Bucking, surging or ??

[96ccdd]

Well towards the end of the summer last year I had posted about my 96 bucking or surging once it got over say 65 mph on a down grade. I replaced the fuel filter, relocated and installed a new pmd ( under front bumper ) and ran a few tanks with a cleaner in it. I thought the problem was gone ............. now that the outside air temp is starting to increase the problem still exists. What is odd is in the am the air is cool and the truck appears to be fine, but when I head home in the afternoon when the temps are up she starts the bucking, surging or whatever you want to call it. This problem feels like when I had a pmd go south but this only happens when the outside air temp starts to creep up. Any ideas ......... oh the truck has 165K on the clock.

Thanks

[Robyn]

Any codes present??

[96ccdd]

No codes,truck runs great other than the issue noted.

[robertb6963]

I'm having the same problem. Does it happen at slower speeds also? On or off the pedal? I thought fuel filter, too, but that didn't clear up the problem either. Man I really hope its not the PMD.

What is yalls take on pulling a PMD from the junk yard?

[94 at Large]

What is yalls take on pulling a PMD from the junk yard?

Isn't that why it is called a junk yard?:D

Just kidding. I bet your chances are less than 50/50 that what you're getting is better than what you got. Best to borrow a known good unit or replace with new and use the one you have as a back-up if it turns out that isn't the problem.

[rameye]

Just for fun...How do you take your intake air?? Factory setup or something else?? Intercooler??

[96ccdd]

I have the factory air filter and intake. I have a new twist to my issue. I have a switch that controls the coolant temp sensor a per a article here. When I use the switch which advances the pp timing and bumps the rpm by about 200 rpms the surging or jerking goes away. Soo my question now is it the inj pp or the injectors ? Any ideas ?

[gmctd]

Could be the advance system is worn mechanically, allowing advance to vary due to leaking fuel pressure around the piston\bore - could even be the camring bore is worn - housing clearances in the max advance position would be less worn than normal operating position

Scantool would show this as Measured timing variations when Desired timing is stable.

Clearances will increase as temp increases.

Prolly time for a replacement IP

[96ccdd]

GMCTD,

Thanks for the feedback. I was thinking it maybe the inj pp but wanted to be pretty darn sure before I pluked down $$ for a new one. I see if I can borrow a scan tool and check it out.

Thanks