98 Cherokee 4.0 Lacks Power

[Fleetmon]

Replaced spark plugs and wires the other day....not because of a problem but simply because it has been approx 8 years, 50k miles. They didn't have Champion plugs so went with Autolite plugs and 7mm wires (the old wires were 7mm also).

The old Champions were definitely worn and the gap was around .070"-.080".....sticker under hood states .035". Old wires were pretty stiff and acted fragile. Since we replacing the plugs/wires, the Jeep is running very smoothly but seems to lack the get-up-and-go it had.

I rechecked the plug gap and they're set at .035" and I didn't drop any of them nor did I bend any of the wires when installing them. I used the old cap and rotor because as said, there were no performance issues. Any ideas?

 

[Zaedock]

It's had twice the spec'd gap for a while. Drive it a bit and the ECM should adjust the timing.

 

[Bottom_Feeder]

Did you use just plain ol' copper (nickel) plugs?

 

[Doog]

I would clean the throttle body, MAF sensor and install a new air filter as well.

 

[Donald]

Could it be subjective? The 4.0 engine in a Jeep GC is not any powerhouse in my mind.

My 3/4 ton pickup with the Cummins diesel has much more get up and go.

 

[05Blazer]

After many years in the parts business, it sounds silly, but I would go get new Champion plugs and put them in. Chrysler and Jeep products just dont run well on anything but Champion and the occasional NGK.

 

[Zaedock]

Originally Posted By: Doog
I would clean the throttle body, MAF sensor and install a new air filter as well.


There is no MAF sensor on his 4.0L.

Throttle bodies typically stay pretty clean on 2.5L/4.0L engines, but it couldn't hurt to clean.

 

[Zaedock]

Originally Posted By: 05Blazer
After many years in the parts business, it sounds silly, but I would go get new Champion plugs and put them in. Chrysler and Jeep products just dont run well on anything but Champion and the occasional NGK.


I have many Jeeps in my stable and they all run well with Autolites.

 

[1345]

This very simple procedure will erase the 1996-2001 “Adaptive Memory” stored inside the PCM and allow a new “Adaptive Memory” to be developed.

After performing this procedure the 1996-2001 PCM will re-learn and store into Adaptive Memory your engines performance characteristics.

Please perform these steps exactly as they are written, in the order they are written. This will cause the adaptive memory in the PCM to be erased and cause the PCM to go into Fast Learn Mode adaptive mode.

Disconnect the POSITIVE battery Terminal and touch it to ground ( not the battery negative terminal ) for 30 seconds. The engine block or the A/C compressor pump will work as a ground. (This is to discharge the PCM capacitors, which maintain the Adaptive Memory).

Reconnect the Battery Cable
Turn Ignition Switch to the “On” position but DO NOT start the engine
Turn Headlight “On”
Turn Headlights “Off”
Turn Ignition Key “Off”


The PCM Adaptive memory has now been flashed, or erased from the PCM.

When you start the engine it will be running off a set of pre-programmed tables that come with the PCM from the factory.

When you get the engine up to operating temperature the PCM will start to collect data for the “Adaptive Memory”.

The PCM will collect data for Adaptive Memory for the first 50 Warm-up Cycles.

 

[SwampSurvivor]

Start replacing the sensors!

The TPS are known to go bad and take away some power. The timing chains stretch after 300k or so miles ... that causes a bit of a loss of power. What about the injectors?

 

[Thax]

Originally Posted By: 05Blazer
After many years in the parts business, it sounds silly, but I would go get new Champion plugs and put them in. Chrysler and Jeep products just dont run well on anything but Champion and the occasional NGK.


OP , Blazer is totally correct here. I would ditch the Autolites , replace with Champion OEM or NGK V-power. I would also replace the Cap and Rotor. Look at the Ignition coil for possible cracking and carbon tracking in the insulation. Clean all sensors , and throttle body. My MAP sensor on my 4.0L was causing hesitation and loss of power, cleaned it and she was all good. But I would surely change plugs to OEM Champion.

 

[Bottom_Feeder]

Originally Posted By: Miller88
Start replacing the sensors!

The TPS are known to go bad and take away some power. The timing chains stretch after 300k or so miles ... that causes a bit of a loss of power. What about the injectors?

So you're saying just to throw money at it until it runs like it did before a simple plug and wire swap?

 

[Nick R]

Originally Posted By: Zaedock
It's had twice the spec'd gap for a while. Drive it a bit and the ECM should adjust the timing.


iirc, from the Grand Cherokee, there is no timing to adjust. It's cap and rotor distributor ignition.

 

[Doog]

Originally Posted By: 1345
This very simple procedure will erase the 1996-2001 “Adaptive Memory” stored inside the PCM and allow a new “Adaptive Memory” to be developed.

After performing this procedure the 1996-2001 PCM will re-learn and store into Adaptive Memory your engines performance characteristics.

Please perform these steps exactly as they are written, in the order they are written. This will cause the adaptive memory in the PCM to be erased and cause the PCM to go into Fast Learn Mode adaptive mode.

Disconnect the POSITIVE battery Terminal and touch it to ground ( not the battery negative terminal ) for 30 seconds. The engine block or the A/C compressor pump will work as a ground. (This is to discharge the PCM capacitors, which maintain the Adaptive Memory).

Reconnect the Battery Cable
Turn Ignition Switch to the “On” position but DO NOT start the engine
Turn Headlight “On”
Turn Headlights “Off”
Turn Ignition Key “Off”


The PCM Adaptive memory has now been flashed, or erased from the PCM.

When you start the engine it will be running off a set of pre-programmed tables that come with the PCM from the factory.

When you get the engine up to operating temperature the PCM will start to collect data for the “Adaptive Memory”.

The PCM will collect data for Adaptive Memory for the first 50 Warm-up Cycles.


+1 THIS ^^^^^^^^^^^

 

[Fleetmon]

C'mon guys...I wrote there were no issues with the Jeep power-wise or smooth running....still running smooth just doesn't feel it has the zip it did before. I've only put about 100 miles on it since the plug swap.

I think I'll switch back to Champions before I do anything else.....heck, I could re-install the old ones for that matter but the gap seems way excessive....actually measured .080"+ (.045" and .035" together and still sloppy) but it was running great!

I only changed them 1) because they were there, 2) it seemed like a long time on the wires and I could see arcing with a spray bottle, and 3) gas mileage could (maybe?) be better...averaging 17.5 mpg on highway.

Thanks for the suggestions though.

 

[Zaedock]

Originally Posted By: Nick R
Originally Posted By: Zaedock
It's had twice the spec'd gap for a while. Drive it a bit and the ECM should adjust the timing.


iirc, from the Grand Cherokee, there is no timing to adjust. It's cap and rotor distributor ignition.


Yes, I know. The ECM adjusts timing.



Originally Posted By: Thax
My MAP sensor on my 4.0L was causing hesitation and loss of power, cleaned it and she was all good. But I would surely change plugs to OEM Champion.


I'd like to know how you cleaned the MAP sensor. It's a sealed sensor that bolts to side of the TB (previous models bolted to the firewall) and has 1 vacuum port reference. In other words, it's not cleanable.



To the OP - I have been messing with EFI Jeep engines for a long time. I have never had any issues with any plug brand, including Champions and Autolites. Every one of my rigs currently has copper Autolites in them and they run fine, even run updside down when I roll over. This design is not picky, but if it makes you feel better, regap the old plugs and reinstall.

 

[jeepman3071]

Originally Posted By: Donald
Could it be subjective? The 4.0 engine in a Jeep GC is not any powerhouse in my mind.

My 3/4 ton pickup with the Cummins diesel has much more get up and go.


The Grand Cherokee weighs about 500lbs more than a regular Cherokee. I notice a difference even when driving a '93-'98 body style Grand compared to the '99-'04 style with the same engine. Get a 62mm bored throttle body, and a better flowing intake and exhaust. That will wake it up a bit.

 

[Chris142]

My 02 runs great on autolite plugs. Id consider a lazy o2 sensor or plugged cat.

 

[jeepman3071]

The '98 with the distributor ignition runs best on Champion coppers. The coil pack 4.0s (late '99 until 2006) run best on NGK coppers. With that said, I would try Champion copper plugs and replace your distributor cap and rotor with new ones that have brass contacts. The brass contacts help prevent corrosion.

I don't think he has a lazy O2 or a plugged cat. He wouldn't have suddenly noticed a problem like that. It would be something that gets progressively worse.

 

[default]

Paranoia and perception? The 4.0 was never peppy, especially in the heavier SUVs but has the torque to climb a wall. A bit different than driving a Honda.