1984 Caarrera stalls EVERY time, after 5-10 seconds

[ianc]

Well, from your problem description, it sounds like the issue is spark.

You say it's not a stuttering death, but like a switch flicked off. If it were fuel pressure, the car would stumble before dying. Ignition would cause it to cut dead.

Coil bad and immediately overheating? Test for spark and try to work from there...

ianc

[bondavi]

Coil resistance checks out on both terminal ends

Would the speed sensor oxygen sensor, or air flow sensor cause stalling? I'm starting to suspect the o2 sensor, anyone concur?

Here's the YT vids if you can discern something by ear.

http://www.youtube.com/watch?v=ig_AhrESDbo

http://www.youtube.com/watch?v=eQ75cVCv1Fc

[ianc]

Coil resistance is not necessarily a good indication. If you have a spare hanging around, try swapping them. It does sound electrical though... How are the grounds on the left intake runner?

O2 sensor is out of the loop until the engine warms up because it doesn't produce a reliable signal until at high temp. You can easily test this by just disconnecting it; it is not required to run.

I would devise some means of testing spark and fuel while the engine is running. Cheap inductive devices to display current in plug wires near them are available at sears, or just go the old fashioned route by pulling a plug wire and connecting to a spare grounded plug. A noid light on a single injector would let you see if the injectors stop getting juice just before the engine dies, or only after. You could attach a fuel pressure gauge to the test port on the left fuel runner and monitor it as the engine dies. You need to narrow it between fuel and spark, then work from there...

ianc

[rick-l]

You have to start eliminating things by end to end tests.

Does the spark disappearance coincide with the stalling?
Noid light to see when injector pulses disappear?
Since someone related a personal experience that said that exact thing happened to him I would look at fuel pressure.

[bondavi]

I have a strong feeling it is now the speed sensor on the flywheel that is doing this after the car is initially started.

However does this sensor also drive the tach? Strange thing is if I unplug the speed sensor but leave the TDC sensor in, the car will not start. I did it the vice versa way too, same result.

[ischmitz]

The speed sensor and the reference sensor are known to be able to fail intermittently. In other words even when they measure O.K. they can still be bad. With your tests it is entirely conceivable one or both cause what you see.

If one is completely dead (or unplugged) the car won't start since the DME has "no idea" about either engine speed or TDC timing. And the tach is driven by the DME.

In general I am not a friend of throwing parts at a probem. However, given the time you spent so far you might want to consider buying one new sensor and see if changing out the speed sensor makes a difference. If not put the old one back in and change the reference sensor. Our host sells them.

Ingo

[bondavi]

I replaced both sensors on the flywheel, swapped in my spare DME relay, retightened ground connections. The car starts and idles without stalling now.

However when I give it enough throttle from idle to 2k rpm it stays steady. While driving under load , each time I apply 1/5-1/4 throttle travel it cuts out immediately each/every time.

ANything else I should be looking at?

Car will be going to the dealer tomorrow.

[kidrock]

I don't have my Bentley handy, but IIRC, the "housing" that holds the speed and reference sensors must be located a very certain distance from the flywheel at installation.

Maybe it's possible that during installation, your new sensors aren't within spec distance from the flywheel? Just a guess.

[nhromyak]

I didn't read it in this thread, but maybe you have already done a full AFM sweep test?

[ischmitz]

Also make sure the idle microswitch is not stuck closed or the cable shorted. It would cut fuel delivery above a certain RPM value.

[3rd_gear_Ted]

it appears there are two GND's that enable external circuits via negative switching logic. Make sure these are not intermittently switching on & off. Use an O-scope with a trace function to see

[bondavi]

Checked all the idle switches, were OK. Also retightened ground between engine compartment and front compartment as well.

I relented and took it in to the dealer today. $$$$$$

I'll let y'all know what it turns out to be. MYSTERY!

[rick-l]

Isn't there a surplus store around you could have bought a pressure guage and some fittings?

[bondavi]

The culprit?

Fuel pump.

Porsche wanted $850 for the part alone. I said no thanks, went out and got myself a Pelicanparts one ($200) and off I go!

[ischmitz]

So in summary you had to independent issues:

- One or both flywheel sensors where marginal resulting in stalling and poor idle
- A marginal fuel pump causing fuel starvation at higher RPM/ engine loads

What are the chances..... Congrats on having all tracked down.

Ingo