reasons why engine is running rich

Hi all,

My emissions testing is just going down the sink. First it was the EGR (which is now fine), now the test indicates the engine is running rich--CO is 1.61ppm at 2500rpm and .99ppm at idle. I searched previous posts regarding the cause of this problem and the following culprits have been suggested:

bad oxygen sensor
bad fuel pressure regulator
bad MAP sensor
assorted vacuum leaks
bad charcoal cannister

I'm not sure where I should begin to diagnose the exact cause of my problem, but I was wondering if one of the aforementioned possibilities is more likely than the others.

I can say that the oxy sensor in my truck hasn't been changed in nearly 140,000 miles (me bad). Also, and this concerns me, my exhaust manifold is leaking--probably a crack. Would this cause a rich fuel situation?

I don't have the money to replace everything willy nilly, so any suggestions on where I should start would be appreciated. Thanks,

Erik
88 Comanche 4.0L

bad oxygen sensor gets my vote. Primary thing responsible for the mixture. BUT of course it may be something else or a combination of things. '88 was 15 years ago. Pretty much all the sensors on my '88 have needed to be replaced. Just the MAT is original and I'm sure it's living on borrowed time already.

sorry I can't be more helpfull.

Thanks, I'll check the oxygen sensor this weekend as per the Chilton method. I suspect I should replace this anyway but since I only get one free smog retest I'd hate to miss any other component that might be contributing to the rich fuel mixture.

Erik

Where in NorCal are you - anywhere near San Jose? I can dig up my FSM set and we can have a confer, see what we can see...

5-90

Hi 5-90, I'm up here in Sonoma. Ahhhh, a FSM--I should have bought one years ago! I think what I'll do first is replace or at least test the oxygen sensor as per Hawaiian's suggestion and see if that works. I know the plastic vacuum bottle near the front bumper must be leaking because of my screwy defroster behavior, so I thought I would check the vacuum system as well. Problem with all this is that I have little time and my truck has to go back for retesting soon. Hopefully, I can figure out what's causing the rich mixture without having to fiddle or replace a bunch of components.

Erik
88 Comanche 4.0L

Also check your coolant temp sensor (CTS). Mine was open, telling the computer that the engine was always cold, causing it to run rich. You can easily test it with an ohmmeter. For a TBI, 2.5L, it should be 3400 ohms at 70 degrees. It should drop to < 1k ohm at operating temperature. If it reads open, it needs replacing.

Thanks Sternbal, I'll check it out.

Erik
88 Comanche 4.0L

sternbal said:

Also check your coolant temp sensor (CTS). Mine was open, telling the computer that the engine was always cold, causing it to run rich.

Click to expand...

Remember that the '88 has two coolant temp sensors. The one for the gauge in the instrument cluster is on the back corner of the head, driver's side. The one Sternbal is referring to is on the driver's side of the block, under the manifolds. The knock sensor is down there, too, so don't get them confused.

I bought a FSM CDROM on ebay for cheap. I think some guy has burned a million of them for the '88. I prefer to have things on paper but for the money it was worth it. I just checked ebay and can't find a listing but check there over the next couple of months and you may get a good deal on one. I could burn you a copy of mine too if you like and if I can find the damn thing

Hawaiian Style said:

I bought a FSM CDROM on ebay for cheap.

Click to expand...

Can someone enlighten me on what a FSM is and how I can potentially obtain one for an 89 Cherokee? I ran into the same issue with my jeep not passing inspection due to hi CO content and appears to also be running rich.

Thanks, Mike

FSM = Factory Service Manual. Options? I know Rosser Jeep (www.thinkjeep.com) has them, they turn up on ebay, and you can see if anyone local to you has a set (I'm in San Jose...)

You can also try www.cataclysm.net/xj/, I think they have the 88 posted there in chunks and that will get you what you are looking for...

5-90