Alright guys, i have a 92 12v in a 77 W100. It has 3/8 supply line to a 2nd gen lift pump and has a 1/4 return line with custom fuel tank. The injection pump looked brand spanking new when i picked up the motor(dismantler). I have not made any mods to the pump and it still has all the anti "tweak" caps still on it (nobody has touched it prior). I have a HX35 with stock everything else. The motor sat for about a year from when i actually started it, who knows how long it sat prior to that. The valves have been adjusted and timing was not touched but is correct. So here is the issue. Up in alaska at 30 degree nights it sits all plugged in and warm, it starts right up every time with no hiccups. Somedays she will have a rough idle but smooths out just above idle(1000rpm) when it warms up, it still has a rough idle. Other days, she will be as smooth as a well balanced 360 cold or warm. Other days she will start off with a smooth idle and rough out again. I want to blame it on the poly transmission mounts but the fact it is hit or miss kinda steers me away from the bushings. What do you think the issue is? Clogged nozzles? Bad injector? Air leak? Not enough info? Thanks in advance.

 

From your description, it seems to happen fairly randomly. I'm not an expert on 91.5-93 KSB wiring, but the symptoms match. If it's shorting out or grounding, the engine vibrations could make it turn on (advancing timing) without any real warning

Good luck

 

So by the KSB opening(letting in more fuel) it would cause it to run a little rough huh? Should I try disconnecting it and see how she runs?

 

Are you running a regulator with the 2nd gen lift pump? The ve's max fuel pressure is around 15psi I think before it blows the front seal out. The 2nd gen pump puts out 25psi+. Too much pressure could be your problem.

 

No i am not, i was unaware of the regulator. What kind do you recommend?

 

I don't have a great grasp on what EXACTLY the KSB does, but my understanding is that it advances timing, which could cause a pretty good rattle. Sketchy wiring could cause it to turn on and off with the engine vibrations.

I have a low pressure piston pump, which is a common upgrade for 1st gens. The P7100 12v's use a higher pressure pump that looks the same on the outside, but as mentioned it puts out alot more pressure. What color is the dot on the pump? You're looking for a purple/violet dot, indicating the low pressure pump. Or is there a P/N on it?

 

Well I am pretty sure it's the high psi pump. I did the mod prob 3 to 4 years ago and I don't think that there was any one running a regulator yet. I did a search last night and saw a bunch of people running regulators. So I went ahead an ordered a FASS bypass regulator. Now as far as the KSB, I am guessing a faulty switch would cause it to go on and off, either that or my manifold never gets above 90*. So with the added high pressure of the lift pump and the KSB staying activated, it would probably give her a nice rough idle. Would you agree? Also, when I install the regulator will my 1/4 return line be sufficient for the bypass return? If so will the pressure overide the injector return pressure? Or should I just run a new return line? Thanks in advance.

 

I believe the later 1st gens used 2 different sensors for the grid heaters and KSB. Could be the sensor, wiring, or KSB itself. I'd hope it isn't the KSB itself, as they are pricey. I'm not sure how high fuel pressure could cause a rough idle, but I could be wrong. To my knowledge, the worst it'd do is push out the front IP seal, and you'd have noticed that a long time ago

 

If you want to rule out the KSB, just unplug it. I left mine unplugged this winter, now granted I'm not in Alaska, but we got into the 20s this year and i had no issue. Plugged it back in because i didn't notice any difference.

You know where it is? Its the cylinder on the bottom of the fuel pump. Easy to get to. There is the fuel shut off with 2 wires on it, then the other wire goes to the KSB.

 

Just unplugged it a little bit ago, seems the idle a lot better. Probably wont drive the truck until i get the pressure regulator. I would like to keep the front seal in one piece. I just have to figure where i am going to run the return. I think i am still going to ditch the poly transmission and transfer case mounts. I am sure those add to the rough idle. That motor already vibrates a ton and those mounts probably transfer a lot of it straight to the cab.