I have a 93 K3500 with a manual transmission. It has an "Intermittent Hiccup" condition only in medium RPMs. I can alieviate the problem by either giving it more throttle, or backing off the throttle. It shows up on heavy, or light load. Empty or towing.

I have used the search function, but, unfortunately, most of the answers involve electronic injector pumps from 94 and later, or problems associated with automatic transmissions. Any ideas?

PS: although I have not tested the fuel pressure, I have ruled out the fuel (lift)pump as being weak, because the problem shows up only in medium RPMs at medium throttle. It does not happen if I mash the pedal. This may be faulty logic, so say if it is.

You may have a bad injector, or your injector pump is starting to act up. Like you said, most of us have the electronic version of your pump, so I have no earth shattering advice for you. But if it only does it at mid range RPM, it sounds like a fuel delivery problem. Yeah, I know, a lot of help I am! Good luck.

So could a single bad injector cause a problem like this? Even though it only shows it's ugly head in a small RPM range?

Are you running #2 in it when it does this, or is this a VO setup also?

Oh, yeah, I forgot to specify. The 6.5TD does NOT have a veggie conversion. The problem manifests itself on both petro-diesel and biodiesel (and any ratio in between).

my 93, manual trans, would hiccup under light load, coast conditions 1500 - 1800 rpm, helped, but not eliminated by using stanadyne lubricity additive. Wanted to try bio-diesel as at only 2% it has better lubricity than additives.

mine[93,ext cab,4x4 ,5 sp man] does this every couple years,and is starting to do it now,everytime so far when it does it not long after i get to put a clutch/pp in :eek:

mine[93,ext cab,4x4 ,5 sp man] does this every couple years,and is starting to do it now,everytime so far when it does it not long after i get to put a clutch/pp in :eek:
OH! That might just be it!

Should do the FAQ checklist and post results so we can see all the data without asking you to Try this, try that, you'll get much more responses, and much ebtter help.

I wouldn't rule out a fuel delivery problem yet either... Do the Checklist.

you might try changing fuel filter before going anything more expensive. They can cause that when starting to clog up.....

you might try changing fuel filter before going anything more expensive. They can cause that when starting to clog up.....
Oh, yeah, I forgot to mention that I changed the fuel filter, and the hiccup happened immediately thereafter.

I pulled the fuel cap off and took a drive. Still hiccupping. I think I will do the quick and dirty lift pump test next.

PS: I am using the diagnostic post as a guide, but unfortunately most of it is tailored to 95 and up, so not all of the info translates to my 93.

am[/I] using the diagnostic post as a guide, but unfortunately most of it is tailored to 95 and up, so not all of the info translates to my 93.

i know what you mean,theres a few other sites i use also for our 93 trucks,if you are interested ill pm them to you,between them all i usually get alot of good info ;)

Oh, yeah, I forgot to mention that I changed the fuel filter, and the hiccup happened immediately thereafter.

Hopefully you didn't get some trash inside your pump when you did the filter change. Did you check to make sure the small white secondary screen was still intact inside your filter housing? It fits around the shaft in the center at the bottom of the housing.

i know what you mean,theres a few other sites i use also for our 93 trucks,if you are interested ill pm them to you,between them all i usually get alot of good info ;)
Yeah, I'm interested. Would posting them here be verboten?

...most of it is tailored to 95 and up...
Oops, I meant to say 94 and up, but I have big, fat fingers

Hopefully you didn't get some trash inside your pump when you did the filter change. Did you check to make sure the small white secondary screen was still intact inside your filter housing? It fits around the shaft in the center at the bottom of the housing.
I did not mean that the hiccup happened initially thereafter. What I meant was that the problem started a while back, and so I changed the fuel filter in an attempt to alieviate it, but as soon as I changed the filter, and was back up on the road, I ran into a hiccup spell.

And yes, I checked the little screen thinger in the bottom of the filter bowl.

Just did the quick-n-dirty lift pump test. There was a very healthy stream of biodiesel out of the water drain on the filter at idle. No air at air bleed.

Here I go again with a chime-in about the oil pressure switch... Think about a relay or control harness to give your lift pump solid power - especially important with biodiesel which is higher viscosity and can cause slightly greater current draw for the lift pump. The stock OPS design doesn't handle a lot of juice and can crap out.

I got some intermittent hicupping a few years back, followed by the 'no warm start / OK cold start / dying OPS' condition. This happened a couple of times. Fried the OPS and took a long time to figure it out. With the OPS relieved of the lift pump current draw, my fuel delivery has been fine for a couple of years. No hiccups, no stalls.

I must be a noob, because I posted a question without giving you guys all the info. Too bad you guys cant read my mind, because I knew what I meant!

The OPS is bypassed with an always hot (when the key is on) source.