'84 Carrera 3.2 shutdown problem

[Synchromesh]

I have a very early Carrera Targa (built 8/83). It's fully stock.

I've been experiencing random shut downs on the engine. Problem is sometimes it happens when the car's been sitting for a few days and sometimes after it ran for a while. But generally it gets better after the engine is hot. The idle seems to dip occasionally and sometimes it dips enough to kill the engine completely.

We've done quite a bit of testing with a friend of mine and figured out it's NOT the DME relay (it was still good but I installed a new one anyway) and it's not fuel pressure related as pressure stays constant even after the engine died. 31 pounds while driving (fluctuating), about 36 pounds at idle. The pressure in the line raises to 39lbs right before the shut down occurs indicating injectors have closed up. ICV valve was removed and cleaned and seems to operating properly. So the problem is most likely electrical.

Head temp sensor is an upgraded 2-wire 964 version. I believe it's working ok because the resistance is going up as it should to 1K ohm as the engine was cooling down. O2 sensor looks very recent as well.

Any ideas what else could be shutting down the injectors or any other tests to throw at it? We're a little stumped.

[Tippy]

Did you check the speed and reference sensors?

[darrin]

what about the dme RELAY? (which sits behind the DME) -- your issues sound similar to issues I had when my dme relay was failing

[Synchromesh]

Haven't checked speed and reference sensors. How do I check those?

As for DME relay - that way my mistake in the original post. We checked the relay and I replaced the relay. We also checked the actual DME Unit and it looks like new inside with no visible damage anywhere. I'll fix it for clarity.

[gomezoneill]

Miliage?

[Hnichols]

Originally Posted by Synchromesh

Haven't checked speed and reference sensors. How do I check those?
.

You remove the plugs located on the left hand side of the engine bay. There is a bracket with three plugs: the top one is white -- that's your CHT sensor, the two black ones are your speed and reference sensors. There are small square wire rings that lock the plugs in place: gently remove these and the plugs disconnect easily.

Each plug will have three pins. Connect an ohmmeter to the bottom two and measure the resistance across it; should be 900-1000 ohms, if memory serves.

If the inside wires have little tags with "BG" and "DG" written on them -- these tell you which is which -- the sensors are likely original. If not, then they've probably been replaced at some point.

There's a more conclusive test that you can run by checking resistance between contects on the DME harness. But checking from the engine bay is easier.

There's a good description of this in the Bentley manual. With pictures.

[scarceller]

Can you borrow a DME from another 84-89 911? I see a lot of bad DMEs these days that act up just as you describe.

I don't think it's fuel pressure but your readings seems to indicate a FP issue as well but not likely the cause of the engine stall.

Fuel pressure with engine running at idle should be 30PSI and then while still running remove the vacuum line from the Fuel Pressure Regulator on the Drivers side fuel rail, when you remove the vacuum line the FP will increase to 36PSI. The fact that you measure 36PSI at idle is not correct it should read 30PSI. Maybe the vacuum line is not properly connected or has no vacuum? You need to go back and check that also.

Basically FP at light loads like idle and cruising should be around 30PSI and it will increase as load increases so that Wide Open Throttle will read 36PSI because the intake has no vacuum. It must remain at 36PSI during wideopen throttle.

Again, I do not believe your FP is the cause of the stalling but it's still not correct.

[mysocal911]

Originally Posted by scarceller

I don't think it's fuel pressure but your readings seems to indicate a FP issue as well but not likely the cause of the engine stall.

A bad fuel pressure regulator or fuel pressure dampener can cause the engine to stall on de-accel,
if either sticks after revving the engine.