86 non-turbo 911 Targa does not start

[remigio]

Starting problem on non-turbo 1986 911 Targa: Car will crank but does not turn over/start. It will, however, start with shot of starter fluid but chamber is still "starved" even though fuel flow is okay thru the filter after replacing both the fuel pump and the filter. Electrical, maybe? bad fuel relay?

Appreciate any and all suggestions.

[911obgyn]

DME relay under the seat next to the brain is the first thing i would check. It is a combo relay and everyone with a 3.2 motronic system should have one in their glovebox.

[steely]

+1 - check or replace relay - don't throw out old one if new one doesn't fix it.
It could also be the reference sensor - they can open up from age and prevent the DME from keeping the relay from operating.
Let us know how you make out w the relay tho'.

[DRACO A5OG]

+3 DME relay then Ref Sensor

[remigio]

thank you, fm remigio

[remigio]

relay shipped fm peiican; will post what happens after replacement & troubleshooting. thanks again. remigio

[steely]

cool - please do, and good luck.

BTW, if you do wind up getting a ref sensor, buy the bmw version from here, it's same bosch part, the cable is a bit longer and it costs less (or at least it used to).

[ischmitz]

If if fires with starter fluid it has spark. That should shift your focus to the fuel system.

- DME relay
- fuel pump
- fuel filter
- CHT if this is a cold-start issue only

Do a fuel pressure and delivery test and verify your CHT reads correct resistance. And if above is fine it’s on to the DME. An easy way to test the DME relay would be to ground pin 20 of the 35 pin DME connector. It should turn the DME relay on with key in ON position. If you can’t hear the pump you found your issue.

The reference sensors will kill fuel AND spark. No exceptions.

Ingo

[remigio]

swapped relays last week, did not solve the fuel delivery issue; and it does start with starter fluid in the chamber.

as i'd said before, we eyeballed the fuel delivery to the output end of the filter. it was good flow. last thing we did last Saturday was to test elec current in the injector wire. test sat, we have power in the wire. question now is, is it just the reference sensor or are there other issues I should be considering? thanks for the help/remigio/

[megerian]

If its the Cylinder Head Temp sensor, here is some info and pictures, since this just happened to me 3 days ago

Stranded out in the country - any guesses?

[ischmitz]

You might not want to pay attention to what I said but fact is: if it’s firing with starter fluid it has spark and it can only have spark if speed and reference sensors are OK. So moving on:

A bad CHT (short to GND) can cause extreme lean conditions at startup. Check it and make sure it’s not short.

A non-working fuel delivery (fuel pressure not present) during cranking can cause the issue - check fuel pressure during cranking.

After that it’s down to checking fuel pulses from the DME with a NOID light. If they are absent the DME needs repair.

Keep us posted

[steely]

Does it even try to 'catch' or start when you are cranking?
Differentiate the symptoms between cranking (starting) and 'running' - sounds stupid I know, but if everything is OK, the DME will provide spark & fuel during cranking.

It is only when the engine starts and you get off of the key (run) that the Ref sensor is used to see RPM in order for spark & fuel to remain.

So you have a small catch-22 of sorts.
You either have a small issue preventing it from starting and a good Ref sensor further disabling it, or your Ref is bad to begin with. Makes for a fun investigation.

Follow Ingo's recommendations, and I would suggest just looking at the Ref Sensor cable itself.
If it is cracked and crumbling, I'd say it's a good candidate for replacement.

[ischmitz]

If the Ref sensor or the speed sensor is bad the DME will not ever issue a single spark signal no matter how much starter fluid you introduce. No exceptions. During cranking the sensors are required to tell the DME when to send a spark.

The only thing that lets the DME work albeit somewhat poorly is an intermittent Ref sensor. It is not needed all the time but needs to deliver at least one pulse to kick the DME into life. There is a famous thread where John Walker found a non-ferrous dowel pin in a replacement flywheel as root cause for a non-start. He was able to start the engine by waving a wrench by the a Ref sensor connected to the DME instead of the Ref sensor in the car. Go search for it.

The difference between key in START or RUN is only how the 2nd stage of the DME relay gets activated. Logically the sensors (flywheel is rotating) see FW rotation and the START signal are OR’ed inside the DME. If either is present the fuel pump and the O2 sensor heater turns on. So during cranking the 2nd stage of the DME relay will turn on simply because the DME sees the START signal alone (even if both sensors are dead). And the moving flywheel during cranking is enough to keep the DME relay activated when you let off the key.

In RUN the flywheel is supposedly still spinning and this is what keeps the DME relay (fuel pump) on. If you stall the engine through a crash the idea is that once the flywheel stops spinning the fuel pump shuts down. This is a safety feature.

Ingo

[steely]

Thanks - I mistakenly thought the coil trigger was bypassed too.

Originally Posted by ischmitz

It is not needed all the time but needs to deliver at least one pulse to kick the DME into life.

Just one? I thought there was a min rpm requirement.

[ischmitz]

The speed sensor needs to see about 100 - 200 RPM to keep the DME relay on (internal status = engine running). The reference sensor really only needs to be triggered one time to get the engine to start. It sets TDC and thus timing.

[steely]

Thank you for that too!
with apologies to you and remigio

[remigio]

per page 209-1 of the Bentley book, tested fuel pressure in the rail. no squirt so I am thinking there is not enough fuel pressure in the line all the way to the rail. comments, guys? thanks remigio.

[remigio]

correction to last post... page 201-9 of the Bentley... remigio

[ischmitz]

How did you confirm the fuel pump was running for your test?

Did you try the jumper test inside the DME connector?

[remigio]

back home this Friday. will review comments and work on the problem this Saturday. will keep u guys posted