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Discussion Starter #21
A couple other notes:

PMD harness ground wire at the engine is good, measures solid short to battery negative. What others should I check? I'll look up the PMD pinouts and make sure it's properly grounded at the PMD.

Fuel cutoff solenoid (vertical, at front of pump) tests 20ohm, is getting power, and one can hear it move when power is applied. Whether it's doing the right thing internally is unknown.

Is there a way to jury rig the fuel metering solenoid (i.e. throttle, back of pump) to force some fuel? I'd really like to see fuel come out of an injector line (got one loose/open).

I'm not sure how the timing stepper motor can affect things.

I'm diving into the DS4 Manual for answers, but please let me know if you do.


Also, if one has some sort of OBD1 Scanner, will it tell you more than the code throwing system?

Lastly, I wonder if this ECM has been reprogrammed, given the odd sequence of codes. I know this truck has been souped up somewhat by previous owner, just don't know exactly what and how.
 

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A couple other notes:

PMD harness ground wire at the engine is good, measures solid short to battery negative. What others should I check? I'll look up the PMD pinouts and make sure it's properly grounded at the PMD.

Fuel cutoff solenoid (vertical, at front of pump) tests 20ohm, is getting power, and one can hear it move when power is applied. Whether it's doing the right thing internally is unknown.

Is there a way to jury rig the fuel metering solenoid (i.e. throttle, back of pump) to force some fuel? I'd really like to see fuel come out of an injector line (got one loose/open).

I'm not sure how the timing stepper motor can affect things.

I'm diving into the DS4 Manual for answers, but please let me know if you do.


Also, if one has some sort of OBD1 Scanner, will it tell you more than the code throwing system?

Lastly, I wonder if this ECM has been reprogrammed, given the odd sequence of codes. I know this truck has been souped up somewhat by previous owner, just don't know exactly what and how.
The DS4 is electronically controlled. there is no rigging nothing. You'll end up turning your IP into a paperweight for the desk...
OBDI systems are bare bones. A tech I or comparable scanner will suffice.

Don't go opening the injector fuel lines you will introduce more air into the system and make things harder for you..
If you must, only open the #1 line. You will only see a dribble of fuel during normal operation.
Look on the ECM if it has been tuned you will see it on the EPROM or maybe a visual sticker placed on the outside of the unit.
 

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On a 94 the LP only runs when the engine is being cranked and then when it has oil pressure, the LP will not run when the key is in RUN and the engine is NOT running, unless it has been modified or has a weird short.
The 12v. batt. circuit that powers the LP also powers the PCM (computer), any wierd stuff going on will also effect the PCM.
A fuel lubricity additive is mandatory.
What DieselPro said.
 

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Discussion Starter #24
On a 94 the LP only runs when the engine is being cranked and then when it has oil pressure, the LP will not run when the key is in RUN and the engine is NOT running, unless it has been modified or has a weird short.
Ah, that's good to know. That's basically what it's doing, though occasionally I can still hear for a few sec's after I stop cranking. Maybe the oil pressure sensor takes a bit of time to fall off.

A fuel lubricity additive is mandatory.
Can you elaborate?

Thanks!
 

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Oil pressure will keep LP running for a bit.
The diesel fuel we have now has practically no lubricity, no sulphur.
These fuel systems need some lubricity, so in my case I use 2-stroke oil.
Opinions on additive type vary but anything is better than nothing.
 
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Discussion Starter #26
Oil pressure will keep LP running for a bit.
The diesel fuel we have now has practically no lubricity, no sulphur.
These fuel systems need some lubricity, so in my case I use 2-stroke oil.
Opinions on additive type vary but anything is better than nothing.
Interesting, I did not know that. What is the "recipe" you use for the 6.5?
 

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On the old mechanical IP's I ran any oil I had, used oil and tranny fluid (filtered) and/or 2-stroke oil.
The DS4 IP fuel needs to be fairly clear due to the OS so I use 2-stroke, 10-16oz. when I fill,
and then add some Marvel mystery oil now and then.
Bio diesel seems to clean things up.
I have not had an IP fail yet, most had or have over 200k on them.
Do not run vegetable oil of any kind.
 

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You can put clean/new non-detergent oil also in the 6.5 mix with diesel.
Amount depends on the tank size or how much you need to fill up.
 

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Bio Fuel, vegi oils, motor oil, ect. Fuel Injection shops love for you to use them...they need the work.

Best to use fuel additives intended for diesel fuel and only use Diesel Fuel in your diesel.... Cost is not even a factor... You just can't beat good ole diesel..
 

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Try a good known PMD. Then if all else fails try DieselPro Gremlin Remover.... Still think it's the armature screw.
 

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Discussion Starter #31
Still think it's the armature screw.
I'm inclined to believe you.

So far:
  • fairly confident the lift pump is providing ample fuel to the IP since pressurized fuel does come out of the right hoses when cranking
  • fairly confident the fuel shutoff solenoid is working since I can hear it move
  • know that no fuel is coming out of the IP
That leaves the possibilities:

  • PMD not sending pulses to the fuel metering solenoid (either bad PMD or no signal from ECM)
  • fuel metering solenoid and/or mechanics associated with it are broken (e.g. armature screw)
  • high pressure pump in IP broken
How can I test any of these three without major surgery? I'm going to try to throw a scope on the PMD output to the metering solenoid.
 

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having issues with TDCO

They say it's listed in the faqs but that's like looking for a needle in a haystack

It's just a way to eliminate pesky quirks that keep the 6.5 from running.... Invented by me
 

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Discussion Starter #35
Thanks, DieselPro. So if it fails the gremlin test, then it's still either the PMD or the pump itself, but that's it.

I'm in electrical engineer, so I'm going to throw a 'scope on the PMD signal from the ECU (or whatever the acronym is), and the PMD signal to the fuel metering solenoid. That ought to be revealing I think.

I found a schematic for the PMD in case anyone is really curious. http://10000cows.com/PumpDriverOriginal.pdf

I'm still figuring out some of it, but mostly it's just a power amplifier go-between between the ECU, which is sending it pulses, and an output for the fuel metering solenoid, with some voltage safety circuits (so that the pump doesn't go nuts when power is shut down for instance, or start pumping fuel before the power is up when you turn the key and the vehicle is still "booting up"). It also has a circuit that measures what's happening on the fuel metering output (which is an high impedance amplifier in the PMD and a coil/inductor and some sort of contactor in the pump). That circuit feeds back to the ECU on a different wire, which uses the information to make sure the metering is what it's supposed to be.

Things like the optical sensor and whatnot hook directly to the ECU and the PMD has nothing to do with that. However, if the optical sensor is disconnected, the system is supposed to operate in some sort of limp mode.

Also, the fuel shutoff solenoid runs off the same wire that provides power to the PMD, but the PMD does not control it. It's "Hot on Run", i.e. key-switch controlled 12V.. The PMD does have a voltage protection diode in case wires get crossed on it's power supply.

My understanding is that there are later versions of the PMD (i.e. different circuits inside the box), but they still are going to be functionally equivalent.

FWIW.

If there's any errors you see in the above, please post so I can correct.
 

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Thanks, DieselPro. So if it fails the gremlin test, then it's still either the PMD or the pump itself, but that's it.

I'm in electrical engineer, so I'm going to throw a 'scope on the PMD signal from the ECU (or whatever the acronym is), and the PMD signal to the fuel metering solenoid. That ought to be revealing I think.

I found a schematic for the PMD in case anyone is really curious. http://10000cows.com/PumpDriverOriginal.pdf

I'm still figuring out some of it, but mostly it's just a power amplifier go-between between the ECU, which is sending it pulses, and an output for the fuel metering solenoid, with some voltage safety circuits (so that the pump doesn't go nuts when power is shut down for instance, or start pumping fuel before the power is up when you turn the key and the vehicle is still "booting up"). It also has a circuit that measures what's happening on the fuel metering output (which is an high impedance amplifier in the PMD and a coil/inductor and some sort of contactor in the pump). That circuit feeds back to the ECU on a different wire, which uses the information to make sure the metering is what it's supposed to be.

Things like the optical sensor and whatnot hook directly to the ECU and the PMD has nothing to do with that. However, if the optical sensor is disconnected, the system is supposed to operate in some sort of limp mode.

Also, the fuel shutoff solenoid runs off the same wire that provides power to the PMD, but the PMD does not control it. It's "Hot on Run", i.e. key-switch controlled 12V.. The PMD does have a voltage protection diode in case wires get crossed on it's power supply.

My understanding is that there are later versions of the PMD (i.e. different circuits inside the box), but they still are going to be functionally equivalent.

FWIW.

If there's any errors you see in the above, please post so I can correct.
The most common failure of the PMD is the amplified portion of the signal
 

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FSO is controled by a ground from the PCM, the blue wire.
 

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Discussion Starter #38
FSO is controled by a ground from the PCM, the blue wire.
So the driver is on the low side? Could be I suppose.

Looking further, it also appears that there is some sort of feedback that happens on that wire at TDC which disables the circuit (i.e. turns the fuel pulse off, in addition to the feeding back to the ECU). In other words, the ECU initiates the pulse in advance of TDC, and a TDC condition inside the pump (some contact?) disables it. I'm not 100% certain though--it's a kind of gnarly circuit.

Additionally, the ECU separately is tweaking the pump's perception of TDC with the stepper motor through some means which also includes the optical encoder. It's possible that it's looking at the optical encoder for that and looking for hiccups of sorts in that stream of pulses.
 

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Mine's a 94 also. Glagulator was exactly on point about the OPS and lift pump run conditions. Never hurts to do the OPS bypass and/or replace the OPS (or carry a spare), but if you're getting decent fuel delivery at the T-valve water drain, your OPS and lift pump are working. You can always jump a couple of pins at the lift pump solenoid plug on the firewall if you need to check lift pump operation. The feed is always hot, so the lift pump should run when jumped no matter what. And it's much easier to hear it ticking when the motor isn't running. (Make sure you jump the right pins!)

I wouldn't eliminate the possibility of the main fuel shutoff solenoid being the culprit based solely on hearing some noise from it at key-on/off. Mine still sounded more or less functional after causing stuttering problems at speed for months while I chased down all kinds of suspected causes, then finally resulting in a no-start/no-fuel condition. I could still hear it clicking at that point, but it sure wasn't letting any fuel through. Started right up and ran like a champ once I gutted the solenoid body. Only took me half an hour to resolve it after many many hours of researching and testing potential sources of the problem. An FSO solenoid failure wouldn't explain your runaway fueling symptom, but it sure could account for the no-fuel symptom. I would suggest swapping in a replacement (~$50) since it's a very good spare to carry anyway, or gutting the solenoid body if you don't want to spend the cash on a replacement, just so you can eliminate that as the source of the no-fuel problem. A helluva lot cheaper than a pump. And that spare will come in handy (even if only for future troubleshooting) sooner or later, if that's not the source of the present problem. Pulling the solenoid off of the pump and observing the plunger/seat movement during key-on/off might well tell you most of the tale without swapping it out or gutting it, but you can never be 100% certain what its doing once it's installed and you can't verify it's operation. I actually keep a new tested spare AND a gutted unit now so I can absolutely eliminate that as a problem in the event of no-fuel/no-start problems. Right next to the spare PMD. Lol.

Then at the very least you can cross that off the list of potential causes with certainty. And maybe you'll get lucky like I did.

If you are seeing no air in the return line, and no fuel when cranking with at least two or three injectors slightly loosened, then your pump is not pushing fuel. Doesn't automatically mean it's a bad pump, but that should address your earlier question about how to determine if any fuel is making it past the pump. Air in the lines is the only thing I've found that can interfere with fuel making it to the injectors if there's fuel at the pump inlet, the FSO is open, and the pump is doing it's job.

Oh, also, the 12-12-12 code only shows up as an "all clear" indication when there are no other DTC's found. And an OBD1 code reader will give you no further info than the manual blinking dash light method.. It's just faster. The GMTDScanTech software and an ALDL cable will give you quite a bit more info/contro however. But its not a magic bullet, it won't interpret the data for you aside from a capsule description of each of the code definitions, which is no more info than you can find in any online table.

I've used 2g of B99 as a lubricity additive per 40g fillup of pump diesel for years now. Works well, and has the added benefit of being an effective solvent, which tends to help keep the fuel system clean without detergents and is hopefully extending the life of my injectors at least marginally. I have had notable fueling issues when using straight B99 or even 50/50, presumably due to OS objections.

Hope some of this helps at least a little.
 

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Discussion Starter #40
Mine's a 94 also.... <snip>
Thanks redshift9. Definitely helpful, thanks for the extensive writeup.

Indeed, I can't rule out the fuel shutoff solenoid just on the sound alone, and I agree as well that it doesn't explain the runaway.

I do get ample fuel coming into the pump body (and out of the bypass), so air can't be it.

Good to know about the 12-12-12. I guess the shop had the GM scanner you mentioned and it must have revealed more.

I still haven't been able to throw a scope on the PMD to definitively eliminate it, but I'm betting it actually is working.

So that leaves a combination of the fuel shutoff solenoid and the fuel metering solenoid's functionality.

...unless there is physically something wrong with the pump itself, but again, that also doesn't explain the runaway.
 
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