doublechaz

Perhaps your plugged exhaust is from a severely out of spec timing chain, or a wiped cam or lifters not opening the exhaust valves on one or more. It sure sounds like a plugged exhaust. It runs like one. At around 4:07 last video your vacuum is fine at idle, it's fine when you first rev it, and then it goes way low right when the problem shows. I still think you have excessive exhaust pressure blowing back through the engine into the intake reducing your vacuum level. When you operate the throttle like you do in that test vacuum should go from your normal idle to slightly lower, and then quickly return to about the same as your normal idle since there is no load on the engine, just RPMs. You instead have a very large reduction in vacuum at that point. Unless you have some kind of vacuum leak that gets worse when you rev the engine, opposite to every known vacuum leak up to this point in history.

Is the distributor driven from the camshaft in this engine like on my Mopar V8? That V8 showed jitter in spark timing with a timing light when my timing chain was shot, allowing the cam to whip around like the tea cups at the carnival. That V8 is a fully mechanical timing setup, but this one being at least partly electronic should not show any jitter at all unless there is a mechanical problem confusing things.

Another fairly simple thing to do for testing would be to do something to hold the engine RPMs steady in the problem range for a couple minutes and after that take the temperature of each exhaust runner with an IR thermometer. They should all be the same. If one or more is an outlier then you can point a finger at them.

Otherwise, you could take off the valve cover and inspect. You could slowly turn the crank back and forth from the nose bolt and see if there is slop between it and the cam. You could burn a bunch of time flow testing using the leak down test tools to blow air through each spark plug hole, through the open exhaust valve and out the tail pipe and check for differences in flow and pressure.

aroundincircles

Taking the valve cover off isn't a bad Idea. It's leaking pretty terrible anyways, so been meaning to replace the gasket. I can inspect the lifters and valve function while it is off.

aroundincircles

Well....... I took the valve cover off, and set up a camera to record the valves moving, and I could tell where I was with each compression stroke, one one of them sounded very different than the others.... So when valves 2b and 3a were pressed at the same time, you can hear a HUGE change in the sound for the compression. so which cylinder is being compressed at the time that that those two cylinders are in an exhaust/intake stroke? would that be cylinder 6?


Here is the video, I suggest headphones:

awg

Please tell me a compression test has been done on this engine

The one with low reading will be the one

aroundincircles

No, of course not. Why would I have done a compression test? I mean, other than it giving me a TON of information, and simplifying my issues.... (sorry a bit of sarcasm, mad at myself). I never thought to do one before I took it to a shop, because It never seemed like the issue was a compression issue. I have a compression tester, sitting on a shelf, right in my garage. But at this point it seems moot, the head has to come off regardless of which cylinder is bad to determine how bad the damage is. If it's a bad piston, cylinder, valve, or something else (or a combination of the above). I'm just an idiot. I've never claimed to be otherwise. Dave51 did ask about my compression number before (and recommended checking rockers/lifters), but I literally have done three things to it since I got it back from the shop over a month ago. I checked the o2 sensor (it was unplugged by the shop and not plugged back in), I checked the EGR valve since I had already bought a gasket, figured... why not, and now this. I've been dealing with other stuff, so just haven't had time till this week to think about it, much less work on it. as much as I am mad at myself for not figuring this out forever ago, I'm glad to be at least going in the right direction. Now I get to shop for a rebuilt head.

awg

Well you got to look for the silver lining in every grey cloud

At least you know the issue is mechanical, can be fixed, and you will be back on the road

sorry if my answer may have seemed sarcastic, I have done my share of not seeing the wood for the trees, thats were a forum can save your backside

..and you still got $500 in your pocket for repairs !

it may be worth doing a leakdown test first , as that will point to pistons or head

indeed a manual vacuum gauge would have showed the problem up right away...digital via OBD is handy, but only hinted at the fault

If you had mechanics look at it, and they couldnt work this one out, they should be disbarred

aroundincircles

Honestly I don't think they spent two hours on it in the over two months it was at their shop... It's renix so it doesn't have OBD, I just have the renix engine management tool. I'm super grateful to the forum. I'll be reaching out to dave tomorrow.

Dave51

I think you should check compression anyway-- all information is good information.

Dave51

I'm kinda thinking it's gonna be -0-.

Dave51

Cause
IMO you need to continue along the diagnostic tree.

aroundincircles

Is there something a leak down test will inform me of that just pulling the head won't?

5-Speed

Yes. Leak down test is very important. If the leak down comes back good, there will be no reason to pull the head. If it comes back bad, it will point you in the direction of it either being valve or ring related.

aroundincircles

So... I have this special talent. I must do everything the hardest way possible.

I went to grab my compression tester. It's just a cheap one I bought at harbor freight forever ago. I used it exactly one time... on my previous XJ I sold nearly 5 years ago so it has been at least 6 or 7 years since I last used it, i have video of me using it on that jeep... but the largest adapter doesn't fit in the spark plug holes of this XJ, they are too small... they are the same engine, right? same spark plug holes? am I missing something?

awg

my nest question would be, does it appear that the apparently defective valve mechanisms on the afflicted cylinder appear to be able to be set to a closed position ?

If so, the rationale behind a leakdown test would be one hopes to be able to exclude some defects, in this case piston/ring issues

If you cannot exclude them this way, you would possible have to remove that piston at least to check, adding extra work

If the valves wont seal, its moot, as Dave indicated

XJ have standard size spark plugs

aroundincircles

Well I got annoyed and did a thing...