Hesitation during acceleration/rough idle

[analogjesus]

My baby is acting up, and I'm not sure where to start with the diagnostics. It idles roughly but at the same RPM (the tach only works sometimes so I don't know at what rpm. . . but its steady, just rough)

On top of that the external fuel pump is making a very loud buzzing noise, and as of recently it will occasionally (its becoming more frequent) hesitate when I accelerate at WOT. Hesitate as in act like its running out of gas, but the tank is always at least 1/4 full.

At first I thought I just needed a new external fuel pump, but after doing some research there seems to be a lot of other things that could be wrong. The in-tank pump could have shit the bed increasing the load on the external pump. I read this could be caused by a rusty gas tank which makes sense since I'm pretty sure its rusty as hell.

So I'm not sure if I should drop the tank, clean it, and replace both pumps. The problem with that being its like $400 for two new pumps from Pelican and I would prefer to rule out any other issues first. I ordered a new fuel filter and I'm hoping replacing that will solve the problem, but with my luck I may need to buy a new tank.

Anyway I know this type of issue can be caused by TPS, CPS, o2 sensor, vacuum leaks, etc. I just want to know where you guys would start, other than the filter which should be here next week.

Hopefully its not too weird to say but if anyone in the northern MA/southern NH area wants to stop by and help out a little I'll buy the beer. I'm not a rapist, promise :D

[SlamedIAm]

I would start by running the suite of fuel diagnostics laid out in the bentley. You can visually inspect your tank by pulling the pump and just looking into it with a flashlight. You can also check for rust by using a magnet but you should be able to tell visually.

A smoke test is the only definitive way to diagnose vacuum leaks. When was the o2 sensor last replaced?

[dnguyen1963]

smoke test, fuel filter, internal pump, external pump, and CPS...in this order. Test them first before replacing. Before dropping the tank, take a look at the pump sock to see how dirty or how much rust you can see. If the sock looks clean then your tank is probably okay.

[analogjesus]

I replaced the fuel filter, found a small leak in one of the hard lines and patched it. That didn't help so I did the internal pump. That didn't seem to fix the issue, although my old one can be confirmed deceased, so that may have been a contributing factor. Just for shits and giggles I pulled the airbox and checked the resitance between pins 1 and 3 while I manually manipulated the flap, seems fine.

I also tried checking the fuel pressure to the rail, my shitty gauge was leaking though so I'm not sure how accurate a number I got but it fluctuated between 35-40 psi where it should be right at 43 psi according to the Bentley.

So I'm thinking my next step is the external pump, although I want to re-check system fuel pressure just to be sure. Oh I was wondering how do our external pumps fail? Do they provide less and less pressure until finally they die? If so I believe it could be the external pump but if they just get noisy until one day they're dead than I don't think its that.

[jlevie]

The failure of the high pressure pump can be sudden and complete, or it may simply degrade. Meaning that it can't supply sufficient pressure/volume. But in this case, the most likely cause will be intake leaks, aged O2 sensor, mis-adjusted valves, or aged ignition system/plugs, in that order. A smoke test will find intake leaks, a new OE/OEM O2 sensor & ignition system parts, and the valves can be adjusted. Use NGK ZGR5A plugs.

[Derptastic]

Like everyone else suggests, get a vacuum leak test done. I had rough idle and hesitation and found leaks in my crankcase vent hose, rocker shaft seal plugs, and through an intake manifold gasket.