Random miss at idle

Snoball the V8 ZJ has developed an intermittent miss at idle and just off idle.  40% of the time it idles just fine, 40% of the time you get a single miss here and there, and 20% of the time you get a miss and then a little series of misses that creates a stumble, but never enough that it's close to stalling. Its only perceptible at idle or just off idle, like easing around a crowded parking lot. If it's doing it at higher RPM or at cruising speed I can't tell. It started this about 2 weeks ago, the day before I put it in the shop to do radiator and water pump, but it's still doing it and I think it might be getting a little worse? The truck accelerates fine and cruises 55-70mph with no problems.

Conditions when misfiring:  Happens on hot or cold idle, no pattern with weather or driving conditions. At hot idle this morning I had a steady 20-21 psi of vacuum (torque app), 13.2 volts, and steady 700-730 rpm. Engine temps around 190 (short drive, cold morning)

Ignition parts replaced: In the last 3 months I've installed Champion Copper plugs (the recommended type), AC Delco wires, Accell cap and rotor, new TPS, and it has been running these for about 2 months with no issues before the miss started. I've done a lot of other parts too, but not related to ignition.

Work so far: last weekend I pulled all the spark plugs and they all look like they're running right. No fouling, no closed gaps, no loose plug wires. I rearranged the plug wires to minimize contact between wires and space them out. There was a TSB about this back in the day. This made no change in the issue. I closed up the garage with the lights off and watched for arcs but saw nothing. I also ran a half tank of injector cleaner through it, but no change (Odd, but we had a Nissan that would behave like this if you didn't run injector cleaner every 6 months.)

Other things I've considered: Crank and cam sensors, but the symptoms for both don't match these issues. Ignition coil, but they are usually a fail-when-hot part. dirty IAC valve and/or dirty throttle body - again, symptoms don't quite match but it's easy enough to clean up. 

It feels like an ignition issue for what that's worth. If I hadn't just done the tune up, I'd think it just needed a tune up.

Any ideas?

Found this issue in my V and it was resolved by correcting grounds that had rattled loose.  Just a thought as it was vexing me and I had chased a similar list of items trying to address it.

I think that's a good possibility. I've cleaned and greased all the grounds I've found so far, but I need to look and see if there are dedicated grounds on the distributor.  Or the coil.

Oh, I should have mentioned in the OP - No CEL lights have come on at the moment.

Champion plugs?  I'd start there.

In reply to APEowner :

That's the best running plug in these Jeeps by a large consensus. Well, those and the NGKs are the two, anyway.

I'm not sure if you can do this with the torque app, but if so watch your throttle position data in a graphing display. With key on engine off slowly go all the way to wide open and watch for a nice smooth pattern. There should be no drop outs. There could be a bad spot in the tps. As I type this I'm thinking that may not be the issue since it also happens at idle, but it' free to check. Also watch your misfire data on the scan tool to see if it is always the same cylinder or random cylinders, that will tell you a lot. And dump a can of sea foam in the tank.

In reply to gearheadmb :

pretty sure I can watch the throttle data with torque, but don't think I can pull misfire data unless it stores a code. The TPS is new, but that doesn't mean it's ok either. I'll check that. I might swap the old one back in (I changed it as a precautionary) and see if there are any changes in the issue.

Sea foam to clean the top end, I'm guessing?

I'd be tempted to pull a couple of plugs and see how they look.  If they look fine and the gaps are still good, it might be a weak coil or E36 M3ty plug wires.  Crank sensor can cause misfires, but usually it's not that minor.  

In reply to rslifkin :

as mentioned in the OP, I did pull them and they do look fine and gaps are good. Wires are new AC Delcos, coil is old...

Does this thing have a ignition amplifier? Might check that if it does. Also would still check the new cap and rotor and maybe replace with the old. Copper plugs also work best on old rovers so those do make sense. 

no amplifier I'm aware of. Not sure I still have the old cap and rotor, but it might be in the shop trash can under the suspension parts, LOL

Wires being new doesn't necessarily mean much.  I tried a cheap set of duralast wires on my ZJ once.  They caused slight misfires like you described.  Better wires fixed it.  

Definitely no coil amplifier, the coil is driven directly off the ECU.  

In reply to rslifkin :

so which wires did you find that worked? 

Can't remember what I ran that worked well when the motor was stock, but I've had a set of the Taylor Thundervolt 8.2 wires on it for a few years at this point with good results.  

ultraclyde said:

In reply to gearheadmb :

pretty sure I can watch the throttle data with torque, but don't think I can pull misfire data unless it stores a code. The TPS is new, but that doesn't mean it's ok either. I'll check that. I might swap the old one back in (I changed it as a precautionary) and see if there are any changes in the issue.

 

Sea foam to clean the top end, I'm guessing?

To clean the injectors.

I was poking around the jeep when I got home and realized the catalytic converter is rattling. Banging on the cat makes the noise stop temporarily. Still have good pressure out the tailpipe and no codes.  

I'm wondering if I should replace the cat and rear O2 sensor before chasing that miss any further.

Damn, another $150.

And again, the project snowballs.

Rear O2 sensor won't / can't cause a misfire.  On the OBDII ZJs it's only there to confirm that the cat is working.  Front O2 sensor is the only one that affects fueling. 

In reply to rslifkin :

I was thinking more about backpressure from a partial blockage, but that should be worse at higher rpms now that I think about it. 

A miss at idle, but not under load, if not generally a spark related problem.  I would be looking for small vacuum leaks that would affect individual cylinders, I would put a can of super-duper injector cleaner in the tank and then drive around like a berkeleying hooligan before I started playing with spark plugs.

Unless you dropped one and have a .005" gap on it.

In reply to Streetwiseguy :

It could be a small miss all the time.  If it's just a light miss, it'll get harder to feel as you increase RPM so it may not be noticed except at idle. 

So I took the truck out at lunch today and was screwing around with it.  I found that if you hold on the brakes and load the engine enough to move the truck slowly, the engine runs fine. I did this in 4lo as well as 4hi and it runs nice and smooth. So, under load at low RPM it runs fine, at idle with no load it misses.

Also, the brand new PCV valve was steadily buzzing like a coked rattlesnake.  Does this mean I have a vacuum leak somewhere?

I ordered a catalytic converter and a set of the Taylor wires that were recommended. I'm not convinced the wires will help, every bad wire situation I've experienced was worse under load. But what the hell.   I don't thin the cat is causing the misfire, but it's definitely bad. Worse, it's annoying, so it needs to go.

With the wires on mine, I think it was mostly high wire resistance causing very slight misfires.  I'm sure it was doing it under load, but it wasn't enough to feel.  At idle, cylinder pressure is pretty low, so it can be tough to light things off cleanly every time.  And the low RPM makes even a slight misfire noticeable.  

A vac leak shouldn't cause a misfire unless it's only affecting 1 or 2 cylinders.  The ECU is MAP based, not MAF, so it doesn't really care where the air comes from (it'll still fuel correctly with a vac leak).  

Okay, that makes sense. Wires will be here tomorrow. I think tonight I will pull the distributor cap and take a look at it - see if anything looks funny. 

Good to know about the ECU base - I can take hunting vac leaks off my list for now.

I p ut the Taylor wires on. It didn't solve the miss, but I can absolutely tell a difference in how the truck runs in general. Smoother, stronger, better acceleration. They're staying on. 

I also sprayed down the throttle body with cleaner and THAT seemed to be making a difference!

Then the straw shot off the cleaner spray nozzle and right into the manifold. WTF?!

Luckily the engine wasn't running. I can see it laying in the manifold. I'll fish it out tomorrow

The next time you approach this vehicle, better bring a dozen roses.