Hello all. I'm having some serious difficulty this morning and need help. I was headed to work, hit the key and she started fine. Then about 15 seconds later, it was like it went into 4 cyl mode. Wouldn't idle and had no power if I revved it. I shut it down and restarted with the exact same results.
Every time it starts fine, but goes to hell after about 15 seconds. I started out looking at the ignition relay, and swapped it out with no luck. Problem still present. My fuel pressure gauge shows pressure, but I guess that the fuel pump relay could be cutting out after start-up. I want to jumper the fuel pump relay out, but I don't remember which slots to jumper.
Does anyone know off hand which slots to jumper??? Any other ideas or advice??? TIA,
Chris Lockhart
'89 928 GT


A friend of mine has had the same trouble with his '90 GT.
The injection management has a special program in case of anti-knock sensors troubles. When this happens the injection is cut on the faulty bank of cylinders. This is design to inform the owner that there's a trouble with the engine and allow him to go to the next Porsche dealer.

I hope this help.

'89 CS


Starting in 1989 Porsche installed head temperature/exhaust temp sensors to all 928s . In addition , as part of the system they mounted a odd relay to the bracket for the LH Brain relay 928 618 175 00 (relay supervise ignition circuit ) . This relay checks the temperature of cylinder head /exhaust ports #4 and #8 in 89 but for 1990 they moved the sensor forward to #3 and #7 to reduce inaccurate readings !! . If the SUPERVISOR relay gets different readings from the two it assumes the engine has had a failure of 1/2 the ignition system . Believing this to be true (to keep the exhaust from reaching critical mass as 1/2 the fuel is passing through the engine unburned turning the catalytic converter into a true afterburner catching the undercoating on fire ) the relay shuts off half the injectors . NOTE it is NOT one bank of cylinders but two on each side , Look at the wire from each distributor cap they go to two on each side . So in this case it may be that one of the tem sensors is reading incorrectly and to save the car the supervisor relay is shutting off 1/2 the injection . It is not a fuel pressure or pump issue but may be a true failure of half the ignition . The 1989 and newer LH brain has fault memory and plugging in the "hammer" tool will down load the faults.

Jim Bailey
928 International


Victory at last!!!!!!!!! I removed, cleaned, and reinstalled the head temp sensors, and low and behold the beast lives!!!!!!!!! Thanks to all who helped out both on and off list. A special thanks to Jim and Mark at 928 International for their wonderful assistance on the phone. Great customer service guys!!!!!!!

Chris Lockhart


>Those sensors near the exhaust headers are temperature sensors. If  they send a significant signal difference they trip a relay down by the LH computer box that shuts off the fuel to half the injectors. The  proverbial 4-cylinder mode. The idea is to prevent a fire from excess fuel pouring into the cats should half the ignition fail. The relay is  clear and has a status light. Green=OK. Red=4 cylinder mode has been tripped.

The LEDs in the clear relay actually show:
Red LED illuminated - Cylinders 1,7,6,4 are shut down.
Green LED illuminated - Cylinders 5,8,3,2 are shut down.
IMR -> Green LED = Driver side problems, Red LED = Passenger side problems.

 It's easy to remember, if you sail or fly - red is the right coil, green is the left coil.

Wally Plumley
928 Specialists


The '89 and up 928s have an ignition circuit temperature sensor system.
This system has a temperature sensor on each side of the engine that monitors exhaust temperatures. If one side has a lower temp, the system kills the fuel injection for those four cylinders. This prevents pumping unburned fuel into the cat, which prevents overheating and a possible fire.
It is possible that you have a late '88 that has the ignition monitoring circuit.

If so, this is one of those situations where you need to catch the malfunction as it is happening. Perhaps the best way of doing that is to remove the carpeted cover from the ECU units on the passenger's kick panel.
Look at the rear of the visible ECU - if you have the monitoring circuit, you should find a clear relay package, about the size of a cigarette pack.
Inside the relay will be two LEDs - one red, one green. If the temp sensor senses low exhaust temp from cylinders 1, 7, 6, 4 (right ignition coil) the system will illuminate the red LED and shut down Injection Circuit I that feeds those cylinders. If the other sensor senses low temps from cylinders 5. 8. 3. 2 (left ignition coil) the system will illuminate the green LED and shut down Injection Circuit II that feeds those cylinders.

This means that you need to leave the cover off and keep an eye on the red/green LEDs. If either of the LEDs illuminates, the problem is probably with one of the exhaust temp sensors. The sensors are located in the bottom of the exhaust manifolds, and are replaced from beneath the car.
Occasionally, the sensor can be removed and turned, and will then work for some time.

If you do not have the circuit, then you have either an ignition system or injection system failure. The first thing that I would check is the coil wires. Remove them, and check the condition of the connectors, and make sure that the wires are not routed against any metal or wire harnesses.

If there is no obvious problem, it's time to start troubleshooting - check for spark on all cylinders, and listen to the injectors on all eight cylinders to see which of the systems has failed.

Wally Plumley
928 Specialists


> It either has an ignition problem (coil, coil wire, ignition switching unit) or  it THINKS is does and the relay on the out side of the Brain box has shut down 1/2 of the injection based on information from the two head temp sensors

Jim, my '89 GT started doing this Saturday evening. When the car is cold, it will run fine, then once it reaches normal operating temp, it kicks over to limp home mode. I'm getting the GREEN LED indicating cylinders 2,3,5,and 8.

I've cleaned the temp probes, cleaned and gapped the plugs, and replaced the left coil, wire, rotor button, and dist. cap, with no change. As soon as she gets up to temp, it turns into a 944!!!!!!!!!!!!! What else am I missing????

Chris Lockhart
'89 GT w/ 131K miles

Hi Chris,

Most likely one of the Ignition Monitoring temp probes is bad. You can replace the sensor ($$$) or since you don't have cats just bypass that system with a Bypass relay. Part # is 928.615.175.00 and we have them on the website for $15.23. I recommend every 89 and later owner to have one in their glove box for when that system starts acting up. In a pinch you can also remove the relay (large clear relay on engine computer console) and jumper pins 2,4,5 together on the relay socket (leave relay out) which should put you back in to 8 cylinder mode.

David Roberts