993turbo

Both my 89’ GT and 92’ GTS are equipped with the ignition control relay.

My GT runs fine, with no issues, but the GTS is idling very rich.

-When I disconnect the relay on the GT, the engine stops instantly.

-When I disconnect the relay on the GTS, the engine idles better. (No jumpers, no nothing)

Why the different behaviour....?

dr bob

The action on the two should be identical, as your GT description where engine stops completely with the relay removed. Look hard for an external jumper on the GTS socket or wiring that's allowing the engine run with a failed ignition on at least one cylinder. You can narrow down the suspect cylinder(s) by first checking to see which injectors are still firing on the GTS with the relay out. The engine will idle smoother on four cylinders (one ignition failed) than it will on seven or six, with individual cylinders failed. Remember that spark plugs are a great telltale of combustion if you pull them immediately after some running. A cylinder without ignition will usually have a wet plug. You can also use your infrared thermometer on exhaust ports to find the colder one after a short engine run from cold.

In the end you'll want to find the jumper that's allowing the GTS to run with the relay removed.

993turbo

There is no jumper.

The jumper procedure is to jump 87 to A1 and A2.

When I measure ohm between 87 and A1 or 87 and A2 I get its open. Meaning no jumper?

With ignition on, ignition control relay disconnected, I have battery voltage at 87 (as it should be), but no voltage at A1 or A2. If it was jumped elsewhere I should see voltage at A1 and A2 as well?

Any ideas....?

993turbo

I wonder if anyone has any ideas about this?

Mrmerlin

Check the code plugs swap the MAF from your GT to GTS disconnect the battery then connect it

993turbo

The mafs are new JDS in all my cars.

The code plug looks like this with the lid off.

John Speake

I don't understand for the different behaviour, but it may be misleading you. I would suspect the O2 sensor on the GTS. Disconnect the LH ECU, unplug the O2 sensor at the 3 pin round connector on the floor in front of the CE panel. Then reconnect the LH ECU and see how the car runs. (with the code plug in place of course)

worf928

@993turbo Read Dr Bob’s post again. With unaltered wiring it is impossible for an IMS-equipped 928 to run, at all, with the IMS relay unplugged. Since your GTS runs without the IMS relay, the wiring has been altered.

993turbo

Sure. Got that part.

And my post after it where I have measured and confirmed that there is no voltage so it cannot have been jumped.

993turbo

It puzzles me as well.

I will disconnect the o2 as described above. I have a new o2 sensor sensor on the shelf, so I might change it anyways. It is about the only thing not new on this car.

Billu

The fact that the car runs with the relay pulled means you have voltage bleeding/crossed wiring somewhere. The engine will not run without voltage to the ignition.
The fact that you don’t read voltage at the relay socket only means there is no voltage there.
Find out where the voltage is coming from first.
Listen to the Dr.

993turbo

Clogged return line caused excessive fuel pressure. 8+ bar. I discovered this when I connected a pressure gauge to the rail. Odd, because I had blown the return line earlier. Found some sediment at the bottom of the tank sensor where the return line ends. Cleared this. Car runs perfect. And stops immediately when I pull the relay.

:-)

dr bob

Scratches head... where hair used to grow. Not sure how the clogged FPR return line is related to a fault in the injection wiring and the protection relay.

worf928

x 928. (Except for the hair part. Presently at least.)

There's a word for it though. https://www.merriam-webster.com/dictionary/coincidence