"Missing" but runs

[trea007]

My 6.5 TD '94 Suburban runs great most of the time but seems to have a "missing" problem. I disconnected the lift pump, which was recently changed, (I think it's the lift pump) in the rail of the frame under the drivers side and there seems to be no change in the "missing". The module on the pump was also changed but the miss is still there. I took out the glove box and checked and cleaned every conection I could find - still there. The fact that the lift pump is disconnected and there seems to be no difference in the way it runs leads me to believe that this may have something to so with my problem. Any Ideas?? Terry trerea@hotmail.com 602 373 2530 Scottsdale AZ

[DmaxMaverick]

Welcome to the Forums!

Missing and rough running issues are hard to diagnose once the obvious things are eliminated. Time and trial/error is the most common process, without the proper diagnostic procedure.

The groundwork and research has been compiled HERE (http://www.thedieselpage.com/members/guide.htm). The 6.2/6.5 Diesel Troubleshooting Resource (http://www.thedieselpage.com/members/guide.htm) can't be equalled, anywhere.

[JohnC]

First thing you need to do is test the lift pump and change the filter.

Reconnect the pump and start the engine. Open the water drain valve on the thermostat housing. If fuel comes out, the pump is OK. Change the filter. If the engine stalls, it's not OK (and you're screwed.) Fix it (maybe bad oil pressure switch) and bleed the air out of the filter.

Changing the filter without a working lift pump is a PITA. Sometimes, on the ODB I trucks, you can get the pump to run while the engine is cranking and bleed the filter that way. On ODB II trucks the pump should run for a few seconds after the key is cycled on.

[Robyn]

94 is easy to deal with.
Open the junction box on the pass side of the engine bay (firewall)
On the right side is the fuel pump relay. On the left is the fuel pump fuse (20A)
With a known good lift pump in the system you can bleed the filter as follows.
Unhook the fuse wire assembly from the little housing its in and touch the center of the fuse where its open to the big hot stud in the junction block.
The fuel pump should run any time this is done.
The starter crank circuit also is hooked up so when you are cranking the beast the fuel pump is energized.
Once the engine is running the oil pressure switch will take over and put power to the pump.
Is this miss at all speeds? If it misses at idle go around and crack open each injector line one at a time untill you find the one that is not working. You may have a bad squirter.
Let us know
Robyn