Bucking while driving.

[BRimpossible]

Hello I have a Vw rabbit 81 pickup with 1.6 hydraulic engine that I just barely swapped from a tired 1.5. 5 speed as well. It runs well and did a lot of switching with new parts such as new fuel injectors and lines. Right now the problem is that when I drive it and reaches the max rpm it would hesitate little bit like slowing down for a sec or 2 then back to normal. Happens quite often. I checked the IP and seems fine no changes after adjusting it. Dirty fuel perhaps? I haven't really checked the fuel tank and thinking might need to replace the fuel filter there. New fuel filter under the hood so that's checked off the list.

[BRimpossible]

Oh also the top speed is about 60-70mph. Some days it drives well other days it would drive weak like it couldn't reach 60 mph but then I live by lots of hills. Those hills aren't steep and yet my truck would struggle sometimes. Sorry for lots of questions not a lot of Vw rabbit owners around here where I live.

[ORCoaster]

Did you put a new fuel filter on it?  It almost sounds to me like you are developing a lack of fuel at the high end.  Maybe it is developing a vacuum and pulling air between the filter and the IP?  You may find that backing the fuel screw in just a bit might give you more even speed at the top.

[BRimpossible]

Yes new fuel filter by the engine but not the fuel tank. It was sitting for a yr with 1/4 tank of fuel. Any ways thought I'll give a update of what o have noticed. When it sits for few hours and I drive it in city it would drive great but once I get to highway it will buck then get worse the farther I drive it. So it does indeed sound like fuel starvation

[ORCoaster]

1/4 full tank for a few yrs sounds like the problem to me.  Pull the gauge sender on top and start cleaning.  Or pull the lower hose and clean.  Either way you are not doing well with whatever is in there and no sense adding new gas to a bad tank situation.

[BRimpossible]

Thanks I was afraid of that. I'll have it cleaned up and use air compressor to clean up the lines as well from tank to filter.

[TylerDurden]

I'd test by bottle feeding first... clear lines in and out of a clean container of fuel to/from the IP.

Don't guess. Test.

[libbydiesel]

Check that all pump mounting bolts are tight. 

[BRimpossible]

Thanks guys. Tyler ill for sure do that test first.
The ip pump bolts were plenty tight but I'll double check. They were tight before and after and still remain the same issue.

[libbydiesel]

To be clear, the bolt of concern would be the fourth one under the metal lines. 

[BRimpossible]

Did the test from a clear bottle filled with diesel and clear vinyl lines for temporary testing. Drove it and it had plenty of power and no more bucking. Time to switch out all the rubber lines and fuel filter along with gas tank cleaning. I'll adjust the IP again because it smoked white at the first start up in the morning.

[BRimpossible]

Oh also when I started the test, fuel was foaming from the return lines and took a while to clear up. The fuel going in the ip wasn't bubbling which is good.

[ORCoaster]

YES it is normal for an IP that has air in it to take awhile to clear it out.  It really needs to get revved up for a bit to make that happen.  Just a lot of places in there that harbor air for some reason.  Sounds like you nailed it.  I thought about the bottle feed long after I posted.  Glad others caught that.  It is so simple to do and isolate the problem to the IP or the two lines connected to it by using that method.

[BRimpossible]

Should I go ahead and drain the fuel or just drive until tank is empty? I have like 1/2 tank full still. If I have to, I'll reroute the line to 5 gal gas tank for temporary until I can get the gas tank clean out.

[vanbcguy]

Personally I'd drain it - I know disposal is a bit of a ***. 

If the fuel is contaminated then you're not doing yourself any favours running it through your pump.  Remember the pump circulates something like 90% of the fuel that goes in the inlet out the outlet again back to the tank so if the fuel in the tank is full of gunk it's going to pass through the IP hundreds of times before you actually use it all up.  The factory fuel filter is something like 30-50 micron, it isn't particularly good at filtering so don't rely on it alone to keep the rest of your fuel system safe.