m923a2 Turbo/Boost problem.

[Lccountryboy89]

Former deuce owner who just picked up a 91 M923a2 with the turbo Cummins in it. Drove it home from Charleston and didn't notice the problem until I started hitting hilly areas around my home state. The truck has a boost gauge and will put out 25-30 lbs of boost. When going into the bottom of a hill truck will have full boost but prob halfway up the boost will fall to 5-10 psi and have very limited power and struggle to keep 40 mph.. Truck will keep constant RPM so don't think it's a engine issue, but rather a turbocharger issue. Will build boost just fine but won't maintain it for some reason... Any thoughts/suggestions?

Thanks

 

[74M35A2]

It is not a boost problem. The truck is starving for fuel. Replace both fuel filters (if you have the frame mounted one). Also check the boost reference line going into the injection pump, some have had leaks which will under-fuel the engine during such conditions. You could also install a fuel pressure gauge at the top port of the mechanical lift pump and check fuel pressure at various conditions. Make sure your tank vent is clear and blow your air filter out with compressed air.

Burning fuel is what drives the turbocharger and hence produces boost pressure. Check the above points and let us know what you find.

 

[doghead]

Agreed, change the fuel filters.

 

[WillWagner]

If the rpm is staying steady and road speed is falling off, you have a trans issue. When road speed falls off, the engine rpm should follow it to the downshift point. if you start up a hill and the rpm is say 2000 and as you climb the rpm stays at 2000 and the road speed falls there is a drive issue. the manifold pressure gauge will fall off because the engine isn't working, the afc part of the pump is seeing no need to increase fuel.

 

[Lccountryboy89]

Sounds good plan to do a full fuel filter change ASAP, hoping that's what it is. Would make perfect sense though! Guess I just skipped over that part since the engine seemed to be running so good all the other times, but that makes sense when it hits the hills and goes under a load and needs more fuel it's starving, which leads to the drop in boost... Will check/change all filters and see how that goes!

 

[wreckerman893]

The air cleaners are very restrictive on these vehicles. There are several threads on modifying them. Opening up the exhaust can help also.