Intermittent misfire...close to being stumped, here

[gull]

Decided to move this to its own thread because I feel like I'm going in circles with it.

The car is an '89 Cabriolet, CIS-Lambda 8-valve, 170k miles.

Symptoms:

- Misfire at all speeds, somewhat intermittent. At highway speed you can feel the car surge and slow down slightly as it cuts in and out.
- I haven't been able to narrow it down to one cylinder by pulling spark plug wires, but it's intermittent enough at idle that I may just be missing it.
- I thought I saw missing pulses with my timing light but I'm now doubting that seeing as I've checked all the ignition components. It could be I was just seeing slower pulses due to the engine slowing down as it misfired.
- Vacuum gauge shows about 16 inHg, with needle flutter down to 15 inHg. I don't know if that's normal for this car 'cause I've never had a vacuum gauge on it before.

What I've tried so far:

- New plugs and wires
- Swapped distributor, cap, and rotor.
- Swapped ignition module with my other car, which runs fine.
- Swapped ignition coil with a known good one.
- Checked fuel pressures, all OK
- Disabling the Lambda system has no effect.

I'm thinking the next step is probably a compression check to make sure I'm not dealing with an internal engine problem, but I would expect low compression to cause a consistent miss on one cylinder.

[Briano1234]

Take your plugs and wires off, and measure the resistance. It sure sounds like a bad plug or wire...... Trust me I was driven bat crappy on one of mine when I changed the plugs and wires and ended up having a bad brand new plug, that if I had checked resistance on it, I would of caught it out of the box, which is now on my pre-install the plug check list.

[gull]

Will try that when I do the compression check. It would be nice if it were that, but I'd have to have developed a bad plug or wire in the old set and gotten a bad one in the new set. I've been chasing this around enough that I'm willing to consider anything, though!

[Briano1234]

Will try that when I do the compression check. It would be nice if it were that, but I'd have to have developed a bad plug or wire in the old set and gotten a bad one in the new set. I've been chasing this around enough that I'm willing to consider anything, though!

That was my WTF moment. I had a bad wire originally, then the new wires resolved that, but installed 2 bad plugs at the same time unbeknown to me... so I was chasing my ass.... I finally swapped wires. cap, rotor, and plugs between the cabbies, and when it started running correctly I put on the new stuff one at a time, and I basically had the Idea that a plug or 2 was bad as they had no indication of ever being fired, like the other 2.

[gull]

If it turns out to be that I'll totally owe you a beer. Unfortunately I won't have time to dig into it again for a week or so.

[gull]

Think I finally figured it out. Bad #1 fuel injector.

I checked all the plug wires and all the plugs for resistance, and they were OK. But the #1 plug was covered with soot, while the #2, #3, and #4 plugs had only light deposits. Hmm. (Remember, these are new plugs that have been run a total of maybe 15 minutes.)

Pulled the #1 fuel injector, and sure enough, it's just peeing fuel out instead of atomizing it.

So, time for a new fuel injector. I think I'm going to have to get a new #1 injector line as well, because man, that sucker is on there tight.

[PDX Cabby]

Soak overnight in Carb Cleaner. It can do some amazing things. I'd not rush out to replace things immediately.

as a side note: I'll be in Auburn (west hill) tomorrow night through sometime monday.

[gull]

Yeah, good thought. I'll give that a try first. It's actually running better at the moment, I think just banging the injector around when I removed and reinstalled it helped clear it out a bit.

[Calimus]

I soaked a set of injectors in seafoam for 3 months and it worked wonders. Not that you want to wait that long though.

[gull]

Seems to be running okay now. I'm going to dump some fuel system cleaner in the next fill-up as insurance.