Hi all,

I have a 1995 Sub that has a rough idle.

In park and neutral it will sit at 700-800 rpm but have a gentle shudder like a small misfire. It will stay running but not be smooth.

Put it into drive or reverse and the revs drop as they should but the shudder and hunting becomes more pronounced and will cut the engine from time to time. Low speed manoeuvring is also near impossible with this issue.

With the brown ecm wire disconnected to set base timing you can feel the misfire/shudder but it is less pronounced and much less of an issue.

It has had a new o2 sensor, Idle control valve, throttle position sensor, coolant temperature sensor, new distributor, cap, rotor, leads, plugs, the egr valve is blanked and I have sprayed injector cleaner everywhere to look for vacuum leaks without any luck.

It smells like it is running rich.

It runs fine when driving.

When cold the idle is higher so it isn't an issue but the shudder is still there. The ECM appears slow to react to increased load from power steering at idle and sometimes the engine cuts out if turning hard to left without any throttle.

It has a slight manifold or downpipe blue as I can just hear a small 'tick' under load but not at idle.

What am I missing please?

There are no mechanics I can take it to as I'm in France and I can't keep chucking parts at it as they take a while to get here from US and aren't cheap after postage and import fees.

Any pointers on what to look at or what may have already been replaced that needs to be checked again will be greatly appreciated.

Could it be the computer shot?

 

Is the check engine or service engine soon light on?

 

Neither but I don't believe it works...

With a hand at the tail pipe to feel the exhaust gas temp it's warm then blows cool when it splutters/misfires. Could that be the slight exhaust blow sucking air in leaning the mixture then o2 sensor riching the mixture to compensate causing the issue?

 

Turn the ignition on without starting it, you should have the check engine light, if not (I've never seen one not working), then it isn't working. With OBD1, you will need to have it functioning as the number of times it blinks the check light will tell you the DTC. Right now, I'd suggest a compression check if you can do that.

 

if it smells like it's running rich, it may not be getting enough juice from the coil. If the coil has a small crack in it, it may be starting to ground it self out when it gets warmed up. Try spraying WD40 on the coil and see if that temporarily fixes the issue. If it does, replace the coil. When you said you replaced the "leads", do you mean ignition/spark plug wires?

 

Hi, thanks for replies.

No the check engine light doesn't work.

Yes by 'leads' I mean ignition wires. Will try the WD40 on the coil trick and post back.

Have ordered a compression test kit, will check when it arrives.

 

I had a similar problem with my 2500 Silverado it ended up being a head or intake gasket leak I decided to try Prestones three in one head gasket seal truck hasn't missed since if you try it follow the instructions exactly on bottle.

 

Sprayed WD40 on the coil, no change.

It appears the slight shudder/misfire is also there when it is cold but harder to feel/hear as it idles higher.

It is also present but less prominent when disconnecting the brown/tan wire to set base timing which I did once again, tried at both 4* and 0*.

Put my timing light on each plug wire individually to check for single flash and could not detect a miss or double spark on any plug wire.

Waiting for compression test kit to arrive although cranked over with coil and injectors disconnected and there was no noticeable increase in speed at any point indicating low compression.

Any ideas where else to look?

Hope does the timing work on these at idle? With the ECM connected again the timing sits all the way past the left of the markings and moves a little as it misses?

 

IMO if it was an electronic ignition coil or anything in the distributor, it wouldn't run or start. I haven't delt with ESC since my 1982. Pickup coil was the last "fun thing" I had with it. What I read is possibly a burnt valve, your compression check will point which way to proceed with your issue.

 

Hi all, compression on all cylinders is within 10% of each other.

What other sensors have I missed that are part of the idle circuit as it is only on idle?