was driving back up into the mnts yesterday and my truck is seriously lagging.
seems to have more or less normal power at low speeds/lower gears/lower rpms, but when I give it wot, esp in 4th and 5th, as i get up towards 2k rpm, it seems like it's not getting either fuel or air... Can't generate more than about 10# boost. Don't have a fuel press gauge, so wondering what are steps for diagnosing?

Last time this happened was lift pump. If it is the lp, this will be about the 4th lp I've changed! Any reason other than being cheap chinese parts that they would fail so often?

 

How long ago was it when you did fuel filter change?

 

My 96 acts like that when the filter is dirty.....

 

not super long, but I'll change it and see what happens...

Also, was kind of a sudden onset, not gradually over time. So unless I got a really dirty batch of fuel it doesn't seem that it would just happen overnight, would it?

 

In that situation where you notice the power loss-

do you have smoke and high EGT?

If smoky and hot + low boost -lack of air, suspect leaks in the charge air system

If no smoke and cool - lack of fuel, suspect fuel delivery.

If the fuel filter and the preheater screen check out to be clean, and you have already changed the lift pump, please take a look at the overflow valve on the P7100- a broken or weak spring in there leads to low fill pressure in the pump and this leads to pump elements not getting filled properly, especially at higher rpm.

The sudden onset points me towards something broken..

Just for grins and giggles do also check out https://www.dieseltruckresource.com/...ration-322212/

HTH

Markus

 

no smoke... and as far as I can see there are no big holes or blown boots anywhere on the turbo/intercooler side of things.

seems to point toward a fueling issue. I have already replaced my lift pump at least twice, maybe three times in the time I've owned it. Is there only only one manufacturer for our lift pumps? Wondering if there's a more reliable part without having to get some crazy expensive electronic thing?

Also have replaced the fuel heater screen and overflow valve, although it's been long enough now that I can't remember how long it's been... Maybe time for another round of new parts.

I am for sure planning the fuel line upgrade as well. I know a lot of people remove the fuel heater as well, but winters here are pretty cold, so I'm thinking I'll just rebuild the thing.

Any opinions on this product?
Larryb - ADJUSTABLE Overflow Valve for Dodge Cummins Diesel with Bosch P7100, 94-98.5

 

If you have reason to lean towards fuel supply as the culprit:

Take a fuel pressure reading- if too low use smooth pliers to crimp the return hose a bit and watch the fuel pressure- if it goes up- overflow valve- if it stays low usually lift pump.
(Shamelessly stolen from the factory service manual for a 97 12V CTD)

About the adjustable valve- never had one and I don't see the need for it at near stock power levels. A standard one will work. And it's easily checked.

If you want to check the overflow valve- I usually use compressed air with a pressure regulator. Just turn up the pressure until it starts to whistle, take a note of the reading on the gauge. Costs 2 copper washers if the valve is good.

HTH

Markus

 

Something else to check is the air line going to the AFC for holes and even the AFC itself, I had a problem like yours about a year ago and for me it ended up that I had a hole in my AFC diaphragm.

 

Also check lever for fuel shut down and make sure it is lifting completely up and not binding. If you have EGR valve make sure valve is not stuck open. Any smoke out tail pipe, white, black or blue in color? Gear maybe slipped on injection pump or injector issue.

 

One more thing to look at is the turbo bypass actuator. If the diaphragm leaks it won's open the bypass and back pressure kills the motor. A leaking diaphragm in the AFC will also affect the bypass actuator (at least it does on my '97) giving you a double whammy.

To check it disconnect the line where it T's, one line to the actuator, one to the AFC. If you can get air, it's the actuator. By the way, the mechanic that showed me that just sucked on the line, no vacuum pump needed.