MJ running rough, erratic idle, lacking power

[acerocknroll]

Well, been lucky for the past year but finally having some running issues with the MJ :(

 

1991 4wd/AX15 transmission. Didn't drive it for a couple weeks then drove it to and from work, all good. Started it up later and almost stalled out right away, could barely get it into first or reverse unless I feathered the throttle a bit. Once into second it seemed better, third seems mostly normal. Coming to stops it has low/erratic idle. Now when I start it up it sounds like a cement mixer or a washing machine with a bunch of sneakers in it for like 10 seconds, then revs way up to like 4000 rpm all of a sudden, then goes back to about 700.

 

Basically, it just feels like it's not getting enough fuel, I have to goose the throttle every time I want to shift, especially from a stop. As mentioned, once into third or fourth gear it feels mostly fine and it cruises right along.

So far I've just replaced the fuel filter and filled it up with fresh gas, it seemed like that fixed it at first but went back to running badly immediately on the test drive. Just looking for some insight on what to try next before I start shotgunning parts at this thing. Once other thing I noticed is there is a new leak either from the oil pan gasket or possibly the transmission (ax-15)... really hoping i haven't been running the trans dry, gonna check the transmission level first thing tomorrow (oil level is fine).

[HOrnbrod]

did you check for fault codes yet?

[Tex06]

Have you done the cruiser tips for ground refreshing yet? These things are notorious for electrical bugs.

 

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[Marine1Texas]

As above said did you check fault codes? Did you do a fuel pressure check?

Unless the manual trans was not releasing disengaging the clutch that would have nothing to do with the way it idles.

I would check your codes first then make sure you have done all cruiser tips, also make sure you have no vacuum line leaks.

[acerocknroll]

Ah, i totally forgot I could check fault codes like that. I get:

 

12 (just disconnected the battery when I did the fuel filter)

 

33 Air conditioning clutch relay

 

55 End

 

Which is weird because... I don't have air conditioning?

 

Will try to check fuel pressure tomorrow. Thanks!

[HOrnbrod]

Normal codes, whatever the problem is didn't show up. Fuel pressure next................

[acerocknroll]

Normal codes, whatever the problem is didn't show up. Fuel pressure next................

 

Fuel pressure test results:

Turn key to prime fuel pump reads about 42

Start engine and idle, reads about 32. Doesn't change much when under load. I've attached a couple of videos if anyone feels like taking a look.

 

Starting up:

It settles into a normal idle towards the end here but goes back into the erratic idle after I stopped the video

 

Going to test ignition system tonight. Could it be clogged injectors maybe?

[HOrnbrod]

Try pulling the TPS and IAC sensors off the throttle body and give them along with the throttle body intake tube.a thorough bath and cleaning with throttle body cleaner. Pay particular attention to the TPS pintle valve.

 

http://www.frontrange4x4.com/forums/showthread.php?18931-XJ-Cherokee-IAC-TPS-and-Throttle-Body-Cleaning

[ParadiseMJ]

 Pay particular attention to the TPS pintle valve.

 

 

 

...and be gentle with the pintle.  They're somewhat fragile.  Don't go at it with a wire wheel... :cheers:

 

Also, I thought when doing the fuel pressure test it was...running, one reading with vacuum to the FPR - and again with the vacuum hose disconnected from the FPR.

[acerocknroll]

 

 Pay particular attention to the TPS pintle valve.

 

 

 

...and be gentle with the pintle.  They're somewhat fragile.  Don't go at it with a wire wheel... :cheers:

 

Also, I thought when doing the fuel pressure test it was...running, one reading with vacuum to the FPR - and again with the vacuum hose disconnected from the FPR.

 

 

Yeah was my first time running that test, may not have done 100% correctly. Gonna have my dad come over and give me a hand with this but will clean those sensors in the meantime.

[acerocknroll]

Didn't have a lot of free time tonight so decided to replace plugs first on a hunch. Here are three of the old ones:



Pretty gnarly. Super burnt and with oil all up in the threads. Now, I put those old ones in before I fixed my leaky valve cover gasket so hopefully the new ones won't get oil in them too.

Took it for an extended test drive after the plugs and... ran totally fine. No hiccups although it seemed to be running a little hot again at times. I think the plugs were just a symptom and not the underlying issue so I'm still going to clean the throttle body/TPS/IAC, replace the ignition coil with a Mopar one (current is an aftermarket one that gets some really nasty reviews online) and check all my grounds and vaccum lines. Also - my battery terminal ends are a total disaster, is there a recommended replacement?

[gogmorgo]

If you just want the battery cable ends, the best ones at the parts store will be the marine grade. Of course, if the battery terminals are bad, you might want to look into replacing the entire cable.

 

A leaky valve cover gasket won't lead to oil inside the cylinders unless it's leaking onto the intake and getting sucked in somehow through a leak in the gasket or otherwise. A leaky intake would definitely cause running issues and possibly overheating if it's making it run lean, but the oil on the plugs is more likely coming from bad rings or valve stem seals, and if you can't see/smell the burning oil it's probably not bad enough to be causing a running issue. 

[acerocknroll]

Yeah I sprayed some starter fluid along the intake manifold to see if there were any leaks and it didn't have any effect

[Tex06]

Didn't have a lot of free time tonight so decided to replace plugs first on a hunch. Here are three of the old ones:

 

 

Pretty gnarly. Super burnt and with oil all up in the threads. Now, I put those old ones in before I fixed my leaky valve cover gasket so hopefully the new ones won't get oil in them too.

 

Took it for an extended test drive after the plugs and... ran totally fine. No hiccups although it seemed to be running a little hot again at times. I think the plugs were just a symptom and not the underlying issue so I'm still going to clean the throttle body/TPS/IAC, replace the ignition coil with a Mopar one (current is an aftermarket one that gets some really nasty reviews online) and check all my grounds and vaccum lines. Also - my battery terminal ends are a total disaster, is there a recommended replacement?

 

The guy I just posted about, Mean Lemons.  My alternator wiring was pretty bad when I replaced my alt for a 137A unit from a dodge pickup. 

[acerocknroll]

Ok, got the TB off to clean out. Couple questions:

 

First, does this look like a fairly normal amount of buildup?

 

 

The gasket on the TB seems to be fully intact still, and there was no residual gasket material where it bolts onto the manifold. Should I just leave it on and reuse it or scrape off and use the Fel Pro gasket I got?

 

 

Should I clean out the inside of the manifold here? And yes I'm going to vacuum out the little mouse nest there lol

 

[HOrnbrod]

Looks about normal for a TB that's been neglected for years.  :yes:   Don't forget to pull the sensors and clean the passageways. Take care removing them from the TB so you don't tear their tiny gaskets.

[Tex06]

If you're having issues, a cheap gasket is better to replace rather than spend days chasing your tail.

 

Normally I reuse them, but if troubleshooting ask yourself how much time will you spend if they are the problem and you overlooked it somehow this go around?

 

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[acerocknroll]

So to follow up on this, my truck is almost back to normal. I cleaned the TB and replaced the gasket, then reset the ECU by leaving the battery disconnected for a couple hours then doing the whole key to on, lights on, off, etc song and dance.

 

However, on startup the engine surges up to 2000 rpm and holds there. It also blows white smoke from the exhaust. Once I put it in gear and drive just a couple blocks, the idle gradually goes down to a normal level and within a minute or two everything is peachy and it's driving great for the rest of the ride, zero white smoke even if I rev it up. I did a compression test recently with all numbers right at 140.

 

I tried resetting the ECU again by touching the battery cables together for 30 seconds and doing the rest of the procedure again, no change.

 

Is this just a result of the adaptive memory beginning the relearning process? I haven't driven it a ton since resetting the first time, should I just keep driving and see if it gets better? Checked vacuum lines for leaks and they seem to be ok. Maybe TPS is not adjusted correctly? I had a hell of  a time getting it off, maybe I damaged it, except it seems to work fine once I've been driving a bit.

[HOrnbrod]

That's not normal after resetting the ECU - it normally idles lower, almost stalling until the computer learns the new settings. You can drive it a little more, but I don't think it will change. There's really no way to check the HO TPS like you can with the Renix except by replacing it. I think it's time to do it, along with it's gasket and o-ring. 

[Minuit]

Agreed with Don. Mine forgets how to idle for a little while but it never does what you're describing. Have you checked the vacuum line from the throttle body to the MAP sensor? Did you clean the idle air control valve when you took the throttle body off? If not, do those before you load up the parts cannon :D

[acerocknroll]

So just to close this thread, cuz it drives me crazy when I'm reading forums looking to solve a problem and the OP disappears without saying if they ever fixed it:

 

Got a throttle body with TPS and IAC already attached for cheaper than either sensor by itself, slapped it on and it's purring like a kitten. Well, maybe a tappy/slappy kitten but yeah startup idle is perfect, throttle response perfect, everything is copacetic. I'm pretty sure I buggered the TPS when I took it off to clean everything. That thing is a MAJOR PITA to get off! They use like a gallon of threadlocker on the screws, which are of course T20 and strip out completely but not before mangling like 3 bits. I ended up cutting the ends of the screws off and just sliding the TPS off, even with the whole screw exposed it was still really tough to turn it out of the throttle body housing, took a lot of heat to get loosened up. But hey, all's well that ends well.

[Hiroler]

This was a nice thread to stumble upon while diagnosing my same issue.  Anyone in DC/MD want to work on my '88 renix ?? :doh:

 

It ran awesome until it shut off on I-695 & wouldn't restart.  One of the hoses fell off the fuel pump.  Replaced the fuel pump, fuel filter, plugs, and distributor cap.  Its drivable but jerks badly under any low-RPM stress.  I'm running fuel system cleaner through and it's getting better - not bad at all in "Power" transmission mode.  I found a couple vacuum leaks which lowered the RPM but the engine still runs like I've got a heavy cam and shudders a little bit under stress in any gear.  

 

I noticed that if I wiggle the injector #2 green wire, the engine bogged down.  I suppose this could end up being a C101 connector type of electrical issue.  Looking for a fuel pressure gauge now.

OH HAPPY DAY! 

[Roost426]

This was a nice thread to stumble upon while diagnosing my same issue.  Anyone in DC/MD want to work on my '88 renix ?? :doh:

 

It ran awesome until it shut off on I-695 & wouldn't restart.  One of the hoses fell off the fuel pump.  Replaced the fuel pump, fuel filter, plugs, and distributor cap.  Its drivable but jerks badly under any low-RPM stress.  I'm running fuel system cleaner through and it's getting better - not bad at all in "Power" transmission mode.  I found a couple vacuum leaks which lowered the RPM but the engine still runs like I've got a heavy cam and shudders a little bit under stress in any gear.  

 

I noticed that if I wiggle the injector #2 green wire, the engine bogged down.  I suppose this could end up being a C101 connector type of electrical issue.  Looking for a fuel pressure gauge now.

OH HAPPY DAY! 

 

Check on the hose from the MAP Sensor to throttle body, was having a somewhat similar condition and mine was cracked right near the elbow where it pokes into the throttle body.