Do ECM's "Break Down"?

[T-Rex Jeepster]

Having a nightmare issue. When I set this motor up I was advised to set the timing advanced 8 degrees BTDC and she ran for 8,000 mile with no issues...Then she started doing this herky-jerky staggering when you would level out the throttle and try to cruise level speed. Pulled the distributor and she was in BAD shape so I replaced it and set the timing back to TDC and noticed right away that while she idled/ran smooth, She just wasn't as responsive and seemed to have lost power while accelerating and the "Herky-Jerky" Problem got worse. So I went back and reset the timing 8 degrees BTDC and she cranked right up and purred like a kitten at idle...But give it any throttle and it would snap-crackle-pop and just wasn't happy. So I went and checked the crank position sensor and it looked really rough to the point I think it's a possible original from the factory, So I changed it. And she was just a joy to start and drive. No "Herky-Jerky" Great acceleration for a 4.0 and seemed to have all it's "Balls" Back. And then I got seven miles down the road and the bottom fell out, Backfiring, Missing, No power and just died. Called a Buddy got it home in about an hour, Got it off the trailer, Fired it up to get it out of the drive...And the M'Fer acted like a brand new ride! I drove it back and forth on my country road dogging it to the red line and it took it like a champ. I'm at a loss...Could it be that the ECM is "Breaking Down" Like a worn out coil? Could it be the coil? It is after market and I bought it with a "High out put" Package that had a distributor cap and button that didn't last 6,000 miles....Any input on this would be GREATLY appreciated

 

[machoheadgames]

For the jerky acceleration, I’d question the ignition coil before I’d question the PCM. PCM usually goes bad in other ways, like capacitors dying.

I am curious what you mean about setting timing 8° BTDC? Are you referring to using a keyed timing chain that lets you advance the valve timing? The distributors in these are not adjustable so I’m not sure how else you’d be adjusting timing advance…as the computer controls all of that. Seems like you’re doing it from the distributor but I don’t see how.

What are the details on your motor and who told you to advance it? Just curious, I feel the detail might help. But probably ignition coil related. I find stock Mopar to be the best ignition coil, nothing like screaming demon or whatever else.

 

[T-Rex Jeepster]

For the jerky acceleration, I’d question the ignition coil before I’d question the PCM. PCM usually goes bad in other ways, like capacitors dying.

I am curious what you mean about setting timing 8° BTDC? Are you referring to using a keyed timing chain that lets you advance the valve timing? The distributors in these are not adjustable so I’m not sure how else you’d be adjusting timing advance…as the computer controls all of that.

What are the details on your motor and who told you to advance it? Just curious, I feel the detail might help. But probably ignition coil related. I find stock Mopar to be the best ignition coil, nothing like screaming demon or whatever else.

There was a guy that is a well known hot rod tuner that tours and races in the big leagues...Knows his stuff. I set the crank mark at 8 BTDC and then set the distributor to #1 at his suggestion. Ran great for more than several thousand miles with no issues and stayed steady at 21 MPG through many tanks. Gears and chain were set to marks. Then changed distributor, Went back to TDC on crank indicator and she just wasn't "Right" Almost like a de-rated setting. Pulled the distributor again and went back to 8 BTDC and she'd start, Idle like silk but wouldn't take any accelerator without missing and spitting. Changed the CPS and everything went perfect....For seven miles and then lost all "Timing" Then died outright. Let her sit, Cranked up but again would take any throttle.... Killed it, Got a tow back, About an hour elapsed and when I cranked her up to get her out of the driveway after getting dropped...She starts right up, Takes all the throttle and goes up and down the road like champ for about five minutes...Parked her and came her to hopefully find out why.... Gonna take your advice and change the coil and see if that stops the delayed action F' Up...And that's what it is behaving like...A coil with bad/thinned oil.

 

[DOWNS]

I don't see a year mentioned, but as mentioned above you have no control over the ignition timing in a 4.0 no matter how much you move the distributor around. The ignition timing is fully controlled by the computer. Usually somewhere between 10 and 14 degrees BTDC at idle. This is a common failure mode of various sensors, they're fine cold but when you get them hot they fail, can make diagnoses very difficult sometimes. This failure mode can also happen with coils. Work fine when cool but get some heat in them and they start to malfunction. You could always put a multimeter on it and troubleshoot it that way. But if it's cold it may show good readings. Maybe drive it till it starts to mess up then shoot it then.

Your experience of bucking backfiring etc sounds like a possible TPS that has a "dead" spot, it causes wildly erratic readings to the computer about where your throttle is actually at. Put your multimeter on it and slowly open and close the throttle by hand (engine off) and watch for erratic or no readings on the multimeter though the full sweep. An analog meter is easier to use for this, but you can use a digital one as well.

 

[machoheadgames]

I think by setting the crank to 8° and then stabbing the distributor, you are basically setting the distributor rotor off by a tooth on the cam gear. I’m surprised that ran well. I also have never heard of anyone doing this on a YJ or TJ 4.0, so I’m surprised this dude recommends it. Not doubting him, I just find it interesting.

Downs makes a good point about the TPS. Those can cause all sorts of wonky conditions. Might be worth testing that first before buying a coil or TPS or anything else. Either of those can make the driving experience very poor, but at least the TPS can easily be tested.

 

[T-Rex Jeepster]

OK...Went and got a middle of the road ignition coil and got it on leaving the timing 8 BTDC...Started right up, Idled like silk and took acceleration just fine... But had a flutter whenever it got up over 1500 and stayed no matter where I held it. I'm thought it was the extra eight with valves not completely seating and causing the flutter. So I yanked the distributor again and reset it back on TDC on the compression stroke, And guess what...You betcha', Running like a brand new watch. Silky from idle all through the RPMS and no flutter when you even out the throttle. Gonna run it to work tomorrow and see for sure she holds. Thanks to MHG and Downs...I changed out all the throttle body sensors a couple of months ago so I guessing I'm good. All my stuff is in storage as far as OHM meters and specialty stuff as I'm waiting for our new house to be finished, So I'm doing this all Ol' Skool. Will buzz back when I know for sure we're dialed in...

 

[machoheadgames]

Great news! Hopefully no surprises and you’re good to go.

I did a wide timing chain a few years ago. At the time I considered opting for the 4° cam timing advance, but I didn’t and left it at 0. Kind of wonder what difference it would have made. I didn’t want it to ping and I didn’t want to have to tear it all back out if I didn’t like it, that’s why I stuck to stock.

 

[DOWNS]

OK...Went and got a middle of the road ignition coil and got it on leaving the timing 8 BTDC...Started right up, Idled like silk and took acceleration just fine... But had a flutter whenever it got up over 1500 and stayed no matter where I held it. I'm thought it was the extra eight with valves not completely seating and causing the flutter. So I yanked the distributor again and reset it back on TDC on the compression stroke, And guess what...You betcha', Running like a brand new watch. Silky from idle all through the RPMS and no flutter when you even out the throttle. Gonna run it to work tomorrow and see for sure she holds. Thanks to MHG and Downs...I changed out all the throttle body sensors a couple of months ago so I guessing I'm good. All my stuff is in storage as far as OHM meters and specialty stuff as I'm waiting for our new house to be finished, So I'm doing this all Ol' Skool. Will buzz back when I know for sure we're dialed in...

Pop down to Harbor Freight and grab a 7 dollar multimeter and keep it in the glove box.