: 93 sts misfires, stalls, very rough idle, looking for cause

06-17-09, 05:52 AM
Hi, I'm new here. I've been searching for solutions at the forum, and I found a few, but I'm still not really getting closer to the cause.
I've got a '93 sts 4.6L here, which used to run fine, but about 2 months ago it started running really rough, stalling and misfiring all of a sudden. Then later (next day, sometimes a few miles down the same highway) the problem would disappear or not seems to be completely at random. When it starts running rough at cruising speed it won't go past 3000 rpm, it just hangs there misfiring until the accelerator is released and rpms go down. Some days it did run fine and other moments the problem would come and go at random times. I never get a code, other than the usual stuff for disconnected battery and so. It did once show 'Traction Disabled' while cruising on the highway and trying to keep the engine alive to find a safe spot to get off the road. Problem grew worse, it looks like when the engine is completely cold the problem isn't there for a while until it warms up. But this morning it started running rough immediately.

I've replaced ignition coils and spark plugs. The strange thing is:
- Started the engine with the old ignition coils, starts just fine and after a minute or so it starts to idle very rough, misfires. When I touch the accelerator, the engine dies.
- Changed ignition coils, start engine, runs fine and then I make a 15 minute test drive. Everything is okay and I thought I solved this one.
Then I park the car, come back 2 hours later and guess what, problem came back. I'm lost. What could it be? PCM? Alternator? Bad wire? Spark plug wires?

I don't think it's the fuel pump or injectors, the problem occurs both on gas and on LPG (car has a dual fuel system, either 8 injectors gas or 8 injectors LPG, each having a different supply system).

Could the problem be connected to the thread about "93 eldo northstar engine missing"?

Thanks for reading, I decided to post here as I'm a bit lost and the car is out of service for over a month now.

06-17-09, 11:08 AM
It could be a fuel pump, but the only way to tell is with a fuel pressure gauge. The other thing that comes to mind is the ICM.

06-17-09, 03:53 PM
If the fuel pressure is good, I would change the spark plug wires (A/C Delco is best).

Mark C
06-17-09, 05:18 PM
Sounds like the ICM (the tray the 4 coils sit on) is going bad. As soon as the engine starts to heat up, your problems appear. Could be bad connections in any one of the 4 connectors on the ICM, or the ICM itself.

06-17-09, 06:28 PM
OK, thanks for your replies. I've got some hope again! Would you by chance know if the 4.9L engine has the same ICM? I know of a 4.9L Seville that's currently going in pieces, if it's interchangable I'll go get the spark plug wires and ICM to try that. But it's a 2 hour drive and the owner doesn't know too much about it either.

06-17-09, 08:53 PM
The ICM on a 4.9 is different. The 4.9 has a distributor and the ICM is inside of the distributor, completely different. The 4.6 N* has a distributorless ignition system.

06-19-09, 06:56 AM
OK, I've taken out the tray and cleaned the coil contacts and the (PCM?) contacts. Within the tray, there is a metal box which is probably the ICM. I suppose you can't take it out, can you? I haven't really tried, afraid to break the plastic. Is the ICM a digital or analog control box? Cleaning the contacts wasn't the problem even though they had obvious corrosion.

06-19-09, 07:25 AM
That whole tray that the coils sit on is the ICM... The PCM is in the dashboard underneath the glovebox. The ICM receives an analog signal from the sensors and converts it to digital for the spark timing.

07-16-09, 12:29 PM
I got an ICM for $50 off E-Bay. I just made a lengthy test drive. And it seems to work!
It took about a month to get here, but I guess that's the price to pay for driving an american car outside of the USA.
I sure hope I've solved this one.

I'm thinking of installing a 300 or maybe 400 ohm resistor on the coolant temp sensor, so the fan will come on sooner.
Maybe the heat under the hood is just too much for the poor ICM.

Anyway, thanks so far!

07-16-09, 12:31 PM
(double post, sorry)

07-16-09, 06:13 PM
I think your PCM is under the dash behind the glove box. The where later moved to under the air filter box so as to be cooled by intake air passing over it.