AnotherOldJeepGuy

Vacuum plug back in place and hose tied off so it doesn't wander... Pulled Dist Cap, the rotor only wiggled back and forth a tiny bit, maybe 1-2 degrees each way, very small. Things looked pretty much the same shape as when installed in Jan. Not pristine, but pretty clean. Still no soap test, but I did give the manifold to exhaust donut the slight poke test. It's not in very good shape, I am pretty sure if I upped the poke pressure I could cause it to crumble. Maybe not 100% but for sure a notable amount. Perhaps it would be worth just replacing it to rule that out? I figure if I end up working on the exhaust manifold I'd be replacing that anyway. I'd thing the cost is minimal for a new gasket. Assuming the rusty bolts will let loose, I am wondering if the donut can be replaced without removing much else? The pipe wanders around from there to the CAT, and I don't see any anchors, and I see maybe an inch of space between the pipe and the frame, would that be enough slack? It's worth confessing that I have been reluctant to crawl around under the truck while leaving it running with no other human in the cab to cut the engine and hold the brakes. But I have stuck my head under a little to try and locate from where the loud comes from but difficult to pinpoint.

Gotcha, I was assuming I would need some kind of tool to check for "inadequate" spark.

AFAIK I don't have a Check Engine lamp, I've never seen one in over the last 36 years that I can recall (which in most cases would be a good thing ). I looked in my manual and it doesn't identify one existing at least not in the instrument cluster. The closest thing seems to be the Engine Maint light that apparently went on at 82,500 miles, that was way too long ago for me to remember it, but I assume it did. I replaced this sensor back in Feb 2025 when I had an idle issue. The issue turned out to be very dirty TB and IAC and was completely resolved after cleaning them and putting in a new TB gasket. Of course it could still be contributing, just not because of age. I'll give that a shot! Have not coordinated with my son on the soap cannon yet for the bubble test but hopefully can get that done pretty soon. I'm guessing I would need another tool I don't have to test this.... correct? Thanks for all the assistance so far!

Got the tool from Harbor Freight and measured the pressure at the fuel rail. FPR hose attached = 30PSI FPR hose not attached = 37PSI

Heading out to get the tools for checking the fuel pressure.....

The battery reset didn't change the outcome, still sluggish response to throttle and running very choppy. My son has a thing called a soap cannon he uses to was his car. I'm gonna see if he will bring in over and let me use it to suds up the exhaust real good and see if any bubbles show up. Think I'll need to do that first thing in the morning when it's as cool as it gonna get since the exhaust is gonna dry out pretty fast once the motor is running, but just maybe I'll see something useful.

I made the "plug" below. The bolt is in very tight to start then snugged further with 2 tie wraps. I don't know that this was a 100% plug but it should be very close to it. (I can redo it with come E6000 glue if it doesn't look like it is OK as is). With this one the vacuum port (but before reading your post about resetting by disconnecting the battery) I saw no change. Sluggish response to throttle, running choppy, and at least once when I got the RPMs up and let go I got a small backfire (similar to the one I posted a video of a page back). I'll go disconnected the battery and repeat the test.

Oh, didn't realize the test went farther than just seeing what happened just when plugging it. I'll come up with something to plug it other than my thumb and see what happens when I drive in a little. As far as cruise, I can't remember the last time that worked, but it is measured in years not weeks!

OK, I don't know if this counts as a V8 moment or not but here's what I observed. 1. Started with hose still connected and listened for a base for comparison 2. Pulled the hose off - RPM increased a bit, not a huge amount but noticeable. 3. Plugged the manifold port with my thumb, engine returned to about the same as with the hose connected. (I didn't really have anything that would qualify as a vacuum plug but thumb seemed to do the job) So the blocked port compared to the hose connected sounded the same. FWIW, as I reattached the hose, just before making contact, I could hear noticeable sucking sound so it is pulling a vacuum. I don't have anything to measure with, but it's there.

I will definitely give this a try tomorrow and post what happens. I got sidetracked today with appointment so I haven't gotten the vacuum tester yet either, but hopefully tomorrow. Haven't tried my soapy water test yet either so plenty of what-ifs on the list to do.

That's "a few" ! lol

How would running rich be noticed? What kind of performance issues might be noticed if the engine is running rich?

Finished with removing and inspecting plugs assignment . They all looked the same. I cleaned them all and put them back in (well new one for the one with broken top), no change in how the truck runs. Planning to get my hands on some kind of pressure measurement gear tomorrow.

If I don't find a loaner, would this from Harbor Freight work? https://www.harborfreight.com/fuel-pump-and-vacuum-tester-62637.html

Thanks @Gojira94, @Porquer. Since I broke a plug I'll go ahead and replace it, there's not a lot of choice on that one. For the others, a pic of one cleaned up is below. I'm thinking cleaning and reinstall is good enough at least for now, the plugs were all new in Jan. I'll be calling the local auto places today to see if a fuel pressure checker can be borrowed or rented so that data point can be added to the collection. As for the elusive yet suspected leak, perhaps the soapy water test (the one used to find a tire leak) might work at least at the donut gasket? Not sure access/visibility is really good enough for that test elsewhere, but maybe I can do it at the donut.

FWIW, Here's the parts catalog page for the 2.5. (Half the time I can't find what I am looking for in these, but might have gotten lucky this time ) Of course having a part number doesn't mean the part is actually available and you may still have to be creative like @eaglescout526!

Here are the OEM part numbers, maybe searching for the 87 ones.

I got mine at NAPA, although I have 1990. But you might check there.

I cleaned that plug up pretty good. Took out the next one and it looked exactly the same. My plan was to take all of them out one at a time, note if any looked different, clean each one up as good as I could and put them back it, and see if anything changes. Unfortunately the 3rd one I removed the ceramic top broke, so that plan doesn't really work anymore so I'll be getting some new plugs. I am expecting 1 of 2 things to happen, assuming they all look the same: 1. The black plugs were blacken by the driving since the time of "the incident", and just maybe all the things fixed at this point will result in running well once all the black plugs are replaced with clean ones. This will be the lucky outcome. 2. Nothing will change and the problem remains as it was before this exercise and I'll still be down.

It's starting to rain but I pulled the easiest to get to plug before getting wet. I am thinking the experts can maybe make something out of just this one to start with and I'll go further after the rain. I looked in my records and I put in new plugs, rotor, dist and wires back in Jan and at that time Jeep was very happy. I'll note in case it is useful that this plug was not very tight. It wasn't finger only removal loose, but it took almost no effort to "break" it loose.

I have not tried to track down a pressure tester yet but I did the quickest one, pulled the dipstick. I don't particularly smell anything from it. It is slippery but could it be "a little thinner?", I can't really judge that. If I hold the dipstick up horizontally with the wide side vertical the fluid slowly begins to accumulate more on the bottom, nothing drips off, that's about as much as I can say about it.

This is NOT TRUE! It is more complicated but perhaps what I noted this morning is helpful. It IS TRUE that pulling throttle just off resting position doesn't cause a stall, IF THE EGR PLUNGER DOES NOT MOVE. BUT, if the EGR Plunger does move, it still stalls. So back to the issue that the EGR moves sometimes, and sometimes it doesn't, when I rev up RPMs. This perhaps means something to folks that know stuff!!

Crap, another tool I don't have! I'll check to see if any of the local auto parts places loan or rent something to measure vacuum with with a fitting that works at the Schrader. I do still have the ballast resistor, but have been considering replacing it because the ceramic outside is cracked. I expect the resistor is still connected just it's house is cracked. But since I am going to disconnect it maybe I'll go ahead and replace it at that time. The journey continues.... thanks @Gojira94 for all you time yesterday!

So since I witnessed no change when I plugged this line, and that suggested a clue, what does that clue suggest?

Sounds like you have made good progress!