I am very new to this forum, found it while searching for some answers to my truck's problem. I am not a diesel mechanic, but am fairly mechanically inclined (do all my maintenance on my m/c, etc) so bear with me please.


I have a '95 Chev K3500 6.5 TD with 102K miles, a Banks exhaust kit & Ottomind chip (been installed for 2-3 yrs). For some time now, the truck occasionally bucks/hesitates under acceleration. It feels like it's shifting back and forth, or power is intermittently being cut off and on. I dont recall seeing any indicator lights come on when this happens. I use it mostly to pull a horse trailer, so there isnt a tremendous load. It had stalled a few times, but always restarted fairly easily, the biggest issue is the bucking.


I've had it in the shop numerous times, but they've yet to fix it. We were at our wits end with this so bad, that I agreed to put the stock chip back in, replacing the Banks/Ottomind, this didnt fix the problem either (I called Banks, they said they quit selling the chip because of problems they had, and it was possible the problems I was having could be caused by a bad chip, which is why I agreed to replace it).


I've searched thru this forum, and believe I better understand the possible problem areas. Seems most common is the PMD/FSD, that's been recently replaced, as well as the OPS (I believe), and the fuel filter. The IP was replaced some time ago under warranty/recall. For grins, I thought I'd try the lift pump test (opening the T valve), and sure enough, no fuel came out, and the engine stalled after a couple of minutes.


So my questions are: Where is the lift pump? Could this be causing the bucking problem? How difficult is it to replace? Is this something I could tackle or should I let the shop do it?


Thanks in advance.

The lift pump is on the frame rail drivers side near the end of the door. It is easy to replace--just unplug, unbolt and put the new one in. Be sure to check and change your fuel filter after changing it out as you may disturb sediment or crud in the changeover. Easy to do!








Tony

Cool, sounds just like replaceing a fuel filter on a gasser.


Does the problem I described sound like could be caused by a faulty lift pump?


Thanks again. http://www.dieselplace.com/forum/smileys/Big Smile.gif

The lift pump circuit, includng the OPS, will cause that exact symprom - no fuel out the air bleed on the filter, or the water drain valve.


Remove the lift pump fuse, connect + battery power to (usually) the driver's-side terminal in the fuse socket, open the filter air bleed - the lift pump should pump fuel out the open bleed port.


Indicates failed OPS - the fuse is on the pump side of the circuit.


If no fuel out the bleed, the lift pump is failed.


Unplug the connector, unscrew the fuel line fittings, don't get Diesel fuel in your eyes or ears, reverse proceedings for the new pump, and you're done under there.


Put + battery power on the fuse to bleed the filter, close the bleed, open the water drain valve to flush sediment, re-install the fuse, and you're done.

Many thanks for the details, will do it this weekend.


Thanks again!!!!!!!

When you're pulling your trailer and the truck seems to shift gears back and forth, it might be the TCC (torque converter clutch) engaging and disengaging depending on load on the engine and throttle position. When the TCC is fully engaged the truck is at it's lowest RPM and seems to be in a lower gear. When you let up on the pedal as you're going downhill the TCC will disengage and the truck will rev 300 RPM higher.


This is greatly magnified (exagerated) if your APP (accelerator pedal potentiometer) is faulty and gives higher voltage readings on any of it's three potentiometer readings. You would find it impossible to use your cruise control as the TCC would engage/disengage constantly.

When you're pulling your trailer and the truck seems to shift gears back and forth, it might be the TCC (torque converter clutch) engaging and disengaging depending on load on the engine and throttle position. When the TCC is fully engaged the truck is at it's lowest RPM and seems to be in a lower gear. When you let up on the pedal as you're going downhill the TCC will disengage and the truck will rev 300 RPM higher.


This is greatly magnified (exagerated) if your APP (accelerator pedal potentiometer) is faulty and gives higher voltage readings on any of it's three potentiometer readings. You would find it impossible to use your cruise control as the TCC would engage/disengage constantly.





The APP was also recently replaced, as it was shifting very hard into each gear, almost slamming into gear, with out any load and in slow stop/go traffic. The APP fixed the shifting issue, but the bucking remains.


I cant use my cruise now at speeds above 65 as the speedo acts like a binding cable. Rpms between 2500 and 3000, puts me bet 75 and 80, yet the speedo will read 65 slowly creap up to 70, back down, etc. Ideas? Seperate issue, just trying to get by one issue at a time.


Your suggestion on the TCC, what about with no load and normal acceleration? The bucking happens whether I have a load or not.


When I last took it to the shop, I told the shop mgr to fix this time, or I'm either gonna burn the *@#() thing, or trade it on a Ford or Dodge. Havent done either of these yet, but am looking at trading for a Duramax. http://www.dieselplace.com/forum/smileys/Confused.gif


Thanks again.

The only things that will make a diesel buck like that is lack of fuel/air.


I've had the lift pump go out, it bucked and stalled occasionally.

I replaced the lift pump today, and the fuel pump relay, same problem. The pump doesnt seem to come on, as I dont hear it nor does anything come out of the water vent line/T handle valve. I know the old one was good as I jumped it across the battery after I got it off. Tried applying + bat to the driver's side of 20A fuse labeled "fuel pump sol", is this the correct one? If it is, didnt hear the pump engage. Looks like the harness in the frame C channel to the pump was replaced during one of my many trips to the shop for this problem.


Suggestions welcomed, and thanks again.


Greg

Lift Pump Fuse is on driver's-side firewall, probably just above brake booster\cylinder, adjacent to Lift Pump Relay - fuse holder looks like small flat relay, two wires out of bottom.


Fuse may be open.


Worn lift pump can run unloaded, but not run with fuel head.

gmctd,


The only thing I can find is directly over the brake booster, flat, sitting horizontal, secured to firewall with 2 screws, but has 3 wires coming into it via green connector, wh, yel/bla, bla (colors slightly faded so may be wrong). Is this it? If so, it looks to be a sealed fuse unit.


Thanks

Greg I can't help you on fuse location in a 95, but another possibility if you finally find it and it turns out to be good, is the OPS (aft center top of engine behind the fuel filter) which sends power to the lift pump as well as sending presurre reading at the dash, OPS has poor contacts which don't hold up under lift pump load, many have replace OPS and then use OPS to control a separate new relay to provide the power to the lift pump.

On that year the fuse is on the passenger side far leftside of the firewall just past the junction block. I would suspect the OPS and bypass it with a relay for good measure.

Shouls have two gray wires out the bottom.


Upper is cover, tab fastened to firewall with sheetmetal hex-head screw.


Lower is fuseholder, unplugs from upper with latching clip, just like a connector

Guys,
I'm really not that dense, but I cant find ANYTHING like what you're describing. There is a 20A fuse in the FUSE-RELAY CENTER labelled "Fuel SOL", could this be it?


I guess I'll have to trace the wires up thru the harness to see just where in the he!! they're going. http://www.dieselplace.com/forum/smileys/Confused.gif


Thanks

Fuel Sol is the main supply to the inj pump - nothing to do with the Lift Pump.


Lift Pump Fuse is in-line type, between OPS\PCM drive and Lift Pump, at 5amps.


One gray wire in from source, one gray wire out to Lift Pump.


Fuse holder plugs up into the plastic cover - fuse cannot be seen until lower is 'unplugged' from upper, which should be fastened somewhere along the top of the firewall, between the electrical buss center and the brake booster.


About 1/2" thick, 1" wide, 2" tall, dark gray or black, tab on passenger-side of cover.


Perhaps QM could do some Show and Tell......

anytime.....


look behind the turbo on the fire wall. remove the black cover over the fuse block.


http://www.dieselplace.com/forum/uploads/37F_fuelpumpfuse.jpg


The OPS is wired before the fuse and located at the rer of the engine behind the fuel filter housing. The Fuel pump relay is on the far right. As far as I can tell, this relay only works during the glowcycle and the OPS powers it the rest of the time, The reason the OPS might be causing the problem and why I suggest ( as I was shown by HowieE)bypassing it with an auxillary relay, where the OPS will switch on the relay to carry the load of the liftpump. I did this on my truck and showed a buddy how to do it when his became impossible to start for lack of fuel pressure. It does the trick because fuel delivery has not been a problem since a new OPS and 30amp relay went in.


Edited by: quantum mechanic

Quantum, thanks for taking the time to post the pic, but there's nothing like that on my firewall. Just to show you guys I'm not smoking crack or something, here's the pics of my f/w. FWIW, I did see inline fuses like on the blue and black wire you posted, but they're stuffed down behind the back of the TURBO POWER cover. Maybe the pump fuse is hidden down there?


Thanks


Lh side f/w:
http://bigbikeriders.com/photopost/data/3047/248truck1.jpg


Rh side f/w:
http://bigbikeriders.com/photopost/data/3047/248truck2.jpg

where would all of this stuff be on a 1997?


my truck has occasionaly been doing this too,


but just on incline's or when i really get on it.Edited by: steiner43511

Thanks, QM - excellent shots, as usual.


You, too, Greg - as you can see, a couple pictures can prove sanity, among other things.


Greg, where did you find the Lift Pump Relay, physically, to replace it?


That engine bay looks more like the OBD-II setup in '96, so I suspect the Lift Pump Fuse is in that box on the driver's-side fender, or somewhere in that vicinity.


That module on the firewall is the Baro Sensor - PCM compensates for varying atmospheric pressures by checking that sensor at each start.


You can drive from Florida to the Catskills to Texas to Pikes Peak, and the PCM can handle the changes in fuel requirements.


But, not without a lift pump. So, you may have to manually trace that wiring to its bitter end.Edited by: gmctd

Greg, where did you find the Lift Pump Relay, physically, to replace it?





Inside the FUSE-RELAY CENTER on the fender well. The only thing close to fuel/lift pump labeled fuse is the "FUEL SOL" 20A fuse, which isnt it.


Thanks

Sorry,


your '95 has the '96 fuse block next to the brake reservoir. everythings a little different, and mostly inside the cover.

i cant find anything like an inline fuse or anything on my 97. in the fuse relay center there is the fuel solenoid 20a and the fuel pump relay. anybody know where the lift pump fuse is on a 97?

I have the same/similar problem with my 97 and have been following this thread for a few days. From what I understand GREGJ has had the truck in and out of the shop with not much luck. I was curios if the lift pump solution/idea solved the problem.


In relation to this I have a question. Can a faulty TCC solenoid make the truck behave like this?

A faulty TCC would stall the engine if stopping, stepping hard on the brake, or fully stopped and stepping on the brake.


It would stall because the TCC locks the converter and can't release when faulty.

Hello all, I just found this forum and look forward to learning lots


I have a '95 3/4 ton with a 6.5 in it and I had the exact same problem for over a year. I went through much of the same proceedures as above before figuring it out. It was a bad oil pressure sender. i guess the sender gives intermittent signals and the computer cuts power to the lift pump when it senses zero oil pressure.


Simple fix, cheap too.


Slapshot

add me to the list.....i have a 98 6.5 stalling and bucking .a mech out in longview has it right now putting on a new fsd...so your saying if that is not the prob, it could be the ops.