6.5L fuel system diagram and PMD/FSD

[BIGTPM73]

Hello all. Does anybody know where I can get a diagram of the fuel system for my 98 6.5L TD. Having never worked on a diesel before, I would like a diagram so that I can fully understand how the complex fuel system works.

When I bought the truck 6 months ago, it turned out I needed a new PMD. I purchased a used one from a supplier reccomended on this site, and it worked great for about 3 months. Now I am having stalling problems again, and all signs are pointing toward the PMD. Fortunately, my PMD has already been relocated with aluminum cooling fins, so changing it is a snap, and this time I ordered a brand new one from the same site. Also, is the PMD really a constant source of failure? It is quite an expensive part that seems to be very failure prone. Does anybody have any suggestions to prevent this from happening again. Thanks in advance for your help.

Tom

[JohnC]

No diagram, but it's really not too complex. From the tank pickup it goes to the lift pump, filter, and injection pump. The injection pump has a return line as does each of the injectors. They are all manifolded together and plumbed back to the tank. The filter has a water drain that is connected to the brass valve on the thermostat housing and also contains a fuel heater element. There is a secondary pickup in the tank a couple of inches above the bottom that comes into play if the main pickup gets clogged.

[DmaxMaverick]

The fuel system isn't complex at all. Relatively very simple, compared to modern gas and Diesel systems:

Tank sender => Lift pump => Filter => Injection pump => Injectors => Return line => Tank. The Injection pump also returns fuel to the return line/tank.


In regards to the PMD/FSD, I don't subscribe to the same as many/most. I don't think it is as failure prone as it is touted. As with any system, if the system in general isn't kept in top health, a weak link will be exploited. The key to a long lasting FSD is a healthy electrical system. Keep feeding it full and clean voltage, and it will generate less heat. Heat that needs to be carried away, and heat that causes early failure. The fuel solenoid (metering solenoid in the pump) is of a specific resistance, so decreasing current will cause the driver (amplifier) to overdrive. Overdrive is excessive heat, and premature failure. Keep it cool and feed it with full clean voltage, and it will live longer. Heat reduces conductance, and reduced conductance causes heat. Keep both in check, and the circle never starts. The GM Diesel electrical system in general is very sensitive to electrical health, not just the fuel system. The quickest way to kill a starter, ignition switch, HVAC, wiper motor, etc, is poor battery/alternator/conductor (cables, wires, connections) health. Grounds are especially important to maintain, and most often neglected. The fuel system electrical is no different. That said, the PMD/FSD is a weak component with a higher than acceptable failure rate, so I offer no guarantees. Some vehicles/owners seem to have a higher failure rate, while others never hear a whimper. There is a commonality, and I believe it is system health related. Even those who try to do everything right (relocated FSD, cooler, extended harness, etc.) still have the problem. Perhaps slightly less often, but the problem remains. Others do no mods, but enjoy long PMD life, on several vehicles. I think it can be attributed to more than just chance.

[BIGTPM73]

Well, I have either good or bad news. I changed the PMD, and no luck. Truck still running bad. What happened was that it would occasionally stall. Sometimes it would start right back up, other times it would take about 15 minutes to "rest" and then it would run fine. Last week, when I first posted this question, it stalled, and it wouldn't start at all until the next day. Fortunately, I was only a mile from home, and it was a nice day here in Philly. So the next day, I went to try and get the truck, and it started, but it ran really rough. The idle would rev, slow down, almost stall, and then catch. I drove it home, bumping and jerking the whole way. I figured the PMD was shot. Well, I put a new one in, and no luck. Any other suggestions??? Thanks for the help.

[blackonblack]

I am currently going through the same thing. I have replaced and relocated my FSD. I had 2 Chevy dealers look over the electical. One dealer replaced my ECM and the other found a lose wire in the ECM wire harness. It ran good after the lose wire was fixed but for only 5 weeks. Now I am back to stalling, no start, and weird reving and choking. Let me know if you find a resolution and I will do the same.

Martin

[JohnC]

Read the codes and go through the stalling thread at the top of this forum.

[CapnAmerica]

"...stalling, no start, and weird revving and choking..."

Have you looked at the fuel lift pump (driver's side frame rail)? The engine will run (sort of...) without one; perhaps it works when you start it up initially and quits or binds up after some time or some heat exposure. I would check to see if it is working just after the key is turned on to ignition (before rolling the starter), and then again when it is doing its "thing". You should be able to hear it from beside the truck (or maybe inside), or on hands and knees, or feel it after you find out where it is.

Lift pump is a "good" problem if you have to have one...it's easy to find and replace and not TOO costly .

It may sound weird, but those are the same symptoms I have had with my 1985 Chevy 6.2NA with, of curse, NO PMD. Mine appeared to be the vacuum advance on the injection pump which had gotten rather dirty over the years...except that after warmup, it ran like a top. A good dousing of WD-40 cured it immediately. The only reason I say this is that perhaps the problem is in the injection pump: an internal link or part broken or damaged? Any idea how old it is? You certainly don't want to change it out without good cause .

As to PMD failures, mine have manifested themselves with unannounced shutdowns when it is on the way out...like turning the ignition off and back on. Then when it's all properly fried, a "no start". And no amount of verbiage or screaming will make it rethink its course of action.

Good luck! Keep us posted.

[Bnave95]

Years back I had just that issue. Replaced the Crank pick-up/sensor. Though it showed a code for it,but not at first. This is over looked many times because the code won't show up and points to other areas.

The idle would rev, slow down, almost stall, and then catch.