No shift, engine miss, and crazy tach!

[OKGMC4]

I dropped the truck off at the tuners place a couple days ago to get the ecm worked over and he calls me last night and says everything went pretty good up until the end where he says it won't shift past second gear, has developed an engine miss at 5200rpm, and he wants to know why my tach shows idle at 1500 and redline at 8000 rpm. The truck has never done these things before, and certainly not in the same day. He's telling me the trans must be shot and it needs a $900 rebuild. Could this be a bad ecm or ground issue? I need to go look at it but I just can't imagine all these things (that are ecm related) happing at one time on a 5.3/4L60E with 65k on it. Any thoughts?

[68GMCCustom]

I hate to ask, please forgive......but how knowledgeable is your tuner?

[BR3W CITY]

....So he just ran it up to 8k, and then noticed that it shouldn't have run that high?

D.A.F.U.Q did he do to it? Did you ask about any codes etc?


TPS can cause issues with high idle and a miss can cause the engine to lock into limp mode which would stick it in 2nd or 3rd (can't remember). It would be throwing some sort of codes....and for god's sake the person PLUGGED DIRECTLY into the computer should be able to "see" something obvious going on.

If the internal ground on the pcm is an issue, you can ground the case directly and see if there is temporary change.

And what "Happened just before the end"? How can it run and tune fine....but then just have problems?


This is why I stand there and watch the tune happen live on the dyno, and then do a road test WITH the tuner plugged in checking shift points and load.
Even if it was a random catastrophic failure, a good tuner should be able to give your a rough idea of what was happening....like "it was running really lean" or "we were having problems with torque management" etc etc etc.... telling you "idk but your ****'s broke" is not an acceptable answer.

[OKGMC4]

Ok I now have answers...for one the tach signal multiplier was set to double. Good if you run a factory v8 calibrated tach but bad if you have an aftermarket tach already set to 4 cylinder mode. Easy fix.

The misfire he saw is random and only shows up in one frame out of ten. So sometimes, for a tenth of a second, above 4200rpm, it shows a misfire. Bad spring or rocker would be constant and run like crap all the time. You can't even feel it but efilive caught it. May or may not be a bad plug or wire. Wouldn't be surprised there. I had to replace two of these NGK plugs new out of the box that wouldn't fire at all.

And lastly the trans is done. I drove it home (only two miles) and it lost second gear as well on the way. The tranny guy says my low mileage means nothing when it sat on the garage floor for 5 years. He thinks the seals dried up and the trans couldn't hold pressure right. He swears the pump must be out. I'll pull the trans myself and save some cash on the rebuild but this sucks. I'm planning on making a rod run from Oklahoma to Tennessee in about 4 weeks and need to get it back on the road.

[BR3W CITY]

Originally Posted by OKGMC4

Ok I now have answers...for one the tach signal multiplier was set to double. Good if you run a factory v8 calibrated tach but bad if you have an aftermarket tach already set to 4 cylinder mode. Easy fix.

The misfire he saw is random and only shows up in one frame out of ten. So sometimes, for a tenth of a second, above 4200rpm, it shows a misfire. Bad spring or rocker would be constant and run like crap all the time. You can't even feel it but efilive caught it. May or may not be a bad plug or wire. Wouldn't be surprised there. I had to replace two of these NGK plugs new out of the box that wouldn't fire at all.

And lastly the trans is done. I drove it home (only two miles) and it lost second gear as well on the way. The tranny guy says my low mileage means nothing when it sat on the garage floor for 5 years. He thinks the seals dried up and the trans couldn't hold pressure right. He swears the pump must be out. I'll pull the trans myself and save some cash on the rebuild but this sucks. I'm planning on making a rod run from Oklahoma to Tennessee in about 4 weeks and need to get it back on the road.

Are u taking the run to Pigeon Forge? I may be down there as well.

[OKGMC4]

If I can get there in my truck, yes. If we have to go in my moms Challenger RT, yes. Been the last three years and have loved it.