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Discussion Starter · #1 ·
When I push the throttle slowly there is no response, even to WOT. If I push the throttle quickly, the engine responds normally and continues to maintain rpm’s normally. This happens when idling or driving. The idle is normal. Put in new batteries in the morning and drove half the day before this started happening. The new battery replacement was because the batteries were six years old and were barely able to start the truck. I was thinking the accelerator pedal sensor? Thoughts?

1997 GMC K3500 6.5L
 

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Any codes present that you know of?
 
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Welcome

Checked/replaced the fuel filter and Air filter?
 

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Discussion Starter · #4 ·
Sorry, I should’ve checked the codes before I posted. Yes there is a code showing up:

P0251:
Injection pump fuel metering
Control A (Cam/Rotor/Injector)

It suggests:
-Open or short circuit condition
-Poor electrical connection
-Faulty fuel quantity solenoid

Fuel filter and air filter have been replaced recently. Truck runs good when driven aggressively (quick gas pedal depressions). It just doesn’t recognize slow pedal depressions, even to wide open throttle. So I don’t think there is an issue with fuel or gas supply limitations.
It’s suspect that I just changed the batteries an hour and 20 miles before this started happening.
 

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Sorry, I should’ve checked the codes before I posted. Yes there is a code showing up:

P0251:
Injection pump fuel metering
Control A (Cam/Rotor/Injector)

It suggests:
-Open or short circuit condition
-Poor electrical connection
-Faulty fuel quantity solenoid

Fuel filter and air filter have been replaced recently. Truck runs good when driven aggressively (quick gas pedal depressions). It just doesn’t recognize slow pedal depressions, even to wide open throttle. So I don’t think there is an issue with fuel or gas supply limitations.
It’s suspect that I just changed the batteries an hour and 20 miles before this started happening.
I would inspect every inch of your engine harness for any rubbed through wire insulation, loose connectors, rodent damage, broken wire, etc
 

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Discussion Starter · #6 ·
Good idea. I relocated my PDM years ago and the cable runs next to the driver battery. Maybe it got pinched underneath the battery. I’ll let you know what I find. Thanks.
No comments on the possibility of the accelerator throttle sensor?
 

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Good idea. I relocated my PDM years ago and the cable runs next to the driver battery. Maybe it got pinched underneath the battery. I’ll let you know what I find. Thanks.
No comments on the possibility of the accelerator throttle sensor?
You would have APP codes pop up if it was the pedal
 
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Good idea. I relocated my PDM years ago and the cable runs next to the driver battery. Maybe it got pinched underneath the battery. I’ll let you know what I find. Thanks.
No comments on the possibility of the accelerator throttle sensor?
The harness from the optical sensor runs under the intake manifold.It’s subject to extreme heat and cold cycles over the years.I would replace it if in doubt.The replacement harnesses made nowadays are made longer so you can route it on top of the intake manifold off to one side away from heat trapped in the valley of the engine.

Low lift pump pressure will also produce the P0251.It happened to me a year or two ago.I checked my lift pump and it was producing very little flow into a container.I replaced it with a new old stock delco lift pump and I never had an issue since.How old is the lift pump on your truck?
 
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Discussion Starter · #9 ·
-Good point, there are no APP codes.
-I will keep in mind replacing the optical sensor harness if it comes to that.
-I replaced the lift pump 5 or 6 years ago along with oil sender. I should have installed a fuel pressure sensor at the same time. Where is the preferred place to check the lift pump pressure? Do you have to tee into the line or can you attach to a vent?

Update: As I said, I replaced the batteries yesterday an hour before this issue started happening. I went out this morning to check if there was a wire disconnected or damaged from the installation and I found the wire running to my PDM relocation was fairly crushed, not showing copper but one wire was really flattened. I think it was damaged by the previous battery install six years ago and has been under the battery the whole time. I moved the wire to get a better look at it then started and drove the truck around without issue. I will cut the wire and examine to see if the copper is broken inside.
 

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-Good point, there are no APP codes.
-I will keep in mind replacing the optical sensor harness if it comes to that.
-I replaced the lift pump 5 or 6 years ago along with oil sender. I should have installed a fuel pressure sensor at the same time. Where is the preferred place to check the lift pump pressure? Do you have to tee into the line or can you attach to a vent?

Update: As I said, I replaced the batteries yesterday an hour before this issue started happening. I went out this morning to check if there was a wire disconnected or damaged from the installation and I found the wire running to my PDM relocation was fairly crushed, not showing copper but one wire was really flattened. I think it was damaged by the previous battery install six years ago and has been under the battery the whole time. I moved the wire to get a better look at it then started and drove the truck around without issue. I will cut the wire and examine to see if the copper is broken inside.
Best way to test the lift pump:
Start and idle the engine, open the T valve for 30 seconds and let fuel drain into a container.
If during that 30 seconds the engine dies or you see no fuel coming from the drain line your lift pump is too weak to support the engine. ( replace it)
 
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Discussion Starter · #11 ·
A few strands of the wire were broken but it seems not enough to affect anything. I repaired the wire. I’ll see if the issue returns and will find my backup PDM and put it in the truck just in case.
I will test the lift pump as suggested.
I have my doubts the issue is resolved but hoping so.
Many thanks.
 

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I would replace the PMD extension harness to be on the safe side. The electrical side of the fuel system is very sensitive to resistance changes. The repair should be soldered ,shrink wrapped and water proof.
Stay away from cheap imported extensions. Leroy diesel sell a good American made extension with quality connectors.
 
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Please don't solder the wire, mechanically fasten it. Solder will break over time and leave you stalled on the side of the road. I agree with the rest of the repair statement. If you are conserned about its ability to work, put a light bulb on one side of the circuit (unplugged) and power it on the other, it should light up. If not the wire is broken. Voltage drop testing would be better way but I don't expect many would actually do this test.

I did wiring repairs at GM dealerships for 30 years.
 

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Discussion Starter · #14 ·
I already soldered and shrink wrapped it before seeing your comment. This is the great thing about talking these things out, it exposes issues that you may not be considering, such as the risks to soldering and puts things into perspective. I will just buy a good quality new PDM extension harness, of course. Not worth the risk of problems in the future.
Thanks again.
 

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Also newer solder has a very low lead content and it makes the connection brittle. GM states you can solder the mechanical connection as a glue(their words). Being its right next to the battery you may be ok as it may not vibrate and shake too much in that location. I'd keep a set of wire pliers and a butt connector in the truck just in case.
 

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Im not here to reign on anyone's parade. 60/40 solders composition hasn't changed in the 48 years I've been using it. A good mechanical connection followed by soldering is far superior with lower resistance and better conductivity. Maybe it doesn't matter in this case. Copper becomes brittle after being stressed, subjected to heat or improperly connected ie improperly supported beyond the joint. That's my learned opinion and we know opinions are like a**holes. Most everyone has one.
 

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Im not here to reign on anyone's parade. 60/40 solders composition hasn't changed in the 48 years I've been using it. A good mechanical connection followed by soldering is far superior with lower resistance and better conductivity. Maybe it doesn't matter in this case. Copper becomes brittle after being stressed, subjected to heat or improperly connected ie improperly supported beyond the joint. That's my learned opinion and we know opinions are like a**holes. Most everyone has one.
Edit: Caldiesel1, you wouldn't happen to be a oscilloscope user would you? I can't seem to find anyone that has waveforms for the 6.5. I would love to see what the PMD looks like. End the edit.

Like I said a mechanical fastener and solder that. I also have numerous types of solder, besides 60/40 there is low lead or lead free which won't hold anything but a circuit board and honestly it won't even hold on a board, terrible solder. Oh I went through SET class too,

I had a friend that used to work for the airlines as a mechanic and he told me that they never ever use solder on any wires because they needed the connections to not fail. I guess its some kind of regulation. I haven't used solder since(on a vehicle) unless it was to fix my Kelsy-hase abs module power relay that well, broke the solder joints. A resolder the relay and back in business.

Anyways, Happy Thanksgiving to all! I hope everyone has a wonderful day and gets nice and full.
 

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I do have 3 oscilloscopes but no 6.5's electronically controlled available to test. As to aircraft electrical connections not being soldered it comes down to two reasons. Primarily weight and service life. Its figured in to an aircraft's life just how long a crimped electrical connection will acceptably last. I too have a friend who worked for Boeing for 40 years. In the electrical connections they found most susceptible to stress failure on re inspection they soldered. A good mechanical connection followed by soldering is as close to a unbroken wire as you can get. The last example I'll give you is about a company who built primarily ambulances called wheeled coach. Their "claim to fame" was a "point to point" crimped wiring system using connection blocks to distribute power to the various systems. It certainly seemed like a good idea, easy to service and change out components but in reality it became a nightmare. Each block contributed to a loss of current and an increase of resistance coupled with a mechanical connection to possibly maintain. More modern vehicles use an unbroken wiring loom with not breaks to the connecting point, there's a reason for that. As I previously stated in my other post, I'm not here to one up someone's opinion. My observations are from my years of personal experience. As the warning says you're experience may vary depending on how much you ingest.
 

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I do have 3 oscilloscopes but no 6.5's electronically controlled available to test. As to aircraft electrical connections not being soldered it comes down to two reasons. Primarily weight and service life. Its figured in to an aircraft's life just how long a crimped electrical connection will acceptably last. I too have a friend who worked for Boeing for 40 years. In the electrical connections they found most susceptible to stress failure on re inspection they soldered. A good mechanical connection followed by soldering is as close to a unbroken wire as you can get. The last example I'll give you is about a company who built primarily ambulances called wheeled coach. Their "claim to fame" was a "point to point" crimped wiring system using connection blocks to distribute power to the various systems. It certainly seemed like a good idea, easy to service and change out components but in reality it became a nightmare. Each block contributed to a loss of current and an increase of resistance coupled with a mechanical connection to possibly maintain. More modern vehicles use an unbroken wiring loom with not breaks to the connecting point, there's a reason for that. As I previously stated in my other post, I'm not here to one up someone's opinion. My observations are from my years of personal experience. As the warning says you're experience may vary depending on how much you ingest.
There are a few threads with scope results. You can find them using the community search feature located at the top of your screen.
Here is one such thread: It lives again!
(y)
 
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