Is this PMD failure?

[Lightyear]

Greetings, all...
Driving home in 100F conditions in ATL, the Burb was already at operating temp. Started bucking, coughing, losing power, wouldn't hold throttle intermittently. Pulled off, shut down and started several times. Sometimes it would stall immediately after start. Waited a bit for AAA, it started again and idled okay but I was scared to drive it on the I-road. Is this PMD failure? The Burb's got 145K on the clock now, and the IP was replaced under warranty around 40K for the original owner. And of course, I'm getting ready to take a new job in Texas, planning to drive to San Antonio with a small U-haul in tow. I've got a little time to make repairs, want to make sure I'm doing the right fixes.

Thanks as always!

[john8662]

It's either air in the system or a PMD failure.

The PMD on my 1996 has bit the dust and it'll run fine under certain conditions but it's failing. I've noticed that I'll get a slight buck (mostly in town driving) and the petal will sometimes get un-responsive and will return back to near idle. Usually when the petal gets un-responsive it's really nearing a total stall, so when I hit the petal again it'll usually just up and die. Most times it'll just restart and run for a little longer, then do it again. Another bad deal is that a failing PMD is no kidding matter, the one on mine has also acted up at a stop light where it wanted to throw fuel at the engine, fortunatly I was firmly on the brakes, hasn't done that but once. I'm putting a new one on this weekend.

I had a fun time getting home one day this week taking residential roads home because it was shutting down so frequently.

So, before coming down here to Texas, I'd get that fixed either by replacing the PMD on the pump with one of the new Grey PMD's or get an external cooler and new PMD from Kennedy.

J

[Robyn]

X2 :)

Myself I would use a DTECH PMD
I have nothing good to say about Stanadyne after the flogging they gave the public for years.

The Dtech will plug right into the harness.

Be sure, if the PMD is still on the IP to get an extension harness and a fresh Number 5 resistor to go into the plug socket on the PMD

The resistor will only fit one way into the PMD and presses down over the pins. The cord plugs in and locks.

Mount your fresh PMD in a cool place, either in front of the header/core support behind the grill or down on the inside of the fold down skid pan right under the radiator.

Be sure to use heat sink paste on the PMD before you bolt it to the finned heat sink.

Good luck

Missy

[ronniejoe]

It's either the fuel solenoid driver or the pump itself. Start with the FSD, cause it's cheapest and easiest to change.

[Lightyear]

Thanks, everyone. I did change he fuel filter about 10 days ago, and bled it out as usual, the truck had been running fine after that. Once I got stuck, I let it cool a bit and was able to start and move about 300 yds before it went berzerko again. I was guessing with about 100K on the current PMD and hot as H-E-2 sticks conditions in the ATL, it's finally given up the ghost. I'll be ordering from DTech today, and will bleed the filter again. Will let you know how it pans out.
Again, thanks so much for your help.

[DmaxMaverick]

Curious.....

If bleeding the filter helps, the problem isn't the PMD. If replacing the PMD fixes it, then why the need to bleed the filter? If you bleed the filter, and it did, in fact, need it, then that is a problem needing attention, regardless of any other problem you may have. One has nothing to do with the other.

If you experience this problem and find air in your filter, I suspect the PMD/FSD is not the problem. You likely have a fuel lift pump not pumping, and a fuel system leak (pulling air into the system). This is not to say you don't need to replace the PMD, because they do fail, but these are two separate systems, with unique failure symptoms.

Or, do what too many folks do, and throw all the parts at it, and never know what the problem actually was, and toss good (expensive) parts away. Good advice is only good if it is accurate. Advice, good or not, will never replace a proper diagnostic process. One is a coin toss, the other is absolute.

[Lightyear]

Good point. I bled the filter from the screw in valve until I got straight fuel without bubbles, and it ran fine for about a week. So that's probably not the deal, since I have never experienced anything like this before. Thanks for the voice of reason...

[DmaxMaverick]

It's simple. If fuel system bleeding results in an immediate improvement, you have likely narrowed the problem to the correct system. Unless you have other, different issues, continue diagnostics on that system until it is corrected. If you correct that problem and continue to have issues, begin a NEW diagnostic process. If you got, and/or are getting air in your fuel system, you MUST correct that before moving on to any other system. No matter what else you fix (whether it is broken or not), the original problem remains, and you will never know what "fixed" the problem, which leaves you open to a more catastrophic failure later. If you are getting air in the fuel, find out why, and fix it. Move on to other problems after, not before or during. To do anything else will likely result in wasted time, effort and money. You can rely on luck, but luck isn't very reliable. I'd rather be lucky than good, but that option is never available, to me.

While a PMD failure can cause the symptoms you experienced, there are other failures that are significantly more likely. This type of PMD symptomology is extremely rare, while it is quite common for fuel system problems. Most problems are much more simple than most people insist they should be.

[Dogbone]

Ok I am new here but came across this thread and have a thought or two for you.

this caught my attention with your problem

" I let it cool a bit and was able to start and move about 300 yds before it went berzerko again."

this sounds like your fuel filter refilled with fuel after you turned on the key which starts your fuel pump before you start the engine. then you drive a bit and your filter empties again.

I would look at your fuel pump being weak so while under a load it empties the filter and then your rig dies. was your rig a bit hard to start when you restarted it? doing this too much can hurt the injector pump.

another thing you said is you changed the fuel filter a few days before. this may sound all too obvious but did you make sure the old gasket did not stick on the mount and then end up with a double gasket on the fuel filter this could lead to air being introduced to the system.

ok off too lurking in my corner again.;)