation of the speed and reference sensors it to crank the vehicle and watch the response of the tachometer. If the tachometer jumps during cranking, the speed and reference sensors are probably good. If it does not jump, it indicates a problem with the speed and/or reference sensor or the DME computer"

Though if you have spark, the sensors must be err sensing good so I'd look for problems with the injector wiring.

If you're up for it, disconnect all the injectors. Connect a test light to the prongs of one of the injector connectors. Crank the car and watch for a blinking light. Blink = ok wiring and DME.

Odd info: The DME isn't Sequential FI. They batch fire all at once and use 2 ground switch feeds. The reason for disconnecting the injectors is to break up factors for your problem