Random miss at idle

I don't see how leakdown could have the truck idling a bit rough but that going away with load.  Seems like it would be opposite.  

There's a few different backyard solutions to cleaning carbon off the valves without removing the heads.

pres589 said:

I don't see how leakdown could have the truck idling a bit rough but that going away with load.  Seems like it would be opposite.  

There's a few different backyard solutions to cleaning carbon off the valves without removing the heads.

A slight compression leak will show badly at low RPM and barely at all at high RPM.  At higher RPM there's less time for it to leak down, so you lose less cylinder pressure and the loss is less noticeable as a result.  I've seen an engine with a slightly bent valve that had 60 psi on one cylinder in a compression test (and 170+ on the others) and almost a dead miss at idle and ran pretty badly at low rpm in general.  Wind it up to 3000+ rpm and the miss was seemingly gone.  And at high rpm, it hardly even seemed down on power.  

After some discussion with the mechanic he agrees it's most likely carbon build up. When I get back in town this weekend I'll be trying a little water injection and a little seafoam injection to see if it helps the problem. 

I may of mentioned, he's a friend of a friend and won't even let me pay him for the testing. But he's a scotch drinker,  so I'm taking a nice bottle when I pick it up.

Any updates?

Last I checked, FedEx says the injectors I sent over should be arriving tomorrow, so I figure we'll see some updates after they arrive. 

As noted in the build thread, yes and no. After the mechanic tested it I ran a half bottle of Seafoam into the intake. It made a huge cloud of smoke but no change in the miss. I checked compression on all cylinders which was all very good. I checked leak down on all cylinders, and it wasnt great but it was the same all the way around. So no sign of valve or Head gasket problems, and nothing that marks #4 as different. Still getting codes for cyl#4 miss and nothing else, so I threw a visible spark checker on that plug. Got a great steady spark when missing, so I can rule out ignition issues.

The only thing left I can think of that would give a cylinder-specific issue is a fuel injector. Everything else in the fuel system would affect multiple cylinders. 

Rslifkin was nice enough to hook me up with an upgraded injector set. As he said they should be here tomorrow and I'll put them on this weekend. If that doesn't do it.....I'm not real sure what to do.

If the injectors don't fix it and you can rule out basically everything else, I'd be tempted to throw a crank sensor at it.  They can do weird stuff when they fail (although they're a PITA to change). 

The spark looked good and regular on the spark checker, but I guess it's still a possibility. A Mopar crank sensor and cam sensor might be in order. And yeah, they are a huge pain to change. well, tha cam's not too bad but the crank is.

Yeah.  Cam sensor is 20 minutes and some bloody hands / wrists.  Crank sensor is 2 hours and wishing you had rubber forearms.  Takes about 2 feet of extensions and u-joints to get a socket on the bolts and you have to work by feel.  No way to see the sensor without pulling the passenger side cylinder head.  And then you have to crawl under the Jeep and reach up around the bellhousing to get to the connector for it.  As long as you know where the sensor is, it's manageable, just annoying and tedious to change.  

For sensor brands, as far as I can tell, the cam sensor doesn't matter.  I've seen plenty of cheap brands used without issue.  Definitely Mopar for the crank sensor though.  Last cheap one I used worked fine for 6 months and then started to get flaky if the engine bay got too warm in traffic.  

Also, in case you don't have it, I'll email you a link to the factory service manual for the Jeep (in PDF form).  

I installed the new injectors and the miss is still there, maybe just a little worse. I put the new upstream o2 sensor in too. Still misses.

When it's not missing it runs much better with the new injectors though. Smoother, starts quicker, just better. So that's cool. 

The closest jeep dealership wants $145 for diagnostics. They credit all of that if they also do the repair. I think I'm dropping it off.

I'm updating this just in case someone finds it while searching for answers on a similar problem.  I hate it when I find an old thread that describes a problem I'm having but no one ever posts back about how they fixed it. The full build thread can be foundhere

To summarize the end result, I left the Jeep at the local dealership while I was out of town for a week so they could diagnose the problem. After banging on it a while they found one of my new spark plugs had developed a crack at the base, very hard to see. They replaced the spark plug and it solved the problem. I didn't find it because I thought I had moved that plug to a different cylinder after the compression tests, but apparently not. I had pulled all the plugs and put them in a shoebox during the test. I reinstalled them randomly and assumed that I wouldn't end up with the same plug in the same hole. Against high odds I guess I got the same plug back in #4 so it only missed on the same cylinder. Bizarre. 

So funny story about ZJs and misfires...  Mine has been a hair rough on cold starts at idle lately, but smooths out after a minute or when you bring it off idle and start driving.  So I threw new plugs in it last weekend.  Had a dead miss on #1 when I fired it up.  Turned out that one of the new plugs was bad and leaking current through the center ceramic and not firing properly.  Threw an old plug back in that cylinder and it ran fine again. 

Now for the roughness at cold idle.  Over the weekend, that started to develop into a noticeable random miss (but only at cold idle).  I've driven myself crazy all week trying to figure out the issue and ruled out the entire ignition system as being fine.  Today it started missing occasionally at hot idle too (but it's on and off and not the same cylinder every time) and it eventually threw a cyl 8 misfire code. 

So it's time to order a crank sensor...  Same symptoms as when the first one failed.  Crappy idle, cyl 8 misfire code.  Now I get to look forward to the fun of getting to the crank sensor (and worse yet, the connector for it)...

out of curiosity...what brand was the new plug that was bad?

ultraclyde said:

out of curiosity...what brand was the new plug that was bad?

 

It was an Autolite.  I normally run the cut-back electrode AR3923 race plugs in the ZJ, as they give a little more margin to detonation than the standard plugs of the same heat range (and the next heat range colder is too cold for DD use and fouls up a bit if the thing idles too much in traffic, as these are already a heat range colder than stock).