Exhaust Manifold Leak?

Searched and found discussions on cracked exhaust manifolds. 2001 Cherokee Sport, 4.0 with auto trans and 150,000 miles. Fuel mileage sucks (around 12mpg combined). Runs great, no codes, operating coolant temp fine. Poured Seafoam in TB to generate smoke to detect manifold leak. Didn't see any smoke in the engine compartment. Changed pre-cat O2 sensors and that didn't seem to change anything. At a loss for why fuel mileage down (plugs changed at 100k miles, idles fine). If air is sucked into the exhaust manifold to alter O2 readings would that mean I wouldn't see the smoke from the Seafoam?

Sorry for the long post but this is really bugging me. Been going on for several months.

2000-01 have cast iron exhaust manifolds that seldom if ever crack. Checked the down pipe at the exhaust donut ? Did you install NTK O2 sensors ? Reset the ECM ?

I suggest testing the Coolant Temperature Sensor. It runs right at 210*, correct ?

12 mpg combined is at the low end of normal, and may caused by your driving style and/or winter blend gasoline.

Tim_MN said:

2000-01 have cast iron exhaust manifolds that seldom if ever crack. Checked the down pipe at the exhaust donut ? Did you install NTK O2 sensors ? Reset the ECM ?



I suggest testing the Coolant Temperature Sensor. It runs right at 210*, correct ?



12 mpg combined is at the low end of normal, and may caused by your driving style and/or winter blend gasoline.

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Yes, NTK sensors. Not certain what you mean by resetting the ECM. I have an OBDII monitor that also resets codes (although no codes ever showed up for this). Could I reset it by disconnecting the battery?

Coolant is usually around 195-200*. I redid the cooling system several years ago and 195* was the OEM thermostat. Even then the mileage was much better than what it is now. Driving style is very conservative.

Did not check the downpipe but I never saw any smoke (from the Seafoam) anywhere except the tailpipe.

I still suggest testing or replacing the CTS.

This very simple procedure will erase the 1996-2001 “Adaptive Memory” stored inside the ECM/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. 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 adaptive mode.

Disconnect the POSITIVE battery Terminal clamp 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.

Bought a CTS and I'll install it tomorrow and reset the the PCM. Thanks for the info. I'll repost when I have some credible results.

Coolant temp runs hotter with the new sensor. Ambient temps around here went from 52 F to 5 F in a few days. Just filled up and got about 14mpg which is better but still not that great.

The dual cat manifold bolts can loosen where they attach to the cat but I have never seen either manifold crack.
Even if there is an unlikely unseen and unheard exhaust leak, it shouldn't effect mileage much, if at all.

A blockage in the cat(s) will cause problems and can be tested for using a vacuum gauge.

I assume the air cleaner is good and the distributor sensor is adjusted correctly.
The 2001 FSM calls for spark plug replacement every 30,000 miles, your plugs have 50,000 miles on them.
With 150K, the compression could be down, consider a leak-down test or compression test.
Winter gas can cause the loss of a couple miles per gallon.

lazyxj said:

The dual cat manifold bolts can loosen where they attach to the cat but I have never seen either manifold crack.
Even if there is an unlikely unseen and unheard exhaust leak, it shouldn't effect mileage much, if at all.

A blockage in the cat(s) will cause problems and can be tested for using a vacuum gauge.

I assume the air cleaner is good and the distributor sensor is adjusted correctly.
The 2001 FSM calls for spark plug replacement every 30,000 miles, your plugs have 50,000 miles on them.
With 150K, the compression could be down, consider a leak-down test or compression test.
Winter gas can cause the loss of a couple miles per gallon.

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Ah, didn't realize the plug change interval. Seems kind of low. I would expect at least 50k. My BMW has 100k intervals (I used NGK Iridiums so they should last). Not gonna put Iridiums in the Jeep though. New air filter. Thanks.

An exhaust manifold leak can allow fresh air into the exhaust system, tricking the ECM to think the mixture is lean. If you did a SeaFoam and saw no smoke prior to the rear bumper, there should be no leaks.

About 30,000 miles is when I change the plugs in my 2000.

BMWGuy said:

Ah, didn't realize the plug change interval. Seems kind of low. I would expect at least 50k. My BMW has 100k intervals (I used NGK Iridiums so they should last). Not gonna put Iridiums in the Jeep though. New air filter. Thanks.

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I usually change plugs every 2 years ~20K in the Jeeps at smog time. Mostly because I'm more attentive to the Jeep and it makes a noticeable difference. I've never changed the NGK's in my Nissan after 100K (I might be due) and only twice in the wife's Benz (Bosch) at roughly 75K intervals.

Changed the plugs. Have seen an improvement in fuel mileage.

Going to change plugs in the BMW also. Apparently Iridiums aren't all they're cracked up to be.