Thank you all for your time!
So my son now owns and drives my old 2003 Chev duramax allison C/C 4X4. He is a student and needs a reliable cheap truck. I bought the truck new in 2002 and serviced it extremely well including very frequent drivetrain fluid changes. It currently has 354,000km.
It recently started stumbling / stalling briefly at about 80km/hr (50mph I think?) under very light load. It won't do it pulling any kind of hill or under a load.
It will do it with tow haul mode and OD cancelled but its more subtle and hard to notice. A hard throttle pull will stop the problem but as soon as you release throttle and allow the upshift it does it again. It does not care if its in 4wd or 2wd. It's intermittent and acts perfectly some days but when it starts doing it, then it does it consistently like every 5 seconds. It almost feels like the the transmission starts a downshift then it decides not to. It will sometimes (very rarely) show a very slight stumble at idle which increases with rpm. This is very rare but it if starts doing it at idle it will act up on the highway. This truck runs otherwise perfect. It starts well and pulls HARD for a bone stock all original truck.
I'm at a loss. We have tried:
1.) Having my local transmission rebuilder test drive it and run it on his snap on scanner. No Codes detected and of course it wouldn't act up for him. He recommended trans-go junior shift kit and a second trans-go kit. He said they fix most Allison issues and he highly recommends them. I personally did them both and can report that the sump was clean and the oil smelled good but was a little dark. The magnet in pan had very light shiny sludge. No gritty stuff. New filters X2 and new oil.
2.) Front driveshaft u joints. They were lumpy. They shouldn't be in play in 2wd but we changed them anyway. Transfer case front output shaft bearing checked while driveshaft was out. Felt smooth. Checked the rear driveshaft u-joints and bearing by turning it while it was up on stands. Everything felt smooth.
3.) Removed and cleaned the grounds at the drivers side frame rail. (this is a known corrosion point and can cause communication errors.)
4.) Replaced the fuel filter and the fuel lines on both sides of the filter. (they were cracked and the truck was losing prime -solved.)
5.) I checked the air filter. The vacuum gauge shows no restriction and the filter looks good.
6.) I ran a diagnostic scan with a friends scanner: No codes found in the system. I can see fuel rail pressures and fuel injector data but I don't know what I'm looking at. If I go into the menu for injectors I can select one injector at a time which seems to shut off that injector for a few seconds causing a cylinder miss.
Known issues maintenance factors:
reman hydro boost
reman power steering pump second one because the first failed. (It whines a bit)
Leaky steering box which caused the failure of oe power steering pump. (I think it is tired and needs replaced)
The brakes have a shimmy and need the rotors turned.
I personally put a set of Bosch reman injectors and all new injector plumbing at about 180,000 because it wouldn't start warm. (high return fuel return rates were warming fuel which reduced fuel viscosity and the engine couldn't build enough pressure to enable injectors) No problem since.
Please help me out. I'm very solid mechanically but I need help with the correct diagnosis.
Thank you again for your time.
So my son now owns and drives my old 2003 Chev duramax allison C/C 4X4. He is a student and needs a reliable cheap truck. I bought the truck new in 2002 and serviced it extremely well including very frequent drivetrain fluid changes. It currently has 354,000km.
It recently started stumbling / stalling briefly at about 80km/hr (50mph I think?) under very light load. It won't do it pulling any kind of hill or under a load.
It will do it with tow haul mode and OD cancelled but its more subtle and hard to notice. A hard throttle pull will stop the problem but as soon as you release throttle and allow the upshift it does it again. It does not care if its in 4wd or 2wd. It's intermittent and acts perfectly some days but when it starts doing it, then it does it consistently like every 5 seconds. It almost feels like the the transmission starts a downshift then it decides not to. It will sometimes (very rarely) show a very slight stumble at idle which increases with rpm. This is very rare but it if starts doing it at idle it will act up on the highway. This truck runs otherwise perfect. It starts well and pulls HARD for a bone stock all original truck.
I'm at a loss. We have tried:
1.) Having my local transmission rebuilder test drive it and run it on his snap on scanner. No Codes detected and of course it wouldn't act up for him. He recommended trans-go junior shift kit and a second trans-go kit. He said they fix most Allison issues and he highly recommends them. I personally did them both and can report that the sump was clean and the oil smelled good but was a little dark. The magnet in pan had very light shiny sludge. No gritty stuff. New filters X2 and new oil.
2.) Front driveshaft u joints. They were lumpy. They shouldn't be in play in 2wd but we changed them anyway. Transfer case front output shaft bearing checked while driveshaft was out. Felt smooth. Checked the rear driveshaft u-joints and bearing by turning it while it was up on stands. Everything felt smooth.
3.) Removed and cleaned the grounds at the drivers side frame rail. (this is a known corrosion point and can cause communication errors.)
4.) Replaced the fuel filter and the fuel lines on both sides of the filter. (they were cracked and the truck was losing prime -solved.)
5.) I checked the air filter. The vacuum gauge shows no restriction and the filter looks good.
6.) I ran a diagnostic scan with a friends scanner: No codes found in the system. I can see fuel rail pressures and fuel injector data but I don't know what I'm looking at. If I go into the menu for injectors I can select one injector at a time which seems to shut off that injector for a few seconds causing a cylinder miss.
Known issues maintenance factors:
reman hydro boost
reman power steering pump second one because the first failed. (It whines a bit)
Leaky steering box which caused the failure of oe power steering pump. (I think it is tired and needs replaced)
The brakes have a shimmy and need the rotors turned.
I personally put a set of Bosch reman injectors and all new injector plumbing at about 180,000 because it wouldn't start warm. (high return fuel return rates were warming fuel which reduced fuel viscosity and the engine couldn't build enough pressure to enable injectors) No problem since.
Please help me out. I'm very solid mechanically but I need help with the correct diagnosis.
Thank you again for your time.
I always wondered if the pedal sensor was not showing WOT. As for doing it on cruise or not I can't say because my son usually drives it. I have recently switched with him and I'm now driving it full time to try and figure it out. 