Strange sporadic hesitation...

[indiemo]

1062186

Here's a good one for y'all. My TJ is developing this hesitation problem that my old YJ used to have as well. The circumstances surrounding them are quite similar so I'd like to see what everyone thinks. Ok, so it does this thing were if I drive it for a bit, then park it and let it rest for anywhere from 10 minutes to an hour, when I start cruising down the road again, it starts to hesitate, and sputter, almost like it's not getting enough gas or something. Almost like a vapor lock. So I have to push the clutch in, let it settle, and then it runs fine. This seems to have started to happen with both my rides around the same time that my catalytic converter started breaking up inside. This led me to believe that the broken pieces of cat were clogging the exhaust and choking the engine. However, when I put a new cat on my YJ, the problem didn't go away. I also replaced the TPS and finally the O2 sensor. Nothing really seemed to fix the problem immediately. It did go away on my YJ about a month or so after I replaced the O2 sensor. But I was also living in NC at the time, and I didn't really ever run the engine that hard since it's all flat down there so whether or not I really fixed it, or it healed itself, I'm not sure. Anyway, now my TJ is doing the same thing. The cat and most likely the O2 sensor are getting replaced next week, so we'll see if that fixes anything. I'd like to go live in NC again and see if that fixes the problem, but that just doesn't seem practical. Has anyone ever encountered this or can anyone offer a suggestion as to what might be causing the problem?[addsig]

 

[dingus]

1062188

my best guess is something to do with the spark plugs... are they old? replaced recently? tried platinum (supposedly run cooler...)? fuel filter? [addsig]

 

[indiemo]

1062191

I actually just replaced the plugs about 5000 miles ago with platinums. I'll look into the fuel filter though. That's not in the tank is it???[addsig]

 

[bluebelle]

1062313

Sounds like one, or more of your sensors is/are sending intermittent signals to the computer. Have you run trouble codes yet? What did that indicate? Fuel and exhaust clogs are not usually indicative of intermittent problems - they just progressively get worse. Yours sounds like it's related to heat, although 10 minutes isn't really long enough for your motor to cool down.

Have you ruled-out all of the basics yet? The old air/fuel/ignition stuff? Have you checked the coil? All connectors? Looked for chaffed/burned/shorted wiring?

Unfortunately, there are a lot of things that may cause what you describe. Methodical, patient troubleshooting will prevail.

Or you can pay the $70 for the dealer to use the computer. But - sometimes that won't show anything either.

Let us know how this progresses. With additional diagnostic information, perhaps we can be more helpful.

Hang in there - mine did the same thing and now she's strong and reliable again!

blue
[addsig]

 

[indiemo]

1062336

I ran the codes and got a 72, which translated says, "catalyst efficiency below required level", or something like that. That and the rattling sound from underneath led me to believe that it was the catalytic converter that was at fault. I guess once that's replaced and the engine codes reset, I'll look into the other things you mentioned bluebelle. hopefully the service manual I just ordered will help with the testing of sensors and other things that I'm not 100% on. Thanks for the direction bluebelle, that helps a lot! I'll be sure to keep y'all updated.[addsig]

 

[bluebelle]

1062359

indiemo,

Given the 'cat efficiency is below the required level', and evidence that your cat is rattling, let's presume your cat is bad. The interior of the cat is ceramic, and it's possible the plates have fractured for whatever reason. You may have some dealer coverage on the cat for the first 100K miles - it's a pollution measure. I've heard of such things. Worth a call - or look it up in your manual.

If we presume that your cat's bad, then less hydrocarbons and nitrates will be 'catalyzed' or chemically transformed. This will result in your exhaust gasses having more un-catalyzed hydrocarbons - just like a too-rich fuel condition.

If you have a second oxygen sensor (likely) in the exhaust pipe just down-stream of the cat, this O2 sensor would sense the increased hydrocarbons, think there's a fuel-rich condition and tell the main computer two things, that the cat's bad (code 72) and that the exhaust is too rich. As a result, the computer may want to lean the mixture down causing what you described.

In other words, your computer may be trying to lean the mixture down when it shouldn't because the bad cat is tricking the second O2 sensor to believe the mixture is too rich because the O2 sensor believes the cat is functioning - when it's not.

When you clutch and idle, the engine needs only an idle mixture - less rich when warm than a power mixture - and will run smooth until you power up again.

Only a theory, but plausible.

Regarding your fuel filter. There's a fuel 'sock' in the tank on the end of the fuel pump pick-up tube. It rarely gets clogged unless something really nasty got into your tank. Typically, there is an in-line filter on the driver's inside frame rail just forward of the rear axle. Factory manual for the 92 YJ recommends changing it every 60K miles. I change mine every 30K - they're $10-ish.

Sounds like you ordered the factory service manual? Excellent choice. Now you won't have to call the dealer with questions, just look 'em up.

blue



[addsig]