Need some help with a rough running 94 C3500 with a 6.5TD

[storminsince73]

The truck has about 167K on it, and it running very ragged. Shakes a lot at idle, and at about 2500 rpm, it will rattle the fillings out of your teeth.

My mechanic, who is NOT a diesel mechanic, thought is was a water issue, since I have been having a lot of trouble with water in fuel this year... 3 different vehicles. Over the past 3-4 months, I have probably added close to a gallon of drygas, with my diesel. I pulled the filter, and flushed out the "well" where the filter goes with fresh diesel, and it got better for a few days. It has reached the point where it is running badly enough to make me wonder if there is something else happening, that I have just missed. I went down to talk to another mechanic, who does work on diesels, and he said that I have a "couple of dead holes", and that is could be injectors, or valves. Well, I know this guy pads his invoices, and does un-necessary work, so I am not running right out to get injectors.

Can somebody give me a legitimate plan to figure out what is going on here? Like, how do I determine which cylinders are making no power? I read 10 pages of threads, and did not find this same problem, but I did see similar problems. Someone mentioned the the if frame rail mounted pump goes out, the engine will still run, but is likely to be rougher. Any suggestions on how I would check it?

Thanks in advance!

[Avec]

First thing to check is the lift pump. It is along the inside of the frame rail at the driver's door. The next thing is fuel filter. After that, check your injectors. From your symptoms, I suspect injectors.

[storminsince73]

Thanks Avec, that at least gives me something to roll with. I too suspect one, or more injectors. Any thoughts on how to pinpoint the bad one(S)?

[Koots]

As for checking individual cylinders, if you can get access to the injector lines nut, try loosening each injector and see if there is any changes to it's idle. Opening up the lines will drop the pressure in the lines and the injector won't "fire", so to speak.

Just like pulling plug wires off of a gasser :cheer:

[Koots]

First things first, Have you changed your PMD?

Has it been changed any time in the last 2 years?

Is it located in the stock location? which is on the drivers side of the injection pump.

If it is, get an extension harness, a new PMD, and mount it out of the engine bay. This will add lots of life to any PMD you buy, which will help them last a while longer. With a bad one, you will get all kinds of strange issues...mostly a stalling though once they are gone really bad.

[storminsince73]

No, the PMD has never been changed that I am aware of. Is it supposed to be changed periodically?

Yes, it is in the stock location. I am looking at getting an extender cable to get it away from the heat.

I think I have had a couple of issues with the PMD. In 2008, I was all over the country in the truck. It did stall a few times. Ever since I've owned the truck (4 years in a couple of months), I will be driving, and it will "stutter", or shudder, like a miss, and then go on down the road. COULD the PMD be my whole problem here? Or do you still think we are looking at an injector issue? I am starting to think that the PMD may be the whole problem.

[Koots]

A PMD of unknown age (especially in the stock location) is on borrowed time for sure. Get an extension harness long enough to put it into one of your bumper holes....its the best location. It still needs a heatsink though, the PMD itself can get very hot from normal operation.

It can cause almost all of your issues, but i wouldnt rule out replacing your injectors. If the truck doesnt sound quite like a diesel at idle, you might have weak injectors. It makes them idle like a pissed off SBC.

[storminsince73]

Nope, it sounds like a diesel. Any recommendation where to get the harness and pmd, is ebay good?

[Koots]

I'm sure this is more expensive than other options...

http://www.pmdcable.com/index.php?main_page=index&cPath=7&zenid=vo98aahi8irdfqcr3425546cf2 (http://www.pmdcable.com/index.php?main_page=index&cPath=7&zenid=vo98aahi8irdfqcr3425546cf2)

but the guy is a good guy, has lots of awesome little diesel parts and he sells the best PMDs and extension harnesses on the net IMHO. He even has an actual replacement for the factory fuel sending unit, that can actually measure the fuel from full to empty.

[storminsince73]

SO, which one is better? The Flight Systems, or the Stanadyne? If I go through these folks, it is just about the same money.

[storminsince73]

KOOTS! Don't let me down now... which PMD is better? The stanadyne, or the flight systems?

[storminsince73]

Alrighty then... new style Stanadyne PMD, extension cable, and 195 degree thermostat ordered. Their claim is that the higher thermostat, and resulting higher operating temp, will increase mpg. Hope they are right.

[Koots]

Alrighty then... new style Stanadyne PMD, extension cable, and 195 degree thermostat ordered. Their claim is that the higher thermostat, and resulting higher operating temp, will increase mpg. Hope they are right.

Being that you are from TN, you might need a 180* stat for summer, but if it isn't hot as hell out and/or you aren't trying to tow 10k, the 195* stat will work excellently.

Otherwise, the 180* stat can make the engine run cold, which can effect performance and MPG. I prefer to keep it stock though in terms of cooling...unless I see a need for something better, that is.

[storminsince73]

Okay, PMD and cable are in. It is a little better when accelerating, but still idles bad. Next thought? On the test drive, it also went to bucking, stuttering, and running very badly. I got home, changed the fuel filter, since it was time anyway... I was hoping, but it did not help.

Soooo, what is next? Try to test the injectors, or look at the lift pump? What is the best way to check the pump? I read on here that the stock pump leaves a lot to be desired, and does not pump the fuel it should.

[Koots]

Yeah, even a well working stock pump is going to be barely adequate for anything above stock levels. Run something like a Carter P4070, Holley Blue or Black (with pressure regulator).

There are high performance (expensive) pumps out there but most of the aforementioned pumps will give you more than enough. I have heard good things about the Carter, so I would go with that. It's cheaper too!