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  #1  
Old August 7th, 2007, 14:26
Drewlee77 Drewlee77 is offline
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Intermittant Poor Running + Poor MPG

I've been gone a long time... I just haven't been able to afford the gas or risk the broken parts to go off-roading in a while, so I've avoided sites like this that make me want to go more than I already do.

That said, I'm stumped. My Jeep is giving me trouble and I can't figure out what the problem is, nor can I afford to just have a mechanic mess around until he finds the problem! Here's what has happened... The check engine came on, so I took it to autozone and had them read the code: O2 sensor is bad it says - which wasn't surprising since I had replaced the other one a year ago so I have been expecting to need to replace the other one. So I replaced it and the light didn't go off, so after a while I pulled the battery cable and reset the computer. 30 miles later, the light came back on so I had autozone read the code again - o2 sensor again, so the autozone guy used his code reader to reset the code. About 100 miles later, the light came on again - but after another 30 or 40 miles, the light went off on its own. It hasn't come back on in a few days. During this time, I have been getting around 10-12 mpg (city and highway mix, but always short trips, AC on nearly all the time - historically I would expect to be getting ~15-16mpg) and it sometimes bogs down as though I were suddenly towing a lot of weight. This is almost always at half-throttle or less and opens up if I hit the gas, but occasionally even full-throttle feels awfully sluggish. This isn't all the time, and I haven't discovered a pattern to it, so I can't reproduce it at will. It doesn't just show up under throttle though - sometimes while idling it will get very rough and start idling very slowly (less than 500 rpm - it dropped to around 250-300 a few times when the AC compressor kicked in.) I don't want to take it to my mechanic until I can reproduce it so he can see for himself what's going on - but right now the only consistent symptom is bad gas mileage.

I can't afford to keep driving with mileage this bad, but I can't afford to fix things that aren't broken. I'm not a very good mechanic, but I can do most basic maintenance. I would really appreciate any help you might be able to give.

My Jeep has nearly 88k miles, with about 30k on the spark plugs. I think all other relevant info can still be found in my signature, though I'm sure I have failed to mention something important. I'm really stumped - I have done a quick search but I haven't found anything similar. I do tend to miss the helpful threads though, so forgive me if this is common!

TIA
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  #2  
Old August 7th, 2007, 14:30
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clean96XJ clean96XJ is offline
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Re: Intermittant Poor Running + Poor MPG

Just a guess but, check the cat.
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Old August 7th, 2007, 14:32
Drewlee77 Drewlee77 is offline
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Re: Intermittant Poor Running + Poor MPG

Thanks for the fast reply!

If the cat were clogged up, wouldn't the problem be consistent rather than intermittent?
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Old August 7th, 2007, 14:41
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Re: Intermittant Poor Running + Poor MPG

I don't think it would cause a check engine light but how about the temp sensor? If it thinks you are not warmed up it will run "closed loop" and give you crappy mileage. Just a thought.
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Old August 7th, 2007, 14:45
Drewlee77 Drewlee77 is offline
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Re: Intermittant Poor Running + Poor MPG

The temp. gauge has never read high - I presume the gauge would read incorrectly if the sensor were toast?
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Old August 7th, 2007, 15:17
Jeremy_Cherokee Jeremy_Cherokee is offline
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Re: Intermittant Poor Running + Poor MPG

Kind of sounds like a fuel delivery issue. Have you changed the fuel filter, air filter, and keep the oil changed every 3k? That could make a big difference. Just switching the air/fuel filter on my fiance's Monte Carlo brought the fuel mileage back up to what it was brand new with 60k on the car.
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Old August 7th, 2007, 20:41
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Re: Intermittant Poor Running + Poor MPG

If the matrix in the cat has broken up, it packs differently after every start and bump. They can be quite intermittent.
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Old August 8th, 2007, 00:09
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Re: Intermittant Poor Running + Poor MPG

What about a vacum leak guys?
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Old August 8th, 2007, 00:15
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Re: Intermittant Poor Running + Poor MPG

Would be giving him some rough idle, but not sure it would affect his MPG....my guess would be bad cat as well...

doesnt hurt to pull a plug and see if it is fouled.....
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Old August 8th, 2007, 00:16
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Re: Intermittant Poor Running + Poor MPG

Forgot to add....get under your jeep and bang your fist (or lightly with a rubber mallet) on the cat....if it rattles, its bad....the ceramic on the inside can break up like Old Man said...

just dont smack it too hard
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Old August 8th, 2007, 09:46
Drewlee77 Drewlee77 is offline
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Re: Intermittant Poor Running + Poor MPG

Thanks for the responses!

When I get home from work I'll try banging on the cat and see if it rattles.

I have changed the air filter and oil regularly, but my understanding was that the fuel filter is inside the gas tank and didn't need to be changed. This has always made me a little nervous - how could it *never* need to changed?

If I had a vacuum leak, I should be able to hear it right? I'm not sure I've listened for one... a vacuum leak would be due to a hole in a hose or a loose hose, right? So wouldn't it be fairly consistent?

(don't take it the wrong way if I seem to shoot down your idea - I'm not, I just question everything - it's the best way to learn. I've not counted anything out yet!) Thanks again!
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Old August 8th, 2007, 12:19
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Re: Intermittant Poor Running + Poor MPG

Cap and rotor age? Spark plug wires?

TPS can cause your intermitent power problem as they wear.....to the limits.

Try running some O2 safe injector cleaner, like sea foam through the gas tank to clean out any trash in the injectors that might be making an injector leak (affecting mileage and O2 sensor and Cat....)

Get an analog volt / ohm meter (multi meter), they only cost $10 to $20 at Wallyworld, Radio shack, auto parts stores, then test all the sensors using the FSM specs that listed in various threads here. All the sensors can be tested and verified.

Bad O2 sensors, and or damaged O2 sensor wiring (from contact with exhaust manifolds) does reduce gas mileage!
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Old August 8th, 2007, 12:31
4xBob 4xBob is offline
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Re: Intermittant Poor Running + Poor MPG

I would check the catylytic converter and the wiring to the O2 sensor, then suspect the TPS. My experience with failing TPS is that drivability suffers at part throttle, with low idle, erratic sluggishness, and tendency to stall- but goes fine at heavy throttle. It all depends on what part of the range fails in the TPS. Easy to change if you have a 1991 and later- 1990 and earlier need a calibration setting (for 4.0 engine).
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Old August 8th, 2007, 12:51
Drewlee77 Drewlee77 is offline
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Re: Intermittant Poor Running + Poor MPG

This is a 2000 model - so no cap, rotor or spark plug wires.

Both O2 sensors are new, but I haven't checked the wiring all the way up, only around the sensors themselves - I'll add that to the list.

I have run a bottle of Techron through the engine, but that didn't seem to help much. I use Chevron gas about 75% of the time anyway.

I will do some looking into the TPS, that sounds like a good possibility.

Thanks!
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Old August 10th, 2007, 09:41
Drewlee77 Drewlee77 is offline
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Re: Intermittant Poor Running + Poor MPG

I've checked the o2 sensor wiring - as far as I can tell it's fine.

The catalytic converter gives only a dull "thud" when I bang on it - no rattling.

I haven't checked the TPS because I haven't found directions anywhere - only advice on RENIX systems. Nor have I found my FSM yet... How expensive is a new TPS?
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