Oldewoodupes

Hey,
I just picked up a rough running ‘90 Gt and it appears to only be running on the passenger side cylinders. Is there any smoking gun that would kill the spark to all the driver’s side cylinders? Plug wires look good, I’m checking the spark using a timing light.

Mrmerlin

are the injectors working on the side thats not working ?
use a long screwdriver placed on the injector to hear if its clicking

Oldewoodupes

Not sure, Stan. I’ll check when I’m home tomorrow and get back to you. Thanks

928Myles

Would an ignition monitoring relay failure be a possible cause?

Myles

Oldewoodupes

I was thinking that too Myles, thanks, but I’m not getting any lights on the relay and I’ve also tried bypassing it without any change in situation. Correct me if I’m wrong, but I also think if the relay is activated it shuts down 2 cylinders on each bank and not all from the same side.

Hilton

Check the king lead from the coil first on that side - it may be dislodged, or if they used the wrong leads (earlier S4's had a different coil top), just a poor loose fit.
Also check the ignition amplifiers (very front of the engine bay by the headlight bar) - you can swap the two plugs there and see if the issue swaps sides.

If its not those two, then you'll need to bust out a multimeter and check the continuity of various circuits - on my '89 I found a former repair which consisted of twisted together wires behind electrical tape which had come apart - it was fairly evident once I'd checked various parts of the harness for continuity - but start with the top two items above as they're more common failures.

GregBBRD

Would have to be a mighty strange ignition system problem:
Each coil, each ignition amplifier, each coil wire, and each rotor and cap contols two spark plugs on one side of the engine and two spark plugs on the other side..
Unless the OP has misdiagnosed that all four cylinders on one side are not firing, it is tough to imagine this being a problem with the ignition system.

The injectors are wired and controlled in groups of four at a time, with all four on one side being triggered at the same time, by the same "ganged" power and ground.

Even though the OP believes the overheat sensor system is not doing this, I'd still remove the relay and install a simple jumper piece to replace it.
I've had the overheat relays "die" without any lights coming on...many times.

The other possibility is deteriorated injector wire, where a ground and a power wire are shorted together, on the side that is not firing.
This will make the entire side not function.

Oldewoodupes

Thanks for the replies Hilton and Greg. So it sounds like a unique issue like I had thought after reading all the other running on 4 cylinder threads. I’ll verify my diagnosis of the spark tomorrow and if it still stands. I’ll start looking for a wiring issue. Greg, I will leave the overheat relay jumpered as you suggested, I just want to verify this is how that is done:

Mrmerlin

The easiest way to verify what’s working is with a IR gun test each exhaust port the working one will be hot

Koenig-Specials 928

Hard to tell correct wiring from the pic.
Check this out.

Rob Edwards

Another view: Y-shaped jumper from the center pin to the red/yellow and red/green wires:

GregBBRD

As others have said, what you've done is correct.
Porsche actually makes what appears to be a "regular square black relay", which plugs into the white plug, also.
I keep these, in inventory, for sale.
Although more expensive than a couple chunks of wire and 3 wire ends, the relay style part is less likely to fall out or have issues with. I use the jumper to test with and try to send cars home, with the "relay style" part....when I can.

Oldewoodupes

Update: checked spark today. I have spark on all cylinders. I think my timing light may have lost its power connection, even though it looked like it was connected, when I was stretching it to the drivers side because I had spark on the passenger side and then immediately checked the drivers side and had nothing. Boner move, sorry. Thanks for all the replies. Rough running is still the same, but I think I can rule out spark for now. I’ll need some more advice, but I’ll start a new thread when the time is right, as I’m not familiar with the newer 928’s and 32v engines. Had enough time before work today to check compression on 6 cylinders, 5 are good above 170#cold, #8 cylinder is down at 50# cold. I’ll be checking the last two tomorrow. It feels like car is running on less than 7 cylinders, so hopefully I only have 1 bad cylinder and maybe a fuel injection issue in the others. More troubleshooting to follow, but I’ll start a new thread as this title is no longer relevant. Thanks again for all the advice so far people.

Couple of quick questions about the 32v motors:

can the cam covers and heads be removed easily with the engine in the car?

would any S4 engine be a drop in replacement for my gt engine, worst case, but just curious?

Mrmerlin

before any more cranking,
please put the engine at TDC and pull the top cam BELT covers off ,
examine the belt for damage,
also check the cam pulleys and what the timing is set to,,
also check the cam pulley bolts by removing the rotors and putting bolts into the cam gears to hold them on the spiders.

Oldewoodupes

will do, Stan. Just curious, if cam timing goes out a tooth or two on these motors is it instant doom? And If I have to make an adjustment on the belt, will my aftermarket belt tensioner work on these 32v belts? This will be my first adjustment coming to an interference motor.