Hey guys....I have a question go the GM diesel techs.
I have an '08 LMM CCSB 4x4 LTZ with 15K miles.

This past May while driving from Pittsburgh to Charlotte I believe my oil pressure began to run low(er) than I had previously noticed. It used to run close to 60psi while driving 75-85mph and suddenly is was running slightly above 20psi. Now I have a "flat" spot when accellerating at highway speeds...like when pulling out to pass....I have to time it because it can, and will, shititself before it gets going.
I made an appointment (back in May) with the dealer to look at it along with scheduled service and reflash. Dealer said oil pressure is "normal" $200 please.

Well I have recently been putting some highway miles on the truck and have noticed a few things:
1.) When the engine is cold and for the first 15-30 minutes of driving the oil pressure will run at 60psi at highway speeds and there is no noticable lag when accellerating at highway speeds. The truck runs fantastic during this period. At somepoint the oil pressure will begin to run in the low 20psi range and the engine seems to run flatter.
2) Running at highway speeds for sustained periods of time will result in very poor accelerator pedal feel. Meaning it seems to take more pedal movement to accellerate. There seems to be a flat spot that I can easily find when cruizing around 75mph. I can feel it hesitate if I slightly press the pedal to slowly increase speed. If I pull out to pass...it usually falls on its face (sometimes it is very bad....and sometimes it is like a quick missfire) before it will accellerate and make the pass. If I ease into it it will accellerate a little better.

Has anyone seen these symptoms before? I love the truck but this is driving me crazy. My two previous Duramaxs never had these symptoms.

Mine runs high when cold too but gradually falls to 30-40 according to the guage and from what I have read here, the guages are not very accuratge, usually reading too low.
Sounds like you have a dealer with no diesel mechanic. did they at least road test it?
Somebody on this site will be able to tell you what it might be.......sounds like it might be in the turbo.

Flat spot on accel sounds like a problem, but the oil psi is prolly normal. Mine will run 65 psi cold and then drop down to about 25 when I'm driving around town and engine hot. There are quite a few threads on the oil psi; all have similar readings. For the power delivery problem, I'd recommend trying a different dealer with, as said above, a real diesel tech

Good luck

Mine is just what Chromer said !!!!!!!!!!

This is normal. If cruising at highway speeds (55mph or over) the good ole Allison doesn't allow to immediatley down-shift like it does on most vehicles. The tranny does this to protect itself.

If u do some searches in the tranny section of the forum, you'll see that it does this is quite common. This is with the vehicles that have the 6-speed Allison (LBZ and LMM) and not the 5-speed allison. (LB7 and LLY)

If u are going to be doing some fast passes just switch your transmission to manual mode and downshift yourself.

Normal Normal..... X2 on the manual downshift - or just roll the throttle into it. Oil psi normal too.

;)

X3 Plan your passes, slowly squeeze the throttle, wait for the turbo to spool a little then go for it.