Jeep will not accelerate past 1500 RPMs

[Bobby Ridgeway]

Good evening guys, I'm a newbie to the jeep TJ world and also to the Forum world. So this is my first ever post but I am begging for help.

I bought a 1998 Sport 4.0L with 5 Speed manual. Test drive went well rode great and run great. I drove it about two hours home. About 30 mins from my house it sputtered during acceleration once and never repeated.
I got home without further issue and decided i would give the ol girl a tune up. I replaced the spark plugs with mew copper Champions (personally checked gap to be 0.35 on all 6) and also new plug wires using a little dielectric grease on contacts. This made the problem worse as the Jeep would routinely sputter during acceleration but it was still driveable.
This led to replacing Upstream Oxygen Sensor with Bosh initially with no change in Jeeps condition(I changed the Bosh out with a NTK after learning Jeeps hate Bosh). I also replaced the MAP, TPS, CPS, and CTS which seemed to make the sputtering worse and turn more into backfire in the manifold upon moderate to heavy acceleration.
I replaced the entire distributor after finding fairly significant corrosion in the cap. The new one went in same position old one came out and I double check position with top dead center #1 with rotor position at #1. I also dropped the gas tank and replaced the entire fuel pump assembly. I found one frayed vaccum hose and replaced it(it probably wasn't losing vaccum but just to be safe). I put 93 Octane in the gas tank mixing it with the standard 89 that was already in it (suggestion from a friend and I am willing to try anything). I caused a vaccum leak and the Jeep isles up appropriately and smoothly then I secured the hose and it idled back down. The exhaust has been redone (prior to me owning it) with a flomaster muffler and not catalytic converter so they tied up the downstream oxygen sensor to the rail of the Jeep out of the way.
I have no check engine light and at no point has it came on. As it sits right now, the Jeep will crank but you have to keep trying on it for a minute or two. When it finally cranks it purrs like a cat. You can accelerate the RPMs to around 1,500 but that's it. It immediately starts popping and bubbling.
I have an ECM on order and if that doesn't fix it, I have no idea what will. Please Help as I'm about $1K into this mess.

 

[Jeepaholic]

Are you sure the spark plug wires are on the correct cylinders? Seems to me the jeep ran pretty ok right up until you changed the plugs. Possibly one or more of the plug wires got messed up during the removal process as well? That's where I would start since that's when the real problems arose.

 

[Mike_H]

X2...go back to the first thing you changed that made it worse. Check you wires. Grab a timing light, and verify the timing.

I'd check compression too, just to be safe. Were all the sensors Mopar? I don't trust the Chinese imports. If you still have them, I'd put the old ones back in.

 

[Bobby Ridgeway]

Are you sure the spark plug wires are on the correct cylinders? Seems to me the jeep ran pretty ok right up until you changed the plugs. Possibly one or more of the plug wires got messed up during the removal process as well? That's where I would start since that's when the real problems arose.

I have verified several times that the plug wires are in proper position and are in good working order.

 

[Bobby Ridgeway]

X2...go back to the first thing you changed that made it worse. Check you wires. Grab a timing light, and verify the timing.

I'd check compression too, just to be safe. Were all the sensors Mopar? I don't trust the Chinese imports. If you still have them, I'd put the old ones back in.

I don't have a timing light and if it's out of time it would be a chain issue and I don't think that I am brave enough for that and I know I can't pull the bottom pulley (Harmonic Balancer?).
The sensors were duralast lifetime warranty parts except the upstream O2 sensor and its NTK.

 

[Bobby Ridgeway]

I got the Jeep out and drove it today hoping to get a check engine light but it did not. The battery was unplugged for about 16 hours with the positive wire ground to discharge any residual electricity .
It crunk easily and idled good but remained a bacfiring mess above 1500 RPMs. I drove it around and it acted the same through 4 gears. Fifth gear seemed to allow it up to 2,000 RPMs smoothly so it tolerated an extra 500 RPMs. Still lost and still waiting on new ECM

 

[Chris]

Oh man, I wish I could be of more help, but given all the variables here (and lack of check engine light) absolutely any suggestion I throw out will be nothing more than a guess.

It sounds like maybe this issue was here all along, but you made it worse by changing some things out? You know what I would do to set off a check engine light? Go drive it hard. Give it tons of throttle, brake hard, etc. That very well may set off the check engine light you need, because I think that's going to at least help to point you in the right direction instead of just throwing money at it blindly.

You could also hook it up to a live diagnostic machine (though you probably don't have one) and see how it's running in real time. That would tell you a lot about what might be going on, without having to rely on a check engine light.

Because honestly, this could be anything from a fuel issue, to a spark issue, to a timing issue, to a bad electronic component.

 

[JP98]

I don't have a timing light and if it's out of time it would be a chain issue and I don't think that I am brave enough for that and I know I can't pull the bottom pulley (Harmonic Balancer?).
The sensors were duralast lifetime warranty parts except the upstream O2 sensor and its NTK.

You can't adjust the timing on it. The ECU controls it even with a distributor. Is it only making 1500 rpms even when out of gear and revving engine?

 

[Bobby Ridgeway]

You can't adjust the timing on it. The ECU controls it even with a distributor. Is it only making 1500 rpms even when out of gear and revving engine?

Yes, only 1,500 before backfiring and such regardless of in neutral or in gear

 

[Chris]

If it won't rev past 1500 rpm at all, that sounds like either an ECM (ECU) issue, or some sort of sensor gone bad (which there's a lot of them).

 

[Bobby Ridgeway]

If it won't rev past 1500 rpm at all, that sounds like either an ECM (ECU) issue, or some sort of sensor gone bad (which there's a lot of them).

I have a new ECM ordered and I think I have replaced every sensor. If new ECM doesn't fix it then it's off to a stealership I guess.

 

[JP98]

Have you cleaned the throttle body and IAC?

 

[Bobby Ridgeway]

Have you cleaned the throttle body and IAC?

I cleaned the throttle body but it was in good shape before hand. I removed the IAC and cleaned it but it was also in good appearance. I can replace it just to be sure I suppose.

 

[JP98]

Cleaning the IAC should be good enough. I had an issue with my '98 kicking and bucking when first starting because of the IAC and it's an odd thing for it to happen but I just wanted to throw that out there.

 

[JP98]

Do you still have your old plugs?

 

[Chris]

I have a new ECM ordered and I think I have replaced every sensor. If new ECM doesn't fix it then it's off to a stealership I guess.

Maybe sure you got an ECM from the exact same year, engine, and transmission type as your TJ, otherwise it's not going to work.

 

[Marc L]

Could be an intake leak? Spray carb cleaner around the gasket while at a high idle and see if motor revs or chocks.

 

[Cisco Kid]

Find some one with a code reader. It can tell you everything you need to know about the problems.

 

[Bobby Ridgeway]

Could be an intake leak? Spray carb cleaner around the gasket while at a high idle and see if motor revs or chocks.

I have done that with no change in idle

 

[JP98]

Not knocking Champion plugs but the TJ can be picky about it's plugs. If you have the old ones try them and see if she gets better. Autolite APP985 is a good choice.