intermitent shudder/violent bucking! help please?

[kevinatd]

I have a 94 ATX with about 130,000 miles on it. Fresh tranny rebuild two years ago with less than 10,000 miles on it. I have had this issue for a while now and have been unable to diagnose.
1. At normal driving speed in town, (40-45 mph, 1500-2000 rpm) has slight shudder. goes away if acelerating. (it has always had this shudder, even before tranny rebuild)
2. At idle (in gear or in park) engine surges between 800-1500 rpms.
3. intermitently starts bucking at level speeds. (I believe it is actually shifting gears) RPMs vary with bucking, sometimes actually going to redline! (shifting to first?) acelerating usually makes it go away. At interstate speeds this can be VERY violent. Usually results in the engine shutting off. After dropping in neutral and restarting, all is well.
4. as bucking starts, usually throttle response goes away and engine does not respond to pedal position. sometimes pumping the throttle regains control and normal response returns.
5. This usually does not result in any DTCs. The only code I have ever got was 551 (idle air solenoid failure?). I replaced IAC against my better judgement. did not fix.
6. I discovered wiggling ECM harness causes engine to rev then die. I cleaned the connector and pins and reinstalled. did not fix. I dont think this is the source anyway, since it does not coincide with bumps.
Ideas, anyone? DIS? TPS? ECM? I have always done my own repairs, but this one has me stumped, and I am not into parts replacing like the local repair shops.

Thanks, Kevin

 

[SuperHO]

replace your cam sensor, reset computer, check for symptoms again.

 

[Bob Gervais]

6. I discovered wiggling ECM harness causes engine to rev then die. I cleaned the connector and pins and reinstalled. did not fix.
Thanks, Kevin

I'd be looking more into this issue first. Maybe a break in a wire(s), shorted wires, etc.

Cam sensor should throw a code.

 

[kevinatd]

Let's look at this a little more scientifically. What should I look at that would cause the following problems:

1. Loss of throttle response without actually stalling. Just seems to go to idle and does not respond to pedal. Seems to rule out ignition or fuel problems? Usually pumping the pedal will restore response....As long as the following doesn't happen!

2. Improper transmission shifting. Will shift to low gear at inappropriately high speed. Something may be telling it to downshift? does the computer not know how fast the vehicle is traveling? I have had it downshift at 70 mph which caused a violent **** and engine revved to red line and immediately died. I then shifted to neutral (while still coasting from speed), restarted engine, and it ran fine for several minutes.

3. This may be related to moisture or cold? seems to happen during either weather condition. Also, these things only seem to happen when driving at a steady speed. I have never had it happen during acceleration.

4. These problems are VERY intermitent! I may go weeks without it acting up. The longest I have had it happen was over about 20 miles on the way home on the interstate. Constant speed and throttle changes kept it running long enough to get home.

So, knowing these things, what should I look at? If it is a break in a wire, where are logical places to look? What wire, if intermitently disconnected, would cause these symptoms? If a sensor, which ones, if failing would cause these problems? Could I have an ECM failing? does it control engine and transmission to the extent it could cause these problems? Would a bad transmission cause no throttle response?

I know this is rather wordy, but I am just trying to be thorough with my descriptions so you experts out there can enlighten me. Help me please, oh Gods of SHO. Thanks!

 

[kevinatd]

OK, finally acted up again today. Bucked, lost throttle, then trannny seemed to be stuck in second(?). Finally threw a new code. 634 - manual lever position (MLP) sensor - signal voltage higher or lower than expected. Ideas?

 

[sickSHO96]

My MTX does this going uphill in reverse.