skipping on deceleration?

[drgerry]

96 GMC 6.5; 275K

Love my truck, but don't drive it much anymore. Last few times I took it out it would occasionally skip (kick - sputter) but only on deceleration. Seems to do it more when I accelerate hard then let of the throttle.

If I put it in N and coast, it quits. If I give it a little throttle it quits.

Doesn't skip at idle, or during acceleration (still has good power).

PCM going maybe? It's been several years since I replaced it, although it's on a heat block.

Any suggestions as to where I should start? Never had this before . . . . I'm at a loss . . . .

[Robyn]

Hello
Welcome to TDP... :)

Has the check engine light come on ???

If the light has been on get the codes.

If no codes showing then the PMD (MOUNTED ON THE COOLER BLOCK) is a suspect.

Any other anomalies ????

Make sure all the battery cables (both ends) are clean and tight.
Check the voltage at both batteries... both need to read the same.

Should read about 14 volts with engine running.

Voltage anomalies can cause issues....

[More Power]

I'd also recommend spending some time in the following forum topic thread on "Stalling".
https://www.thedieselpageforums.com/tdpforum/showthread.php?t=2938

A lot of what is mentioned there will apply to your situation.

Jim

[ronniejoe]

This can happen if the FSD is failing.

[More Power]

If this truck is a 1500 series, the problem could also be related to some failure or poorly performing component in the EGR system.

[trbankii]

I was recently experiencing "hiccups" and stalling. Pulled the MAP sensor and charge temperature sender, cleaned them both with MAP sensor cleaner (https://www.crcindustries.com/products/mass-air-flow-sensor-cleaner-11-wt-oz-05110.html), cleaned (https://www.crcindustries.com/products/lectra-cleanr-3000-electric-parts-cleaner-19-wt-oz-retail-1750536.html) the electrical connections, coated the electrical connections with dielectric grease (https://www.crcindustries.com/products/technician-grade-dielectric-grease-3-3-wt-oz-05113.html?___SID=U), installed them and plugged everything back together, and haven't had any issues since. Something to try before going too deep into PMDs and such.

[drgerry]

Thank you all for your suggestions. I've been unable to get back here since my post, I apologize.

Since my original post date:

I drove it one night and left it idling while I was talking to someone. It began to kick and sputter, then suddenly began to rev up, 4000+ RPM. My wife was in the truck and turned it off.

when I got back to the truck, it cranked perfect but really kicked and coughed as I eased down the road and then again it began high rev's! I put it in N and cut it off and coasted for a while.

Then, re-cranked (perfect crank) and kicking and sputtering I moved on back home. But I noticed that I had NO Throttle a all. Pushing the pedal did absolutely nothing. I had to only use the erratic kick/sputter at idle to get me back home.

My thought was a PMD (FSD) also. But wanted my expert friends to reassure me. Thank you.

Cheap ones on eBay from Ricky's Performance Parts . . . . but no brand name. Not sure it they're any good. But he has 99.8% after 46K+ sales. Must not be a bad person.

Is Stanadyne still any good?

[Robyn]

The high reving is the least common failure of the PMD, BUT THE MOST EXCITING.

Get a fresh PMD ON THE BEAST

I prefer the the DTECH

[drgerry]

Thanks!
Yeah . . . . it'll get your blood flowing! I'm just glad it didn't blow up! HA!

Looking for one now. Don't see much DTech's. I wonder if Rudy's is a DTech?

[Robyn]

The engine will not reach dangerous speed.
Once it reaches RED LINE the ECM will shut it off.

The wild part is when it's in gear and takes off and you try to corral it with the brakes and it starts brodying around smoking the tires :eek:

This type of failure is the prime reason that the round tall shut off solenoid is on the injection pump.

That sucker shuts off all fuel flow to the pump.

I have had these take off while I was under the hood..:eek::eek::eek:

They howl, smoke up the area big time and then shut down.

https://www.ebay.com/itm/Flight-Systems-GM-6-5L-Diesel-PMD-LIFETIME-WARRANTY-Free-Resistor-2-Day-Air/131944270501?hash=item1eb87dcaa5:g:J3YAAOSwrx5UYTQ R

[trbankii]

Cheap ones on eBay from Ricky's Performance Parts . . . . but no brand name. Not sure it they're any good. But he has 99.8% after 46K+ sales. Must not be a bad person.

The PMD isn't something I'd cheap out on. Even the good ones seem to fail after time. The cheap ones aren't likely to last long - but at least a few months or years, which is long enough to get a good review.

[DmaxMaverick]

Cheap ones on eBay from Ricky's Performance Parts . . . . but no brand name. Not sure it they're any good. But he has 99.8% after 46K+ sales. Must not be a bad person.

Is Stanadyne still any good?


The PMD isn't something I'd cheap out on. Even the good ones seem to fail after time. The cheap ones aren't likely to last long - but at least a few months or years, which is long enough to get a good review.

Ebay seller feedback ratings are almost exclusively based on the seller and the process of the transaction, not necessarily the items they sell. As trbankii said, even a bad PMD will likely last longer than the feedback period is allowed, which is fairly short. Usually, if they work once, they'll work for a while. Even new from reputable retailers can be bad right out of the box, so it's really a mixed bag, and a risk in any case. If you read the feedback, you'll see that the vast majority of them mention nothing of the longevity of an item that's correct, undamaged, and delivered on time. I buy a lot from ebay sellers, and likewise, if I get the item I ordered on time and undamaged, I submit the feedback as soon as I know the item works as advertised. As far as seller-stated warranties go, unless it's a manufacturer's warranty, none of them are good after 30 days. Ebay only allows complaints within 30 days of delivery, and the seller isn't obligated to respond after that. Also, unless the seller is also a reputable retailer outside ebay, many manufacturer's don't accept ebay transaction receipts as valid for warranty service. I've seen it go both ways. Buyer beware.

[drgerry]

I ended up having to get a Dorman PMD from a local auto parts store. I need the truck and couldn't wait.

It solved the problem . . . .

Thanks to all for sharing!!!

[Robyn]

At least you know what the situation is now and in the interim you can get a Dtech or a flight systems and have in the glove box...

Which resistor did you use in the PMD plug ??

[john8662]

The cheap PMD's that are unmarked are Dorman clones. Dorman made their copy of the PMD module by having them made in China. Typical China once you make something there, they tend to make your product and sell it as their own, this is where the cheap ebay modules are coming from.


The Dorman PMD has been something I stay away from after experiencing high failure rate on customer rigs, and when they fail they do the runaway condition described in this thread by Robyn and indicated by the thread originator. They're scary and not worth the risk of wreck or engine damage to save some bucks.


Only really two options for quality PMD. The Flight and D-Tech units and Stanadyne (either black or gray).



Only issue with the Stanadyne gray unit is that is is less resilient to heat (not as thick) so it must be placed on a good heat sync out of the engine by or on the pump as designed to last. When these fail, they just do the classic stall and start back up situation.

[arveetek]

I have had very good experience with the D-Tech units myself. That's my personal preference.

Casey

[Robyn]

I will not use the Standayne units for one reason.

Back in the hay day of the 6.5 there was pretty much ZIP ZILCH NADA help from anyone when it came to fixing the PMD failure issues.

GM did little and Stanadyne was little better.

The aftermarket figured out that the PMD was the "Fuse" and the only solution with the available parts was to keep the thing as cool as possible by whatever means available.

Sure seems to me that the big boys at Stanadyne could have fixed the design flaw and made a top notch replacement available for a decent price.

Most 6.5 owners would have happily sprung for something good....

GM hatched this abortion and could not wait to off it in favor of it's next hatchling...

I will support Flight systems or DTECH on this one.

GM could have done a common rail on this engine, but went with the cheap fix.

[trbankii]

I will not use the Standayne units for one reason.

Back in the hay day of the 6.5 there was pretty much ZIP ZILCH NADA help from anyone when it came to fixing the PMD failure issues.

[snip]

I will support Flight systems or DTECH on this one.

GM could have done a common rail on this engine, but went with the cheap fix.

Pretty much my analysis of the situation as well. Further, Flight Systems is just down the road from me (Lewisberry, Pennsylvania) - actually, within a stone's throw of the high school I went to - and I'll happily support a local business that I know to be a quality operation (I know a number of people that work there). I cannot say if any of them are specifically involved in the 6.5TD PMD - I'll have to ask around as I run into people.

I also found this paper prepared by DTech about the DTech/Flight Systems PMD and Stanadyne's response to them: http://pmdfailures.com/files/DTechDSPumpPMDInformation1.pdf

One item that I noted from that paper:

GM 6.5L engine owners should be made aware that most of the issues encountered with this application are actually caused by other sources such as: faulty ignition switches, electrical connectors, oil pressure sending units, PCM problems etc.

And DTech's response:

It is curious that now that there is a an alternative product available, Stanadyne now embraces the idea that "other sources" are the problem, yet continues to build large quantities of PMD replacement modules?

So, before replacing a PMD, should one typically go through and replace the ignition switch, OPS, etc to make sure that any such problems are resolved? I know a lot of people just keep a spare PMD and switch over to it when they experience issues - which seems to rectify the situation.

[DmaxMaverick]

Pretty much my analysis of the situation as well. Further, Flight Systems is just down the road from me (Lewisberry, Pennsylvania) - actually, within a stone's throw of the high school I went to - and I'll happily support a local business that I know to be a quality operation (I know a number of people that work there). I cannot say if any of them are specifically involved in the 6.5TD PMD - I'll have to ask around as I run into people.

I also found this paper prepared by DTech about the DTech/Flight Systems PMD and Stanadyne's response to them: http://pmdfailures.com/files/DTechDSPumpPMDInformation1.pdf

One item that I noted from that paper:


GM 6.5L engine owners should be made aware that most of the issues encountered with this application are actually caused by other sources such as: faulty ignition switches, electrical connectors, oil pressure sending units, PCM problems etc. And DTech's response:


It is curious that now that there is a an alternative product available, Stanadyne now embraces the idea that "other sources" are the problem, yet continues to build large quantities of PMD replacement modules? So, before replacing a PMD, should one typically go through and replace the ignition switch, OPS, etc to make sure that any such problems are resolved? I know a lot of people just keep a spare PMD and switch over to it when they experience issues - which seems to rectify the situation.

I tend to agree with Stanadyne's response, at least in part. Problem is, the design is faulty, in that, it is so sensitive and dependent on the system components mentioned. As common as PMD failure is, it really isn't all that common. Those vehicles that have issues, usually have the issue ongoing, and not limited to the PMD, specifically. I know of several trucks (including one I had, and a bunch of commercial fleets) that still have the OEM driver in place, and happily keep on truckin', with very few, if any, abnormal issues of any kind. I don't know of anyone who complains about their unbroken truck. The trucks I know of that have continuing issues, usually have other issues, almost always among those suggested by the response or related in some way.

Stanadyne could have designed a more durable driver, but they didn't. Like a lot of engineers, they assume their design will be operated under ideal conditions all the time. Problem is, the real world disagrees, so we are here.

[trbankii]

Like a lot of engineers, they assume their design will be operated under ideal conditions all the time. Problem is, the real world disagrees, so we are here.

"In theory, there is no difference between theory and practice. But, in practice, there is." - Jan L. A. van de Snepscheut

;)

So, as mentioned, a certain number of people carry a spare PMD. Should they be going through and replacing the switches, sensors, connectors, and such? And, if those are the actual problem, why does a new PMD rectify the situation - at least for some period of time?

[DmaxMaverick]

"In theory, there is no difference between theory and practice. But, in practice, there is." - Jan L. A. van de Snepscheut

;)

So, as mentioned, a certain number of people carry a spare PMD. Should they be going through and replacing the switches, sensors, connectors, and such? And, if those are the actual problem, why does a new PMD rectify the situation - at least for some period of time?

Fault tolerance: Durability.

I think the underlying cause(s) are rarely identified, let alone corrected. A more "durable" component will often continue under the same conditions while a less durable component will fail earlier. Stanadyne's (and most other's) solution, albeit hardly a solution, wasn't/isn't to correct causal conditions, and it shouldn't be. Stanadyne answered GM's requirements. The system was designed and tested in a laboratory environment (to include prototype and test vehicles). The system was then applied to a flawed practical environment, and left to the random environmental affects of the public consumer. The two systems (the fuel injection system, and the vehicle chassis), are incompatible in practice, but it worked often enough to be called a success and marketed to the consumer. Bean counters caused the problem. Bean counters continue the problem. It is us diehard fanboys who tolerate their ineptitude.

Should all the (supposed) causal components be replaced or upgraded? I dunno. How are we to know which components, even when new, are marginal in performance, and how many and which of the system components need to be marginal before we create the perfect storm that leads to inevitable failure?

[trbankii]

Bean counters caused the problem. Bean counters continue the problem.

I've often hear that engineers should be required to use the products they design - the same should apply to bean counters by a factor of ten…

Many thanks for the input. It gives me a better understanding of the situation.

[Robyn]

I bought a brand new GMC 3500 DUALLY 4X4 in 1994.

Did not even get it home and it quit with all the indications we usually see with a dead/dying PMD

The ign. switch, connections, grounds and such were likely not the issue that could possibly show up on a rig with a few years under its belt.

We bought a new Burb 4x4 in 95 and it had the same problems.

5 pumps later under warranty we offed the beast....

I agree that in a perfect world (Test lab with unreal conditions) the system works..

Not out in the real world with BUBBA at the helm.

I suspect that voltage variability due to ????? Possible ground issues and gawd knows what else causes unnecessary stress on sketchy components that can't take the abuse and die.

A diode in the alternator that's causing a slightly hinky current.
A faulty plug pass through in the firewall.
A ground bus connection deep under the dash that has one wire that is not making a good connection.
A chassis ground cable that's not gittenerdone.
A very small short/fault in the transmission electronic solenoids.
NO WAY TO FIND THIS STUFF

Trying to wring out any of these subtle anomalies is nearly impossible considering all the components/wires...yada yada yada involved in the entire electrical system of these rigs.


Using components in the PMD that were a LOT more ROBUST likely would have cost a couple $$$ more per unit, but would live and shrug off slight issues with no worries.
During the Reverse engineering process it should have been possible to see what components were failing and then figure out WHY and build around that problem.
Build the thing hell for stout and forget the subtle issues of the real world..

Our 95 Burb would run about 3K miles and the PMD would die.

Clockwork... every 3K a fresh pump under warranty and good to go.

Once were were getting close to 3K mark we would not take the rig out of town.

Pretty soon ya had to drag it home from the grocery store.

A fresh pump and good to go.

Dealer made a million excuses and even accused us of NOT KNOWING HOW TO DRIVE A DIESEL VEHICLE :confused:

Once I joined this Forum back in 01 I think, and learned a few things we bought another 6.5 and by then the aftermarket was alive and on the ball.

Stanadyne does not have my respect AT ALL....

GM knew they had a mess on their hands and simply tried to sweep it under the rug.

LET THE FIELD SERVICE FOLKS DEAL WITH IT :eek: