1994 GMC 2500 6.5L TD - weird start
 

I'm pretty new to all this stuff. I have taken a foundation course for Heavy Duty Commercial Transport Mechanics earlier this year, but other than that my knowledge is pretty limited.

I purchased this truck, 1994 GMC 2500 6.5L TD, to work on and fix up a bit. The guy gave me a list of things that could be wrong with it but so far hes diagnostics have turned out to be false.

The pump mounted driver has been changed a few times, so i have a couple of them. And i recently changed the pickup pump, and put new batteries in it. The batteries were swollen and causing a short, which was acting like a ground. The pickup pump wasn't working so i replaced it. I thought the pickup pump could have been the cause. Ill continue with the issue now.

I turn the key and the engine will almost start by itself, sometimes it does start, but it idles at such a low RPM it seems like its about to stall. If it turns over i must step on the accelerator to maintain 650 RPM for 10 - 20 seconds while it creeps up to 1000 RPM, BUT if i release my foot before 1000 RPM it goes back to very very low idle. IF i hold my foot on the accelerator all the way to the floor, approximately 10-20 seconds, it will reach 1000 rpm, maybe 950, and will act as if it has full fuel and Rev without hesitation. Only after this sequence can i take my foot off the accelerator and it will idle at 650 rpm or a bit higher, normally. It has lots of power and sounds beautiful after this. There's no smoke and very little blow by, not puffing, pretty steady.

If i disconnect the battery for 30 seconds, also clears the fault codes, the 10-20 seconds on the throttle during my starting sequence the time i need to hold the accelerator drops to about 1-2 seconds before going over 1000, at this point i can take my foot off the accelerator and it will idle normally.

I have a code 36 - ignition pulse width fault (long?), and i read someplace that it could be the ground from my relocated PMD to the IP.

- The previous owner fiddled with this quite a bit, and upon inspection it looks like the PMD is from china, with a heat sink, and the connectors are slightly damaged. It doesnt look like the relocated PMD was grounded to the IP.

- Tested two PMD's , the one from china and an original stanadyne, and the starting sequence remained the same. Also tested one faulty PMD as a base test and the vehicle wouldnt start.


I hope this ground thing works, but there's always the chance it wont. I'm asking for help now. If there's information i'm missing just ask and ill see if i can get at it. The pump looks like its a stanadyne. Not sure if its the DS4 or the other one.

Thank you

Will be giving this some thought

Thank you
 

I appreciate your help. I've read lots of your posts prior to asking for help. I'm glad you've responded.

So the wire on the PMD harness was fiddled with so much one of them came loose inside the connector. soldered the wire and tested the PMD. The Ignition Pulse Width code is gone and the engine revs past 1000 when i hit the accelerator now. Have a short someplace now, something to do with the glow plugs, or something else shorting and turning the glow plugs off early.



I hear a ticking, not sure if its 10-13 times a second like a fuel injector. ohh boy. starting to feel like i'm winning. :jump:

good job man! sorry i took a day of from the forum for my birthday yesterday :choochoo:
i would check the glow plug wires, and maybe even oull them and test them. i do not think that truck is going to tell you when one craps out.
They are pretty east to test, touch one end to the pos batt terminal, take your volt meter and place black on the neg post, and red to the other end of the glowplug. it should read 12.xxV whatever the batt is. if it checks out then its good :tu:

give all of the wiring a good once over, and While you have glow plugs out do a compression check. it only makes sense
Great deal on MityVac MV5532 at ToolTopia.com
It is worth the purchase. you can also get this tool at your local carquest i believe.

I cannot point you at a specific wire to check, i am not that proficient with a 6.5, nor have i encountered this issue. however i will still give it a little more brain power

Any progress? :humm:

6.5 have two big issues
 

I've been working with 6.5L for a long time. Most people have issues with the PMD and the glow plugs. I personnally changed out my glow plugs and went to a starter relay because I was always shorting out wires. The glow plug control would crap out or I would melt a wire. Fixed that issue. When it comes to the PMD I have never had an issues with the one from ss diesel supply GM 6.5 Diesel FSD Heat-Sync kit (includes PMD and resistor) :: SSDiesel Supply :: GM 6.5 TD Specialists

more progress
 

well, that was not the end of my problems. The starting problem resurfaces. Intermittent little bugger.

I replaced the Coolant temperature sensor and the start problem went away. The computer controls idle at start, which is dependent on the information received from the sensor. Also, before this, i connected my lift pump to a switch and fuse because it wasn't working. I have the feeling the lift pump wasn't working because of this sensor too.

Didnt do the glow plug wire test in this way. The glow plugs are 6 volt.

no longer need to worry about putting the truck over 1000 rpm to get it to idle normally. it does it all on its own when i turn the key

Starts and runs great now. ill update if there's anything else

thanks

:tu:

Okay. so problem persists
 

I've tried two engine coolant temperature sensors and the start problem occurs with them both. Only when it is unplugged does the engine start and idle at 950 rpm when I turn the key, without being assisted by the accelerator.

tracing back - back steping
 

So i placed the PMD right next to the injection pump, eliminating variables, and the code 36 has went away when idle and when accelerating and decelerating. Code 36 still pops up until engine idle RPM on start has been interfered with by accelerator actuation. If i step on the accelerator the code 36 goes away almost immediately, but if i let it idle the code 36 will stay and the 'service engine soon' light stays on.


With the PMD relocation wiring harness the code 36 is very intermittent.

- tried a brand new relocation wiring harness and its still the same thing

Tested resistance in both of these harnesses and tested both harnesses on the truck and the results are the same with both harnesses


Is there a specific PMD i need which is meant for relocation?