Big Fuel Injection problem

Kekec944/84 · 07-29-2004, 06:19 PM

Well I have an 84 944 with no mods.

My problem is the car will not start with all four fuel Injecters pluged in if I unplug 2 injector plugs it doesnt matter wich two the car runs bad but it runs. I know that the DME has a saftey if too much current goes through the injectors but I replaced the Fuel pressure regulator recently so I don think the fuel presure is the problem. I dont know but mayby there is a bad ground from the fuel injetors making it difficult to get enough current to run all 4 of them. If someone beleives that could be the problem then can they give me some advice on which fuses and relays are involved with the fuel injectors and other spots where I can look for a bad ground. Earlier I was talking about the DME shut off to the fuel injectors when there is too much current through the injectors. So, does the DME cut out when the combined current to all the components conected the DME become too high or is the cut-out system only isolated to the fuel injectors? Because for a month before I had this problem the idle would somtimes drop to 500 rpm an then recover back to 900 so i was wondering if a faulty engine speed sensor or any other component connected with the DME is drawing more current then its suposed to making the DME saftey kick in shutting off all four injectors.

I really would apreciate some help with this one thanks guys.

SoCal Driver · SoCal Driver's Garage

DME does not have a "safety" persay. The four injectors fire at the same time. If the fuel pressure is too high the voltage is insufficent to open them. Remove the leads to one or two and there is sufficent voltage through the DME "switch" (scr?) to open them against the fuel pressure.

The grounds for injectors go back to the DME. They don't go through the chassis first.

I would pull the plug on the DME and strip back it's cover to see if any of the wires have pulled off of the pins. Also open up the DME and check for loose connections/solder joints on the printed circuits from the plug to the driver for the injector.

Here's the electrical diagrams for your car:

http://www.pelicanparts.com/944/electrical/944_electrical_diagrams.htm

Kekec944/84 · 07-29-2004, 09:59 PM

Does anyone have a better electrical diagram i can refrence from?

And thanks SoCal I'll do that and let you know how it goes

SoCal Driver · SoCal Driver's Garage

The electrical diagrams are from the factory manuals. Don't think they get better than that. However I have to admit it's sometimes difficult to read the wire color codes.

Kekec944/84 · 07-30-2004, 07:33 PM

I was reading some other posts and they said that tapping the DME fixed there no start problem, I tapped the DME with a ratchet and sarted up, so like you said probly an old tired joint on the circut board im just a little scared to resolder can i damage anything or is there areas i should stay away from? or is there a procedure for doing that I herd that Danno's racer-x site has a procedure but i dont know the URL.

SoCal Driver · SoCal Driver's Garage

Nothing trick to it. Just get a $15 soldering iron, open the DME up and start resoldering the connections from the plug to the driver for the injectors. The factory electicals will tell you which pin numbers the injectors connect to. The pin numbers are also cast into the connector.

Chris_924s · Chris_924s's Garage

SoCal, a good diagnosis.. but I really dont recommend the DME resolder as a first time project for anybody..

It's very easy to say it's easy.

I've resoldered several (Been building circuits since the old breadboarding days.. 1975 to be exact.) and there are hundreds of contact points. Too hot an iron, too long contact, too much heat will cause damage to components and the ribbon cable.

When I resolder DME's I also remove components and test if they look like they have heat or cycle stress.

It's not completely visual if a cold solder is present, so continuity checks will have to be made and the traces on the boards will have to be inspected by a trained eye.

By resoldering a DME without proper instruction you can do more harm than good - actually introducing ne cold solder joints and bridging circuits with excess solder.

PM or e-mail me if you need help on this project Kekec944/84.

SoCal Driver · SoCal Driver's Garage

Guess I've been soldering too long and just have the hang of it. I have to admit that putting together the Heath Kit CB radios in the early 60's was lot's easier with the wire and lug than the newfangled PC boards and their discrete components. Then there is surface mount!

Kekec944/84 · 08-01-2004, 09:57 PM

ive soldered before I'll do ok I just want to know if there are any heat sensitive areas that I should look out for I picture would be great. But for now the car is running good so I will leave it as is but if the problem becomes persistent I'll probobly bring out the iron. I did the dme relay it came out ok but I just dont want to screw around with the DME a new one would cost me a fortune.

Porsche944 · 08-02-2004, 10:55 PM

SoCal:

You had it, then you let it go!
If the fuel pressure is too high the voltage is insufficent to open them.

My first check would be fuel pressure as a bad Fuel Pressure Regulator will cause this exact problem.