Fixed engine high idle cold/dying warm problem! --Troubleshooting & Repair Writeup
 

In case this happens to anyone else, here's the experience I had:
1997 TJ 4.0L

Running trail at 12,500 ft+ on Saturday. On engine-compression
controlled descent, check engine light comes on. At level ground with
the vehicle out of gear, the Jeep tries to loose idle and die. For the
most part, though, it drives out of the trail without issue. This
happened once or twice to me last year and has happened to friends
with 97 TJs before, so I didn't think it too serious at first.

On the highway home, it had no power. Power in 1st felt as if I was in
3rd, and so on. Also, when I would push the gas pedal down to
accelerate, the vehicle would slowly accelerate, then lurch, as if
multiple cylinders in a row had failed to fire, then would continue
accelerating slowly. Check Engine light still on. When parked, I tried
"goosing" the gas, and letting off immediately; this would be a sure
way to kill the engine. It was like it couldn't figure out where to
add gas in order to maintain the idle again as the rpms dropped, and
would allow the engine to slow down too much, to where it would just
die.

At home, I check the codes: 12, 24, 14, 55. 12 means the battery has
been disconnected sometime (no doubt). 55 just means "end of codes".
So, 24 and 14 are my error codes. I look them up; 24 means the TPS and
MAP values received by the computer do not correlate to one another
correctly. 14 means the voltage reading from the MAP is out of spec
range. Ok, will have to look into buying a new MAP sensor in a day or
so.

Monday, driving to work, it seems to work fine again. Check Engine
light is still on, though. Odd.

Monday, driving home from work, it idles at 2000 RPM when I initially
start it up in my work parking lot. Not good. I decide maybe it'll
slow down when it warms up. Nope. When it warms up, it goes from
high-idle to low-idle, and attempts to die at every light on the way
home. When I get home, I have a friend drive me to buy a new MAP
sensor ($80!!! Note also, there are two possible part numbers for the
MAP on a 97 TJ; your stock map should have the number. If it doesn't,
it means the previous owner already replaced your MAP, and I don't
know how you would determine which part you need. I needed the one
that ended in "405".) The MAP was extremely easy to install. After
installing my new MAP sensor, it seemed to drive ok on a quick turn
around the block.

Tuesday, driving to work, it seemed ok, although it was still idling a
few hundred RPM higher than normal (about 1100 vs. the normal 750-800
I usually see). Check Engine light is off now (I unplugged the battery
for a few minutes to make the computer reset the Check Engine code).

Tuesday, going out for lunch, the Check Engine light comes on again.
Damn. I immediately check the codes, and sure enough, 24 (TPS and MAP
disagree) is still there. However, code 14 (MAP out of whack) is gone.
Time to go buy a new TPS. I hurry over to a nearby auto-parts store,
and buy a new TPS ($47). I stick that sucker on (old one was hard to
get out, as its screws have threadlocker on them, plus my air
compressor tank is mounted in the engine bay above the engine, and
made it very difficult to get at the screws). I found it odd my stock
one had no gasket at the inlet port, while the new one did. I made
sure to check that there wasn't an old gasket still stuck in the port
at the throttle body, but there wasn't. The TPS would've been
extremely easy to replace if I didn't have the air tank in the way,
for anyone wondering if it's difficult. I install the new TPS, and
again disconnect the battery for a minute to reset the computer.

Tuesday through now: The jeep drives just like normal again. So, it
seems I fixed the problem (I sure as hell hope so; I don't know what
I'm going to do if damn Check Engine light comes on again! >:-O ).

Total cost: $130 for two new sensors. That, vs. the cost of an engine
diagnostic at Pep Boys ($188). You could say I came out ahead ;-) .
Stressful, though. I don't like it when the engine has problems; I am
still pretty much a greenhorn when it comes to troubleshooting engine
performance issues.

So, if it ever happens to anyone else, hopefully they'll find my
little story helpful in resolving the issue for themselves. My biggest
complaints in the replacement of the sensors was (a) the price
(especially for that damn MAP sensor), (b) the screws holding the
sensors on use torx (T-20 IIRC), and (c) my custom installed air tank
made it a royal PITA to get at the torx screws holding the TPS on.

Most of what I knew for how to troubleshoot and fix these problems I
have gleaned from reading random posts on this group over the course
of the last year or so, making it possible for me to fix this without
having to ask for "HELP!" on this forum. ;-) So thank you all for
providing the consistent wealth of knowledge for various problems when
people post to this board.

Later,
Bob