Still running rough with poor MPG

Backstory: bought a 2000 XJ with a cracked head, replaced that with a remaned 7120 head from a 91-95 4.0 HO. The guy that did the swap bored out the 2000 exhaust manifold to match the bigger exhaust ports on the 7120 head. It seemed to run ok before the swap, from what I recall.

The Jeep idles really rough cold or warm, almost like a drag car but it smooths out upon acceleration. I have also been getting 5-8 mpg since last October with a 3" lift and 31" tires, everything else stock. No check engine lights. I do get the heat soak CEL on cylinder three once in a while. Here is what I have done:

Compression test - all cylinders w/in spec
Fuel pressure test - normal, 48-49#s
new coil pack
new Bosch oxygen sensors, all four of them
new NGK spark plugs, gapped to .035 and torqued correctly.
new idle air control valve
new throttle position sensor
ran a can of seafoam and then a bottle of techron through two diff tanks
checked that all injectors were firing by listening with stethoscope

I am at a loss. Should I replace the injectors? Would having the different head cause this if the exhaust ports and the exhaust manifold don't exactly line up? I do get a pending code for random misfire, P0300, and other cylinder misfires once in a while (P0301, P0303, P0305) It starts quickly every time, within a second of turning the key. I'm getting ready to park it on a dock over Lake Michigan and hope that it falls into the water like Tom Cruise's Porsche in Risky Business.

EDIT: I forgot to mention I have not checked the camshaft position sensor sync yet. Although, I had it hooked up to a comprehensive scantool and the guy said it looked ok.

Are you 100% absolutely positive that the camshaft position sensor is aligned properly? Talking about the one that sticks out where the distributor used to be. It only takes a little time and a toothpick to get it right but having it out of synch causes all sort of random issues, especially misfires.

TerraWombat said:

Are you 100% absolutely positive that the camshaft position sensor is aligned properly? Talking about the one that sticks out where the distributor used to be. It only takes a little time and a toothpick to get it right but having it out of synch causes all sort of random issues, especially misfires.

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Never should have been pulled for a head swap, so if this seems to be a new problem i'd look elsewhere.

Get some carb spray and check for vacuum leaks along the manifold. Maybe his port job was overzealous. Or he didn't get a dowel lined up and cracked the manifold?

Oh, might want to run a leakdown and make sure the valves are closing all the way. A head swap is very easy to have valves hanging open a couple thousandths and it will not show on a compression check.

I've had TPS's come brand new out of the box bad. I don't know which years, makes or models will cross-reference with yours, but I've had better luck with factory items from the yard. Dittos on the previous post. Rockers that are snugged down too tight can cause a crappy idle. Also, I didn't see mention of a new Coolant Temp Sensor. The CPS can prevent closed loop operation. Not too spendy, easy to replace.

heyhar said:

I've had TPS's come brand new out of the box bad. I don't know which years, makes or models will cross-reference with yours, but I've had better luck with factory items from the yard. Dittos on the previous post. Rockers that are snugged down too tight can cause a crappy idle. Also, I didn't see mention of a new Coolant Temp Sensor. The CPS can prevent closed loop operation. Not too spendy, easy to replace.

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Maybe when it warms up a little more outside (so much for Spring!) I will pull the valve cover and re-torque the rockers. I have not replaced the coolant temp sensor. I have an obdII tool that I hook up to my laptop and I can see it go into closed loop once it warms up, right around 190-200 or so. Should it close before this?

Also wanted to mention when I pulled the old plugs, which were replaced when the head was replaced, approx 1000 miles ago, they looked completely normal, just a bit of tan around the white ceramic.

I will check the cam position sensor this weekend.

Retorquing the rockers wouldn't be an issue, if they were loose the valves would stay closed longer and you'd have tappet noise...too tight would be the issue. Like from surfacing the head which would make the distance from the lifter to the rocker shorter causing the rockers to stay under pressure even when the cam is on the base circle.

Just to update, I replaced the coolant temp sensor and I don't know if it was that or the end of winter blend gas, but my mileage seems to be up slightly.

I also replaced the stock injectors with flow matched Mustang injectors, so that may have helped with better fuel economy but not the crappy idle.

The rough idle is still there. I am about at my wits end with this thing. I was searching under hood, for the umpteenth time, for a vacuum leak, and decided to unplug the MAP sensor for the hell of it. The engine roughness did not change at all. If I gave it a little gas it would seem like it wanted to stall but never did. I plugged it back in and unplugged it and got the same result. It will idle like crap forever unplugged until I touch the throttle.

Does this sound like a bad MAP? Or would an engine continue to run fine with the MAP unplugged as long as the throttle isn't touched? When reading the posts on how to test stuff with a voltmeter it sounds like Greek to me.

I do get a check engine light with it unplugged, but no lights prior to that.

If you have access to OBD-II data, you should be able to check that all the sensors and other parameters are within specification and functioning correctly. I am not trying to be a horses behind, but you have access to all the live and stored data and all we have for data is what you post.

Low mileage like you have means you are probably not entering closed loop. The PCM is probably running the engine in warm-up mode all the time.

Purchase a Factory Service Manual and follow the trouble shooting charts and test the sensors as described. Test all the sensors, including the O2's.


You should be getting 13-15 city/hwy mpg and +10% better with warmer weather. A stock XJ gets 15-18 city/hwy mpg's.

It could be your ignition/coil rail. My son had somewhat of a similar issue (2000 XJ/2wd/auto) and he noticed a crack in one of the boots on the rail. Replaced it (coil rail) and all was well.

The bad coolant temp sensor would cause it to stay in open loop/warm-up mode--maybe why you saw some mpg increase when you replaced it. Definitely do the toothpick indexing of the CMP, as a misaligned one is a culprit for misfire codes, and it's always good to know firsthand that it's right than to assume it's been right.

Sorry, I have forgot some info here. I should have included in the first post that I replaced the coil pack with a new one on the advice of a mechanic. No change. I have also indexed the camshaft position sensor, with the no change to the idle.