Can a dirty fuel filter cause the engine to run too rich?

Lately I have been trying to figure out this running too rich problem and failed, so I made an appointment to take it into a shop. Last week I ran a can of BG44K through it and could tell no difference. Exhaust still smells nasty like unburnt fuel. Well tonight I had my son change the fuel filter, which has never been replaced since new. (now has 226,000 miles on it) So I notice that when the old fuel filter is turned up to pour gas out the exit hole, the gas looks clean. When I turned it the other direction the gas coming out of the intake hole is black-real black. So I took it for a 1 mile test drive and pulled it into the garage and got out while it was still running and smelled the exhaust. For the first time in several weeks it smells normal ! Could a dirty fuel filter have caused it to run too rich? I read several old threads on dirty fuel filters and did not see any mention of it.

The answer is no, a plugged filter will not cause it to run too rich.

Stupid question...how is the o2 sensor?

jeepdeepfreak said:

Stupid question...how is the o2 sensor?

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The upstream O2 sensor was replaced several weeks ago by the Jeep dealership. I recently replaced the cat and wondered if the O2 sensor that goes into the cat needed replaced, but the dealer tech said no, it's fine.

old_man said:

The answer is no, a plugged filter will not cause it to run too rich.

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Thanks. It did not make any sense to me, but stranger things have happened, so I had to ask.

Check your plug wires. Is it running a little rough or stumbling at all? Mine was also recently running rough and I could smell unburnt fuel ... when I opened the hood I could see arching of the plug boot to the block. Changed the plug wires, plugs, distributor cap and rotor and the problem was solved.

I never have smelled un-burnt fuel smell on a engine with a working CAT.
I'm guessing that for un-burnt gas to get by the CAT your mileage would be very very poor.

Some things that have been replaced recently by myself or the Jeep dealer:

New cat from Summit Racing
New upstream O2 Sensor (OEM)
Plugs -Champion, same as OEM
Plug wires - NAPA
Cap and rotor - NAPA
Fuel Filter - Autozone (just last night)
Air Filter - NAPA Gold
Drilled several 3/4" holes in the front of the airbox
BG44K fuel and injector cleaner


There may be more but that's all I can think of right now that is related to the engine performance. Oh, yea, the dealer tech put a fuel pressure guage on the fuel rail and it was 38 lbs, and dropped to 30 for an instant. After shutting off the engine, the pressure held fine for a long time, hence, no leaky injector. FSM says the pressure should be 49, plus or minus 2, IIRC. New fuel pump 2 years ago. Dealer Tech says low fuel pressure could cause the rich condition, although I don't recall the details of why.

Yea, I expected the gas mileage to be bad too, but if it is down from normal, it's just a small amount. It runs great, except when idling for more than a few seconds when it is warm out. Then it starts missing and acting like it has too much fuel, as in too rich. Just normal driving clears it out in 1/4 mile to a mile. But even after it's cleared out and running normally again, I can still smell that too rich odor in the exhaust. It was real cold here yesterday, 10 degrees F or less yesterday morning. It's supposed to be warmer this weekend. I'll drive it some and see how it does in warmer air. After reading a lot of posts in this forum, I'm wondering if the MAP sensor is bad, or there is a vacum leak to it. Where will I find the MAP? I don't know what it looks like.

Problem Solved !

It was a bad fuel pump. Cost $502.47 installed.

Thanks for the replies.

Interesting - what was the problem with the oil pump? Low pressure?

grahamtwatson said:

Interesting - what was the problem with the oil pump? Low pressure?

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It was the fuel pump, and it did not have full pressure and it was pumping air bubbles in the gas.

Re: Problem Solved !

96 Cherokee said:

It was a bad fuel pump. Cost $502.47 installed.

Thanks for the replies.

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ouch!!!