Misfire felt only during idle

[gavinl]

The car's a 1997 Subaru Impreza 2.2L. The idle is misfiring and very lumpy, but once I get on the gas the car drives fine without any misfiring CEL coming on. Also the ignition timing is at factory spec.

I used the ignition timing light and checked all four plug wires with the strobe light. Seems like #1 and #3 (fires together, waste spark system) has intermittent "blackouts" compared to #2 and #4. Could an ignition coil be almost dead and fires intermittently consistently or am I facing ECU troubles?

 

[surfstar]

recent tune up? plugs wires and everything? if they're original wires, it would be time to chage for sure.

 

[Chris142]

Usually a miss at idle is a low compression problem. At faster engine speeds it's harder to feel.

Bad plugs/wires usually show up at higher speeds or under load.

 

[BlazerLT]

Go get the codes scanned.

The CEL mens there will be a code telling you what the problem is.

 

[gavinl]

Plugs and wires are new. Engine is new rebuild with 0 miles currently, no CELs (yet).

I'm thinking along the lines of a bad ignition coil that is obviously intermittent when idling but as the firing rate goes up with the rpms the miss is less obvious.

 

[kc8adu]

next step is looking at the output of ignition module for the "blackouts"you describe.you need a scope for this.they can be gotten cheap on ebay.
and if yes look at the crank/cam sensors too.
play swaptronics and swap the coils to see if the problem moves with the coil.

 

[gavinl]

Just ordered a new coil, old one is 140k miles old so it's probably a good time to change.

However all the sensors area also 140k miles old, they were working right before a bad timing belt sent the valves into the pistons.

 

[Tosh]

Gavin, don't start throwing money at it.

 

[Kestas]

I once had a miss only at idle that turned out to be an egr valve with a leaky seat. One way to diagnose this is to block off the flow of egr gases with a steel plate at the mounting flange and see if the idle improves.

 

[mechtech]

Fuel injectors may not be spraying equally, cam lobes may be slightly different from wear, valves could be leaking, deposits in the C.C. could be varying from cyl to cyl, and a dozen other things.