I have a 97 Cummins that developed a problem with the idle that seems to be weather related. In the morning or evening when air temps are cool the truck runs just fine. When the air temp gets over 75 degrees or so then the truck idles rough and low, often dying at stops. If I shift into neutral then the idle smooths out and it runs normally.

This weekend things took another turn and the truck has no power. On a hill it will not rev. As one guys says - "this thing couldn't even get out of it's own way"! My mechanic suspects the lift pump and is concerned that in failing it could have damaged the injector pump.

The question(s) - could the idle problem and the power issue be related? Is the mechanic correct in suspecting damage to the IP caused by failure of the lift pump?

Thanks.

 

The mechanical lift pump is pretty much bulletproof. It is also very easy to do a volume test on the lift pump if you really suspect it is the problem. Do that before replacing it.

The most frequent cause of low and rough idle is that the injection pump timing has slipped. It will run better at lower temperatures because the fuel is denser and has greater viscosity and will generate a bit more power. Once it gets warm the fuel thins and the idle drops and the power goes south. Although generally not enough to create real bad drivability problems.

There is also an overflow valve on the inside of the injection pump that sometimes gets worn or breaks a spring. On the 97 that valve has a small wrench fitting that can be loosened and the valve carefully dissambled for inspection.

With all the above said, I would look to the fuel hoses or the fuel heater as allowing air leaks with the symptoms you list.

The usual cause of really hot weather fueling problems, and lack of power, is microscopic holes in the flexible fuel lines. You will not see any wetness from fuel, but air leaks in old fuel lines and mixes with the fuel. Some people do not realize that there are six lines that have to be replaced every few years. Two from the engine to the frame, one between the lift pump and the fuel heater, two on top of the tank that join that to the fame lines, and one in the tank on the pick-up.

The other thing to check is the fuel heater electrical connector. They develop air leaks in the insulation and will allow air into the fuel system. Unplug the connector and fill the socket with dielectric grease and replug the connector. If the problem goes away, you need a new electric heating unit.

Other leaks can be the primary filter is loose, or the fuel heater has bad seals.

There is also a screen in the fuel heater that requires periodic replacement. If that picks up algae and crud from the fuel you will run out of power. Although, that is nearly always found only in cold weather.

If your fuel feed is low it will seldom damage the P7100 injection pump. That is one tough pump and the standard by which all others should be judged.

Do the flow test to verify the lift pump is good, then all the inexpensive and routine maintenance issues and the problem will likely go away. Even if the lift pump is bad, do the maintenance items anyway to avoid problems in the future.

 

I've run into this several times, the spring in the overflow valve gets weak and it's springiness (for lack of the correct word) is reduced resulting in low to no fuel pressure as the fuel temp increases.

The overflow valve is the single part that wears out most on a 12 valve and is always a good place to start. 5 minute, one wrench job to replace it.

You won't find a better price
Precision Diesel Injection and Turbo Inc.
Marty Tompkins
888-734-7349
$35 + $7 shipping and handling.

Object at lower left is the oil fill cap

 

Thanks for all the tips! It's in the shop getting the once over now. I'll post the mechanics opinions.