One wonders, without a complete history, if one of the POs did something askew, like installed a cam wrong, put in a new cam with no zinc (in which case it would last about an hour), wrong or cracked flexplate... who knows.

IIWM, and

I think it's time for teardown and inspect lifters, cam, timing chain and flexplate.

That said, perhaps there is still some fiddlement you might waste your time with try:

Pop distributor cap and turn crankshaft back and forth. Observe rotor to see how much play (IDK what WNL is tho).

Check existing preload on lifters. Then reset preload.

Get entry level oscilloscope and check CPS/CKS relationship (again, IDK if it's the same as coil rail, but I think it is and besides it's testing and can't hurt). If you're low on coin I can loan you my stuff if you pay for postage and give me a massive security deposit, or you can get a basic 2 channel for about 100 bucks. Get it on Amazon and if it doesn't work out...

Mismatched (hopefully PO would have seen a crack/warp) flexplate checking is going to be an issue. Perhaps pop the dust cover and see what you can see.

 

Here's a couple more videos just of how the Jeep is running, and how it bogs under load. In the first video it also randomly shuts off out of nowhere while trying to rev It Up.

Running:

Driving:

 

Check fuel pressure, with a gauge, I know it sounds stupid but I had the back fire through intake and lack of power hard to start. I fought this for a while and thought there was no way fuel pressure was an issue because I had a brand new pump and didn't know any better lol. I tore my engine down found nothing, swaped engines and same issue but it came down to a cheap autoparts store fuel pump that wasn't doing the whole fuel pump thing. Went to the junk yard got an oem used pump and its been perfect for like 8 years now.

 

Yes, I did the fuel pressure test, and it was good. I have all the readings listed in post #28

 

So thanks to your video of manifold pressure/rpm and timing.....YOU HAVE A VACUUM LEAK. I made a video of mine with my phone but when I try to post it it becomes too blurry to see. AT idle I run rpm of 670-680 when warmed up...190, timing of approx 10ish give or take +/- 2. MY MAP is almost a dead steady 10. Yours is 17-20. Thats WAY too high. AN engine not running is 29-30. Wide open throttle map is probably mid 20's. Thats probably why your idle is near 800, and your timing is also 17-20. Is your throttle plate fully closed?...there is an adjustment screw under the linkage arms but the adjusting part is underneath the throttle body leg casting...difficult to adjust or mis adjust.. Bottom line...you are getting air in there from somewhere, throwing off your mixture curves. The only simple test I know for a map sensor is to turn off engine and see if it reads 29-30 . What does your throttle valve sensor read at idle/closed...I will check mine later

 

Ok, good to know. I can check and see what the map sensor is reading with the engine off. If I remember right, my throttle position actually reads around 13% at idle. There is an aftermarket throttle body installed by the PO, but I do still have the old throttle body. I need to check and see if the throttle plate is completely closed at idle. Also should probably make sure they actually installed a gasket under the throttle body. I had not thought about those things. I'll look at that stuff and then update

 

Ok, so I made a diy smoke machine and smoked the intake. I got plenty of smoke in the intake manifold, and the only place I can see it leaking from was where it was seeping by the rag I had plugging the top of the throttle body. And also, when I would pull the oil fill cap off, smoke would come out the top of the valve cover and also through one of the PCV hoses. I spent a good hour looking around with a flashlight keeping smoke circulating through there the whole time, and I was not able to see any leaks around the intake manifold. I was able to see the two dowel pins, and the intake manifold does not appear to be hung up on them at all. They look like they're slid right through the holes they're supposed to be lined up in. I dont know what else to do. I guess I can see what the map sensor reads for atmospheric pressure with the engine off, and what throttle position sensor reads with the throttle closed. Should I reconsider a mechanical issue? Stretched timing chain? Pull the exhaust off? Oil too thin causing the lifters to not move the valves far enough? That may be way off, but Im just throwing stuff out there. Im starting to get the feeling that ive either missed the issue entirely in my testing, or that its some obscure freak thing that im not going to discover using common sense and logical testing procedures.

 

NO with "the oil being too thin" and not moving the valves enough.. A wiped cam lobe will act as you've been describing and has already been thrown out there...I'd be checking that as well as cam timing And even possibly what cam it has as a replacement if any.....As stated many posts up I'd go after the mechanical engine checks next...

 

Ok, how do I check the cam and cam timing?

 

Also, I just checked the MAP sensor readings with the engine off, and it reads 29.5 InHg which is correct. However, the TPS is reading 13% throttle with no throttle input, and it reads 75% throttle at wide open throttle. This seems a bit narrow to me. I know Its wishfull thinking, but could a faulty/miscalibrated TPS cause the abnormally high MAP readings while running, and MAP that drops with throttle input?

 

Yes, that is wishful thinking...mine shows almost the same. My scanner gives 2 readings for TPS. ($18) and ($D1). They differ and I don't know what they stand for. Bottom line is that your manifold pressure is WAY to high at idle and probably too high most of your rpm settings too. Either your map sensor is off or you have an air leak or idle control valve stuck open. Hook a vacuum tube to the map sensor port on TB and suck on it while reading it...you should be able to suck close to 0...but thats not calibrated. I think mine has a little flex elbow..maybe yours is loose or cracked...wouldn't show up in a smoke test. Put your old stock tb back on...15 min and 2 dollar gasket.

 

Ok, I'll check and see if I can get it down around 0 by sucking on that port. I did swap the IAC at one point from a cheap Chinese one that was on there, to the one that was in the old throttle body. It may or may not be a good part though.

 

IMO if you're interested in vacuum then you should direct measure vacuum:


Many XJ owners have noted vacuum at 15-16 inHg in normal running 4.0s (not just me) so that should be your minimum.

Anyway, this is also a good way to confirm MAP readings as MAP + Vac = PB (in this case 12.3 + 16.0 = 29.1, or close enough with an old beat-up vacuum gauge).

As timing, worn cam, valves and plugged exhaust can also cause low vacuum, at some point you should simply be confident that you have effectively ruled out the stuff on top.

 

Measure push rod travel. Got a dial indicator?

 

In re: timing chain

although pulling the timing chain cover should really be on the agenda at some point.