Aggressively Rough Idle / Driving
- Thread starter Ol' Bessie's Keeper
- Start date
I kind of felt like trying out this scenario too, ha. Once I get time to pull the piston and thoroughly inspect the cylinder wall, I'll report back.
I had a hole like that in one of the middle pistons of my 4.0L in 2018. It had been running rough and slowly losing power for a couple days, ending with needing 3rd gear and wrapped up tight to keep up on the highway (5-speed manual), then a sudden cloud of blue smoke on the highway and lost all the remaining power. Yes, I should have stopped driving it when I first noticed, but I wasn't exactly thinking straight.
I ended up with a short block from RockAuto. And replaced the synchros while I had it out anyway. Best I can figure is either a bad radiator cap or a cracked radiator (replaced both of those too), so that I couldn't keep water in it, and the temp gauge on the dash didn't show over-temp with air/steam. Thus, overheating without knowing, and eventually melted a hole in the piston. It looked a lot like the pictures here. I'm fuel injected, and the check engine light never came on, so I doubt I had a bad mixture. It did come on when an exhaust coupling came apart, so the light does work.
Don't know if that was the actual cause or not - I wasn't particularly scientific, just wanted to get it back on the road - so I'm interested to see what you come up with.
I ended up with a short block from RockAuto. And replaced the synchros while I had it out anyway. Best I can figure is either a bad radiator cap or a cracked radiator (replaced both of those too), so that I couldn't keep water in it, and the temp gauge on the dash didn't show over-temp with air/steam. Thus, overheating without knowing, and eventually melted a hole in the piston. It looked a lot like the pictures here. I'm fuel injected, and the check engine light never came on, so I doubt I had a bad mixture. It did come on when an exhaust coupling came apart, so the light does work.
Don't know if that was the actual cause or not - I wasn't particularly scientific, just wanted to get it back on the road - so I'm interested to see what you come up with.
Oh man....I think you may have just hit the nail on the head! Very glad you chimed in!
When I removed the head, I noticed the heater hose pipe that sticks out of the water pump was loose. I had a very small coolant leak (white cooked coolant on the downpipe) and couldn't find where it was coming from, but this is definitely the source. My check engine light very much works also, haha, but never got one for overheating. Temp always showed 210* also. The piston must have also gotten cooked like yours, which means I'm no longer gonna just pop another piston in and send it; might as well just redo all four pistons at this point.
I've been enjoying my ol' Comanche in the meantime, even had to steal a couple parts from the TJ to fix her up recently...never ends!
I'll update whenever I actually get around to getting those pistons in.
-Update-
Finally pulled the block yesterday and ripped it apart today. As far as where the oil came from, I really only see one potential culprit but I'd love to hear others' opinions on the matter. I don't see any cracks, all though I guess small cracks could contract when the block isn't hot and be unnoticeable to the eye? There are two main sources of oil buildup/covering on the block, one on each side- on the driver's side, below the rear freeze plug, and on the passenger's side, up underneath the distributor and near the dipstick tube.
Not really sure why oil is caked on the driver's side, but when I pulled the dipstick tube on the passenger's side, I was assuming it wouldn't just slide out of the block like it did; I thought they were pressed in? Or needed to be chilled then inserted and expand into it? Either way, it just slide out, so I'm assuming when the piston had a hole bust in the center of it, it forced oil out the dipstick tube? Would that make sense or would it not create a vacuum like that?
Finally pulled the block yesterday and ripped it apart today. As far as where the oil came from, I really only see one potential culprit but I'd love to hear others' opinions on the matter. I don't see any cracks, all though I guess small cracks could contract when the block isn't hot and be unnoticeable to the eye? There are two main sources of oil buildup/covering on the block, one on each side- on the driver's side, below the rear freeze plug, and on the passenger's side, up underneath the distributor and near the dipstick tube.
Not really sure why oil is caked on the driver's side, but when I pulled the dipstick tube on the passenger's side, I was assuming it wouldn't just slide out of the block like it did; I thought they were pressed in? Or needed to be chilled then inserted and expand into it? Either way, it just slide out, so I'm assuming when the piston had a hole bust in the center of it, it forced oil out the dipstick tube? Would that make sense or would it not create a vacuum like that?
KennethS
TJ Expert
I would not expect it to create a vacuum, but rather have the combustion chamber pressure vent through the hole in the piston and pressurize the crankcase.
Any wear and tear experts out there that can look at this cam & respective lifters (that are somewhat new by the way) and let me know if it should be replaced or not? I only ask because I do get some solid top end noise/lifter tick/chatter/etc and have swapped out the rockers, lifters, and push rods before, but not the cam itself. Looking at around 185K on it I believe and I can't tell if this wear looks OK or not. A couple of lifters seem to tell a bit of a story, but I can't read, haha.
Here's pictures of the 97 XJ 4.0 I tore down and sent to the machine shop. I don't know what I'm doing but they look ok. Concerned about the lines on your cam lobes.
-Mac
-Mac
KennethS
TJ Expert
I would measure the cam lobes to see if the intakes and exhausts are all the correct heights (all intakes the same, and all exhausts the same). That would verify any excessive wear and tear particular lobes.
Alright lads, finally put her back together around May. She is running ROUGH. Haven't driven her yet because of this, and she died on her maiden voyage at a stoplight. I finally had time to get back at it, checked compression; all cylinders above 150. I'm gonna send a video. I feel like it's either a timing issue or a fuel injector issue?
Actually yeah, I'm an idiot. I usually post via Google Drive. Whoops!
View: https://drive.google.com/file/d/1wk3_qGe-91LGUWv2Hd_MWcGduwE4gRbk/view?usp=sharing
View: https://drive.google.com/file/d/1wk3_qGe-91LGUWv2Hd_MWcGduwE4gRbk/view?usp=sharing
Bit of a mixed bag- With just priming the system, it barely moved the needle, but upon startup it stayed a constant 47psi.
Well because of how poor she is running, I only let her run for a minute or so. She's still in her break-in period also so I'm trying to be mindful of all of that since she won't really drive.
But ultimately, sounds like she starves for fuel on initial turnover and have to pump the accelerator to get her to fire. Then runs like shit. Held 47psi for a minute or so before shut-off, revving constantly.
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