Western_PA 944

Okay, this is related to another post of mine asking about lowering the fuse box. I wanted to check the back of the box to see the connections for the DME relay, which first seemed to be the problem. First thought was fuel pump, and after some testing and time, it seemed to be the DME relay. Got a new one, popped it in, and no dice, BUT when I pushed upwards on the relay, I was able to get her started several times, even drove it a little, but alas, it stalled out and I limped home with it Long story short, I pulled down the fuse/relay box and found one of the spade terminals for the DME relay was pushed out. I reattached it and plugged the relay back in while the box was still down to be sure the terminal stayed in place and it did, and the relay does click when I turn the key, but still the car won't start. I don't hear the fuel pump humming at all as it normally would. I have tach bounce. I pulled the DME main harness connection and tested the TDC and RPM sensors with a meter, and they both read in an acceptable range. (9xx ohms)

What else could be causing the lack or power to the pump? Of course, I checked the fuse for the pump before anything else. Since the relay doesn't appear to be the cause, could the ECU/DME itself be toast?

Rensb

I could be a ground. I'm awful with the electronics with these cars, but in a similar situation I ran power directly to the fuel pump using the 3 spade jumper described on the Clark's Garage site to test the pump. If the jumper sent power to the pump, and the pump continued to not work / get power, then you could use a lead to test the pump to a grounding point. If the runs when connected to a ground, then that might be a clue there is a bad ground someplace. On my 84 there is a ground under the hatch carpet, close to the spare tire well (on a horizontal surface) that I had to clean with a dremmel and then my car started right up (issue similar to yours).

Western_PA 944

Okay... here's the latest update! I pulled the connectors to the speed & reference sensors and sprayed then with Deoxit, and cleaned them with a small, thin file. Plugged them back in, and now the fuel pump is humming,,, but she still won't start. A few days ago I checked the resistance of those sensors from the DME harness, and the readings seemed right... but when I test them with a meter from the sensors themselves, only one of them gave any kind or reading, and that was only 170 Ohms, far below what they should read. After that, I sprayed starting fluid down her throat tonight, and she sputtered to life until the fluid was gone, so now I know it's a fuel delivery issue, not a no spark issue. So, my question to those on here with far more knowledge than myself - which sensor would cause the fuel issue (injectors not firing?) I'm guessing the reference, or could it be both? Also, are the ref. and speed sensors identical? I've read that they are.

Any and all help is appreciated!!

T&T Racing

There could be two issues, one is the fuel injectors flow rate are not in spec, lower flow rate yielding a lean F/A ratio exacerbate on a cold start and/or an issue with the coolant temperature sensor circuit harness wiring to the DME or even internal to the DME where the injector pulse duration is not increasing for a cold start and thus too lean a F/A ratio.

I have a similar problem, the injectors were sent to SouthBay for cleaning and testing and returned within specs. They were installed and the engine did not start. Sprayed fuel into the intake, and the engine started

On vacation, so I will report what is happening with the coolant temperature sensor circuit, but the coolant temperature sensor stand alone tested OK

Western_PA 944

Updates since the last post - put in new speed & reference sensors, still no change. Finally threw my hands up and decided to take it to my Porsche guy, and he only looked a it but not in depth, and seemed to think it was the DME. Well, I got a rebuilt one from ECU Doctors in Florida, and still, no start. We sprayed starting fluid into the intake and it *almost* fired up, then nothing, even while spraying in the intake while cranking it.

He has been backed up in his shop and hasn't had the time to do more in depth diagnostics yet.

T&T Racing

I would suggest pulling the fuel rail with injectors from the intake manifold. Place each injector in a cup. Crank the engine for x duration. Determine how much fuel is in each cup. If there is not equal amounts in each cup, significant differences, then the injectors are not pulsing properly or some or all are partially clogged or clogged.

If one injector has no fuel, then switch it with one that has good flow to determine if it is the fuel harness or the injector.

My current situation is the engine does not start when cold. I sprayed gasoline fuel from a spray bottle, plastic compatible with gasoline. Separate the J boot and the AFM, crank the engine and spray fuel into the J boot inlet. If the engine starts keep it running at idle by spraying fuel for a few seconds and then turn off the engine. Spray a couple of mists into the J boot and then reconnect. Crank engine and see if it starts.

If the engine does not stsrt or runs rough, the the injectors are the problem.

Note: with the J boot and AFM disconnected, the engine stalls if you depress the throttle pedal.

T&T Racing

After performing all of the tests following Clark Garage and then pulling the fuel injector rail to determine the fuel injectors had different fuel flow rates, I sent the injectors to SouthBay Fuel Injectors for cleaning and testing.

My first attempt to start the engine was not successful.
Replaced the ECU with another unit, the engine started. Waited for 30 minutes, and the engine restarted.

Conclusion: test all subsystems following Clark Garage procedures and verify that the injectors are pulsing uniform flow rates.

If not, clean and test the injectors by sending them to SouthBay Fuel Injectors.

If the engine does not start, then it is probably the ECU

Porsche 83NA

Yooo player! I am having the same no start condition, so it turned out to be your DME? (ECU) your car is running fine now?? I too have an 1983 944! So its critical to know if the DME is the culprit??!

T&T Racing

THANK YOU FOR THE ACCOLADES, but have to report that I am back to a NO START condition with either ECU and the spare ECU did start once after putting back in, and ran for 4 minutes and shutoff as if the ignition switch was turn off or the injectors stopped pulsing.

So back to looking, cannot say what is the issue except it probably is electrical.

T&T Racing

I need to caution that it might not be the ECU. The race car has a AIM MXL Pista datalogger and it interfaces with the ECU in only one connection. The AIM MXL connects to the ECU/DME pin for the OEM tech and takes a square wave signal to produce the digital display.

I need to investigate if the AIM MXL has a circuit failure which adversely affects the ECU/DME to cause the engine to not start.

That is my next test by isolating the AIM MXL from the ECU/DME.

Keeo you posted

Swenny

"So back to looking, cannot say what is the issue except it probably is electrical."
It's that East German wire that Porsche used on the wiring harnesses.😁 Besides having my headlamp switch catch fire I've the same no fuel supply (runs with starting fluid pretty good!). I have done a visual check on the DME (not fried to the eyes) and changed out the DME relay and pump fuse but I have no power to the pump after testing with meter. I've made the jumper from Clark's page, I have yet to check out (injury). My pump does fire up after I pulled it out and applied power. I would like to check power from under the dash to pump. So the trouble can be only a handful of things: Bad pump,DME,relay, wire(unless I'm overlooking something). I've knocked off 2 of 4. I'm curious to read your future posts.

Porsche 83NA

Replace both reference sensors player, rotor ,distributor cap, ignition wire, new ignition coil. First thing first, check fuses, and replace fuel pump/dme relay, then as follows

Porsche 83NA

Replace the fuel pump player! I had a 7 psi one when i should have had a 40 psi one or more! I will keep you updated! I will purchase what I said to purchase, in high hopes that i will get my car running!

T&T Racing

My latest tests and analyses led to the DME harness is OK; reference and speed sensors are OK; coolant temperature sensor is OK; AFM was rebuilt and OK but probably did not need to be rebuilt; TPS is OK; DME relay is OK; fuel rail pressure is in spec; 3 of 4 fuel injectors were bad by testing (removed the fuel rail with injectors from intake manifold and placed each in a cup and observed quantity of fuel in each cup after cranking engine x seconds); switch the good injector with a bad injector in each bad injector position and the good injector fired in all 4 positions(verified the fuel injector harness is OK; removed distributor cap and verified no playing in rotor with camshaft and cheese head screw was tight, rotor OK, and distributor cap OK; remove one spark plug and placed the plug attached to spark plug wire, cranked engine and there was spark (BUT this does not verify the spark plug is firing when installed because the cylinder compression pressure lowers the resistance between the electrode and ground between especially if the plug is fouled with carbon deposit on the entire porcelain insulator or a crack in the insulator(there is an internal short and no spark);there was tech bounce when cranking the engine (good reference and speed sensors);

All of these tests except the fuel ingector flow rate test indicate and good ECU.

sent the injectors for cleaning and testing. Reinstalled the cleaned injectors and the engine started and then it did not start. On these no starts, there was no indication by change in noise that the engine was trying to fire, ie combustion but no run as previously before the injectors were cleaned.

Removed all spark plugs. There was uniform carbon deposit on the porcelain insulator for the entire length encapsulating the electrode of each spark plug. My assumption is in place in the cylinder head, the carbon deposit tracked the spark to ground and thus no spark for combustion. And yet, the removed spark plug produced spark when grounded to the intake manifold.

Replaced spark plugs and the engine started. Repeated the restart twice within 30 minutes and the engine fired and ran.

Tomorrow, retest to see if engine starts. If it does, then the mystery is solved.

The bottom line is complete all if the tests in Clark's Garage.
This data sets what is next, do not be a parts changer, expensive and frustration.

If you added fuel injector cleaner to the fuel tank, which I did a couple of times, is not a good thing. The cleaner cleans the supply line after the filter and deposits varnish deposits in the fuel injectors. Probably an outcome that caused my injectors to foul.

IMO, the tortoise wins the endurance race every time, patience, test, analyze the test data finds the "brass ring."

I thought I had it 2 days ago but ignored the carbon deposit on the spark plug electrode insulator as a non-issue. Tomorrow's cranking the engine and if it fires, then the problem was solved.

Post outcome tomorrow 10/19

Swenny

Sounds like you may have found the problem.The Porsche service manual recommends not using the plug wire or the plug to ground because of serious DME damage. I just read that within an hour ago. I'm guilty of checking for spark that way also. There's a couple of tests in the service manual that one can do to check the DME w/o a oscilloscope. The manual has one troubleshoot the speed/reference sensors also. With tach bounce my guess is that the test(s) can be blown off unless one gets desperate. You can also start the fuel pump at the firewall with the 9 pin connector by supplying power to term 4.




Suppy power to term 4 to run fuel pump