Low throttle surging?

[Silver Burner]

My neighbor's 89 non-turbo is having an issue that i'm confused about. He says his rig is surging when hes light on the throttle but goes just fine under WOT and sits there and idles just fine. Just wondering what you guys think might be going on. I told him maybe an injector going out or the IP. He just put a new lift pump on but it was doing this with the old one as well. Thoughts?

 

[79jasper]

Could be something to do with the advance in the pump. Maybe you could take a gander at it or even better, get a video.

 

[international]

My truck does the same thing. If you hold it at say 1200 rpm it will move on its own up to 1500 or so then back down to 1200 etc. figured my pump was on its way out.

 

[franklin2]

I would put a fuel pressure gauge on it, just to make sure you had a good constant supply of fuel to the injection pump.

 

[Silver Burner]

Thanks for the replies. Ill see about getting a video of it up when I can. Hopwfully we can figure this out..

 

[Wicked97]

I would put a fuel pressure gauge on it, just to make sure you had a good constant supply of fuel to the injection pump.

X2 start with that.

 

[C.J.]

My truck does the same thing. If you hold it at say 1200 rpm it will move on its own up to 1500 or so then back down to 1200 etc. figured my pump was on its way out.

The wife's diesel beetle did the same thing. I've always wondered what the cause of it was. The car has recently had a complete fuel system replacement since Fred Meyers filled her car with unleaded. It no longer does it so I'm inclined to think it is a pump related issue.