Random surging while driving

[ISPKI]

Mornin' folks. Just had an issue come up with 94: While driving the truck will surge a little bit, acting like I suddenly hammered the throttle down. Usually lasts a handful of seconds and then goes back to it's normal sluggish self. Seems to happen once the truck is warmed up a bit, as in, it usually doesnt come up until ive been driving it for 10-15 mins, then starts happened every few minutes.

Doesnt seem related to speed or RPM or throttle position, does it at idle sometimes, RPM will increase by a couple hundred and become rough, again lasting for some 5-15 seconds time frame and then levels out again. Almost sounds like it starts missing on one cylinder but then why the sudden gain in power?

I havent been able to dig into it yet but trying to establish a place to start. I was going initially thinking air intrusion - truck has 210k on it and I have no idea about the age or condition of the fuel lines from the tanks or the lift pump, or any of those items. Figured I would start there.

However, some part of me has a sneaking suspicion if it could be an issue with injectors due to the randomness of it. If there was a faulty seal somewhere letting air in, I would expect it to be consistent. However, is it possible that one injector has some build up on it that is just enough to not stick until it warms up, material expands slightly and that expansion is enough to make it stick every so often? Seems like a stretch now that I mull it over.

 

[Cubey]

air intrusion, a failing lift pump, or a very blocked fuel filter

 

[franklin2]

Yes, the lift pump is sucking air in from somewhere. Do not worry about anything on top of the engine, it's going to be on the suction side anywhere from the pump back to the tank. Most common spot is the sending unit hard lines are rusted and have pinholes on top of the tank, but it could be anywhere you have a minor leak on the suction side of the pump.

The engine is surging and seems to have more power because the injection pump uses the incoming fuel pressure from the lift pump to operate the timing advance mechanism inside the injection pump. When the fuel pressure goes wacky supplying the injection pump, it affects the timing advance and makes the engine surge forward randomly.

 

[Jesus Freak]

They're on to something! Definitely air, but if you read @Datjosh thread, we guessed air though and it was loose lifters......WHO KNEW!!!

 

[KansasIDI]

Yes, the lift pump is sucking air in from somewhere. Do not worry about anything on top of the engine, it's going to be on the suction side anywhere from the pump back to the tank. Most common spot is the sending unit hard lines are rusted and have pinholes on top of the tank, but it could be anywhere you have a minor leak on the suction side of the pump.

The engine is surging and seems to have more power because the injection pump uses the incoming fuel pressure from the lift pump to operate the timing advance mechanism inside the injection pump. When the fuel pressure goes wacky supplying the injection pump, it affects the timing advance and makes the engine surge forward randomly.

My 1990 did some real goofy stuff when the IP died. I put all new fuel lines on it and actually put 18-20 psi in them to see if it leaked anywhere, blocked off the inlet to the IP to do this. No leaks at all. Ran fine for a while, but then the IP started going wacky and would idle up and clatter randomly, advance piston was messed up I think, replaced some O rings which helped but it ultimately failed. Of course, check lines and lift pump and filter, maybe even lifters too, before IP pump replacement, unless it hasn’t been done in a long while

 

[XOLATEM]

Yes, the lift pump is sucking air in from somewhere. Do not worry about anything on top of the engine, it's going to be on the suction side anywhere from the pump back to the tank. Most common spot is the sending unit hard lines are rusted and have pinholes on top of the tank, but it could be anywhere you have a minor leak on the suction side of the pump.

The engine is surging and seems to have more power because the injection pump uses the incoming fuel pressure from the lift pump to operate the timing advance mechanism inside the injection pump. When the fuel pressure goes wacky supplying the injection pump, it affects the timing advance and makes the engine surge forward randomly.

I am impressed and taken aback most every time you weigh in here...

Your knowledge and insight and the way you explain things...you would make an excellent auto and truck, diesel and gas vehicle teacher.

Gotta give credit where it is due...

 

[IDIBRONCO]

Yes, the lift pump is sucking air in from somewhere. Do not worry about anything on top of the engine, it's going to be on the suction side anywhere from the pump back to the tank.

Yes. This is most definitely the issue.

 

[onetonjohn]

Had the exact same type of symptoms, but much less frequent when I first got my first (86 see avatar) IDI. Turned out the truck had a racor water separator (that I didn't know about) and filter was really old. Once I replaced that, never saw the issue again.

 

[ISPKI]

Thanks for the info folks! The truck had been overhauled back in 2014, wheel bearings, transmission service, new torque converter, I believe a new IP (log says "fuel pump"), and only has ~15k on it after that work. I also put a new fuel filter on it at my last oil change earlier this year. Suppose i'll start with the fuel lines from tank to switching valve to lift pump and maybe just do the lift pump so at least I know its a new unit.

 

[ISPKI]

Well I started looking into this issue this weekend a bit (with what little time I had). Lift pump is on order so figured id once over the whole system. The fuel lines from tanks to FSV are brand new stainless, still with labels on them and the FSV looks relatively new, not an OEM unit, its plastic. Looks like shop work since I dont know any DIY mechanic who labels their fuel hardlines like this. Lines from return to FSV and FSV to lift pump arent new but look solid and are metal most of the way. Going to replace the lift pump once it comes in.

Popped the line from the top of the IP to the front right injector off and found that the ends were ovalled out pretty bad and it was completely dry of fuel. Seems like I might have a drain back issue somewhere in the return system. I dont recall, does that top line feed return fuel back to the IP or does it direct excess fuel from the IP to the return system? either way I replaced it with some clear fuel hose I had temporarily to check for air in the system.

Also - if I recall correctly, there are seals in the hardline from lift pump to filter head. I may convert that hardline with a softline with a check valve at the head.

 

[IDIBRONCO]

it direct excess fuel from the IP to the return system

This is correct.

 

[chillman88]

Also - if I recall correctly, there are seals in the hardline from lift pump to filter head. I may convert that hardline with a softline with a check valve at the head.

The bottom at the pump is a standard inverted flare fitting. The top is a parker vibra-lok fitting with the "olive" seal. As are both ends of the hardline between the filter and IP.

Return system leaks cause drainback and hard starts/stalls. Surging/stuttering while running will be a leak before the fuel pump (or at the pump) unless it's a MAJOR leak elsewhere.

After the lift pump is under pressure so that will be a fuel leak while running, before the pump is under vacuum so that will suck air even if it's not leaking fuel.

I just fought with this myself and it was a bad quick connector at the sending unit in my case.

It is possible for the issue to be internal to the FSV so if you can't find it elsewhere maybe try to bypass that? I don't think that's very likely but it's worth mentioning just in case.

 

[IDIBRONCO]

Well said, sir. I approve.

 

[ISPKI]

Awesome pointers as usual. My local parts store had a lift pump in stock so I picked that up since I have no idea how old mine is. Next step is going to be replacing that, then replace all the return lines since I have no idea how old those are. I did replace the O-rings in the caps when I first got the truck with #75 Viton rings.

I was also looking at doing the upgraded feed line from filter to IP from CDD. Not sure if anyone has experience with it but heres a link to it. Changes the fittings over to -6AN and adds a fuel pressure gauge port. Also - looking to see if there is a cost effective way to convert the hardline from lift pump to filter head over to a -6AN setup.

 

[franklin2]

Hold on to your old mechanical pump. I have been hearing rumors on here the new pumps don't last. That's what you get with parts changing now days. The new stuff is not very good quality, and you end up with more problems than you started with.

You were stating all the new stuff on your truck already replaced. I would not be surprised if your problem was with something new on the truck. For instance I think you mentioned you had a new switching valve, and someone just mentioned a faulty switching valve could be a problem. Might be worth bypassing it to eliminate it as the problem.