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put in new pcm does it need to be flashed ?

4K views 46 replies 11 participants last post by  AnotherRedZj 
#1 ·
Im still having stalling issues with my 5.9, been like a year now and still does it.. Replaced sensors, new battery, new alternator.. Wiggled cables and i cant replicate it.. Does it randomly.. One week its fine, the next it stalls like 20 times and then once its really warmed up it drives fine all day

everytime i let off the gas the rpms drop to 100 and it stalls unless i keep my foot on the gas.. Even if im rolling 2-5mph itll stall and imediatly when it comes to 0 mph it goes back to 600rpm or so idle. Its wierd..

I have a spare PCM i took from a 5.9 in a Junkyard, does it matter if the part numbers are diff ? Mine is 56041 519AE, this other one is 56041 519AC. Both are Federal Premium. Not sure what the diff between AE and AC would be, besides revision ?

So is this PCM going to change anything, besides i guess inspection.. When i get the emission inspection it will read a different vin number right ? Should I just swap in my original one and drive 100 miles or so before inspection and then swap the other one back in. I installed it just now this morning and it starts and drives fine so far


Im really hoping this PCM swap fixes my stalling, its getting so dangerous.. I litterally was making a right hand turn today and when i let off the gas i stalled in the middle of the road and lost power steering.. It starts right back up though.. I have trained myself to drive with both feet now one for brakes one for gas.. Its rediculous..

Im starting to hate this Jeep
 
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#2 ·
It's my understanding, and the common wisdom as well, that the letter suffix denotes software version. It should work fine, providing the unit is in working shape, which is hard to ascertain on a JY module until it's installed. Good luck.
 
#3 ·
Letter suffix is just revision, no concern there.

It's unclear if the inspection will actually fail based on the wrong VIN in the PCM, but your swap back plan would work if the original doesn't die on the shelf (they do that).
 
#4 ·
It's unclear if the inspection will actually fail based on the wrong VIN in the PCM...
It's clear to me that a PCM from a different vehicle, with a different VIN, will pass the smog test in California, the strictest of the "smoggers".

I'm guessing that the bar-code at the top of my renewal notice contains the VIN of my ZJ, as it is printed at the top of my test results. It is not a VIN that was "pulled" from the PCM. Virginia may be different, but for purposes of smog-testing, I doubt that they're stricter than California. That said, they may check the VIN for some other type of inspection. I just know that you'd be OK in Cali with the PCM from a different car, and without having it programmed.

The results are from my '98 ZJ, with a JY PCM of the exact same part number, un-flashed/un-programmed before or after installation.
 

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#5 ·
"Revision" doesn't include death flash does it? I don't think it does but thought I would mention. You've checked the normal stuff Candy?
 
#6 ·
"Death flash" would have a code change sticker under the hood since it's a change not just little fixes.
 
#7 ·
What do you mean a pcm goes bad sitting on a shelf ? Is there like a 3v lithium battery inside keeping the memory intact or something ? The one i swapped it with i had for like 6 months and was probly in the yard for another 2-3 months.

As for checking the usual, yes.. New alternator even though it is a 136 not 150.. New Battery.. I took off the battery terms cleaned them, cleaned the wires on the terminals. new TPS new IAC.. Ignition coil looks fine and is only a few years old, i havent replaced the distributor in probly 4 years or so though. The plugs are maybe a year old..

The odd thing is this behavior goes away when i unplugg the battery and reset the pcm.. Once the PCM learns my driving after what 75 miles or whatever it is.. The problems starts all over again...

It drives fine once its HOT.. like meaning if i drive it for like 10 minutes.. But during that 10 minutes when its cold it stalls non stop... As soon as i let all the gas even at 40mph the rpms take a nose dive and drop to like 100-200rpm... Even if im rolling at a crawl with the foot off the gas itll drop to 100rpm and die.. However once i hit the brakes on a crawl roll and it stops completetly it idles back up to normal rpm..

Its weird, and its dangerous and making my daily driver extremely unreliable. The PCM is the only thing i can think of thats doing this.. But its wierd how its fine when i unplugg the battery and once it learns itself then the problems happen.

The Intake manifold was fixed 3-4 years ago. I dont have any vacuume leaks that i know of. All the tubing is new, as well as that plug on the nipple on the intake drivers side. Why would it stall even when its crawling at 2-5mph and then go back to normal idle when its stopped ?

This leads me to believe it isnt a wiring issue, because the engine isnt flexing up and down because no throttle is being given.. Plus it drives fine with my foot on the gas.. I can WOT and drive casually and no issues.. Then boom come red light i let off the gas and it dies.. I lose all power steering and brakes and thats why i have to use my right foot on the gas and left on the brakes when im stopping so i dont hit someone due to no brakes.

In fact i found a video of a 5.9 on youtube with the same exact issue . I recently recorded mine too I'll upload if you want.. I was driving in a parking lot and you can see the rpm gauge going crazy and dropping like a fly when i let off the pedal. I dont have any codes.. The only thing i havent replaced is the Crank shaft sensor or whatever.. But it isnt acting like that sensor is bad. The 4.0 had a bad one that sensor on the bellhousing and this is nothing like when that went bad. You can imagine my frustration.. This is my work vehicle for my landscaping and its been doing this for a year and its getting so bad its now doing it every day 20x a day when its cold.. Before it would do it like maybe once a week or so. Why does it only do it when its cold ? It takes like 10minutes of driving for it to finally go away

So far i have driven only like 30 miles on the new PCM so i have probly another 50 to go before it learns itself.. Lets see if this new pcm fixes it..

 
#16 ·
What do you mean a pcm goes bad sitting on a shelf ?
I remember Ratmonkey saying this once and not knowing the reason, but just that it happens. It could be one of those things that happened a few times and just got repeated, or it could be a real phenomena - hard to say.

...it drives fine with my foot on the gas.. I can WOT and drive casually and no issues.. Then boom come red light i let off the gas and it dies..
When I had this problem cleaning the IAC & throttle body solved it. I know you replaced the IAC, but when I removed the throttle body to clean it, the passage from the IAC through to the bottom of the throttle body was clogged with carbon. This may not be your problem, but it's free and doesn't take very long.
 
#8 ·
Sub'd due the fact I have the same issue but not as bad. I too have learned to drive with 2 feet. When it pisses me off enough I pull the vacuum line off the pcv to let it idle high and the rpms won't drop enough to stall but the idle is high as hell in park obviously.... not calling that a fix but it gets me to work.

Sent from my HTC One using Tapatalk
 
#9 ·
are you saying that when it's warm it NEVER stalls?

are you saying that when you lift off the gas it ALWAYS drops to 100 rpm idle?

that sounds like two entirely reproducible problems, no?

have you cleaned out your throttle body (specially in the IAC area)? replaced your IAC? IAC gets sticky with the carbon on old throttle bodies.

get a vacuum gauge and run it into the cabin.

drive and see if the vacuum is good all the time

have you done the vacuum test in the haynes manual?
 
#10 ·
You can get vacuum readings from OBD2 with a good reader or a bluetooth adapter and any number of apps.
 
#15 ·
I have a catch can yes but i bypassed it and ran the hose directly to the intake and it still does it, and as far as cheap hoses its regular vacuum line so i dont see how it would collapse. I could try switching the hoses. But that still doesnt explain why its fine when the pcm is reset and after it re-learns it does it again.

I suspected it might be an O2 sensor, but the sensor is also only a few years old and NGK one too. Could an O2 sensor cause this ? Are the O2's used when the PCM is reset ?..

Why is the Jeep fine when the PCM has been reset (unplugging the bat), and then after it re-learns after x amount of miles it starts its act up.. In a way it doesnt seem like it could be the pcm but maybe a sensor that isnt active yet right ? until the pcm re-learns.

And yes the Jeep has to be really hot for it not to happen.. I dont mean once it idles for 10 min and get to 200f.. The engine bay litterally has to be hot and then it stops doing it, so it has to be driven for like 5 miles or so and then itll be fine all day long and never do it again until the next day after it cooled down completetly. And yes when i lift OFF the gas it drops to 100rpm the engine litterally wants to stall and 50% of the time it will unless i tap the gas.. but if i just tap it and let off again itll drop back to 100 rpm when its rolling at most speeds.. I have to keep my foot on the gas even if its slight for to run normally.. And then once i drive 5 miles or so.. Its fine and never does it again that day. The throttble body and intake are spotless, but i guess i can take the throttle body off and clean it to be sure

No it isnt reproducable.. It happens some days, other days it doesnt do it.. Its one of those problems when you bring it to the dealership and they go "we cant reproduce it" and then the next day when you have it, it will do it again. I havent done a vacuum test on it. I dont have any tools for that. The IAC and TPS are new.. they didnt help at all.. At first they seemed like they did but thats only because i had to unplugg the battery to reset the pcm.. Then 2-3 days later when the PCM re-learned it started to act up again..

And if this was a vacuum issue wouldnt it be doing it regardless of the pcm being reset or not ? And if there was a vacuum leak why arent the RPM's high ? My understanding is if there was a leak anywhere the RPMs would be higher.. This issue feels more like plugged or clogged vacuum line then having a leak. Like if you unplugg the PCV and then put your thumb over the vacuum and the engine stalls.

And also WHY would it do it even when you roll at 5-10mph.. I will upload the video later but i was in a parking lot because it started getting dangerous to drive. And i recorded myself rolling at less then 10mph barely even moving and the Jeep was at 100rpm.. and when i stopped completetly it went back to normal idle like nothing happend. I dont understand how rolling at such low speeds will do this and when its stopped it doesnt do it.. Its wierd..

So.

1. It only does it when its cold, once its driven for a few miles it stops and runs fine

2. It stops doing it completetly for 2-3 days if you unplugg the battery and reset the pcm

Ok here is the video, sorry i was spinning the wheel alot. It was a small parking lot i had to back up alot.. But about half way in the video you will really see im coasting at 10mph or less and the rpms are pretty much not even reading.. it was about to stall, i didnt have my foot on the brakes i was just rolling.

If the video doesnt work give it a little bit to finish processing on youtube

.
 
#14 ·
O2 isn't used in open loop, which gives the sensor a chance to warm up.
 
#17 ·
A vacuum gauge is one of the cheapest diagnostic tools and the first thing you check when you have any engine problem. Get a vacuum gauge like right now -- there are a million things it can diagnose.

I mean it probably has nothing to do with vacuum but you always wanna check your vacuum.

My dodge was doing exactly what your Jeep was doing and it was a tiny tiny ring of carbon in the hole where the IAC servo goes in. The carbon was causing the IAC plunger to stick. (My dodge has the same throttle body.)

 
#21 ·
well it isnt the pcm.. I was driving home from work and im at the 50-70 mile mark to where the pcm re-learns and as soon as i got to that mark i had let off the gas when on the way home and the Jeep rpms at 50mph dropped to about 500 and started stalling again except thsi time it was doing it when the Jeep was hot

WHAT THE HELL IS GOING ON WITH MY JEEP...

What about the PCM re-learning itself is causing the stalling.. which sensors activate when the pcm relearns, and which ones are dormant i should say when i unplugg the battery and reset it.. Clearly this is some kind of sensor doing this.. If it were vacuum related wouldnt it be doing it all the time and not when i reset the electronics... Could it be the O2 sensor... Tomorrow when the rain stops and the Jeep is cool ill take the throttle body off and clean it if its dirty.. but im at my wits end here.. Im close to thinking about selling this unreliable pos.

Should I unplugg the O2 sensor and see if this fixes the stalling ? Would this be something to try ? I guess when i get the money after i pay my bills here soon ill buy a pressure gauge and check the vacuum
 
#22 ·
Ok i took the throttle body off. It realy wasnt all that dirty.. there was some carbon in the IAC area where the nipple rests against so i took a screw driver and soaked a rag with brake cleaner and got a good amount of black stuff out , and also some hard deposits on the butterfly areas.. I took the map sensor off and sprayed brake cleaner in the sensor tubes and the tube for the throttle body.. So far its driving ok tonight, but well see tomorrow and the next few days...

The intake manifold is pretty spotless since i did it like 3-4 years ago. No oil anywhere on it. it looks like i just put it on.. Soo yea..
 
#23 ·
No. This won't do. You have to clean in that IAC hole until it is completely silver color in there. Seriously, mine was sticking and kept sticking until I went in there with a tooth brush for 10 solid minutes.

Sent from my SAMSUNG-SGH-I317 using Tapatalk
 
#25 ·
I'll bet you a dollar the car is fine now.

Did you do a wiggle test of all the wiring while the car is running?

Can I confirm this: you never ever have a problem at wide open throttle?
And: you never ever have a problem at half throttle?


Sent from my SAMSUNG-SGH-I317 using Tapatalk
 
#26 ·
Never had a problem with any throttle, its when i let off the throttle it stalls or tries to stall.. I can keep my foot on the gas very lightly and itll be fine. Yea wiggle test did nothing i wiggled every wire harness and plug i could find, didnt do anything. Tomorrow i have 4-5 houses to mow so i will be driving alot. Well see if it stalls or not. I hope not.

And here is inside the intake, as you can see 4 years after my plenum fix its pretty darn spotless, minus some dirt which probly got passed my k@n filter somehow. So i highely doubt their is a intake leak or i should be leaking oil i believe ?
 

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#28 ·
Whenever I disconnect my battery, I notice a lot more rpms and engine braking when off throttle at speed.. As it relearns and the more I drive the rpms settle down and the engine braking goes away when coasting. My bet is the carbon you cleaned out of the iac will fix it..
 
#30 ·
Na i used my other Jeep today. I'll drive my 5.9 today when i go to work.

As for the engine throttling. Yea mine does it too when i get to 50mph the vehicle drops in rpm until it relearns but thats normal. I think thats the torque convertor
 
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