He's my question, been searching for days without finding anything similar.

Truck starts and runs well, fairly decent idle occasional hiccup, no smoke. this started after sorting out an air infiltration issue with rubber hoses and old fuel filter. At least I started noticing it then. Runs better now than it has in the 15 years I've owned it.

1987 GMC V2500 TH400 tans, 4.10 gears 33inch tires, 3/4 ton 4X4 runs well but taking foot off of the go pedal at higher speeds (say 50 to 70 is where you notice it most through engine braking) the first few seconds there is a notable surging before it tapers off and stops.

While I was replacing the rubber hoses and fuel filter I also installed a low pressure electric fuel pump on the engine side of the 6 port valve as getting the engine running after a fuel filter replacement was an exercise in futility and always needed a second vehicle and jumper cables and about 20 minutes of trying before it ever ran again. So it would self prime. That resolved the hard starting and rough idle I'd suffered with for years.

Tuck only has 74,XXX actual original miles on it. Any ideas...I'm completely stumped.

 

6 port valve?

 

Yes...think I found out whats causing this since I posted..the electric pump I put on it back by the 6 port valve is over 9 psi (it will supply up to 14 psi) and the IP doesn't like it. Apparently it advances the pump and forces fuel past in on a suden shutdown decellerating from a higher speed, thats $60 down the drain unless I can resell it to recoup part of the cost. Cheaper to buy a new pump than a pressure regulator, fewer failure points too. As much research as I did before doing it...I think I should have done a little more first.

Was sorting out a lot of various issues that piled up over the last 15 years I've owned this truck (the fact I've driven it less than 9,000 miles in 15 years), the worst of them was no power, somewhat rough idle and it being a bear to get started, was all traced back to air getting in the fuel lines, and why I only used to to haul stuff from home depot or when the weather was really bad, I still have to do the left side tank and the hoses under the hood to do, Sorted out the fuel guage not working and the tanks not switching as well. Had to order a lower pressure electric pump (so as few of the lines will be under suction as possible and so it will self prime a new filter).

 

I understand,no previous indication that it had dual tanks.

The ideal electric lift pump to use would be from a 6.5L 93 model year AC Delco EP158,with that you could eliminate the stock mechanical lift pump and push fuel from the tank to the filter,mount it between the 6 way valve and filter.

Idealy you would use a setup like the 96+ dual tank trucks had including a fuel balance module.

With that setup the balance module controls 2 pumps and averages the gauges from both tanks.

The difference is you would eliminate the 6 way valve and redo the fuel flow,all fuel would come from 1 tank and the other tank would then using the second electric pump transfer it's fuel to the main tank via commands from the balance module.

A fair bit of work is inviolved to update to that system though.

 

I'll research that. I would like to keep things as simple and reliable as possible. I hang on to things long enough that if I make major changes I will eventually forget exactly what changes I made. This thing ran so bad I was considering selling it or putting in a LS1 engine...but it runs so well now (though I don't need enough power to smoke the tires) I want to fix its minor issues and use it for years to come.

 

I checked all I was told and knew: wire continuity tests to ECM, sensor itself, all grounds. Stil keeps THROTTLE FAILURE in dash. When pushed brakes goes to iddle after relesing brakes all ok. My cuestion and doubt is : If I buy an ACCELERATOR PEDAL PAD ASSEMBLY say for a 2003 GMC ($$45 ) ( connector for sensor looks the same 9 pins) will it work on my 1995 C3500 6.5 L TD ? The delco OE pedal assembly for this costs $$ 270. ANY AND ALL SUGGESTIONS WELCOME.

 

Seems I forgot to follow up on this thread on my problem.

I replaced it with a 4-7 psi pump so the system will self prime without draining the batteries. And voila...problem went away. And it hasn't done it again and I've been driving it a lot lately. If I do something else later remains to be seen. Its too close to real Winter weather to be doing anything major. I really HATE working outside in the cold as the truck doesn't even come close to fitting through my garage door. Even without a lift. This works well for now so I'll leave well enough alone for now.

I'll rig up that other pump to use as a small transfer pump so I won't have to siphon anything anymore since its bought and paid for and can't be returned. Waste not want not.

 

OK..changes and updates made to the truck since this last post....had to rebuild most of the front axles due to a ball going going bad....Put lift kit, new springs, shacks tire wheels etc...AND I put in a Banks sidewinder....

Truck has been running phenomenally ...until it got really cold again recently.

Started surging while driving at idle (basicly coasting on residential roads up to stop signs). Did it a few times yesterday until it warmed up....but today it did it (heavy surging) fairly violently on the way to work....same point in trip. Under 1 mile from startup. and for about another mile or 2 then it stopped doing it.

So its NOT excessive fuel pressure...it no longer has the mechanical lift pump on it...the banks manifold would not fit with it there, and it had the electric pump back at the 6 port valve.

I do have to pick up a piece of clear fuel line to put on the top of the pump to see if there are any air bubbles to eliminate a new problem with that.

I was researching and I see how people made comments about the fuel RETURN line causing that...I have no idea how. The left tank line does have a return line leak. More like a drip...a slow one at that. Can't see how that could possibly be it, feel line yes if it was that but not the return line.

I'm going to replace those with some new brake line since nobody sells replacement hard lines for these....and I would prefer lines with the expanded flare vs..one without.

It just has to wait until the weather gets a little warmer than it is now.

Any Ideas on whats causing that? I do have a new DB2831-5485 (milspec with the arctic low viscosity hardened parts) rated similar to a 4911 I couldn't find) going in when its warmer and I get motivated to replace the original DB2829-xxxx pump in it.

Can't get enough fuel out of the stock one (turned up all the way) to get the EGT's over 900f full throttle acceleration from a standing start to 75 mph with the turbo (a modest 10 psi boost spiking to 12 psi at transmission shifts) I don't want or need more as its got stock head gaskets and no head studs. And I'm happy at that level of performance.

If I can get rid of this surging....

 

A fuel line restriction would cause surging,not a leak.
The IP fuel pressure increases when return is restricted.

Not that familiar with mechanical IP diagnosis since it has been since the mid 90's that I worked on any but issue seems temp related-how is timing advance controlled on that IP?

 

I only have basic knowledge of this particular injection pump. I know the MW pump of the Mercedes OM617 Turbo diesel significantly better.

As I understand there is a sensor for an electrical cold warm up...and the rest is mechanical. Both a ramp on the accelerator linkage and internal.

Might all be moot as I hope to have a warmish break for a couple days so I can put the new IP on. I know its supposed to be about 4 hours work....I'm expecting and planning for a lot longer before I start. I need this for crappy weather. and its not worth working in bitter cold to do it right away.

I can't see how its a return restriction since its a recent event and hasn't been cold enough to thicken the fuel here. Fuels only about 2 weeks old so Can't see having any non-winter fuel. BUT...it won't hurt to try and check it for a return restriction.

It does go away fairly quickly. and doesn't do it when its warmer.