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Old 11-22-2010, 06:58 PM   #1
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Cold start issues

Got my TJ back from the dealer, $1544 later, and the problem persists. When starting the vehicle after sitting for a while, overnight or even an hour, the engine idles fine but if you put it in gear, it has no power and will miss and puke unless you floor it and get it revving, as in low gear. Not desireable. After the temp reaches operating temperature, runs like a charm. (97 2.5L auto)

Possibilities: 1) Coolant temp sensor 2) TPS 3) Idle air control motor 4) 02 sensor 5) MAP 6) inlet air temp sensor

They installed new 02 sensors, upstream and downstream. If the TPS was messing up, it would mess up when hot as well ???? Idle air control motor would mean bad idle, not the case. Even cold it idles fine, just won't go. I'm thinking the other three could be the cause. But I'm new with TJ's, not new with motors, I'm a gearhead. Temp gauge works fine, reads cold when cold, reads hot when hot. Are there two coolant temp sensors ? One for the gauge, one for the ECM. Not sure about the MAP, not sure if it would cause cold run problems only. The only other likely possibility would be the inlet air temp sensor. All you gearheads out there with all the TJ experience..............What do you think ?

By the way, dealer didn't just work on this. They did a complete tune up, removed and resealed the tranny and transfer case, and outsourced the installation of a new cat converter and 02 sensors. For this, they charged the $1544, not cheap but not unreasonable. But it didn't fix the problem. As I said, after it warms up, drives like a dream.

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Old 11-22-2010, 07:24 PM   #2
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I don't have any 2.5L experience but you may want to check for leaks around your manifold.

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Old 11-22-2010, 07:37 PM   #3
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No engine light is on. The MAP being messed up can really make it run like crap. You said it is a 97. Should be obd 2. Plug it in and see. Does the engine light come on when your key is on?? If not, bad light. Do the key off and on five times fast and leave it on fifth time and watch the odometer. It should do something. At minimum it should go thru all the gages and do a show to let you know the ECM and PCM are fine.. Check vacuum lines. There is one to the MAP and if it isn't on it will run like crap.
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Old 11-22-2010, 07:59 PM   #4
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Originally Posted by JEEPSTER988 View Post
No engine light is on. The MAP being messed up can really make it run like crap. You said it is a 97. Should be obd 2. Plug it in and see. Does the engine light come on when your key is on?? If not, bad light. Do the key off and on five times fast and leave it on fifth time and watch the odometer. It should do something. At minimum it should go thru all the gages and do a show to let you know the ECM and PCM are fine.. Check vacuum lines. There is one to the MAP and if it isn't on it will run like crap.
The "check engine light" was fixed/replaced by the dealer and does work. No, no light when running. I can take my scanner tool with me next time and check for codes but the dealer said after they did the work, no more codes (I know, check it yourself). I agree with your MAP assessment but wouldn't it run like crap when warmed up too ? I'm not sure. I'll have to check for vacuum leaks. I just thought the dealer had as well........maybe not. I keep the car up in NC but live in Florida. Going back up in December so I'll take the scan tool. I thought it was OBD I being a 97. So five times with the key on and off will bring codes to the odometer...didn't know, not a mopar guy. Thanks for the help.
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Old 11-22-2010, 08:06 PM   #5
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Most vehicles started obd2 in 97. It was a federal law. If not the obd1 will still kick codes. The MAP reads changes in atmospheric changes and barometric pressures and adjusts accordingly with the TPS. There is also a sensor on the TB that you can pull out and clean. I can't remember what it is but it gets all carboned up. Clean it with throttle body cleaner, not carb and choke cleaner. Sounds Wierd but your fuel filter and a bad or going bad fuel pump can behave like this. Good luck.
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Old 11-22-2010, 08:18 PM   #6
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clean your idle control valve.
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Old 11-22-2010, 11:31 PM   #7
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If I shelled out $1500 for someone to fix my problem and they didn't, I'd be taking my Jeep right back to the dealership/mechanic and demanding they actually FIX the problem that you paid for them to fix...or refund your money in full. That's my $.02...
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Old 11-23-2010, 01:47 AM   #8
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1996 and later all cars sold in the US were required to be OBDII - Federal Law.

Obviously the dealer wasn't much help.

Map - would be bad all the time - not temperature sensitive
TPS - same as above - not temperature sensitive
IAC - idle was fine - it only works at idle.

Vacuum leak - would radically affect idle more than higher speeds.

EGR - maybe - simply unplug the EGR vacuum line - if so equipped - should be 0 vacuum on it. Apply a vacuum to the nipple - watch it move, or engine running it should make it run terrible at idle. Could be the vacuum amplifier feeding it - or the CTO valve - the temp valve inhibiting EGR vacuum till it's warm. Vacuum line routing?

Inlet air temp - does not make that much difference, not enough to do what you are experiencing.

Block temp - bet that's the area of suspect. Sender, wires etc - even a bad battery to engine ground. Headlights on make any difference?

You need to see more:
If you have a laptop computer, or can get your desktop close to the Jeep - go to Ebay, get a scanner that interfaces with your laptop. Cheap!
Something like this:
AUTO SCANNER OBDII OBD2 OBD II 2 CAN BUS USB SCAN TOOL - eBay (item 370457987908 end time Nov-23-10 11:27:34 PST)
Hooks to the OBDII interface connector and a USB port - simple.

I'm not recommending this one, but it looks like it'll do the job.

What you want is more than a code reader. You need to see the same information the computer sees - the "data stream."
Notice in with this one you can see the temperature the computer THINKS it is. They are mostly interpreted values. But looking at them cold vs hot you should be able to see what sensor is lying. For example if you see the temp says it's 600 degrees, yet the engine is cold --. Sensor, shorted sensor wire etc.

The dealer should have done that, but maybe they don't know how - their DRB ($4000-6000) tester can do it - but again, they didn't or don't know what it means.
This will not take you deep into the prioritized Chrysler information, but should be enough in your case.
Some of the better interfaces even let you see signal patterns too. And aftermarket software abounds - cheap - on Ebay too. You can even see things like the Crank and Cam Sensor pulses, alternator diodes etc - handy for troubleshooting.

Simply look for something that does not make sense, then find out why before throwing parts at it.
Today's computers are pretty amazing - if you learn a little about them.
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Old 11-23-2010, 05:48 AM   #9
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1996 and later all cars sold in the US were required to be OBDII - Federal Law.

Obviously the dealer wasn't much help.

Map - would be bad all the time - not temperature sensitive
TPS - same as above - not temperature sensitive
IAC - idle was fine - it only works at idle.

Vacuum leak - would radically affect idle more than higher speeds.

EGR - maybe - simply unplug the EGR vacuum line - if so equipped - should be 0 vacuum on it. Apply a vacuum to the nipple - watch it move, or engine running it should make it run terrible at idle. Could be the vacuum amplifier feeding it - or the CTO valve - the temp valve inhibiting EGR vacuum till it's warm. Vacuum line routing?

Inlet air temp - does not make that much difference, not enough to do what you are experiencing.

Block temp - bet that's the area of suspect. Sender, wires etc - even a bad battery to engine ground. Headlights on make any difference?

You need to see more:
If you have a laptop computer, or can get your desktop close to the Jeep - go to Ebay, get a scanner that interfaces with your laptop. Cheap!
Something like this:
AUTO SCANNER OBDII OBD2 OBD II 2 CAN BUS USB SCAN TOOL - eBay (item 370457987908 end time Nov-23-10 11:27:34 PST)
Hooks to the OBDII interface connector and a USB port - simple.

I'm not recommending this one, but it looks like it'll do the job.

What you want is more than a code reader. You need to see the same information the computer sees - the "data stream."
Notice in with this one you can see the temperature the computer THINKS it is. They are mostly interpreted values. But looking at them cold vs hot you should be able to see what sensor is lying. For example if you see the temp says it's 600 degrees, yet the engine is cold --. Sensor, shorted sensor wire etc.

The dealer should have done that, but maybe they don't know how - their DRB ($4000-6000) tester can do it - but again, they didn't or don't know what it means.
This will not take you deep into the prioritized Chrysler information, but should be enough in your case.
Some of the better interfaces even let you see signal patterns too. And aftermarket software abounds - cheap - on Ebay too. You can even see things like the Crank and Cam Sensor pulses, alternator diodes etc - handy for troubleshooting.

Simply look for something that does not make sense, then find out why before throwing parts at it.
Today's computers are pretty amazing - if you learn a little about them.
The dealer claimed that when they ran the car, it didn't act up. Not sure they ever ran it cold, however.

You have the same conclusion I got on the MAP, IAC and TPS. If they screw up, they screw up all the time. I thought about the EGR but figured it would act up hot as well and cause me problems when trying to accelerate, hot or cold. I've seen them hang up open with a piece of carbon jamming the valve which really screws up the idle. The coolant temp sensor was my guess but seeing the temp gauge work somewhat correctly makes me think otherwise. Does throwing on the headlights make a difference in ground routing for the TJ ?

Thanks for the tips. I'll check out the OBDII tool you show on ebay. I have an Actron scanner but it doesn't interface with my laptop. I appreciate your knowledge and helping me out with this issue. I'd rather the dealer have taken care of this but if not, I'm not afraid of doing it myself.
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Old 11-23-2010, 06:09 AM   #10
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If I shelled out $1500 for someone to fix my problem and they didn't, I'd be taking my Jeep right back to the dealership/mechanic and demanding they actually FIX the problem that you paid for them to fix...or refund your money in full. That's my $.02...
Not that I'm sticking up for the dealer but here is the deal. I bought the TJ from a consignment dealer in GA. The prior owners had removed the cat converter, then went to the trouble of removing the CEL as well. When I discovered the issues, the reseller offered to refund 100% of my money. I thought about it but here was a 97 TJ in very good shape, no rust, pretty good paint, excellent interior, a very clean chassis, engine bay, etc., and only 100K miles at a cost of $5650. I've been looking for a while and that was $1500 to $2000 less than comparable TJ's. So I take it to the dealer to fix the issues. They took off and resealed the tranny and transfer case that had leaks. They did a complete tune-up. They sent it out for a new cat converter and 02 sensors to save me what mopar wanted for a cat. They took the dash off and fixed the CEL light and then checked for any codes. None popped up. I don't think they screwed me in any way. And, they're the most pleasant people you could ask for. No, I'm not mad at them at all.
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Old 11-23-2010, 06:36 AM   #11
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Sounds like your temp sensor is not sensing to cold engine so the PCM is not richening the fuel mixture for the cold engine.
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Old 11-23-2010, 07:38 AM   #12
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Exactly! As Hawk said. That circuit's probably the cause.

Even though it didn't act up when they looked at it, something is still wrong.
They obviously DID NOT look at the data stream. You need to.

A simple scanner only reads and resets codes - it does not read the data stream.
Similar to going to a doctor with severe chest pains and having lots of tests done - then only hearing you don't need an operation - yet.
Codes are same like informing you "the pot of gold is in California, go dig it up", but it does not tell you enough to start digging.
Simple scanners can wipe out the codes, they cannot erase or clean the deep memory. They only scratch the surface.


When the PO took off the converter and the bulb, the ECM went into "limp mode" - giving poor performance. They had to do something else to "trick" the computer into working right - that something is probably still there. A resistor, a cut or grounded wire etc.

The data stream will probably show what's not working, reading wrong, or not changing and can be fixed.

Or take another $1500 to the dealer and let him not fix it again. Someday someone will find the trouble - may as well be you.
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Old 11-23-2010, 09:08 AM   #13
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Exactly! As Hawk said. That circuit's probably the cause.

Even though it didn't act up when they looked at it, something is still wrong.
They obviously DID NOT look at the data stream. You need to.

A simple scanner only reads and resets codes - it does not read the data stream.
Similar to going to a doctor with severe chest pains and having lots of tests done - then only hearing you don't need an operation - yet.
Codes are same like informing you "the pot of gold is in California, go dig it up", but it does not tell you enough to start digging.
Simple scanners can wipe out the codes, they cannot erase or clean the deep memory. They only scratch the surface.


When the PO took off the converter and the bulb, the ECM went into "limp mode" - giving poor performance. They had to do something else to "trick" the computer into working right - that something is probably still there. A resistor, a cut or grounded wire etc.

The data stream will probably show what's not working, reading wrong, or not changing and can be fixed.

Or take another $1500 to the dealer and let him not fix it again. Someday someone will find the trouble - may as well be you.
I went on ebay and bought the scanning tool you recommended. It was cheap enough, $22 with shipping. Thanks for the tip. I was looking on the wiring schematic of my service manual and could only find one coolant temp sensor. Are there two or just the one which provides for the gauge as well as the PCM. According to the manual, the sensor is located at the back of the head on the 2.5L engine. Sensors are only about $23 each, might be worth just replacing it. However if the wiring is bad, that wouldn't fix the problem. Still don't understand how the wiring or sensor can be bad and the gauge seemingly works properly.
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Old 11-23-2010, 11:15 AM   #14
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I'm not intimate with the 4 banger. Look at the schematic for the PCM, the inputs and outputs. You should see the temp input to the computer. Trace that wire back through all the pages - to see if it's the same one that is the input to the gauge.

That may not be the trouble, but it's a good place to start.

Hopefully you'll see something on the data stream that doesn't make sense.

Wondering if the guy that did the previous mods may have changed or rewired the temp gauge to read normal, but did something to put a false signal to the PCM. That temp sensor may even be the wrong one - but the data stream should show strange readings if it is.

Normally removal of the O2 sensors would not give a proper input signal to the PCM. Beyond setting an O2 code it may go super lean or rich - don't know without trying it. The PCM will fall back into "emergency mode." Runs but very bad performance. I'm not sure how it runs when cold, it may be as you describe.

Removing the bulb is not enough to get it to run very well. It just hides the problem. He probably did something else to trick the PCM into thinking it's normal - richening it up. He somehow introduced something to give it a fixed mixture. But now when it's cold it still stays at the fixed richness setting it used when warm. Cold it needs lots more fuel, and it's not getting it.

Watching the data stream - PCM inputs and outputs while it warms up hopefully you'll see something that's not changing as it warms up - that's the place to start.

It may be as simple as a diode or something buried in the wiring harness under the plastic.

Maybe try searching the web for things like "tricking the PCM," or "Cat removal," "O2 sensor bypass," etc. He may have followed someone's advice as to how to do it.
There is all kinds of crazy armchair racer's advice out there - most of it wrong.

Wish it was here, I'd love to play with it - a good challenge.

Another thought - try disconnecting the battery overnight - at least 8 hours. That erases all old memory. Just disconnecting for a few minutes only erases the codes, it does not erase the deep memory. It essentially "reboots" the PCM, just like you do with your desktop when it acts strange.

There are some folks that think you can do it by touching the cables together - once you see the data stream you'll see that does nothing.
8 hours minimum, preferably longer.

Some better adapters with the software enable you to actually see the stored memory, like tire size, speedo calibration, timing charts etc.
But this one should get you where you want to be.
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Old 11-23-2010, 06:43 PM   #15
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I definitely think you're on the right track with your help. It makes sense. The guy rips off the cat to save a few bucks, $200 at most, then has to make up for it with the PCM not getting the right read. I believe the upstream 02 sensor is on the exhaust manifold but the downstream is on the cat. I could be chasing this for a while. However, worse case scenario, it has to warm up then its fine.

I'll put the scanner on it when I get it from ebay and read the downstream data on my lap top. If I don't see anything there, I'll disconnect the battery for 24 hours and do a reset of the PCM. I may just buy a new coolant temp sensor and replace that as well. As you point out, it may well be the wrong one to trick the PCM. If the guy monkeyed with the harness, I should be able to see different tape wrap on it or a bulge if he inserted a resistor or diode in the circuit. According to my wiring schematics, there is only one coolant sensor. It feeds back to the PCM which in turn feeds back to the gauge cluster.

And no, I'm not paying the dealer another $1500 to do nothing. I don't think they tried to screw me, but they didn't fix it either. Driving 24 miles to drop off the Jeep, having my wife follow me, then later re-doing the same thing gets old and wastes time. I'll try to find it myself. Thank you for your immense help. This forum is great, helps an idiot like me to figure things out.

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