94 suburban 126,000 miles

[joe_murphy]

I initailly had a real sparadic miss on acceleration- I took it into the GM dealer. They hooked it up, and said it was the injection pump-so we replaced it, and it still missed. So we started replacing what might have been the prolem, so far we have 1) installed a rebuilt injection pump, with I think it was called a resistor 9 2) we put new injectors 3) new fuel lift pump 4)visual check on the fuel lines for air leaks 5) new fuel filter 6) new vacuum hoses and a new waste gate selinod. The diesel mechanic at our local GM dealer is stumped- nothing shows up on his computer. He had it hooked up while I drove, and nothing showed up on his tester when it missed. We even went thru a few drills while it was hooked up, and he said everything came up looking great. It misses most of the time on acceleration around 2300 to 2500 rpm, and normally the last gear just before it goes into the overdrive gear. It does occasionally miss right at the start of acceleration in 1st gear. It misses when it wants to- sometimes I can drive for an hour with out it doing it, and sometimes it misses on every accceleration. The miss is not a big miss, it's just a few soft misses.The diesel mechanic @our Gm dealer said it could be one of the three- O.P. sensor, the crank sensor, or the T.P. sensor- but none of them are showing up on his computer. So- other than changing those out we are at the end of our rope. Is there a chance that the new parts are bad, or we put them in wrong? I have had the truck since it was new, and I really like it, I don't want to give up. So can you give us some advice? THANKS JOE

[JohnC]

Have you verified that the fuel lift pump is running when the missing happens? Might be worth it to replace the oil pressure switch for grins if you haven't already.

[joe_murphy]

No, I have not checked the lift pump. I will ask the mechanic to do that. And for the oil pres.solenoid- the mechanic thought that if it were the O.P. solenoid, that when it did miss the oil pres. gauge would drop, and it doesn't. Is that your thinking as will?

[bcbigfoot]

The gauge can work fine (and usally does) and still have a bad OPS. You can also have a OPS that can work when you start the truck and stop working while your driving, guess what I'm saying is sporadic operation.

[JohnC]

One would hope that whoever replaced the lift pump would confirm it works correctly afterwards.

That said, try this. Pull the drain hose on the water separator valve located on the thermostat housing out where you can see it. (Pay attention to where it was so you can put it back.) Start the truck and open the valve. You may want to direct it into a tin can or something. If fuel comes out steadily, the lift pump is running. If it doesn't, it isn't. Replace the oil pressure switch. The gauge part is separate from the lift pump switch and generally doesn't fail. Repeat the test.

If fuel does come out, I would plumb a pressure gauge to the hose and put it up on the windshield where you can see it and drive. Go for a ride and note what the gauge does when the engine misses. If the pressure goes negative, I'd replace the oil pressure switch. Note that under heavy loads, the pressure can drop to nearly zero, but shouldn't go negative.

Or, you could just change the oil pressure switch and cross your fingers...

[rjwest]

OK: I'm gonna say it again, If you own a 6.5L engine.....

Remote mount the PMD,
and install a fuel pressure gage on the dash.

It will save many hours of screwing around...and ? money!!

[joe_murphy]

The PMD is remote, and I am having the mechanic come over this afternoon to change the O.P.switch. I appreciate your input, and will post the results! I am keeping my fingers crossed

[joe_murphy]

Well it doesn't miss- been going now for a week. It must have been the O.P. switch. THANKS for your input.