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rrental's Avatar
 
Join Date: Aug 2011
Location: Kamloops BC Canada
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kinda confused....(electric gizmo's)

When I bought the car the porsche dealer noticed the car shutting off randomly. They changed the big yellow relay and checked all the wiring. Car started and ran good. We took it home and a return trip to Salt Lake City.

In salt Lake the problem appeared again and a local Porsche shop made a quick fix to get us going again. ( by passed the fuel pump relays in the front) We figured out there was a ground issue.

Back home we started to look a bit closer and tried to figure out what's the problem. At first we thought the over boost switch, but we by passed the switch and nothing really happened.

Last week I took out and replaced the black relay at the rear (next to the yellow one) and cleaned the connections. Things started to change. The car now runs rough when cold, at very low rpm. When warm, rpms vary between 600-1200 stationary. I also noticed we have no power after 6000 rpm. Still boosts to .8. Funny thing is, it used to boost too 1.0.

Now the weird part, when I remove the black relays, all is fine! Smooth idle at 1000 rpm, pulls well into the red, and boosts till about .8

So.....I will look into this more, but is it ok to drive without the black relay? Any ideas of whats going on?

Maybe airflow sensor is shot?

So recap:

Replaced red fuel relays
Replaced big yellow replay
Replaced rear fuel pump
Replaced Black relay in rear ( and now driving without)
By passed overboost switch located under intercooler (direct to ground)
Checked and cleaned connectors of speed relay under seat

Old 06-22-2013, 09:14 PM
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That black relay in the back powers the 02 sensor, just in case you were not aware.
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Old 06-23-2013, 05:59 AM
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Sounds like you've checked most everything except the factory alarm system controller and it's plug under the cowl nearby the glovebox. It's one more thing in the fuel pump ground loop.

You don't say what year your car is but I guess it's an '86 or later if it has the factory lambda emission system.

In the motor compartment check the multi pin plug on the left rear relay panel. It powers the ignition and everything else back there and also check the lambda harness plug on the left side of the shock tower crossmember.
Old 06-23-2013, 07:04 AM
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So....left the house this early morning, drove down the highway at reasonable speed and after about 100 miles it quit on us. We put the black relay back and it ran again.. after 20. Miles or so it quit again. Removed relay and ran fine for another 100 miles and did not quit on me.

Yes it's a 1986 930.

Thoughts?
Old 06-23-2013, 10:00 AM
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Quote:
Originally Posted by oilonly View Post
That black relay in the back powers the 02 sensor, just in case you were not aware.
Isn't that relay powering the safety switch? That's what it looks like in the drawing provided in the "my car doesn't start" thread. (thanks for that btw!)
Old 06-23-2013, 10:07 AM
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Quote:
Originally Posted by rrental View Post
Isn't that relay powering the safety switch? That's what it looks like in the drawing provided in the "my car doesn't start" thread. (thanks for that btw!)
No - it supplies power to the O2 sensor's heating element (and possibly something else to do with the Lambda circuit).

There is quite a bit of information (some of it wrong, however) on that subject in the following threads (and in others if you search "black relay") -

Relay ID - engine compartment

where is the enrichment relay at?

Last edited by Ronnie's.930; 06-23-2013 at 10:46 AM..
Old 06-23-2013, 10:42 AM
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what year is it?

there is a speed relay under the seat that tends to have bad solder joints. with it randomly cutting off, it could be this. this controls the fuel pumps and the O2 sensor.

i know the fuel system very well but i dont have any drawings for anything else.
the speed relay also controls the O2 sensor in that it cuts it out at a certain RPM, i think 3k, dont remember.
someone said the black relay you pulled is for the O2, if it is running fine without it, i would look into that circuit.
check your fuses too. they can be very intermittant.

all this has been pulled from my car so i only know bits a pieces of the 930. if someone could send me a diagram of the igniton system that would be a big help.

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Old 06-24-2013, 08:23 AM
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