Thread: Strange shifting & Idle issue

Hey Guys,
This is the 2nd time this has happened in the past 7 months. Not sure where to start.

Car is warm. Driving casually at 50-55 and the car starts shifting from OD to D to OD to D every few seconds while under a slight load.
Pull over and the Idle is high. 1200-1500rpms. Shut the car off and restart. Idle is up to about 2K.
If I shut the car off and let it sit for about 10+ min. it will fire right up and Idle perfectly as well as shift as it should.

All tune up parts are AC Delco and have about 12K on them. Plugs wires, PVC & Fuel Filter.
TB & MAP cleaned, IAC, & TPS , replaced with AC Delco as well.
Trans fluid is full and clean. Serviced 2 years ago. 152K on car.

Has anyone had a similar issue?

a bad maf could cause this. or a cracked vac line

Thanks for the info Scotty.
OP should read:Cleaned the MAF...Yes, I used MAF Cleaner.
If it happens again, I can try unplugging the MAF connector to see if anything changes.
Will check for vac leaks as well.

Had a broken vac line at the MAP sensor cause some odd trans activity.

Vac lines checked out good.
I've had no problems for the past week until yesterday. Started doing the same thing again.
Strange, and could just be a coincidence but when this happens, the ignition wont turn to the off position to remove the key.
I usually tilt the wheel up and push the release button.
Not sure if or how this could be relevant to the idle/shifting issue...???

I actually did not change the TPS or MAP on the GP like I thought. It was on my 94 Cadillac Deville Concours N*

Could be the TPS or MAP or even a bad Temp sensor. Maybe the PCM is flaking out....
It's possible the tranny is just getting tired..
I noticed yesterday when it happened that the Air Intake sounded as if the car were cold. The shovel over cement sound that I usually only hear at cold start up. That leads me to think of the Temp Sensor. Never a CEL either.

This is from today. Seems like any more than 30 min of driving now and this occurs. Idle will be anywhere from 1K to 3K. I tried lightly tapping on all sensors when it started (maf, map, iac, tps,ait...) to see if anything changed. Nothing. Unplugged sensors one at a time..no change. Unlugged all sensors...no change. Not even a CEL. Even unplugged the EGR.

Stopped and spent a couple of hours on the beach. Fired right up, idled and shifted fine for about a half hour.

Here's a vid of the erratic shifting.
https://youtu.be/VmKVkOBIA_s


12-24 sec mark and 1:16 to end.

Tps sensor issues causes issues with the trans as well

TPS tested good.

Today I took the TB off for a more thorough cleaning. I had only cleaned it from the front a while back. Damn was it caked up. Also found that the gasket for the TB/intake was leaking slightly at the bottom. Scrubbed all and installed a new gasket.
Cleaned the MAF again and the EGR. Installed the upstream O2 I had for it as well (AC/Delco)

This issue only happens after 30-35 min of driving. Otherwise the car runs perfectly.

I stopped for gas today shortly after it started shifting funky and had been driving for about 40 min. Shut the car off long enough to pump 11 gallons of gas. About 5 min. Fired up, idled fine. Shifted correctly the remaining 10 miles home.

Where to go next....

Originally Posted by Turbocharged400sbc

You're going to need to scan the ECU while driving to simulate the condition my guess is you're going to find all sensory inputs going sporadic while it happens probably due to a bad ground somewhere and it's very possible that when the radiator fans/high loads/built up heat at resistance point... are required.
that it is killing the ground circuit for the ECU and causing a voltage drop which causes all sensor inputs to be wrong.

Good idea. That's where I'll try next. I know a Trans Specialist about 40 min from me that should have the scanner. Should be acting up by the time I arrive.

Would a bad Coolant Temp Sensor prevent the cooling fans form turning on?
Fans don't turn on with the AC either. System is charged and the compressor kicks in.
Just found this out this morning. I haven't tested the relays or fuses yet.

Pulled and checked the fuses and relays for the fans. Put them back in place. Both fans are working now when AC is on....Weird...
Temp sensor still needs to be replaced. Looks like it's been leaking a bit. AC Delco Temp sensor and 195 T-stat on the way form Rock Auto.

Originally Posted by CrazyGuy03

Tps sensor issues causes issues with the trans as well

This was it CrazyGuy03

Replaced the thermostat & temp sensor. Flush and fill. Temp gauge never goes above 210 and fans kick on at the correct time now. However, not the problem.

Let the car run in the driveway for over 30 min. Went to pick up a pizza and it started shifting screwy with high rpm's within a quarter mile. Got it home, rpm's at 2500 and shut it down.
Had some za with the family then started the car again. RPM's still over 2K. Opened the hood and pulled the TPS connector. Idle drops to about 800... TPS tested good a week beforehand. Went out for a drive. Taking it easy of course and no idle or shifting issues. Over 1.5 hrs of continuous driving and still no issues.
When this happened at the beach last week I pulled the TPS connection and no change. Intermittent sensor failure I guess? TPS on the way from Rock Auto. No one local carries AC/Delco except for the dealer.

TPS came in the mail yesterday. Installed and cleared the codes from unplugging it. Fired right up. Runs, drives and shifts so much better. It's like a totally different car.

This story is not over yet.
After replacing the TPS the car ran great for about a week then the erratic shifting again. Mother flucker..son of a biache.,,,,grrr...
I decided to replace the PCM. Found a stock 03 GT PCM that ZZP had updated with the latest from GM for cheap.
Car fired right up after install. Did the quick 10 sec on 3 sec off key learn I found on this forum. Thanks to Scottydog I think it was, who posted on it.
Running very well but....the damn Trac Off light is on all the time now. Pushing the button does nothing.
A mechanic at work let me use his SnapOn scanner. Came up with one code: P1670 that wouldn't clear. PCM not communicating with the EBCM. No ABS light or CEL either.
No erratic shifting issues at all but no TC. I swapped my old PCM back in and TC was as it should be. Swapped back to the ZZP TC off light always on
A replacement PCM from ZZP is on the way and will be here tomorrow.
Ready to do a trans fluid & filter and a TransGo kit this weekend as well.

A big Thanks goes out to member rottonj for his help and guidance over the last couple of weeks....Much appreciated.

You'll need to bring the car to the dealership and have them update the bcm to Pcm by the sounds of it. Or if you know someone local with a tuner have them out a stock file back on your stock Pcm.

Also check the wires from the tps sensor, they might be frayed or melted somewhere and when they get hot enough they ground out.

[QUOTE=Swiggles;1432384]You'll need to bring the car to the dealership and have them update the bcm to Pcm by the sounds of it. Or if you know someone local with a tuner have them out a stock file back on your stock Pcm.

My stock PCM works but doesn't have the latest GM updates. Their is no problem with the ABS or Traction Control when it's installed.

The TC is inoperable with the ZZP PCM I purchased, which is why they are sending out a replacement.(no charge) Will be here this afternoon.

Well I wouldn't be surprised if the new one from zzp does the same thing. The latest gm update has nothing to do with it. It's the way the TC modules, Pcm and bcm communicate, the Pcm and TC module don't recognize each other. At least that's how my 04 Impala works with the airbag module. If I put the spare Pcm I have that's got the same file on as my stock one the airbag light stays on. When I put the original back in the light shuts off.