Hi . New to this forum and new to 928s .Just purchased this . Intent is to drive it as a almost daily . Car is currently triggering red light for ECU . so shut down of cylinders . I have jumpered relay to get it to run on 8 as I work out how to fix it .Worked good . Now wont run at all . If I leave it over night and plug back the connection to red/ green light . Starts up great and runs good for about 20 seconds then progressively gets worse ,to a point of occasionally firing , fuel smell. I noticed that the brown wire that runs to this plug is pretty much toast , burnt and charred . Anyone know what this wire goes to or runs .Much help appreciated . Thanks In advance . Michael

 

Forgot to add I get Black soot @idle when its idling and lots of it

 

MAF is my 1st guess.

 

Hi Michael, welcome to this site - lots more traffic here so should get lots of help at all hours!

For those who don't follow reutterwerk, Michael's recently bought a LHD Japanese 89 S4 which has been converted to RHD, and he's in Australia. So the car has O2 loop and cats.

RW thread here: http://reutterwerk.com/forums/showthread.php?t=42260

The brown wire is something new you didn't mention before - I'd stop driving the car and disconnect the battery.

Brown wires are ground wires, which means you've got something which is shorting power to ground, and has fried an unknown amount of your elec harness. Unfortunately, its probably on an un-fused circuit (or some idiotn previous owner fixed problems using bigger and bigger fuses).

First step after disconnecting the battery, is to follow the burnt ground wire, and check for damage to any adjacent wires along the way, in the way of melted insulation, breaks etc. The ground points for the ECU/ignition system are on top of the engine (through the firewall boot), but I don't have the wiring diag handy at work to see whether the Ignition Monitoring system shares those same ground points. Then you can replace the burned wires, and track down the short with a multimeter.

As noted on RW, your running symptoms sound like an O2 loop related problem - however in light of the fried ground wire, you need to sort out your wiring and fix the short, before you can get on with diagnosing the running issue.

On the bright side, your car hasn't burnt to the ground!

My "wild random guess without diagnosis or more details" is that you've got a short in the LH harness which has fried the O2 loop part of the ECU, causing the rough running symptom (LH is un-fused and powered directly from the battery terminals).

 

Totally the MAF

 

These go slowly and most never realize they are going. i just had all i mine rebuilt. Rich up in Mesa does some of the best at a great price. PM me and i will give you his contact info.

 

Red/Green leds? Are you talking about the CAT protection relay by the ECU's?

Please be aware of a possible CAT fire! !!
I would strongly recommend you check the coils and wires for each 4 cylinder bank.
By jumping the relay you may be dumping raw fuel into dead cylinders.

 

MAF or Temp II. Check connections for those along with the parts.

 

You've got multiple things going on.

a) The 928 S4 and newer engines have two separate ignition circuits. Ignition circuit 1 is comprised of cylinders 1,4,6,7, and the right side coil and distributor. Ignition circuit 2 is comprised of cylinders 2,3,5,8, and the left side coil and distributor. If there is a failure in one of the ignition circuits, raw fuel is pumped out of the cylinders and into the exhaust system. When the raw fuel
hits the hot catalytic converters a fire may occur – OUCH!

From model year 1989 onwards, Porsche added the Ignition Circuit Monitoring Relay as a safety feature. This circuit is completely independent of the LH injection system. It is identified in the workshop manual wiring diagram as the Ignition Control Circuit. The function of the system is to turn off the fuel injection to the failing ignition circuit. The inputs to the relay are two exhaust
temperature sensors that provide input to the relay. If one of the ignition circuits fails, the “Ignition Circuit Monitoring Relay” shuts off the pulse signal to all of the fuel injectors of the affected circuit. As you might imagine, the symptoms of a shut down ignition circuit are a significant reduction in
power and a rough running engine. For trouble shooting, you need to determine if the shut-down is due to a failed ignition circuit or a failure of the Ignition Circuit Monitoring Relay itself.

b) Black soot when engine is idling. The air/fuel ratio is too rich. There can be several causes:
- leaking fuel pressure regulator/dampener. Pull the vacuum line and look for fuel flowing out. If flowing, the item needs to be replaced.
- TEMP II sensor failing (telling the LH module to apply cold start mixture)
- failing MAF
- failing LH injection module. One of the failure modes is running way too rich.

 

No offense to all above - but re-read the last line of the original post. He has a burnt ground wire in the wiring harness off the ignition monitoring relay.

I agree that the symptoms could be related to the possible causes above, but IMO his first priority should be tracing the burned ground wire and inspecting/fixing all the damaged wires inside the harnesses.

If the ground is one which passes through the firewall boot, then it could be the cause of his symptoms, as the relevant LH signal wires for MAF, Temp II etc. (except O2 sensor) pass through that boot too, and a hot ground could have caused other shorts/damage.

 

Circuit Monitoring Relay circuit signals to test are:
1. Terminal 31: ground
2. Ground must be present at terminal AL when the ignition switch is in the off
position.
3. Battery voltage must be present at terminals A1, A2, 15 and 87 when the ignition
is in the on position.
4. Battery voltage must be present at terminals AL and 61 when the engine is
running.
5. A Voltage value of approximately 2.7 V must be present at both terminals E1 and
E2 when the ignition is in the on position.
6. The resistance between E1 and E2 is approximately 5 – 10 Ohms (measured at the
disconnected relay socket).

Rich
'93 928 GTS cover girl
'87 928 S4 with refreshed AT
'79 928 5-speed rescue
'79 928 5-liter track beast