What the heck is my problem!
 

Ok, so I thought I had it narrowed down to CDI unit (see this thread, 1989 930 shuts down when hot - http://forums.pelicanparts.com/porsche-911-technical-forum/102345-when-i-get-her-hot-she-just-shuts-down.html ) but it turns out I may have jumped the gun.

I had a few free minutes last night so after work, with the car warm I took some miles of my brakes simulating stop and go traffic conditions under which the car has previously shut down. I got the temp up to above horizontal, higher than I have ever seen it but she kept on running fine, without a hiccup. 30 minutes of simulated traffic later I gave up. This morning, after 30 SECONDS of traffic, she died! She was barely even warm this time! Here's what I noticed...

If she's cold (CA cold, 65 degrees) and I switch on the ignition I hear the CDI whine. Good

If shes just been shut off (by me) and I turn the ignigtion on I hear the CDI whine AND a pump (fuel?) running even though the engine hasn't been cranked. Is this right?

I she shuts herself down and I turn the ingnition on I hear the CDI but do not hear the pump. After as little as 1 minute I can restart and continue on my merry way until the next time.

The new observations are:

-CDI whines and seems to be consistent in operation.
-There is a pump that seems to be inconsistent in operation.

The new questions are:

-What is the pump?
-Should it be on when the engine is not running but the car is warm?
-If it should, and is not after she shuts herself down, what are the possible causes?

There you go, 1989 930 that is really trying to be a good car after some neglect by PO. Help!

Thanks!

Sounds like it may be your fuel pump relay.

and...930's have 2 fuel pumps?

I think the pumps should run only when cranking and when running. When it dies check and see if you have a spark

Which poses the question:

If I'm driving around and it dies how the heck do I check for spark? Not so easy to crank the motor while checking the engine compartment!

Any solutions to that conundrum?

Thanks from the 'roadside mechanic',

If you can get a friend to ride with you, have them crank it while you check the coil lead. Get some spark plug pliers to hold the lead while cranking. You also might replace the two fuel pump relays just incase

there's a yellow relay on the rear electrical panel that will shut off the fuel pumps if it isn't firmly pushed into it's plug. the two fuel pump relays are mounted on the top of the front fuse panel. they're the ones closest to the windshield. remove all three and carefully spread their pins just a bit with a sharp pocket knife for better contact into the plugs. (a couple of new red relays would be good, because they're probably old). also check the holes in the plugs for overheating damage and clean as necessary. if the sensor plate switch is unplugged, the pumps will run with just the key on.

Yellow relay in the engine compartment, you say? I'll check tonight. I've got 2 reds and two blacks coming via Pelican-mail but didn't have the time to check what was actually in the back there.

The pump runs when the ignition is on if the car has been running but not if it's cold. Where is the switch on the sensor plate of which you speak? Seems to me this could be the culprit! Which is the 'pumps on' condition, continuity or open (eg: could I uplug it and see if my problem goes away or do I have to jump it)?

Thanks JW, you've given me a lot to go on!

Cheers!

The airflow switch is on the back top of the airflow meter. If it is unplugged the pumps will run all the time.

Ok, now for some diagrams, here is the engine compartment electrical (not mine, just an example) and legend as I got it:

http://forums.pelicanparts.com/uploads/engcomp.gif

LEGEND

1 Tail fuse box

2 Regulator plate

3 Air flow sensor / charging pressure relay

4 Delayed ignition cut-off relay

5 Two stage rear window defogger relay (not applicable to 1978 )

6 CDI control unit capacitor

7 Radio shield

8 Voltage regulator

9 CDI control unit


So what we're talking about is relay number 3, correct? Not having additional information with me it seems that this relay controls 'pressure' of something, my guess based on JW's comments is the pressure of the fuel system. The control is based on 'Air flow sensor' input, which means if this relay (which was removed and put back during recent engine swap BTW making it a prime suspect) was to go wonky, the symptoms I have described might result (fuel pump on sometimes when engine not running, fuel pump cutting out when engine heats up, etc.).

Is there a way to bypass is the next question as now when she shuts down I can test the theory!

Cheers (again!)

Thanks Dean!

I'm assuming if the sensor is unplugged, the pumps will run all the time IF the relay is good, correct? Appreciate all the help, you guys are great!

I belive the #3 relay is for the overboost circuit

Hells bells, I think you're right about the overboost circuit relay (although I suppose it could still be tripping and shutting fuel supply down).

Where's the magic yellow relay JW is talking about?

I don't know where that relay is. And yes if the airflow sensor is unplugged the fuel pumps should run all the time. I bet the airflow sw. is unplugged. I would also inspect the relay recepticals like JW said

Think I've narrowed it down:

The relay in the image above, #3 (the round one) on my car is a yellow box relay, like JW mentioned. It is apparently and overboost relay but appears to do a bit more that just that. The interesting part is that when I twist the relay and socket with ignition on but not running I can get the fuel pump to come on or turn off. With the car running I can twist and if I get it right the pump will shut down, along with the engine.

Sounds like a short with said circuit somewhere very near the engine fusebox. Now I can fix it this weekend!

Thanks to all who contributed to yet another of my dilemas!

Cheers!