The case of disappearing boost

This one has me stumped.

The truck:
'95 6.5TD KD chip, gear drive, KD exhaust, air box opened, clean air/fuel filters, and the required boost/egt gauges. New vacuum pump and belt/tensioner, and newer OPS and lift pump (both seem to be working fine), oem original turbo with approx 200k miles.

It seems that when I'm running under 2,000 rpm I have almost 0 boost. It never had a lot but would easily pull 5-7# or more if I leaned on it at all. Once I get closer to 2200-2400 rpm boost pressure shoots to what I'll call normal, 11-12# with a bit of the skinny pedal applied. Obviously, with the KD chip and no boost it rolls the black smoke like an old Mack dump truck so I would think the fuel is there, just no boost. The actuator rod on the turbo is nice and tight at an idle too as it should be.

I haven't tested the new pump but the old finger on the hose trick says that it's pulling pretty hard and smooth. Testing is in the plan for tonight.

Thoughts?

Thanks in advance
Big Green

Remove the actuator on the turbo to see that the actuator moves properly not binding and has no excessive play pulling the flat part towards you.

Check for exhaust leaks, especially on the turbo crossover pipe, a blown donut can cause these issues.

Speaking of leaks, check for intake leaks as well, especially the boot between the turbo and the upper intake manifold. Then check the gasket between the upper and lower intake manifold, these can blow out.

J

Thanks John,
I'll check the items you suggested.

I would have thought that a blown donut would make enough noise that you'd notice it but I'll check it none the less. The crossover is oem original too so while I'm under there I'll give it the good once-over.

The boot was replaced when I added the gauges, still worth the double check. Same goes for the upper to lower intake gasket.

I'd tee into the vaccum line and watch vacuum at the wastegate and compare to the pump itself. Sounds like a waste gate solenoid. It might not be regulating well to make the lower boost but can at higher rpm and more boost call out. Gotta know for sure the pump can hold steady 26"Hg iirc all the time and at the actuator it varies. Stock at 2000 rpm light throttle boost falls off and so will vacuum at actuator. KD program might be slightly different but is still PCM controlled I believe; so the solenoid must function and so must the vacuum pump / lines.

Well, I check over the crossover, donuts, vacuum lines, actuator, etc and all appears well and good.

Looks like the issue may be the vacuum pump. Gauge shows 23-24 inches at the solenoid.

So out to the garage to pull the brand new pump and back to the junk store (local autoparts) it goes.

Will keep you updated...

Thanks

23-24" is a GOOD pump. Optimum is about 25-26", but they more typically produce 22-25". 20"+ is more than sufficient. The modulated vacuum at the WG actuator is only 15" (or should be). If it's showing 15" at the actuator, your pump is not the problem.

Well new vacuum pump helped but still seems low to me.

I wd-40'd the solenoid and cleaned the manifold pressure sensor while I was there.

So, the search goes on...

Under 2000 RPM the PCM doesn't monitor the boost. It runs the solenoid at some default duty cycle (maybe 70%) and takes whatever it gets. If your wastegate was leaking or had some deposits holding it off the seat this could account for the low boost. Once it reaches 2000 (actually 1800) and the fuel rate is high enough that it could overboost (20mm3, IIRC), the PCM starts paying attention. If the wastegate leaks it will crank up the duty cycle until it sees the desired pressure. (Also, the leak becomes less significant.)

So, I'd try to get an eyeball on the wastegate seat and valve and confirm it is closing all the way.