TJ 4.0 engine gurus

[mrblaine]

Yes it would. Now I'm trying to think of what would allow the valve to travel further into the cylinder, or the piston to travel further up.

Also, we are assuming this is a stock engine with no significant decking, higher comp pistons or stroking.

As stated above, 100% bone stock as it came from the factory.

 

[Apparition]

Look like this?

I'm looking for suggestions on how my #4 exhaust valve and piston came in contact with one another. The 4.0l 266k miles has had a slight ticking noise for some time that I believed to be lifter tick. Last week it started running real rough at highway speed she heated up real quick but I managed to get home without ever exceeding normal operating temp of about 210. I found cylinder #4 to have no compression and missing coolant so I assumed it was head gasket. After getting the intake and exhaust manifold off I found the rear half of the block to be wet. So I pulled the head last night and also found bent #4 exhaust valve and a clear mark on the piston top.

I baby this thing because of the amount of miles and just needing it to get me by as my DD so I don't rev it hard enough that valve float should be an issue IMO. Being a non interference motor I would think timing is not the issue either. I did have some overheating issues a year or so ago but I'm not sure if the noise corresponded with any of that. I've never received any misfire codes until this latest episode. It doesn't look good but not terribly bent either. I guess what I'm asking is do you think the valve has been like that for awhile or did the valve and gasket damage happen at the same time?

I don't want to just replace the valve and piston only to have this same problem. What else could cause this interference?

They didn't really figure it out either.

Seems to be best theory "Chunk of carbon got between valve and piston most likely."

https://www.cherokeeforum.com/f2/piston-valve-contact-how-246524/

 

[Peppergat]

I'm more thinking a small burr or piece of dirt caused the valve to stick open. Its the exhaust valve on number 3, which would be one of the hottest places in the engine (so most thermal change). Kind of a perfect storm, if you have a valve on the large size of tolerance, guide on the small side and a little bit of contamination. It really wouldn't take much... Once it hangs, that piston comes up, smacks the valve and bends the stem, locking it in place. That would also explain why the rest of the valve train looks decent, because its now not doing anything. The pushrod might be slightly bent just because its now loose between its points and if it misses the rocker pocket or lifter pocket, it might put a slight bend in it.

Also not a 4.0L guru, but I've been in my fair share of old tech pushrod engines...but I might be completely wrong about this and that is OK!

I also was thinking this. How did that plug look?

 

[mrblaine]

Look like this?View attachment 560693
View attachment 560694
View attachment 560695




They didn't really figure it out either.

Seems to be best theory "Chunk of carbon got between valve and piston most likely."

https://www.cherokeeforum.com/f2/piston-valve-contact-how-246524/

The divot in the piston was much deeper and very well defined. Plus the whole mess is a lot cleaner in there. We moved the piston up and watched how close it got to the valve and it was not very close at all. My bore scope doesn't look up very well, but we could see the valve pretty easily.

 

[mrblaine]

I also was thinking this. How did that plug look?

Hard to say. The way we found where to look was 2 codes, 0175 and 0174 (iirc) which are highly rich condition on Bank 1. We went down the diagnostic tree which is considerable and near the bottom it said that if none of the other problems existed, there is probably an engine mechanical issue.

In other words, the plugs had been changed before we got it, it exhibited a -32% fuel trim with a 63% engine load at idle. You could smell raw fuel dumping out the exhaust which set us on the normal path for a misfire, plugs, coil rail, injectors, injector o-rings, O2 sensors, harness chafing, PCM check, PDC check for O2 heater circuit TSB, all sensors, OPDA timing or cam/crank synch. We AB swapped a lot of known good parts to no avail. The plugs were clean and shiny. Kinda like they were washed with a lot of fuel. Oddly, Bank 2 plugs were completely fouled each time we checked them. 3 super clean, 3 super black and covered in carbon.

 

[Mike_H]

Oddly, the push rod is not loose at all. I don't disagree with any of this, I just can't figure out how the valve gets down far enough for the piston to smack it on a non interference motor other than valve float.

It would have gotten hot and expanded more than normal...I don't know what the valve to piston clearance is nor do I know how hot it would have had to be to make the valve touch the piston. With a 150,000 mile motor, carbon build up and a chunk breaking off and getting somewhere its not supposed to be (including in the valve seat and having all that flame front flow through to the backside of the valve) is possible...It could have gotten very hot in a hurry.

 

[Peppergat]

Hard to say. The way we found where to look was 2 codes, 0175 and 0174 (iirc) which are highly rich condition on Bank 1. We went down the diagnostic tree which is considerable and near the bottom it said that if none of the other problems existed, there is probably an engine mechanical issue.

In other words, the plugs had been changed before we got it, it exhibited a -32% fuel trim with a 63% engine load at idle. You could smell raw fuel dumping out the exhaust which set us on the normal path for a misfire, plugs, coil rail, injectors, injector o-rings, O2 sensors, harness chafing, PCM check, PDC check for O2 heater circuit TSB, all sensors, OPDA timing or cam/crank synch. We AB swapped a lot of known good parts to no avail. The plugs were clean and shiny. Kinda like they were washed with a lot of fuel. Oddly, Bank 2 plugs were completely fouled each time we checked them. 3 super clean, 3 super black and covered in carbon.

I was thinking along the lines of it was possible that the tip of the plug broke off and tried to escape via the exhaust valve. Just a thought. I’ve seen quite a few diesel engines do something similar when an injector tip breaks off.

 

[macleanflood]

Still thinking it's nothing more than a worn out valve spring.

-Mac