98 Cherokee 4.0 Lacks Power

[wsar10]

He is probably refering to the IAC valve.......

And I gotta say, if anybody has been around a few 4.0's its pretty common knowledge that the 4.0 generaly operates with optimal performance with the cheap champion copper cores


Is the JEEP an XJ or ZJ ??

 

[Zaedock]

Originally Posted By: wsar10
And I gotta say, if anybody has been around a few 4.0's its pretty common knowledge that the 4.0 generaly operates with optimal performance with the cheap champion copper cores


It's pretty common knowledge that it doesn't matter. I've dyno tested intakes (log vs horseshoe), throttle bodies, air filtration, headers vs manifolds, plugs (all kinds including Champs, Autolite, Splitfire...), etc. There was no difference between brands of copper, platinum or iridium.

 

[Fleetmon]

Geez folks....there wasn't a problem before and it's not a big problem now.

I'm going to put the old plugs back in and see what it does and how it feels. I can't see how one brand of plug would have a noticable difference but there is definitely something lacking.

It's not a perception thing.....I know how this Jeep runs.

Oh yeah, before I forget....yes, I know it's not a Honda.....not only do I not own one, I probably never will....except for the engine on my power washer. This Jeep is actually pretty peppy....to say differently tells me you haven't driven them much.

 

[CharBaby]

I too have had this happen! Changed the plugs on a Dodge 2.2L and the new plugs failed to perform. Yeah, the engine started and idled/ran fine but there was no power for passing.

Put the old ones back in and all was OK again! Champion Copper was what I took out and replaced with the exact same#. Funny how the old ones were better.

Finaly I just changed to NGK!

 

[wn1998]

I kinda read through the thread fast but did you replace the cap and rotor? If not, I would do that next. I swap plugs every 30k and everything else every 60k or so (wires, cap, rotor).

I can tell you it's not your TPS. When mine when out, the jeep wouldn't start.

 

[meep]

Originally Posted By: Fleetmon
Geez folks....there wasn't a problem before and it's not a big problem now.

I'm going to put the old plugs back in and see what it does and how it feels. I can't see how one brand of plug would have a noticable difference but there is definitely something lacking.

It's not a perception thing.....I know how this Jeep runs.

Oh yeah, before I forget....yes, I know it's not a Honda.....not only do I not own one, I probably never will....except for the engine on my power washer. This Jeep is actually pretty peppy....to say differently tells me you haven't driven them much.


Thumbs up brotha'! The ZJ 4.0 I had was a hoot! I agree they have some giddyup in them.

I've noticed this same thing too--- smaller gap runs smoother, larger gap feels like it has more oomph. It could be illusion, the larger gap may make things a bit more rowdy under load, like barely perceptable, giving the illusion of more power??? I don't know but I've felt it myself in other vehicles.

A wider gap actually retards the spark by a cpl of microseconds... as it takes just that much longer to build the higher V needed for the spark to jump the gap. this gets worse under high load, low rpm conditions. it also subjects the ignition system to higher voltage, and can eat rotors, caps and wires more rapidly. and coils. BUT, a wider gap also ignites more fuel at once. some think that = more power. IDK. I can't tell. And I don't think there's much to it, because if that was the case, every oem out there would use big gaps.

If it were my, I'd do the ECU reset and just drive it. If it bugged me, I'd go over to champion plugs.I would not put the old ones back in.

M

 

[wsar10]

they did not get the nickname 4.slow for nothing.......

they are very "torquey" but not snappy.

 

[jeepman3071]

Originally Posted By: wsar10
they did not get the nickname 4.slow for nothing.......

they are very "torquey" but not snappy.



I think only people with Grand Cherokees call them the 4.slow.

My 4.0L Cherokee can get my buddy's 5.3 Chevy Silverado off the line and keep with him until 60. Its not a powerhouse or a V8 but it has some torque. I also have a freer flowing exhaust and a bored throttle body for snappier acceleration. I think it is a very enjoyable engine to drive because it doesn't require much pedal to get going. I can easily accelerate with traffic while keeping the shifts under 2000 RPM. I hate engines that have to be wound out until they sound like they will blow apart.

When (if) my 4.0L dies I'm putting in a 4.6L stroker motor.

 

[Zaedock]

I'm currently building a stroker for my kid's XJ.



6 = 8

 

[Fleetmon]

Sounds like a neat project!

I'm satisfied with the power of the 4.0 in THIS Cherokee.....merely noticed a difference once plugs were changed. Like you, I don't see any difference Autolite plugs would make but will probably go back to Champions....just because.

I've done a lot to this thing lately (it's hers and she bought it new BUT allowed her two teenage sons to drive it for awhile)...new shocks, junk yard fuel tank (clean and spotless though), new ac system including evap, new door speakers, new stabilizer bushings,"new" junk yard fog lights, new water pump and fan clutch, new alternator and battery, new tires, new neutral safety switch and tailight assy to pass inspection (no reverse lights), etc, etc.

 

[Doog]

Originally Posted By: Fleetmon

I'm satisfied with the power of the 4.0 in THIS Cherokee.....merely noticed a difference once plugs were changed. Like you, I don't see any difference Autolite plugs would make but will probably go back to Champions....just because.


I had issues with Autolite plugs in my motorcycle. They cause the CEL to come on and throw codes. Put the factory plugs back in (Champions). No CEL. Go figure.

 

[another Todd]

Are you positive the plug wires are exactly where they were, as in all six not rotated one post off on the distributor or two crossed?

 

[Fleetmon]

Yep....I did think of that and rechecked the firing order and wires and all is as it was before. It's running too good to be any thing really off just doesn't seem to have the pep it did before....I don't have the Autolight numbers handy right now but I suppose they could have given me the wrong plugs although they "looked" the same....the only difference I could see was the name.

It runs well enough to drive it as is too so...well, we'll see.

Thanks for all the help and suggestions folks!

 

[440Magnum]

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.


And make SURE that the plugs installed are the "ridiculously long" extended-tip type like the OEM Champion plugs were. Some plug manufacturers cross-reference a much less-extended tip for that plug, but the 4.0 (and the Chrysler 3.5 v6 which uses a similar if not same plug) REALLY like the spark moved way out in the chamber away from the cylinder head.

 

[Fleetmon]

they are some long one's aren't they? Yep, checked them alongside the old ones....everything looked the same.....thanks though!

 

[1345]

Try a premium distributor cap with brass tips. Champion copper plus RCY12LYC spark plugs makes my 97 cherokee xj 4.0 run the smoothest.

 

[wsar10]

It's also worth mentioning that when I brought home my last
XJ it had a hesitation from 2nd to 3rd gear.....it eneded up being an 02 sensor. The hesitation acted like a miss or a fuel delivery issue (makes sense).

 

[CharBaby]

One thing that I've noticed over the years(long ago) is, when I pulled the old plugs and the gap was, Oh let's say, .060...but the new plugs were set to spec, Oh let's say, .035.

Some engines liked and ran better after the gap had spread on the plugs even though we should think the oposite!

Larger spread in gap also allowed for a better fuel burn...even though we should think that the spark had to travel/jump further, causing the engine to be less efficient or having less power. When in fact, it was the oposite!

Lot's of many good pointers from members as-far-as what to do. But, I think that the OP should try a different set of plugs(NGK) maybe!

I have had the most trouble with Champion plugs...even when changing from Champion right back to another set & same number Champion. Autolite are similar IMHO. Sure, I have had decent Champion & Autolite but, have always had good LUCK with NGK or Denso(never an issue). I've always had good success with AC Delco as well even in Mopars

Due to experience, I don't have much faith in the standard(copper) Champion or Autolite as the consistancy from plug-to-plug isn't the best quality but, I do believe each company makes better higher line plugs from Platinum on upward!

 

[CharBaby]

And too, if you could actually measure the resistance of the new plugs, you may(may) notice the the Ohms reading on the new plugs(may be) HIGHER. For what ever reason, IDK!

And do check your wires, cap/rotor as other members has said

I have always liked premium wires. Something along the lines or MSW/Pretronics or better! But, you don't necessarily need racing wires with lower resistance, but you may!

Also, and this could be a PAIN! But, have you put back the old plugs...I've done this and found what I mentioned about the plugs-The old plugs allowed the engine to perform better. Again, for what ever reason, IDK!

I believe it's resistance but, have only had these issues with Champ-O-Lite

 

[84zmyfavorite]

I currently have 7 Cherokees, 5 of which have the 4.0, They All have AutoLite 985 Copper Core plugs. From a 1st year 1987 4.0 to my Beloved 64,820 mile 2001 4.0, they ALL get the same.