1984 911 Carrera rough idle - won't accelerate

[shafferl]

I drove my 911 to town and parked it today. A half hour later I went to start it and it started right up and idled for about 5 seconds and then suddenly quit. Just like someone had shut it off. It then turned over and over but it wouldn't fire again. I tried on and off for a half hour or so. I then read through the forums and tried jumping the fuel pump fuse and I could hear the pump run. Maybe the DME relay? I got a ride home and got a few tools and came back 4 hours later. Tried to start it, and it started right up....but was idling very rough with some backfiring. I stepped on the accelerator and nothing happened. I shut it off and tried several more times with the same result. I then pulled the DME relay, opened it up, and looked at the soldered connections inside and they looked fine. Reinstalled the DME and started it again, but got the same result. Very rough idle and no engine acceleration. Any ideas?

[ischmitz]

Can be a multitude of things: My gut guess: Mixture off, too much or not enough fuel.

- CHT not working (failed open). It will tell the DME the engine is stone-cold and enrich the mixture like crazy. With a cold engine you get it to start and run rough but once warmed up there is way too much fuel for it to run. It'll feel like someone pulled the plug.

- Lack of fuel pressure. A fuel pump on its way out has done to me what you describe (very hard start, rough idle and pressing the accelerator makes doesn't do much and engine wants to die)

- Gross air leak (not so likely in your case as it would not fix itself after 4 hours)

Ingo

[shafferl]

Should the fuel pump build up pressure and shut off, or does it just continue to run when the fuse is jumped? I can't hear the fuel pump when I turn the key on, or when I am cranking it over. But it may be hard to hear when cranking. I jumped the fuse and the fuel pump was running and then started it, but it ran the same way. Other vehicles I have the fuel pump will build up pressure when the key is turned on, and then shut off once pressure is met with the engine not started.

[dentist90]

Pretty sure our fuel pumps only run when the DME senses the flywheel is turning, during start or normal running.
If key is in run position,but car not started, no fuel pump. When running the fuel pump runs continuously. The excess fuel is returned to the tank by fuel pressure regulator. If you jump the fuse (ie, send 12v directly to pump) it will indeed run continuously and you can hear it.

[Nick Triesch]

Try Techron. Could be really clogged fuel injectors. Will clean them in a few minutes for $8.

[shafferl]

The car ran great for 40 minutes. (Has been running very smoothly these last few weeks since coming out of storage) Shut it off for about a half hour. Started and was idling normally before it unexpectedly shut off and then would not fire. Sat for 4 hours and then would restart but idles very rough, backfires, and stepping on the gas pedal doesn't make a bit of difference. Is there a way to check the pressure coming from the fuel pump? Does unhooking the battery do anything as far as resetting anything?

[ischmitz]

Battery unhooking doesn't do anything. If pressing the gas pedal does nothing is it possible the throttle linkage came undone. At least it should open the throttle plate and either accelerate the engine or kill it. Verify motion at the throttle body while someone steps on it.

Measuring fuel presume require a fuel pressure gauge kit. It can be bought online. Google it.

[911obgyn]

Check cap and rotor I had a similar issue on my 85 with 150K+ miles and had to rebuild distributor,the bushings had worn so bad the rotor was hitting the cap.

[Hugh R]

As ISCh said, CHT was my problem.

[shafferl]

Rented a trailer this morning and went to get the car. Jumped in and it fired right up and ran like a top.

[Nick Triesch]

A bottle of techron dude. really.

[Walt Fricke]

CHT - they don't last forever, and get frustratingly intermittent. PITA to change, but bite the bullet. Other suggestions worth following up on - Techron won't hurt, and checking the distributor is easy.

[911 sc]

fuel filter plugs wires Dist cap rotor , was it raining or damp out when this happens?
Yes cht change it mine just gave me trouble.

[shafferl]

CHT makes sense. Car was running great and now runs great again. Hard to see it being clogged fuel injectors. I did put a bottle of Sea Foam in a few weeks ago. Running non-ethanol premium gas. Was a warm sunny day when it happened. Certainly acted like an electrical issue.

[gomezoneill]

Reference and position sensors. Always create an intermittent situation.