Hi guys- this is my first time posting. Ive got a '97 3500 dually 360K w/ NV4500 that im having a low power issue on. Its stock- used to be turned up a bit but fuel plate is back to stock. Ive installed vacuum and fuel pressure gauges, and all new filters, all new fuel lines. Pulling about 5 in Hg of vacuum, 40 psi out of my stock lift pump (just replaced it 2 days ago, my old LP was pushing 55 psi into the IP) Ive converted it for vegetable oil use, but have only put 700 mi on veg oil. The low power problem im having is on diesel, but is more noticeable on veg oil (no surprise theyre supposed to have lower power on veg)
So heres what its doing. Gears 1 and 2 seem normal in 3 it starts to lag a bit, very slow in 4th and 5th gears UNTIL you get to 1800 rpms (in any gear) the thing launches you back into your seat. so for instance youre pulling through 4th gear and you can stomp the pedal at 1200 rpm and very little response, it hits 1700/1800 rpms and it throws you back and takes off. Sometimes, about 1 of ten times accelerating, theres a surge of power for about 50 to 100 rpms at 1200 rpm. youre not doing anything with your foot, holding it steady, it just decides to confuse you by having a huge jump in power then goes back to being slow at 1300 rpm. weird huh? ive spent hours on every forum, cant seem to find much. could my stock injectors be coked from that little of veg oil driving? the oil is super heated and im religious about super long purges (full op temp on diesel before i switch over, long diesel purge at shut down) any ideas??
thanks!

1997 3500 12V DRW straightpiped NV4500, vacuum, fuel pressure gauges. 2 tank veg oil conversion. otherwise stock

 

My thought before I even finished reading your post was coked injectors.
Easy enough to pull one out.

Something is very wrong if you're getting 55 or even 40 psi out of the stock lift pump like a plugged up return line or overflow valve. Spec is 20-25 psi at idle 25-35 at 2200 psi.
Return fuel is essential for cooling and lubing the injection pump.
With the fuel pressures you're seeing the injection pump isn't going to last long.

 

have you moved the afc cover toward the back of the motor when you changed the fuel plate, or did you turn the starwheel at all? It sounds like you either have a boost leak in the boost line going to the afc cover or you moved the afc foot.

 

ive replaced the overflow valve with a brand new bosch one- wasnt it. ive relocated the position of stock fuel filter to the vacuum side of the lift pump (better access, no more $30 stock filter, used a great $10 goldenrod 10 micron water/fuel diesel filter canister style with goldenrod head) so could the stock fuel filter be restricting the flow to 22 psi instead of 40? Ive also sent both return lines back to the tank- before, the injection pump overflow line was the only one that went to the tank, and the hard line that connects the injectors was put back in between lift pump and injections pump. could the introduction of that tiny bit of fuel that drizzles from that hard line be enough to drop the pressure to 22 psi from 40?

 

I havent touched the afc housing or fuel plate since i got the truck. I had the previous owner (a self proclaimed "diesel mechanic" whos tinkering I dont trust one bit) put it all back to stock for me- i didnt want it turned up.. I want to go far, not fast) thing is it seemed to drive fine (ie responsive) for months after I bought it, and its losing power seemed to get progressively worse with the colder weather (northern indiana were having days in the teens low 20s, single digit nights) any way the cold could do that? Ive thought about sliding the fuel plate A BIT, or advancing the timing a bit to get some more power but I dont want to artificially fix the problem ie cover up something else thats going on.. Id really like to find the root cause.

 

The way your post reads it seems like you think your FP is too low.
What I'm saying is it's too high. The pressures you're seeing are like what I see when blocking off the return line completely to test the lift pump.
Where the injector return line returns shouldn't make a difference, it barely dribbles but I've often wondered if proper injector operation depends on the return line being on the pressure side of the lift pump.
Something is definitely going on with your high fuel pressure that needs to be addressed.

 

Infidel- I wince every time I get in my truck and see 40 psi, imagine how I felt when it was running 60.. I wish I could find out why its not at ~20 like its supposed to be. I replaced the overflow valve hoping the new spring rate was lower (which seems counter intuitive as the valve wears) Im going to pull the hard return line from the back of the IP and do the old blow test for restrictions (thats the only part of my return lines that werent replaced) and Im also heading out the door to get Lubro Moly to do a diesel purge. From my reading it sounds like coked injectors/varnished and sticky IP could be my low power problem so Im going to see if a purge helps that... That probly wont fix my high fuel pressure I have a feeling thats a whole nother issue. letcha know what happens! thanks for helping out

 

I did a diesel purge with Lubro Moly (the best) using 2 500ml bottles. truck blew some carbon soot onto the driveway, but not a ton. I stuck the supply and return hoses into an Aquafina bottle and took it for a drive. Fuel pressure was at 20 at idle, and the truck was fast! back to its old self.. I re-routed my lines, put the injectors hard line into the pressure side of the lift pump, and simply looped the return into the suction side of the lift pump. I tried to get both of those far away from my vacuum gauge line and FP gauge so as not to influence them. installed a tee with two ball valves so i can use the primer pump to get air out and not be looping the air pockets around and around. primed it and took it for a drive- seems ok! Id recommend a diesel purge to anyone with a high-mileage truck. (mine has 360K)

 

If I'm reading you right your main return line tees into the suction line to the lift pump rather than going to the tank.
No problem with that, I have several tractors that are plumbed the same way.
The only problem I see though especially with veggie is that the return fuel is extremely hot and will help heat your fuel tank up.

Few years ago out of curiosity I put the infrared temp gun to the bottom of my fuel tank on a nice even 0 F day, It was 0.
After driving 57 miles I measured again, it was 86°.

 

wow thats a big change in temp! Ive got my main veg tank (120 gal Freightliner alum) in the extended cab part of my truck (took the back seat out) so it picks up some heat there, and its gravity fed into a smaller tank that sits on the frame rail right below it which is coolant heated, so my truck is always drawing hot veg.. do you run veg in your cummins? ive noticed low power on veg in mine (at least at 15 degrees) so Ive asked around and talked to a pump shop here that specializes in Bio and they said mix my vegetable oil 1 to 10 with gasoline. so 100 gallons of veg oil and 10 gallons of gas... to put some explosive power in the fuel.. do you see any problem with that? I havent tried it yet but plan to

 

There was an additive called Diesel Secret used to make veggie oil more similar to diesel fuel.
Turns out it was just gasoline and Power Service and total ripoff.
Was nowhere near 10% gas though
Check it out www.dieselsecret.com

 

HA! yeah Ive been into veggie for 5 years now (owned and converted 4 early 80s Mercedes diesels, this is my first truck) But I remember seeing Diesel Secret, and remembering it was too good to be true, in which case it usually is.. I almost got suckered into buying some, but I didnt. Now Im glad I didnt

 

There's a claim on the Net somewhere that Diesel Secret is a common fuel additive called xylene. The "secret" recipe also includes ~5% regular unleaded gasoline, ~10% kerosene (winter diesel) and a jar of diesel conditioner. Variations of that recipe adjust for weather, fuel system mods, etc.. I've read that ethanol (up to 7%) can replace the gasoline in the summer and warm states.

If your aluminum fuel tank wasn't cleaned out real well after a few years of carrying diesel fuel, then that could cause power problems. Diesel isn't a detergent, but veggie oil and blends are more like biodiesel that way.