Tech Notes From Dan: FSD / PMD

In 1994 GM came out with the 6.5 turbo diesel engine. This is roughly the same engine used in the Hummer (H1) from 1996 to 2005. Here at Predator
we get a lot of calls from people that are having trouble with their engines. The main complaints seems to be a rough running engine, will not start after shutting down, dies as they are driving along, etc. The list goes on and on. Usually we ask if they are getting a check engine light and if so, do they know what code is coming up (this usually takes a Tech2 Scanner to analyze). If you go to a dealer and he gets a code of 1216, 1217 or 1218, they will probaby tell you that you have a bad fuel injection pump and you need to replace it. This is because in the repair manual, these codes relate to the injection pump. In 99% of the time you have a good pump and a bad Fuel System Driver (FSD), also known as a Pump Mounted Drive (PMD) which is mounted directly on the side of the injection pump. There is no single code to differentiate between a bad fuel injection pump or a bad PMD. If you elect to have the injection pump replaced, this will usually fix the problem because in the overhaul of the pump a new PMD is installed. This will usually cost around $4,000.00. The real problem is in the design.

GM decided to mount the PMD on the side of the injection pump where it would be cooled by the fuel going into the injection pump. It sounds good and works most of the time in the pick up truck lineup however in the Hummer, the engine lacks airflow over the top of the engine. Additionally, the PMD unit is heat soaked as soon as the engine is turned off . So by changing the injection pump you are still faced with the same design flaw. It isn’t “IF” it fails but “WHEN” it fails. Several years ago I was in Indiana at a Hummer home coming and got to talk to one of AMG instructors who told me he always carries a spare PMD in his pocket when he goes off-roading. He said it has saved him many times. The real solution on the market is a PMD Isolator Kit which relocates the PMD onto a heat sink. There are a number of units on the market that accomplish this however most all are designed for the GM Pick Up Truck and don’t actually relocate it away from the engine. This is ok for GM Trucks however with our lack of airflow across the engine, it is a serious problem for the heat sink to disipate the heat it absorbs. When we began R&D for a solution for the H1 Hummer, we looked at a number of options before coming up with our system. We did not like the cooling fins because they tend to clog up in dirt and mud. Also we see the same problem occurring if you mount the new PMD inside the engine bay.

The solution for us was to relocate the PMD onto a flat plate heat sink and relocate it in the fender well area, far away from the engine. This solution has proved to add many years of trouble free service to the H1 lineup. If you suspect a bad PMD, shoot us a call and we will be happy to help you diagnose the problem over the phone.

Dan Wilson