Badmunky

Okay so I bought a 1998 Grand Cherokee Laredo about 2 weeks back.
It ran fine on the test drive, but it did seem to be down on power just a little. The Plug wires and cap looked old to me. I thought it just needed a good tune up.

So, that's what I did. Found it had 2 different kinds of plugs in it.

Front of motor---------------------->

And they were gaped wrong. The 4 short-tip plugs where all about .055. The 2 long-tip plugs where gaped about .065.
Cap contacts where burnt, and the rotor was burnt.
Plug wires where dirty, but looked okay to me.

So, I changed every thing. I put in new correctly gaped plugs.
I reset the ECU, and then took the Jeep for a test drive.
I found that it ran better. Had more power, but still seemed to be down a bit from where I think it should be.

I did that all yesterday, along with a brake job.

Today, it's a different story, it's back to no power again. It will not spin the tires on wet pavement. On dirt it will spin both rears for 3 feet or less before it no longer has the power to keep them going.

And the other problem is it has a hesitation / stumble.

After it has been parked for a bit (say 3+ hours), and you start it up, it will start right up, and run fine. But about 2 min or so, after you take off it will hesitate, and stumble, lose all power, and backfire through the intake if you give it more gas. If you let off, it will sort its self out and run great after a few seconds. And will not do this again until it has been shut off and allowed to cool for a bit.

I don't know what is causing this.
And the power just is not there like it should be. I know its a Jeep, not a sports car. And I don't expect it to be able to white smoke the tires, or hit 60 in under 6 sec. But, right now it can not even get wheel spin. And 60 takes about 16-18 seconds and around 1/4 of a mile at FULL throttle.

There has to be something wrong here.

Things I do know about this Jeep:
It has a cracked header.
It has a newer cat, but it looks like its been cut open and then welded shut again.
Its got a new radiator.
Its got a new thermostat and housing.
Its got new hoses.
Its got a newer alternator and shows 14v on the dash.


Things I have done so far.
Seafoam'd the motor through the TB, and put half the can in the gas tank.
Changed the plugs.
Changed the wires.
Changed the cap.
Changed the rotor.
Cleaned the TB.
Cleaned the IAC.
Power washed the motor.


I'm at a loss here.

Thanks guys.

Bustedback

I bet those plugs were gapped right when they were installed. Those are some extremely worn out plugs! Check the fuel pressure, and the cracked header can cause the O2 sensor to read incorrectly and cause a loss of power. You won't see a 0-60 time in the six second range, it's a 4.0 I6 moving a pretty heavy vehicle. It the tires are larger than stock the Jeep will feel like a pig. If you want to pick up a few horsepower and have a smoother running engine, do this. Swap the intake manifold for a 99+ intake, swap in some 703 Dodge Neon injectors, and relocate the IAT sensor to the air cleaner assembly. Those changes and a new un-cracked exhaust manifold will make a huge difference in how the engine performs. But I do think you have a fuel pump problem.

dave1123

Hey Busted. Will relocating the IAT sensor work in cold weather?

Bustedback

It should be ok.

Badmunky

Right now the ZJ is totally stock. Tires, wheels, every thing. For now any way.

My plan at this point is this:
Replace cracked header with new one. (Not sure what to buy yet, little help here?)
Replace injectors with 703 Neon injectors. (Knew about this and was planing it already.)
Replace up stream O2 sensor. (Current one looks to be stock. Jeep has 152k on it)
Replace failing fan clutch and fan with electric fan from 99 GC, and a fan controller.
Replace drive belt.
Replace TPS.
Do a compression test.
Test fuel pressure.
Check for vacuum leaks.
Change oil and filter.
Service transmission.
Change oil in both diffs and replace leaking cover seals.
Change oil in T-case.
Replace leaking rear wheel seals and install new wheel bearings while I'm in there.
Replace leaking real main seal.
Replace leaking valve cover gasket.
Replace leaking oil pan gasket.
Replace leaking oil filter adapter seal.
Attempt to force the ECU to throw a CEL by unhooking engine temp sensor to be sure that the CEL circuit in the ECU is working. (Have been told Jeeps have a problem with this, and not throwing a CEL or code even when there is a problem.)

And I will look at relocating the AIT sensor as well. Why do people do this?
Where is it now? And will this work okay in extreme cold such as ND in January? Extreme hot such as NV desert in August? I ask because I may move at the end of the year, but I don't yet know where I will be going.


I understand that this thing is not going to do 0-60 in under 6 seconds. LOL
But stock it was rated to do 0-60 in just about 11 seconds. I would be happy with that. Right now I'm closer to 17 seconds, give or take a second.
And passing is just out of the question.

I will admit I am used to driving a much faster car then this is. But I have owned slower cars before. And its not a problem. As long as they run right.

My current DD is a highly moded WRX pushing about 460hp at the crank.
I don't expect the Jeep will ever keep up with it. LOL
But I do expect I can at least keep up with my sister driving a fully loaded Dodge Mini van pulling a trailer!

Yeah she smoked me, her and 4 kids, 2 big dogs, and bunch stuff for my brothers baby shower, PULLING A 1,400lb TRAILER!!!!!!!!!!!!!!!!!!!!!!

She drives like that mini van is a sports car. I was trying to follow her and had to call her and tell her to slow down or she was going to lose me. I could not keep with her in the city, or on the high way. (She was doing like 90mph on the high way with the trailer!!!! Yeah shes nuts!


If what I have planed dos not fix my problem where should go from there?

I have herd that there is a TSB for the ECU not throwing codes. Is this true?

And is there any thing I left off my list that I should just do?


Thanks for the help guys!

Badmunky

I'm going to order header for it next Thursday after I get paid.

Right now I have it down to these headers:
OBX. (I have run them in the past with no problems and they have a life time warranty)
APN. (Don't like the way they route the #6 primary as it will hit the intake manifold on the 99+ GC and I am changing to that IM when I do the header. So if I run this one I will need to cut the header up to fix the problem.)

Other. Recommend something.

dave1123

Why would you cut a brand new header to get it to fit? I think you answered your own question.

Badmunky

From what I have found I will have to cut the number 6 primary of every header on the market.

I am currently looking for a flange so I can make my own header.

Bustedback

I've heard the stock tube style exhaust manifolds flow pretty good.

Badmunky

Ok so last night we drove the Jeep 8 miles to the movies.
For no reason (I have not changed any thing yet.) its back to running great! Its got good power, and I was even able to pass a slower car with no problems.

It still tries to stall, stumbles, and backfires a minute or so after a cold start. And the problem seems to be getting worse. It now dose this even if the motor is still worm. And the stumble/ hesitation/ stall/ backfire is lasting longer before it sorts its self out.

Also last night after the movie on the way home I floored it from a stop to see how long it would take to get to 60. It did much better then the 17 or seconds from the last time. Took it about 12-13 seconds. (Still a little slower then it should be.) But the trans did something I was not expecting. At full throttle its shifting out of first gear at 4,000rpm. I thought it should hold to close to 5,000rpm?
Is this normal?
I checked the trans cable. It is adjusted correctly. The trans is due for a service. The fluid is a bit darker then it should be and smells a little burnt. I am going to have the service done on it the next time I get home around the 4th of July. Until then it will not be driven much.

I don't know much about autos. This is really the first one I have owned as an adult. (Had a few beaters when I was a kid that never lasted long.)
I have been driving manual trans cars for the last 15+ years. (Could not find a manual Cherokee that was not rusted out, so we settled for the ZJ.)

Thanks guys for the help!

reikiwarrior

hi mine does the same it runs rough on cold start but some times its ok ive changed most of the electrics in the engine bay , its worth trying to
remove negative cable for 3 seconds on battery and that reboots ecu, but the electrics on these jeeps are terrible, regards RW

bruni86xj

try a tps, mine acts like that seldomly and thats what its throwing a code for, they're only about 40$

Badmunky

Its on my list of things I am going to replace.
Its got 152k on it so I expect some sensors are going to go bad.

CalgaryGuy

I'm interested to see how this ends up...

I have a similar hesitation/backfire issue and I've:It doesn't pull codes, and it starts all year like a champ in Calgary, AB, Canada. It's got 231km on it and doesn't burn oil or leak onto the ground, the gearbox leaked, but it was from the adjustment screw and now it's simply a non-leaking 1" play steering gearbox, like a lot of Jeeps (hehe).

I put "new to me" injectors in from Precision Auto Injectors in today and if that doesn't help I'm then pretty confused. I really don't want to go much farther as my problem isn't that bad for me and putting in a new fuel pump or more sensors and parts seems irrational as I'd rather buy some fun stuff instead.

CalgaryGuy

Took it out tonight... Got 11.17sec from 0-60 stopped at a light on quite flat ground, might be a tad slow as I'm hitting start while flattening gas and watching road... Original new car time posted for my 98 4L was 9.8sec so if I'm getting 11.7 with me testing in a Jeep with original transmission and engine at 231,000Km I'm happy. I also adjusted my EV linkage and that made an improvement on shifting during acceleration - just seemed more accurate related to the throttle position...