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Wierd cold start stalling issue 97 TJ 2.5L New engine


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Ok, so i only get this issue during a coldstart (sitting overnight or an hour ish). So here's what its doing, when i start the rig everything idles good, as soon as I hit the gas it revs up and then starts to stall out, i rev the gas a few more times and it bogs and does a small backfire like sound and then starts idling great again. I can drive it around for hours on end or all day and it runs tip top, no stalling, bogging, nothing, just runs perfect. I can turn it off after it warms up and back on and nothing... runs great. Wait 10min start it up.... runs great. This only happens on a coldstart and after I rev the engine for the first time. No problems any other time. The engine turns over immediately (No cranking) so its not a fuel pump issue, i dont get constant throttle issues so i dont think its the TPS, i dont get black smoke out the tail pipe so i dont think its leaky injectors.... Possible IAC? But it idles fine at start and after wierd hiccup.... Possible MAP sensor? Looking forward to any insight on this. I have a OBD reader on the way to me. Oh and no codes pop for this issue. No engine alarms of any kind so my code reader is useless. Hopefully i can read values with the OBD reader to help narrow it down.

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TJ47...This sounds like a cold start circuit related problem.  The cold engine needs a cold mode signal to enrich mixture.  Troubleshoot accordingly. 

Make sure the fuel rail damper's vacuum circuit is routed properly.  Check the Coolant Temperature Sensor ohms readings (cold).  Check the Intake Manifold Air Temperature Sensor ohms readings (cold).  Make sure the oxygen sensor is heating up properly when cold.  Check connections.  Ohms readings for sensors are available in the FSM for your TJ Jeep. 

If you do not have an FSM, factory service manual CDs are available at eBay—cheap.  I never troubleshoot these problems without consulting the official FSM.  My FSMs are bought and paid for paper editions or CDs.  I have an official manufacturer's shop manual for each vehicle we have owned and every vehicle I worked on professionally.  My library of pro trade books dates back to the 1920s.  When we ran a shop restoring pre-war, postwar and muscle era powertrains, steering gears and manual/automatic transmissions, I bought used FSM copies or reprints for each make/model I serviced.  Those vintage manuals alone fill a six-foot tall by two foot bookcase.

Folks waste money on parts changing solutions and internet anecdotal opinions.  The FSM is fact based and cost-effective in the long run.  Another source of information is the factory TSB releases (Technical Service Bulletins) from Mopar, available online for free.

Moses

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Hey thanks for this info by the way Moses! I would have never thought of it. Im going to break out my multimeter this week and figure this out! Much appreciated! Hope I find the smoking gun. Will keep you posted.

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Ok so update.... i did all the tests on those sensors and found them all to be in range but they were really worn out so i replaced them anyway. Thanks for leading me in that direction tho moses. I ended up doing alot of research on it and stumbled accross a discussion for an issue very similar to the one i WAS having. A guy in this conversation said something simple and i tried it and located my issue immediately! So if you read my original post i mentioned i had my OBD tester coming. Well i got it in. Played with it for a few days and got familiar with it and for the life of me ... couldnt find a thing wrong haha.. go figure. Checked my O2 sensors and they were reading within range ( i checked everything but this sensor is involved with the story ) The simple thing this guy said was "unhook your upstream O2 sensor and see if any thing changes. If the problem goes away then a simple O2 sensor swap" ...... so being a fan of easy tests I decided to do the check (even though all numbers were within parameters) and sure enough... no upstream O2 sensor and NO stalling or rough idle!!!! B-Lined it to the parts store and the rest is history. Thanks again moses to getting me in the mindset of it not having a rich enough mixture. 

The part i through at this issue...

Map sensor, manifold air pressure sensor, coolant temp sensor, multiple new vacuum elbows and fittings. All of these were worn and i do not regret replacing them at all.. except the MAP sensor. But i kept new one and will shelf it for the future.. just in case. 

Also thoroughly cleaned the throttle body which is something i will be doing once a year from now on anyway... good preventative maintenance never hurt anything! 

So if you are having this issue do this simple check- unplug it, start it up, if all good... get new O2 sensor lol. Now i remind you... i have no O2 sensor codes AND all the numbers were within range, this O2 sensor showed no signs of being the issue so dont count it out. Super easy test. If this did not fix your issue keep at it! Good luck everyone and thanks. 

 

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TJ47...The misfire codes can be nebulous, the worst code is a #1 Misfire.  Many malfunctions will throw this code.  A faulting O2 sensor is one of those ballpark triggers.  This is why the PCM and scan tools are neither fail safe nor 100% reliable.  As a result, consumers read codes and piss away endless amounts of money on parts they don't need—without solving the problem.  Frankly, with all the good intent, OBD-II fell way short of the troubleshooting mark.

At the dealership or a deeply immersed independent shop, experience turns up ideas like let's test the O2 sensor by taking it off line.  Your method is very sensible!

On that note, I always use OEM (Denso, NTK, etc.) sensors.  I bought an aftermarket sensor as a preventive care measure for our 4.0L years ago, using a Brand-X from AutoZone.  The engine immediately ran poorly.  I crossed over the OEM Mopar part to NTK (same part number as on the OEM sensor) and purchased the NTK for far less than Mopar's packaged unit.  Replaced the Brand-X, and the engine has run flawlessly ever since.  

If your new sensor is Brand-X and not OEM but works very well, great!  If not, the issue with the aftermarket Brand-X sensors is their generic fitment.  A given sensor will fit a broad range of applications.  It may be slightly out of spec for many of these applications.  The OEM part number is always within spec.

Yea, you fixed it!

Moses

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