I have a 1986 F-250 (351W, carbureted) that suddenly died from fuel starvation. Testing revealed no fuel available at the carburetor so I checked the dual-tank selector valve and associated plumbing and it was found to be ok (and FWIW this vehicle does not have in-tank pumps.) Evidence was now pointing at the mechanical fuel pump so I tested and found that it had low suction (was barely able to lift fuel more than about 6" from a test container), so this confirmed a bad pump (or I guess a bad eccentric in theory, but that wasn't the problem.)

So... the replacement pump works fine, but now I have a new problem... the engine starts and runs normally for about 15 seconds, then will barely run, and the carb seems to be venting fuel everywhere so seems like I have way too high fuel pressure. I have not positively confirmed with a pressure gauge and will when I get one in a few days, but from the symptoms I have it seems pretty likely the pressure is too high. This pump has a fuel return line but I checked and it is not blocked.

The pump was from O'Reilys (I know, I hate 'parts store' parts but I needed to get the truck running) and is it possible it's just a crap part? If so I'm kinda pissed because the pump replacement was not an easy job on this vehicle, and would not look forward to doing it again.

Anyway... if I do confirm that pressure is too high, one option is replacing the pump again using a higher quality part and hope I get a better one, or I've also seen recommendations to just use a pressure regulator because mechanical pumps can be all over the place and a regulator is the best solution. What is your experience and what would you do?

 

More than likely the pump is good, and you just have a piece of dirt or chunk of rubber hose caught in the float needle in the carb. What carb do you have? If you have a holley 4bbl, most of those you can pull the needle and seat as an assembly out of the top of the bowl and inspect it and clean out any thing caught under the needle. If you have a Motorcraft 2100 or 2150, you can easily pull the top off the carb and then pull the float and needle assembly out and check it for dirt. Since you have been messing with the fuel system, I suspect this is your problem.

 

Great idea, not sure why that didn't occur to me. It's a Holly carb, and it looks to be OEM judging by all the emissions stuff on it. I was a GM guy back in the day and could assemble a Quadrajet blindfolded, but not much experience with Holly carbs. I assume that if I disturb the needle/seat assembly I will have to readjust the fuel level?

 

Yes you will, you can find instructions online for doing this. Maybe even a video on how to get the fuel level to the edge of the sight plug. If you haven't done this before it's confusing.

 

If you're going to start disassembling the carburetor you MAW do a full rebuild. Like you, I started on Quadrajets and GM carburetors (4GC). If you've rebuilt a Quadrajet the Holley will be easy. I still haven't gotten my hands on a Thermoquad.

 

I'd have to disagree with the first part of your statement. You have a new problem that only appeared after replacing the pump. Call me Mr. Vegas, but I'm willing to bet your reputation that your new pump is no good. It sure sounds like it is putting out too much pressure.

I wouldn't bother with a pressure regulator. A quality pump is fully capable of maintaining consistent output pressure.

Does your carb have a filter at the fuel inlet? That should have caught any crap trying to get into the carb. Replace that filter and cut it open to look for debris.

You are the second person in recent days to have mentioned how difficult it was to change the mechanical pump on a 351W. I've converted my 351W to an electric pump, but I don't recall access to the mechanical pump being all that bad from underneath. Looks miserable from the top, but it was no big deal from under the truck.

 

There is a filter in-line so not sure how I would have gotten any crap into the carb, but then again a new pump that was this bad out of the box would be a pretty sad affair and hopefully that's unlikely. I'm definitely going to do a pressure check first before going into the carb, and I guess the results of that will tell the story.

On my vehicle the power steering pump prevents access to the fuel pump and I couldn't see any practical way of doing the job without moving it, even from underneath. But it's not that difficult to unbolt the pump and bracket and move it out of the way.

If it is the pump I might just return the POS, block off the opening and replace with an electric unit and be done with it. What make/model pump did you use? Although for this application I guess they're pretty generic.

 

Au contraire, I think there's a very high probability of getting bad parts nowadays. I cringe every time I hear somebody say, "the (insert name of part) must be good because it's new."


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Great write-up, thanks. And no, I am not assuming that the new pump is good... hence this thread

 

So saying if the auto parts store has a few pumps that work on your engine, spend the money for the high price one... Seems now day's, you get what you pay for...

 

I did

In my experience 'parts store' (AutoZone, O'Reilly's, etc.) brands are always a crap shoot at best.

 

Oh dear, You might try tapping the carb bowl with a small hammer. Sometime that may help.

Holley's are easy to work on. You can remove the four long screws and pull the bowl off and inpsect the seat. I'm sure asking Google, How to ... will tell you how.

 

Do you have the factory fuel filter? Most of those have a spring behind the filter, if they get clogged the fuel pump pressure pushes the filter out of the way and lets everything go into the carb. You can also get it running, and while the engine is running, squeeze the fuel line shut, if it is over fueling it should run better for a little bit, and then stall. Then let go of the pliers. The float will be all the way down because the fuel bowl is empty, and when you let go from pinching the line, a burst of fuel should enter the carb. With the float all the way down this sometimes will dislodge what is under the needle.

 

Another good idea. I'm first going to verify fuel pressure when I get a gauge tomorrow and if ok then it's on to diagnosing the carb. The filter is just a simple generic in-line unit so no bypass, but if fuel pressure is ok then something must have gotten in there somehow because this is not just a coincidence and must be in some way related to the work on the fuel system. The filter is of unknown age and I probably shouldn't have re-used it even for a test as it's possible that the media is not sound, although output into a glass container looks clean. Anyway this sounds like a good technique since with the float bottomed out and the needle valve fully open an inrush of fuel might clear it unless it is a big chunk of something. I appreciate all the tips, it's been so long since I've worked on this old school stuff it seems I've forgotten 90% of what I knew.

To be continued...

 

It's highly doubtful that a mechanical diaphragm pump would be putting out too much pressure. I think your needle valve is not shutting off the excess fuel as Franklin suggests.