"Miss" under moderate to heavy load

[HarrySanders]

Have a 95 2500 Suburban with the 6.5 Turbo and 4L80E. Only mods are Amsoil bypass filters w/ all Syn lubes, big exhaust w/o Cat and amsoil air filter. Run stanadyne Blue. We just had a warranty replacement of the injection pump at 117,000. Ever since we have had a "miss" that basicly causes a sharp deceleration for a very short period of time. Reminiscent of a high speed miss or electrical breakdown in older gas cars.

Any thoughts? This never ocurred before the new pump.

[rameye]

Just happened to me this summer...

Failing lift pump due to bad oil pressure sender.

exact symptoms.

[patrick m.]

i vote oil press sender as well
most of us have had this happen.

since your truck is a 95, you may also have an ign switch problem at some point (or already)

[HarrySanders]

Ok I hear what your saying. But why did the problem coincide with the new pump? I thought I remembered something about a bypass valve for the fuel with dirt or something???

[rameye]

I don't know about a dirty fuel bypass but, I will tell you there is no rhyme or reason to when things break..they just do...sometimes with strange coincidence. :mad:

[DKeen]

Harr- I have had the same problem for 6 weeks. Changed OPS, PMD, electronic filter and I also have an added fuel filter. Still got it. Driving me nuts!

[rameye]

If you want to rule out fuel delivery to the IP then T into the fuel line right before it enters the IP and run a mechanical temped gage inside the cab.

See if this sharp deceleration reminiscent of poor fuel delivery is caused by intermittent loss of fuel pressure. I guess you could have a cruddy screen in your fuel tank, momentary loss of 12v to lift pump, or sucking air or some other restriction.

make sense?

[Heilander]

I replaced the short hose from the fuel drain valve with one long enough to reach into the cab, connected a fuel pressure/vacuum guage to the other end, & opened the valve. Fuel pressure would rise when cranking, but would drop to a vacuum after engine started - clearly indicating that LP was OK, but OPS not working.

Check fuel pressure under normal running and hard accelertion - should stay between 3 - 4 psi.

Good Luck. ;)

[99gmccrew]

If your oil pressure sender goes bad, wouldn't this be indicated by your oil pressure gauge?

[Heilander]

Not necessarily, according to my wiring diagram, they are two separate circuits. The gauge circuit provides a variable resistance between pin A and ground, whereas the fuel lift pump circuit is a hard contact switch between pins B & C. My gauge worked fine, but the switch wouldn

[99gmccrew]

Great explanation, that makes perfect sense. I had the same miss. It seemed to go away when I replaced the fuel filter. I had not used the entire tank of bad fuel yet, and the miss is starting to come back, but not as bad as it was. I'm going to try one more fuel filter after I cycle some more good fuel through. If it doesn't go away I'll try what you all have done. I can hear my lift pump working when I switch on the ignition. Do they have a typical life span? I've got about 97k on this rig.