Poor gas mileage and rough running

My 2000 XJ has been running rough for about a month. I bought it with a bad head, had that replaced along with new spark plugs, etc and it's still running rough. Oil pressure is decent. The RPMs are not jumpy, it just runs rough while sitting at an idle, whether in gear or not. Once I start driving it almost seems to smooth out. It's almost like sitting in a car with a souped up V8, to where it vibrates interior plastic pieces.

I also got about 9 mpg on my last fill up. Granted, this was city driving with one expressway trip, but this just seems deplorable. I do have a 2.5" lift and 31s on the Jeep, but this still seems low.

I have got CEL codes for the throttle position sensor twice, but they went away. Would the TPS contribute to the running rough and poor mileage? Would failing oxygen sensors also exhibit these symptoms?

There is no missing or power loss. I can go check a few things but it is 18 degrees out and I don't want to go crawling around in the snow, and, before I take it to a mechanic I would just like a list of things to check, so they're not throwing parts at it willy-nilly.

Get the engine fully warmed up, after 20 minutes of driving. Throw a tach on the engine--don't try to use the IP tach if so equipped--then pull the injector connectors off one at a time and watch the drop in RPM when each cylinder is done--the cylinder with the smallest or zero RPM drop is the issue.

thanks. although I am not a mechanic I am mechanically inclined , but I have never installed a tach on an engine before. how is this done?

I suggest simple routine maintenance and a new TPS from Chrysler/JEEP. Fresh Champion or NGK spark plugs, some fuel injector cleaner in the gas tank, clean the throttle body and Idle Air Controller. If the engine is staying in warm up mode excessively you will have poor MPG's. You should get about 11-13 city, 13-15+ city/hwy with cold weather and winter blend gas.

shaman1204 said:

thanks. although I am not a mechanic I am mechanically inclined , but I have never installed a tach on an engine before. how is this done?

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On your distributorless coil-rail system using a scanner to read off the OBDII port is the best way.

Tim_MN said:

I suggest simple routine maintenance and a new TPS from Chrysler/JEEP. Fresh Champion or NGK spark plugs, some fuel injector cleaner in the gas tank, clean the throttle body and Idle Air Controller. If the engine is staying in warm up mode excessively you will have poor MPG's. You should get about 11-13 city, 13-15+ city/hwy with cold weather and winter blend gas.

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I had new Champion plugs with the head swap, ran a bottle of seafoam through a few tanks of gas ago (did not pour into the throttle body), new air filter, different brands of gas. I removed the throttle body and all the sensors a few months ago and gave it a thorough cleaning. Did not clean the IAC though, not real sure on how to do that and didn't want to mess it up. So for the most part tune up hardware is fresh. I may try pulling the plugs and to check for fouling and make sure they are gapped.

To Joe_Peters: I have a scan tool, just a basic one. Do I need a more elaborate scanner to read the RPMs going through the OBDII port?

What was the bad head replaced with? The original 0331 head is a dog, with the casting defect between cylinders 3-4.

Keep an eye on your coolant reservoir for any coolant loss if you don't have a beefed up head when that work was done.

The throttle position sensor can be tested with a manual and an analog meter.

Absolutely check plugs; they can tell a lot about how an engine is running. A compression test is a good "snapshot in time" of the internal condition of the engine. For the 4.0, the spec is 120-150 psi, with no more than a 30 psi variation between cylinders.

Purchase sensor safe Throttle Body cleaner spray.

1. Remove the Throttle body from the intake manifold.
2. Remove the IAC with a TORX 15 driver (2 bolts)
3. Gently wiggle out the IAC from the throttle body
4. Clean the IAC with Throttle Body Cleaner (not carburetor cleaner). Use cleaner, a rag, and a toothbrush. Be gentle; don’t twist or pull on the pintle as it is fragile and can be damaged easily
5. Also clean where the IAC seats in the throttle body with the same throttle body cleaner
6. While you have the Throttle Body off, give it a good cleaning also.
7. Reinstall IAC, the Throttle body, and check idle quality.

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Does kinda sound like a vacuum leak.
Also have you tried changing the fuel filter?

You might also try.
Chevron Techron Concentrate Fuel System Cleaner

birchlakeXJ said:

What was the bad head replaced with? The original 0331 head is a dog, with the casting defect between cylinders 3-4.

Keep an eye on your coolant reservoir for any coolant loss if you don't have a beefed up head when that work was done.

The throttle position sensor can be tested with a manual and an analog meter.

Absolutely check plugs; they can tell a lot about how an engine is running. A compression test is a good "snapshot in time" of the internal condition of the engine. For the 4.0, the spec is 120-150 psi, with no more than a 30 psi variation between cylinders.

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head was replaced with a 7120 HO head. new radiator, water pump and termo at the same time. coolant level is good.

xsubarudude said:

Does kinda sound like a vacuum leak.
Also have you tried changing the fuel filter?

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to check for vaccuum leaks, do i just check all the vaccuum hoses on top of the engine, or are there more?

fuel filter will be last resort, i would have to drop the whole tank for that.