How to prime a mechanical fuel pump

So, your vehicle ran in the loss. You take it out of storage after a winter snooze, and crank and also crank and also crank it, and it won’t begin. What’s up? If it’s a carbureted auto via a mechanical fuel pump, it may have actually a priming concern.

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Nearly on all fuel-injected cars have an electrical fuel pump. The pumps on at an early stage injected cars switch on immediately when you crank the vital to ignition. Later ones have actually a safety and security interlock that frequently needs the visibility of air flow right into the engine so that they won’t continue to pump fuel if the vehicle is in an accident. But in either case, electric pumps provide gobs of fuel pressure almost instantly, which is just one of the points that helps fuel-injected cars begin even if they’ve been sitting for a long time.

In comparison, most carbureted cars have, or initially had actually, a mechanical fuel pump through a rubber diaphragm inside that’s relocated in and also out by a lever before or a pushrod generally moved off the camshaft. Tbelow are numerous reasons why this regularly creates a situation wbelow the vehicle is tough, sometimes impossible, to begin after it’s been sitting all winter.

First, electric fuel pumps on fuel-injected cars deserve to easily output 60 psi, often regulated down to 30-ish psi and then the excess fuel dumped earlier right into the tank via a rerotate line. That pressure pushes fuel very quickly. In comparison, the fuel press produced by mechanical fuel pumps is much less, typically in the 1–4 psi array.

Second, this is vital bereason the fuel device needs to be primed—the lines in between the tank and also the engine have to be filled via fuel. On a fuel-injected auto through an electrical fuel pump, this happens in mere secs, but it’s different in a carbureted auto with a mechanical pump whose fuel press relies on engine rpm. If there’s still fuel in the float bowl, you have the right to mash the accelerator pedal a couple of times, and also on a carburetor through an accelerator pump, it’ll squirt gas from the float bowl dvery own into the throat of the carb. That’ll help the engine to start, and also when it’s running, the higher rpm will make the fuel pump run much faster, making it much easier for it to prime the device. But if the car’s been sitting long enough that the fuel in the float bowl has actually evaporated, the fuel pump will have to fill it. So you have the right to get into bootstrapping worry wbelow, when the engine is being cranked off the starter, it’s spinning just a fraction as rapid as it is as soon as its running, and also combined through a weak battery, an old starter, an original mechanical fuel pump in which the rubber diaphragm has actually gotten old and also shed some of its elasticity, it may have trouble priming.

The third issue is that, in enhancement to the press distinctions in between mechanical and also electric fuel pumps, they work-related in various ways. Mechanical fuel pumps suck fuel by producing an oscillating vacuum on the inlet side. In comparison, electric fuel pumps, at leastern the rotary ones, produce a relatively constant fuel press, and also while they obviously need to attract fuel from the tank, they push even more than they suck. This is why mechanical fuel pumps are installed on the engine whereas electrical fuel pumps are placed at the rear of the auto, frequently in the gas tank itself.

Lastly is the problem of vacuum leaks in the fuel lines. As rubber fuel hose gets old, it dries out, and it’s feasible for air to be drawn through the sides. On the vintage BMWs I own, there’s a short area of rubber fuel line in the trunk that connects the pick-up tube in the job tank to the steel or plastic line that runs the length of the car. That line is under a screwed-down trunk panel, so it’s prevalent to discover that it’s the original 47-year-old item and also it’s become dry-rotted. If the car ran last winter however won’t fill the float bowl in the spring, this is a prime culprit. If it’s original, rearea it. Be specific the hose clamps are tight. If the fuel line is plastic, examine the finish of it thoaround for cracks and also splits.

If the fuel system won’t prime, the first area to inspect is the connection to the gas tank. Rob Siegel

Of course, it’s feasible that the mechanical fuel pump really has actually died—that the rubber diaphragm has actually cracked or torn, or that the little inspect valve inside is stuck open. So just how to you test it?

Obviously, first make certain that the difficulty actually is fuel and also not spark. Take the facility wire out of the distributor and host it via rubber gloves or insulated pliers one-quarter inch from a great ground while someone cranks the engine. If you view spark, good. Then, if you prefer, you can host open the throttle, offer a good blast of founding liquid, and attempt to begin the automobile. If it runs for a couple of seconds, then dies, you definitely have a fuel shipment difficulty.

If you have a low-press electrical fuel pump, or even a pump bulb (prefer for a small gas boat engine), you have the right to temporarily splice it in and fill up the float bowl. If it won’t fill, then you have actually some other problem prefer a fuel line obstruction or a clogged fuel filter. If as soon as the float bowl is full, the auto still won’t begin and also run, you have actually some various other difficulty in the carb such as plugged jets or accelerator pump nozzle. The irony, though, is that if you prime the device and also the auto then starts and also runs, you’ve made it harder to find the root reason of the inability for the device to prime itself.

Here’s what I perform to test the mechanical fuel pump directly:

Uncarry out the gas lines from the fuel pump and unbolt the pump from the engine. A lever of some sort will be located at the base. The one on my Lotus Europa protrudes by a number of inches and straight contacts the camshaft, yet others may usage an intermediate pushrod and also have actually a much smaller sized lever before mechanism. It might look choose a small lever or it might be a spring approximately a plunger. It may be recessed inside the base or it may protrude. If it is recessed, usage a wood or plastic probe to move it. If it’s exposed, you have the right to just push the fuel pump down on it on a block of timber. Listen for a sound vaguely like inhaling and exhaling out of pursed lips. If you hear nothing, the diaphragm might be torn, and also the pump is likely bad.

The integrated lever before on my Lotus’ fuel pump is massive enough that it deserve to sindicate be ordered and pumped. Rob Siegel
Pushing dvery own the fuel pump spring/plunger from my BMW 2002 on a block of hardwood. Rob Siegel

If the pump passes that test, repeat it while placing a finger on both the inlet and outlet fittings. You must feel suction on the previous and pressure on the latter. If you don’t, the examine valve inside the pump can be stuck open up.

Next, test that the pump actually moves liquid. You can perform this via water, yet bereason of issue over gaining water right into the float bowl, I do it with gas (be certain to have a fire extinguisher at the ready). Put a brief length of rubber gas line (that you’re specific isn’t porous) on the inlet and also outlet sides of the pump. Placed the inlet hose into a container of liquid and the outlet hose into a catch container. Work the lever of the pump back and forth. It usually doesn’t take much effort to relocate liquid and convince yourself that either the fuel pump actually pumps fuel or it’s dead. If it is dead, whether it’s rebuildable or worth restructure varies significantly model to design. Sometimes the diaphragm can be easily reinserted, yet various other pumps have actually permanently crimped height and also bottom sections. As with many type of vintage vehicle parts, if originality is important to you, tright here are specialty shops that redevelop fuel pumps.

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The standalone liquid test. Rob Siegel

If the pump appears healthy and balanced, repeat the test through it mounted on the engine. That is, mount the pump back onto the engine but proceed to feed it gas or water using a hose right into a resource container, and also the output sent right into a catch container. It’s simplest to perform this via a helper that cranks the engine, or to usage a remote begin switch hooked to the starter. Be specific to disattach the wire from the “+” or “15” side of the coil so the engine won’t try to begin. If you like, you have the right to rerelocate the spark plugs to make the starter spin quicker. If no liquid pumps when the pump is attached yet it did when it was tested alone, you may still have actually an air leak you haven’t discovered. If not, the difference could be that loss of elasticity in the diaphragm, or in excessive situations, wear in the pump rod/lever—or also the lobe of the camshaft—has tipped it over the edge to non-functionality.

The in-automobile test. If you’re doing this through gas, be really specific to have actually a fire extinguisher nearby. Rob Siegel

But if it does pump fuel from a little container, re-install one link at a time. Try the inlet side—the link to the fuel tank—first. If it doesn’t pump, trace it dvery own. You’ll most likely uncover a porous rubber gas hose, or a cracked plastic one. Then reaffix the output side. It’s feasible that the needle valve in the float bowl is stuck closed.

So, through all that, I have the right to tell my mechanical fuel pump story. I just went through whatever above on my 48,000-mile 1973 BMW 2002. It had actually a tough time beginning as soon as I bought it last loss, however that wasn’t surpincreasing because it had sat for a decade. I primed it via an electric fuel pump, and the trouble greatly went away; after that it started and also drove fine. But once I roused it from its winter snooze, I confronted the same trouble and wanted to resolve it when and for all. I went via the above actions, discovered that the mechanical fuel pump functioned fine both in standalone testing and once reattached to the engine and also fed gas from a deserve to, yet didn’t job-related once the gas lines were hooked ago up to it. There was a fuel filter on the inlet side of the pump, as tright here normally is on a BMW 2002. I removed both hoses and also the filter and also reinserted them via a solitary brand-new section of fuel hose run straight from the plastic fuel pipe coming with the firewall to the fuel pump inlet, and it pumped fine.

What was going on?

The difficulty was the fuel filter.

No, it wasn’t clogged. No, it wasn’t leaking.

The cause shows up to be that mechanical fuel pumps aren’t really designed to have actually filters on the inlet side—displays, yes, however not canister filters through any kind of considerable volume to them. I would’ve sworn that this was the stock area for a BMW 2002 fuel filter, however on digging into it, I learned that these cars have a display at the bottom of the pickup tube in the trunk, and also a 2nd one inside the fuel pump, but that, when delivered brand-new, had actually no exterior fuel filter. They were added by repair shops and also DIYers and also ended up being so common that I and also the majority of various other folks assume they’re original.

There’s the institution of believed (which I always followed), that the filter should go before the fuel pump because that method it keeps rust and also sediment from the gas tank out of the pump. The other argument, by the means, for having the filter on the inlet side of the fuel pump is that if the filter splits or leaks, the pump will simply suck air and fuel circulation will speak, whereas if it’s on the outlet side, fuel will certainly continue to be pumped right into it and also it’ll leak in the engine compartment. While I don’t discount either of these disagreements, it turns out that, with a mechanical fuel pump, having actually the filter on the inlet side have the right to be problem bereason, if it’s not full of fuel, the fuel filter real estate then holds a pocket of air that the oscillating pressure of the fuel pump can’t suck out. As I shelp above, integrate this with an old fuel pump and also a weak starter motor and it can be sufficient that the system won’t prime. This appeared to be the case through my auto. Fuel filter on inlet side of pump = no prime. No fuel filter at all or filter on outlet side of pump = prime and start.

For years, as soon as resurrecting long-dead cars through rusty gas tanks, I’ve cleaned the gas tank by throwing a chain into it and also shaking the tank, getting to into it (if I can) and also scrubbing it with a Scotch-Brite pad, then splicing a canister filter into the gas line in the trunk to capture any type of residual rust and sediment prior to it gets into the electric fuel pump and damages it. However before, I currently realize that this has actually been on fuel-injected cars, whose rear-placed high-push electrical fuel pumps aren’t as most likely to have the priming problem described above.

I’m not saying that every fuel filter installed in front of a mechanical fuel pump will reason problems. I’m just saying that, if you’re enduring a priming concern, add it to the list of possibilities.

Go and find your prime suspect.


Rob Siegel has been creating the column The Hack Mechanic™ for BMW CCA Roundel magazine for 34 years and also is the writer of 5 automotive publications. His brand-new book, Resurrecting Bertha: Buying earlier our wedding automobile after 26 years in storage, is obtainable on Amazon, as are his various other publications, favor Ran When Parked. You can order personally inscribed copies here.