Have a '97 GC Laredo 4.0 with ~150k miles

It suddenly started idling rough a few weeks ago: dark / black exhaust, avg mileage down to ~6mpg (ouch), but feels fine running >1000rpm down the road. Had it coded locally and told "PO171 = bad fuel filter / pump". Checked the key codes and received 12, 51, and 55. Figured I'd browse the forums before paying for a new pump.

Most articles suggested O2 sensors or exhaust leak prior to upstream O2. My converter and muffler were loud, so had both checked & replaced. Had manifold checked for cracks / leaks while there - no issues. Mechanic said O2 sensors didn't look terrible - recommended I give it a day or two before replacing. Now sounds much better, but codes returned within an hour, and mileage is still horrible.

With exhaust repaired, believe I now hear a vacuum leak (slight whistle), so took it to another shop. They replaced both O2 sensors (no fix, so removed), checked for vacuum leaks (no external leaks found), checked the exhaust for leaks (none found), etc. The manager told me compression tested great... and then told me I have a "tired engine, and should probably get a used one from a junkyard -- or invest many $$$$ having us tear it apart". They believe I have an "internal vacuum leak".

I'm a little concerned: don't understand how compression can be great yet I have a "tired engine"? If the head' cracked, wouldn't it show on the compression test? I'm not a mechanic, so pardon my ignorance -- just doesn't seem to fit.

Is it possible that this is something as simple as a bad O-Ring on the injector on the 1st cylinder? I've read that the TPS and CPS sensors could be suspect, etc. -- just not sure where to start?

Don't know if it helps, but I understand that the keyed "12" code means that power was disconnected to the PCM during the previous 50 starts. The mechanic reset the codes with the scanner, but unless that process disconnects power to the PCM, the battery hasn't been disconnected for >8 months.

Thanks in advance for any help!

Tired engine my ass.

Take a look at code 51. You've hit all the high points.

http://www.allpar.com/fix/80s-codes.html

Have a '97 GC Laredo 4.0 with ~150k miles

It suddenly started idling rough a few weeks ago: dark / black exhaust, avg mileage down to ~6mpg (ouch), but feels fine running >1000rpm down the road. Had it coded locally and told "PO171 = bad fuel filter / pump". Checked the key codes and received 12, 51, and 55. Figured I'd browse the forums before paying for a new pump.

Most articles suggested O2 sensors or exhaust leak prior to upstream O2. My converter and muffler were loud, so had both checked & replaced. Had manifold checked for cracks / leaks while there - no issues. Mechanic said O2 sensors didn't look terrible - recommended I give it a day or two before replacing. Now sounds much better, but codes returned within an hour, and mileage is still horrible.

With exhaust repaired, believe I now hear a vacuum leak (slight whistle), so took it to another shop. They replaced both O2 sensors (no fix, so removed), checked for vacuum leaks (no external leaks found), checked the exhaust for leaks (none found), etc. The manager told me compression tested great... and then told me I have a "tired engine, and should probably get a used one from a junkyard -- or invest many $$$$ having us tear it apart". They believe I have an "internal vacuum leak".

I'm a little concerned: don't understand how compression can be great yet I have a "tired engine"? If the head' cracked, wouldn't it show on the compression test? I'm not a mechanic, so pardon my ignorance -- just doesn't seem to fit.

Is it possible that this is something as simple as a bad O-Ring on the injector on the 1st cylinder? I've read that the TPS and CPS sensors could be suspect, etc. -- just not sure where to start?

Don't know if it helps, but I understand that the keyed "12" code means that power was disconnected to the PCM during the previous 50 starts. The mechanic reset the codes with the scanner, but unless that process disconnects power to the PCM, the battery hasn't been disconnected for >8 months.

Thanks in advance for any help!


Like bringselpup says.:grinyes: :lol:

You need to forget the key codes and get a real scanner on it with a real mechanic working the scanner.
It could be several things.
The code 51 only points to system lean.
A real engine capable scanner with the 0bd11 codes will help narrow it down.
Check and watch them 02 sensor readings and fuel trims. Check that fuel prerssure and fuel filter. Also any leaks or extra backpressure on exhaust system will cause problems on 02 sensor reading to.
How is the tune up on this thing?
Find a good repair shop with a good engine capable scanner and let them have at it.
If you get any good OBD11 codes post them back.
MT

First thing I did was install plugs / wires / cap / rotor, so tune up is done.

Just called another shop and am taking it in tomorrow morning. They're saying o-ring or head (I'm obviously pessimistic) -- will ask them for the codes you mentioned, and will post the info here before authorizing anything.

A sincere thanks to both of you

First thing I did was install plugs / wires / cap / rotor, so tune up is done.

Just called another shop and am taking it in tomorrow morning. They're saying o-ring or head (I'm obviously pessimistic) -- will ask them for the codes you mentioned, and will post the info here before authorizing anything.

A sincere thanks to both of you

You are welcome
If your compression is good the engine is good.
The O ring thing I can not figure out for sure how they are figuring that.
Let us know how it goes. And post back them OBD11 codes.
MT

I have the same problem with my 95 GCLaredo. I have replaced the 02 sensors, relay switch, battery etc. and ll the same things you did. I read in Consumer Reports that older models were reporting a defective idle air control motor that was causing rough idles. I wonder if this could be the cause. I also have a seal at the top of my gas tank that is leaking. Could this cause enough vacume loss to create a problem? Haven't got the car fixed perm. yet. Seems like everything I do to it is just temp. fix. The problem reoccurs.

Help!

I wasn't able to be as verbose as I might ordinarily have been yesterday.

:grinyes: :lol:


I meant JD listed black exhaust, rough idle, and the o2 sensors weren't found to be bad. That link I listed in the earlier post said code 51 was MAP sensor and talked about all the stuff JD was seeing. Seemed to me to be the next place to go. My Haynes Grand Cherokee book has a pretty concise writeup on how to test and replace.

Just got back from the shop -- still amazed by the way this is going

Told me my "vacuum leak" was an exhaust leak (I'm deaf in one ear - oops on me, I suppose). Took it back to shop that did exhaust, and they confirmed -- did a little welding, and sounds normal.

Coded and confirmed PO171 again, so reset - figured might be ok now that exhaust is fixed.

Drove ~4 miles (ran great, by the way - instant mileage read 14 to 28), then light came on again, could smell gas again, began idling rough at a stop again, and mileage was back to ~6mpg avg.

Had it coded again at local auto store -- again showing PO171 (nothing else).

Took it home, pulled battery cable and cleaned posts, let it sit for ~10 minutes, hooked it up -- again ran great... for ~4 miles. Then? Boom. Light on, rough idle at stop, smell the fuel, etc.

This time, though, noticed the "engine block" (red square on command center) flashed red for a moment? Not there now, though. Took it back out and again codes PO171.

So I assume I now replace the MAP sensor? Hate to sell this stupid thing to pay for all of the time I'm spending at the shop. I'm not a mechanic, but I'm a pretty good PC tech if you can help and want to barter... lol

Thanks again, all

Look at 171 here.

http://www.ptcruizer.com/computer-codes.html

I going to take a flyer here and guess that codes 171 + 172 = Upper O2 and 173 + 174 = Lower O2 so this code is probably for the upstream O2.

Put that with the comment from the other code page, "Webmaster note: MAP sensors seem to die regularly." and another symptom that you had listed in the beginning - poor mileage, yeah I think the first place to look is that map sensor.

It also referenced note #5: "the breather tube leading into the catalytic converter had rusted and become detached. This some how would cause the sensor to read an over rich condition and run crummy"

Check that too. I hope their little bit of welding isn't because the breather tube wasn't in evidence so they just sealed the hole on you.

Just got back from the shop -- still amazed by the way this is going

Told me my "vacuum leak" was an exhaust leak (I'm deaf in one ear - oops on me, I suppose). Took it back to shop that did exhaust, and they confirmed -- did a little welding, and sounds normal.

Coded and confirmed PO171 again, so reset - figured might be ok now that exhaust is fixed.

Drove ~4 miles (ran great, by the way - instant mileage read 14 to 28), then light came on again, could smell gas again, began idling rough at a stop again, and mileage was back to ~6mpg avg.

Had it coded again at local auto store -- again showing PO171 (nothing else).

Took it home, pulled battery cable and cleaned posts, let it sit for ~10 minutes, hooked it up -- again ran great... for ~4 miles. Then? Boom. Light on, rough idle at stop, smell the fuel, etc.

This time, though, noticed the "engine block" (red square on command center) flashed red for a moment? Not there now, though. Took it back out and again codes PO171.

So I assume I now replace the MAP sensor? Hate to sell this stupid thing to pay for all of the time I'm spending at the shop. I'm not a mechanic, but I'm a pretty good PC tech if you can help and want to barter... lol

Thanks again, all

Sorry I missed the code P0171 in the first post.
The code P0171 points to lean fuel/engine running lean.
Several things can cause that.
One of first things to check is fuel system and fuel pressure.
Also that welding up on exhuast does not sound like a proper fix.
As Bringsepup said it need them air pipes back under there.
If they are not right it will throw off the front 02 sensor and make it read lean and give the P0171 lean code.
I think you need to check around for a better or good repair shop or even a good 5 star cry dealer..
A good repair shop can get a scanner and fuel pressure gauge on it and read out all sensors and check fuel system and find out what is throwing the code and no running right.
You said you heard a vacume leak look the engine over good for leaks. Check Pcv valve and all vaume hoses ,intake gaskets and stuff like that.
The intake you can squirt with a little carb cleaner and spot a leak a lot of times.
One other thing or first test on a lean code is make sure it has not run out of gas and make sure you keep over a 1/4 tank of fuel in it.
Check that fuel pressure and filter and fuel pressure regulator.
If it has a fuel pressure regulator on engine with a vacume line control pull that vacume line and check for gas in it.
Her is some tips on finding a good repair shop.
Let us know how it goes. MT

Check around Ask around
Word of mouth.
Family friends coworkers neighbors business people and delivery people.
Mailmen and parcel delivery people get around and notice a lot of stuff.
Even a good parts house knows what shops are good and not good.
Chamber of commerce and better business bureau and city hall.
If you find a good referral to a repair shop go look them over and talk to them.
Look for a clean looking busy place with nice people running it.
Ask a few questions and ask about their qualifications and training.
Not all places have trained tech/mechanics.
If they do not find a place that does.
Even all dealers do not have all trained techs/mechanics.

I had the same set of symptoms a few years ago with my 1997 Grand Cherokee Laredo. The technician quickly nailed it down to the idle air control unit, and if memory serves me it cost less than $150 to diagnose and replace, including the part. The tech had a hard time removing and reinstalling since the valve is in a very constrained area with very little room to turn a wrench. Maybe the Jeep dealers have a special tool, but mine was replaced at one of the "quick" tune/lube shops. Since the replacement, my Jeep's idle has been as smooth as when it was brand new.

Hope this helps.

Mike

I'm a little poorer at the moment, but happy to report that my problems are finally resolved.

After having the new exhaust leak repaired, I took my Jeep to a shop recommended by my father. Two hours later, they called and told me their tests showed the front / upstream O2 sensor was bad. I was pessimistic, but they assured me they would remove the part if it didn't fix the issue, so I authorized the repair.

They took it out on the road, hooked it up and checked performance again, and gave it a clean bill of health. I picked it up last night, and it's running like a champ -- great mileage, no codes, no black exhaust, etc.

So $200 later, the O2 sensor ended up solving the problem. Too bad I didn't visit this shop first -- may have saved me several hundred $$'s and a lot of frustration...

Thanks again for everyone's help!

Sorry I missed the code P0171 in the first post.
The code P0171 points to lean fuel/engine running lean.
Several things can cause that.
One of first things to check is fuel system and fuel pressure.
Also that welding up on exhuast does not sound like a proper fix.
As Bringsepup said it need them air pipes back under there.
If they are not right it will throw off the front 02 sensor and make it read lean and give the P0171 lean code.
I think you need to check around for a better or good repair shop or even a good 5 star cry dealer..
A good repair shop can get a scanner and fuel pressure gauge on it and read out all sensors and check fuel system and find out what is throwing the code and no running right.
You said you heard a vacume leak look the engine over good for leaks. Check Pcv valve and all vaume hoses ,intake gaskets and stuff like that.
The intake you can squirt with a little carb cleaner and spot a leak a lot of times.
One other thing or first test on a lean code is make sure it has not run out of gas and make sure you keep over a 1/4 tank of fuel in it.
Check that fuel pressure and filter and fuel pressure regulator.
If it has a fuel pressure regulator on engine with a vacume line control pull that vacume line and check for gas in it.
Her is some tips on finding a good repair shop.
Let us know how it goes. MT

Check around Ask around
Word of mouth.
Family friends coworkers neighbors business people and delivery people.
Mailmen and parcel delivery people get around and notice a lot of stuff.
Even a good parts house knows what shops are good and not good.
Chamber of commerce and better business bureau and city hall.
If you find a good referral to a repair shop go look them over and talk to them.
Look for a clean looking busy place with nice people running it.
Ask a few questions and ask about their qualifications and training.
Not all places have trained tech/mechanics.
If they do not find a place that does.
Even all dealers do not have all trained techs/mechanics.

Thanks for posting back on how it went.
Glad you got it going.
Well the old check around tip worked out.
Family and freinds sometimes know best.
Now that you found a good shop you can tell someone else about them.
And do not forget where they are you may need them again.
Good Luck Mt

Sweet!

Hey all that money wasn't a waste, like mt-2500 says, now you know of a first rate place to go.

If it isn't too hard try to find out what brand of 02 sensor they used.