FSD Masking Bad IP?

[DA BIG ONE]

Any of you know how to tell if FSD change is really masking an IP going bad?

Had a stalling issue, checked lif pump was ok, then plugged into one of my spare FSD's w/cooler and runs fine now. There is an occasional surge at idle, and just off idle with what seems to be a miss on rare occasions at speed. Truck runs fine.

[Robyn]

Really tough to tell without yanking the pump and having it run up on a test bench.
The optical sensor could be getting flakey and or the fuel solenoid itself.
How many hours on the IP since it was shopped??
Generally from what I hear from the diesel pump shop here in Portland that usually DS4's dont wear out but usually suffer the PMD failure and the smaller shops and GM dealers trade them for a fresh one and the IP itself is usually in good shape.
Seems someone posted a tid bit recently of a wiring/connection issue at the PCM or some other point that was causing an issue that would mimick the same symptoms you are seeing.
Poor grounds will certainly foul things up

Robyn

[jspringator]

Mine has always had an occasional surge in idle. If I recall correctly, it got worse before I had to replace the FSD due to stalling.

[rjwest]

my failing IP , along with 2 bad PMd's gave me fits.

Finially I had enough good PMD's to determine my IP was
causing quits/stalls when it warmed up from below freezing
temps....also it was rough running at low rpm, right after the TC locked up, above 1700 rpm ok...

Gomers Diagnos over the phone proved correct,
Rocky mountain diesel replaced the pump, and
BIG differance,& no more stalls with 2 PMD's that had been running on the old pump.

So , in my case, IP was cause of 'quits' similiar to PMD
failure.

[JohnC]

If I understand your question correctly...

A good PMD can't make a bad pump run better unless the PMD was bad also.

[DA BIG ONE]

If I understand your question correctly...

A good PMD can't make a bad pump run better unless the PMD was bad also.

Well, this morning I startup here (temps in upper 70's) runs like crap like major misses, when I put into gear it surges (seems to be smooth) to 1000+rpms then bogs when I give it fuel, take it out of gear and runs like crap.

Yesterday, I changed out a #9 PMD resister to a #6 then did a top dead center relearn ( forgot I had the #9 installed from some tinkering awhile ago). Thruck ran fine when I test drove it. Wondering if I screwed up the TDC relearn would this happen? Turned key on after warm up, held throttle to floor for 45 seconds, released throttle, turned key off for 30 seconds then restarted, it actually ran better than before the resistor change out, returned home after test drive, shut it down for the night.

STUMPED

[DA BIG ONE]

I decided to go back the #9 resistor in the FSD/PMD truck starts and runs ok.

[DA BIG ONE]

After changing out the FSD I find out the unit that would not work on my motor was ok, was even installed on another truck and has clocked 3,300+ miles w/o any issues.

My burb has been running fine, great highway milage when driving conservatve, poor milage but awsome performance when I put my foot into it.