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OK, I've searched the forum, and I'm a bit lost. I see lots of guys with parts bazookas just blasting parts at their trucks, and I've gotten alot of conflicting info.


Ive got a 98 C3500 6.5. Bought it with 120k. At that time it had a goodwrench engine in it, unknown when replaced. Done normal maintenance, and misc repairs along the way.


Ever since we got this truck, I have thought the throttle was touchy - I'll try to describe the best I can. Empty truck, no trailer - going down a level high way, if I press on the throttle just enough to come off idle, the truck wants to gain speed - I'm going to say 2% throttle application. If I lift ever so slightly, it goes to "idle" (RPMs fall off, engine isn't making power beyond idling). Kind of a pain in the arse to drive. Pretty much acts the same way when pulling an empty car trailer. Side note - this truck is MARRIED to a car trailer, that's all she does for a living is haul cars around.



IF I load a car on the trailer, the truck becomes more palletable. The 2% throttle application will hold or slowly lose speed. If I apply a bit more throttle (say 5-10%), it will hold / start to gain speed - also the TCC will lock. If the vehicle on the car trailer is heavy (say a Trailblazer or fullsize pickup) I can actually keep the TCC locked. If the car is light / midsized, once the TCC locks up, the truck will start gaining speed, untill I remopve enough throttle application to slow down, but then the TCC unlocks.


OK, I have always just dealt with it. But now it has developed more symptoms (or a different problem). Now the truck is surging. Quite bad when cold, but still noticeable when warm. The throttle seems to be touchier now than ever - now slight application of the throttle (approx 2%) will start accelerating the loaded truck. If I ignore the speed limit, the TCC will lock up with the appropriate amount of throttle.


More history - Roughly a year ago, the LP went out. I replaced that. She gets a new fuel filter every 6000 miles. Today, I tested the LP by opening up the top vent on the screw on fuel filter, and fuel pumps out when the LP starts up(key on initially). Roughly a year ago the vacuum pump went out, so the turbo wasn't working, that was repaired (I believe unrelated). Just replaced thermostats (again, unrelated). Turbo seems to be functioning properly. Transmission seems to be functioning fine.



The APP reads what appears to be normal on our scanner (see video), so I have dismissed that. As I said before, I have searched this, and other forums, and all I can seem to find is guys with their parts bazookas just blowing parts at it - I'm undecided as to...
1)Air in fuel (haven't noticed any, probably ruling that out)
2)Bad PMD (Haven't found a DIFINATIVE reason, besides it hasn't been changed yet....:shake:)
3)IP. (see above)


I remember when these were new, dealerships were just SLAMMING IPs in these, then later found out changing the PMD would solve some problems, so that began to be the 'end all' to non-diagnostics.


ANYWAY....
Here's a couple videos. One is myself explaining the problem (sorry, long winded). The second is a test drive. There IS a stop sign along the way, but I really tried to hold a steady throttle if I could



 

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Crankshaft Position Sensor (CPS)? It should give you a DTC, though sometimes it is not.
 

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Change out the PMD with a "Known Working" w/ resistor installed.
 

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What did you find out about this problem Crazybry17 ? I have one doing exactly the same thing.
I've been scoping the inputs/outputs from the PCM and PMD, but haven't yet seen what I think may be a cause.
It seems that most of the suggestions I read are to throw a part on it.....I'd like to diagnose it and make a proper repair the first time.
 

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What did you find out about this problem Crazybry17 ? I have one doing exactly the same thing.
I've been scoping the inputs/outputs from the PCM and PMD, but haven't yet seen what I think may be a cause.
It seems that most of the suggestions I read are to throw a part on it.....I'd like to diagnose it and make a proper repair the first time.
Welcome to DieselPlace.

Unless you have an Oscilloscope the only test you can do for the PMD is to replace it with a "Known Working" PMD.
Two known conditions cause uncontrolled RPMs:
Faulty PMD
Armature within the IP is worn= time to replace the injection pump
 

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Thanks for the response OkDually.
I do have a scope and have been looking at waveforms, but this is my first experience with the 6.5.
This is waveform of the PMD output to the armature at idle...
639200


If you have experience with this, I'd love to hear your thoughts!

Mark
 

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Thanks for the response OkDually.
I do have a scope and have been looking at waveforms, but this is my first experience with the 6.5.
This is waveform of the PMD output to the armature at idle... View attachment 639200

If you have experience with this, I'd love to hear your thoughts!

Mark
The part of the PMD that will fail is the amplification portion of the signal within the transistors
 

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Seconded, looks like classic spun armature failure to me.
Hi Texas Diesel Guy...thanks for the input.
Can you explain spun armature? Does something bind on the poppet valve or armature pin and cause damage?
It also has a slight surge at idle. Almost sounds like a misfire, but if you load it just a little (engage the clutch slightly with a gear engaged or even just push the shifter against the 4th gear synchro with the clutch pedal released) it smooths right out. Seems like it's having a little trouble regulating injection quantity at idle?
Again, thanks for your input.
 

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If idling in park. Can you hold a steady 1200 RPM without surging?
 

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If idling in park. Can you hold a steady 1200 RPM without surging?
Hi Tommygunner.....no. It's manual trans. If you touch the accel. pedal lightly to raise RPM a little, as soon as it leaves idle it runs away unless you release the pedal immediately, then it'll come back to idle.
It does have a surge at idle.....feels almost like misfire except if I load it very lightly by engaging a gear and releasing the clutch pedal a little or simply pull the shifter against the fourth year synchronizer to load it s little, it smoothes right out.
 

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Hi Tommygunner.....no. It's manual trans. If you touch the accel. pedal lightly to raise RPM a little, as soon as it leaves idle it runs away unless you release the pedal immediately, then it'll come back to idle.
That is usually a sure sign the armature in the IP is bad. I think it is time to replace the IP.
 
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What happens is the screw that holds the armature plate to the pin gets loosened and allows the pin to rotate and the plate to wobble around. After that, fuel delivery control becomes erratic. In my experience, this is only happens with extreme high mileage or water in fuel.
639267
 

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What happens is the screw that holds the armature plate to the pin gets loosened and allows the pin to rotate and the plate to wobble around. After that, fuel delivery control becomes erratic. In my experience, this is only happens with extreme high mileage or water in fuel.
View attachment 639267
Man, I really appreciate the detailed info. I'm shop foreman at MB dealership and always want to know the root cause and make a proper diagnosis first time if possible. You sharing your knowledge is greatly appreciated.
This thing has close to 200 on it, a retired work truck with an altec bucket on it, so I don't know history. Belongs to a neighbor/friend who uses for projects at his place.
My next question (if you don't mind) is can you recommend a dependable source for a replacement pump. There are bound to be shoddy rebuilds out there and I don't want one. Also can you advise what else I should consider (additional repairs accessible at that time, etc.) as I go about replacing the pump.
Kind regards,
Mark
 

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Man, I really appreciate the detailed info. I'm shop foreman at MB dealership and always want to know the root cause and make a proper diagnosis first time if possible. You sharing your knowledge is greatly appreciated.
This thing has close to 200 on it, a retired work truck with an altec bucket on it, so I don't know history. Belongs to a neighbor/friend who uses for projects at his place.
My next question (if you don't mind) is can you recommend a dependable source for a replacement pump. There are bound to be shoddy rebuilds out there and I don't want one. Also can you advise what else I should consider (additional repairs accessible at that time, etc.) as I go about replacing the pump.
Kind regards,
Mark
You can browse our supporting vendors:
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Other sites that supply 6.5L parts:
Leroy Diesel
Badger Diesel
Quadstar Tuning
 

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Did a new IP fix your issue or no ?
Got a picture of this failed armature part ?


Having a similar issue, appreciate if you followed up with the help given here.


Thank You
 

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1995 Chevrolet Silverado Z71 6.5 TD
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Two owners same issue on one post.
 
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