Hi guys I have a 92 sc400 without about 130k miles.

It runs super strong 95% of the time, but I've had an intermittent misfire issue. The idle is rough enough to feel the car shake. Also I can get it up to speed okay, but the car will fluctuate between bucking/hesitating, with random surges of power.


It seems to do it the most often when the car sits for a week or so. If I drive it regularly it doesn't seem to happen as much. However today it randomly decided to do it. But then I go back outside an hour later and it took it for a drive to have it run full power again.

It's had a full tuneup: plugs, wires, caps/rotors, coils, etc. The injectors and fuel filter are also new. Good compression. I'm stumped, and not sure where to go from here. There is also no check engine light.

 

could be a bad ecu or even egr related.

 

Would EGR cause hesitation/stumbling above idle? I also passed emissions not that long ago with no issues.

Supposedly the ECU has been rebuilt before. But I have no idea how to test it.

 

i believe there is a pinpoint test to see if the ECU is bad. i dont have a copy of the manual in front of me to look now.

i would look there before EGR..IM NOT SAYING IT CANT BE EGR. just giving my 2 cents

 

Yeah I still think it could be the ECU as well. I have no idea how well it was supposedly rebuilt.

I thought maybe it could be an intermittent issues with a cam or crank sensor, but you'd think that would throw a code.

I even went out there shaking random connections to see if the car would act up again, but still ran great.

 

Any other ideas? :/ Drove it today and it ran great. Seems to be at its worse if it sits for several days w/o driving.

 

bump... what I don't get too is how I can come home and it's running great...

to have it run horribly the next time it's started :/

 

Fuel Pump?

Fuel pump ecu?

 

had a tune up recently?

 

small blown head gasket

 

fuel pump ecu is on my list to check

yes.

naw, good compression all around, oil and coolant both look good (and not mixed) lol.


I've pretty much confirmed it only does it when it sits for prolonged periods of time. Drove it somewhere a hour each way today and it ran great. Guess I'll just keep driving it, and maybe it will work itself out lol.

 

Thought I would update this. The weather has started to cool down here a bit, and seems to have made the rough idle issue appear more often.

Basically now I start the car when cold and it fires right up, then rpms chug and idles poorly, and puffs some white or greyish smoke for a bit. However I've realized all I have to do to make the rough idle go away, is simply rev it a couple times. Then the car smooths out like nothing happened.

I did decide to change the Engine coolant temp sensor, and it did not fix the issue. However the car does seem to have picked up a bit of power. The car runs great! I just need to figure out this cold start idle issue.

My next things to check are the EGR and IACV for carbon blockage. Any ideas/advice?

thanks

 

with the smoking on startup and going away with revving (assuming good compression), it makes me think valve stem seals but i dont know if this would make it run rough unless they are extremely shot.
maybe pull a plug after its been sitting for a while and see if it looks drenched or if you can see any oil in the cylinder with a light.

an interesting read:
http://lextreme.com/valve_seals.html

 

It's more of a rich fuel smelling smoke though. I'm thinking the air/fuel mixture is bad at start up, and poorly burnt fuel vapor is coming out the exhaust. Also in warmer weather I'd still get the rough idle occasionally, but no smoke.


compression is also good. All fluids look good too.

 

Rich fuel smell definately helps. fuel pump ecu is more common to having a no-start condition but I wouldn't rule it out completely. have you tried resetting the ecu? you should try doing that before your next cold start nd let us know how it goes.