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90 2.5 running rich

15K views 7 replies 4 participants last post by  InEmberClad 
#1 ·
I've been searching the forums for a couple days trying to decide what to do but I haven't seen any threads with the same symptoms. My 2.5 is running rich (you can smell the exhaust fumes) but it's not running rough. I gave it a full tune-up when I bought it a couple weeks ago and it's running much better but still a little rich.
Most of the threads I found mentioned their engine running rich and really rough but mine is running very smoothly. The only thing I've noticed is every once in a while when it's idling for a little bit the idle will drop down really low.
Any suggestions? Does it sound more like a bad o2 sensor or a bad map sensor? Any other ideas? Thanks

btw, I have no dash at all right now (bought it without a dash) so I can't tell if the check engine light is on.
 
#2 ·
There is no check eng light on the 90 2.5. That came later on in the YJ. The only light is an emissions maintenance light that comes on via a mileage trigger (80k). My jeep is an 89 w/ a 2.5 & i have been going through the same thing. Same corrections so far, new T-stat, new temp coolant sensor, plugs, wires, cleaned the TB. I have a MAT sensor on the way & I will change that & reset the ECU at the same time when that comes in.
For you i would suggest checking all the sensors & making sure that the T-stat is a 195 deg. The engine running rich is usually due to something making the ECU think that it is not warmed up to operating temperature. When it's cold, the fuel-air mixture is much richer. Start with simple things & work towards the more complex & expensive. Hope this helps a little.
 
#3 ·
I thought I read something about it not having a check engine light but I wasn't sure. I don't have the indicator overlay so I have no idea what lights it had.

Can I check to make sure the thermostat is 195* or do I have to just get another one?
I understand why the jeep would be running rich but I'm not sure what I should replace next. I can't make up my mind between the map sensor or the o2 sensor. I doubt either one has ever been changed because the previous owner was horrible with maintenance (at least he changed the oil). Thanks for the help.
 
#4 ·
My 87 with a 2.5 has a check engine light. Doesn't work, but it has a bulb, socket, and wiring. And it's running rich because the IAT sensor died horribly. Still haven't found one, so I have a resistor in there for the time being.
 
#5 ·
I would try testing the O2 sensor with a voltmeter if you have one. There was a thread on here the other day about a jeep that was "burning through O2 sensors." search the forum for that & you'll get the instructions. That might save you $60. A voltmeter at harbor frieght only costs about $8 if you don't have one & you would definitely get your money's worth out of it. As for the IAT (intake air temp) sensor, aka manifold air temp (MAT) sensor, aka air charge sensor, that could be a factor. it's located aft of the throttle body on the intake manifold & has two wires about 6" long that connect to a plug next to the valve cover. Same style plug as the CTS. That sensor also sends inputs to the ECU, just like the O2 sensor. It's cheaper (~$30) but discontinued & extremely hard to find one. When I took mine off to check it, it was fouled so bad that it smelled like the rich exhaust odor. It also had an impenatrable coating of carbon on it.
 
#6 ·
Yes, the IAT is a nightmare to find. I've started looking for a suitable thermistor to use to rebuild the one I have.
 
#7 ·
Thanks guys. I hope I don't have to start searching for an IAT sensor if it's really that hard to find.
I think what I'm going to do first is pull all of the sensors mentioned and clean them up with carb cleaner and hope that makes a difference. If not I'll just start buying new sensors until I find the one that fixes it.

I've read through the thread mentioned above about o2 sensors but I don't know how to use a voltmeter. Last time I tried I fried the thing.
 
#8 ·
I had a very similar sounding problem and it turned out I had a bad CTS and the vac hose to the MAT sensor was leaking.

In terms of the IAT, I found one for a later year that had the wrong kind of terminal but gave the same resistance values, so I just used a dremel and some solder and spliced it directly in to the wiring harness. I think I got it from quadratec or rockauto for 20 bucks or something.
 
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