84' 944-No start until pull/replace DME Relay

[Jfrahm]

I have not been paying close attention but fuel pressure seems unlikely.
Have you looked at the plugs after it cranks but does not start?
Did you try a shot of starting fluid when it cranks but won't start?

Feels like a typical wet plugs, blown FPR or damper diaphragm issue to me (or wonky temp reading skewing the mixture rich).

[Monkey Wrench]

Id hook a small 12 V test bulb across the primary coil so you can see if it's blinking/ flickering when you have a condition of no start. If the light bulb stops flickering then it's a spark issue. work back through checking the spark path through the powerstage transistor and to the DME.

Id also pull the fuel rail and watch the fuel flow, see if it looks like the injectors are always spraying normally by trying ot start a few times, but do that outside and take precautions of the gasoline explosion or fire risks. cold engine , no sparks dont collect any substantial amount of gasoline. gasoline rated fire extinguisher at the ready.

maybe buy a noid light so you can see if the injectors are getting their pulse during this time when you have a no start condition. a fuel pressure gauge may help show the pressure if the raw gas makes you nervous.

i think either your fuel delivery or the spark is being interrupted intermittently. Knowing which will tell you which direction to go in. no point confusing the issue without making that differentiation
a fuel delivery issue could be either no pulse at the injectors or no fuel pressure to them.

I don't think you have bad injectors because they wouldnt all fail so it would still start even if rough. you drove it so the fuel pump can operate, that does not mean you cant; have intermittent power to the pump. Id put a small test lamp in parallel to the pump, permanently, so you can easily watch that lamp to make sure it is on when the problem occurs. it will always be on when the pump is then. Just use a little bulb like a dashboard illumination lamp. the power draw is insignificant then.

sometimes pumps come with new wires and those crimp connectors, id make sure your connections there are really good and solder them. a bad crimp there can stall you in a intersection or when pulling out in traffic , That can cause a car crash.

the lamps on the new relay might do that but can you see them when you are cranking? thats why I'd put them in view of the drivers seat. - so you can note if it happens but intermittently. I think yours , like mine is down under the drivers side and out of view when cranking unless you are a contortionist. I suppose you could extend the relay to a better position if you wanted but using an old relay as the "plug in" and then spade connectors on the end of your extension cord. thus move that relay in view of the drivers seat.



if you remove the fuel rail, also check any related ground wires, a bad ground could cause this and that's as likely as a no power to it situation.

you can wiggle wires when it's running , if you found a bad wire or connection and wiggled it, it might cause it to die or sputter.

maybe the immobilizer is preventing spark, my car has a key-switch just to the right of the drivers door key.. you can jump it, but I'd find out first if it actually has an intermittent no spark or no fuel condition, or not.

when you find the cause it will then be safe in traffic and this will probably be an easy repair, the trick is isolating the actual problem and then you can relax and feel better, until this is resolved it's a danger. You really don't want it to quit suddenly so until it's resolved refrain from depending on the power as it might stall. no pulling into traffic suddenly , etc.

if you swap a part and then it starts up ok and you think you have it resolved, Id recheck by putting the bad part back to verify, the reason is that, for your safety, you want to know what it was so you can be confident that it's fixed and not just hidden for a while, only to return at a bad time.

Good thing you have patience.

[originalowner]

Originally Posted by JD1990

Observation -
the red - Fuel Injector led - was "on" when I plugged it in, and remains on after the ignition is turned off.
the green - Ctl Fuel Pump led - turned off after start when at idle. But the Blue Fuel Pump led remained on.

I don't have one of these relays, so I cannot answer if this is correct. You should read whatever instructions came with it to interpret.

More importantly...

What do the LEDs indicate when you are cranking and it does not start?

[djnolan]

Originally Posted by Jfrahm

I have not been paying close attention but fuel pressure seems unlikely.
Have you looked at the plugs after it cranks but does not start?
Did you try a shot of starting fluid when it cranks but won't start?

Feels like a typical wet plugs, blown FPR or damper diaphragm issue to me (or wonky temp reading skewing the mixture rich).

How do you test for a blown FPR or FPD? It is discussed extensively here: Fuel Pressure Regulator - faulty?

[Monkey Wrench]

one problem with the shotgun approach ( replacing multiple parts to try to solve a no start issue) is that you can introduce a problem , like if that speed sensor or one of the other parts you replaced is bad. If I do that I'll often try the part note the difference and then if it doesn't; solve anything , remove it or the duration of the problem solving, because there is probably just one reason and you really dont want it to not start for more than one reason at the same time.

when it does start, does it fun perfectly well or is it running a bit rough? if it runs perfectly when it starts you might rule out a vacuum leak and look for a bad ground or similar wiring issue. a bad wire connection somewhere can easily go from running fine to not at all, a bad adjustment or vacuum leak etc might cause it to start only sometimes but not run quite right. That's why how well it runs when it does start might be a clue.

last night I had a problem starting my volvo 240 after replacing the head gasket. turned out I had a unplugged temperature sensor wire and that was causing it to start about half the time and switch back and forth between starting sometimes ( along with a check engine light) and not starting sometimes.. . I think the ECU saw that there was no temp reading, it got all confused and said no way I'm shutting down..

once plugged in right, problem solved. Maybe the porsche is similar where a bad temperature sensor or bad wire to it might cause a no start issue too. If I took a more analitical approach I may have found that he spark was not there sometimes and went down that path to find it was because the ECU was unhappy. on the porsche thats the DME, but similar ..

someone else here might know if a bad temperature sensor could cause a no start issue . I dont think the temp sensor would have been the first thing I'd suspect to cause this sort of issue. My volvo has two , I think one for the ECU and one for the dash gauge. I bet it would run with out the dash gauge sensor, bit not without the one that reports to the ECU

[Jfrahm]

> someone else here might know if a bad temperature sensor could cause a no start issue

Yes it can cause the ECU to over-fuel and wet the plugs. They tend to fail suggesting it's very very cold.

[djnolan]

JD, can you recheck this and make sure it is correct? Also what lights are on when it is running?

Originally Posted by JD1990

Observation -
the red - Fuel Injector led - was "on" when I plugged it in, and remains on after the ignition is turned off.
the green - Ctl Fuel Pump led - turned off after start when at idle. But the Blue Fuel Pump led remained on.

[JD1990]

Apparently I haven't responded in a while. Life got in the way - graduation and other family items all on the good side.
I've been trying to get the right fitting for the fuel rail. the M16 is too big for what I have.
I didn't notice that this thread got to a 2nd page. Thanks for all the input here.
I've got time this weekend to hit a few of the recommendations. I'll run through them and let you know what I get. I think the near term focus is on the fuel and spark suggestions. Tried a start from cold this morning and took 5 or more attempts then a rough idle. It did restart, but only after a few tries. So, I'll keep troubleshooting. More later...

[JD1990]

I did continue troubleshooting. Pulled fuel rail and set up a safe environment to test the injectors. They all appeared to spray about the same, and no drips after we stopped.

I pulled the plugs and all looked about the same - half black the other half clear. Thinking they're slightly fouled I replaced them. Still takes 3-4 tries to get a start.

When it does start, it will run a little rough in the 800-900 rpm range. Smooths out the closer to 1,000 rpm you get.

The solid state DME relay with the 4 colored diodes - top to bottom
Blue - Ctl. Fuel Pump - on in run and start position
Green - Fuel Pump - off in run and start position (didn't rev or put under load)
Red - Fuel Inj. Power - on in run and start position (also remains on constantly)
Yellow - Ignition SW - on in run and start position

The Blue - Ctl. Fuel Pump relay will be on when you turn to the run position. You audibly hear the fuel pump run for 3-4 seconds. It then cuts off as does the light. Upon turning to the Start position the Blue light comes back on (can't hear fuel pump over starter). It remains on when you let the key go back to the run position. The Green "fuel pump" diode does not come on at all during the starting process.

Going to pull up the temp sensor check and also try some of the vacuum system tests. There is a new FPR on the return side of the fuel rail. I need to find the correct adapters for a fuel pressure gage. the M16 mentioned earlier was too big.

[walfreyydo]

You dont "need" the adapter (although its best that you do) to do a fuel pressure test as shown in this video:
https://www.youtube.com/watch?v=Dq6anWRYlGA&t=150s

Have you run through the tests on Clarks Garage?
https://www.clarks-garage.com/shop-manual/ts-01.htm

A quick google search reveals the fitting is a M12x1.5...
https://www.google.com/search?q=porsche+944+fuel+rail+fitting&rlz=1C1OPNX_enUS940US940&oq=porsche+944+fuel+rail+fitting&aqs=chrome..69i57j33i160j33i299.4176j0j7&sourceid=chrome&ie=UTF-8