I need some solid information about vapor lock in an automobile fuel line.

When I moved down from Alaska, I brought all of my belongings in a 38 year old school bus that I bought for $1.00. (When I finished getting it roadworthy I had spent so much money that I made the guy who sold it to me give me half of my money back!) The bus has a 360 cubic inch Dodge V-8 engine feeding through a single two-barrel carburetor. It has a five speed transmission with a two speed rear axle, and in low gear/low range it would drive straight up a wall if it had enough traction. (Less than walking pace at 2000 RPM)

The bus has a complex fuel system -- dual tanks (25 gallon & 50 gallon) running to a selector valve, and then through two in-line filters and an electric fuel pump to the carburetor. (See diagram.)

If I drive the bus in warm weather (65 degrees or warmer) under load at speeds slower than 20 MPH, it vapor locks and quits. I have about 20 seconds warning that this is happening as the fuel pump is mounted under the floorboards and I can hear it starting to go "thocka-thocka-thocka" when it tries to pump vapor and I have until the float bowl in the carburetor empties to get the bus off the road. Opening the hood and letting excess heat escape for 10-15 minutes makes it work again. The engine is not over-heating; only the fuel system is too hot.

It was always my understanding that vapor lock could only occur on the input side of the fuel pump; that the pump couldn't pump gasoline "steam" and the fuel flow stopped; but that if the fuel were boiling in the output line between the pump and the carburetor the pump would keep pressure in the line and maintain fuel flow even if some of it were boiling.

I am beginning to wonder if this is a misunderstanding on my part.

The fuel lines are well away from any heat source, from the fuel tanks all the way through the selector valve, fuel filters, and fuel pump. From the pump to the carburetor is a different story -- the steel line is uncomfortably close to the exhaust manifold and exhaust pipe. Logically that is the most likely place for the fuel to boil. Arguing against that is the line should be pressurized, and usually when it vapor locks it is during a high fuel flow situation (crawling up a steep hill at maximum throttle in third gear low range at 15-20 MPH) so the fuel shouldn't spend enough time in the hot fuel line to boil. Also arguing against the boiling being on the output side of the pump is the fact that the pump is going "thocka-thocka-thocka" which strongly suggests that it is not receiving fuel from the input side. But the input side is not subject to much heat...

So, I don't know what is going on. Symptoms point to vapor lock on the input; logic points to vapor lock on the output side; [possibly faulty] knowledge says output side vapor lock shouldn't happen.

If somebody out here in bbs-land really knows and understands vapor lock from hands-on experience (lastdan, are you listening? It's been 10 months since we heard from you) I would love to hear from you.

tanstaafl.

Can you quickly wrap the fuel line with aluminum foil?
That might add enough insulation to keep it cool.

Sudden bursts of high pressure between the carb and the pump will push backward on the pump. That's how I'm thinking of your situation, anyway.
Also, at higher speeds the fuel flow rate might be high enough to keep the line cool.

Dunno- something to think about, anyway.

It's those 2 filters in the pump suction. What you've got going on has more to do with vapor pressure than heat. Prior to the fuel pump, you should only have a strainer, which is usually integral to the pickup line in the tank. Paper filters are high pressure drop items and should be in the pump discharge. One in the suction line and you'll probably get vapor lock. Two and you'll have it for sure.

This thing doesn't happen to have two fuel pumps, does it? I don't know anything about buses, but I know that some cars have a second fuel pump inside the gas tank, probably to prevent just such a situation.

Yes, aluminum foil may help out at least sort term. Old Chryslers are known for this.

Another thing you might want to check is if you are getting ANY restriction on the input side. If so the sucking of the pump will lower the pressure of the fuel. As you probably know lowering the pressure of a liquid causes it to boil at a much lower temperature. Another reason why most cars have in-tank fuel pumps.

Another possibility is that your pump is going bad. It may be generating its own heat and causing the gas to boil internally.

You might temporarily run a rubber fuel line from the inlet on the fuel pump to a gas can sitting in the bus (make sure it's not going to fall over) and then go for a ride on a hot day. That will at least rule out the input system.

For drag race cars they make something called a "cool can." You put ice in it to lower the fuel temperature so you get a denser air/fuel mixture. It's not necessarily anything you want to deal with but thought I'd bring it up.

Also gas is formulated differently for winter and summer driving. You may have some “cold Alaska gas” in the tank and that may contribute to its easier boiling point.

Only fuel injected stuff has that second in-tank pump (and not even the older fi stuff did). This bus probably originally had an engine driven pump that was replaced by the electric one.

Correct, just the one electric pump. The pump is new (replaced before leaving Alaska) and the fuel system was gone through thoroughly before leaving, to the point of pulling and steam cleaning the inside of the fuel tanks and under anything other than low speed high workload situations (climbing a long steep hill) everything works properly.

I know why this causes vapor lock (lots of heat generated by the engine working hard coupled with lack of airflow to dissipate the heat due to low vehicle speed) but what I really need to know is this: Is the vapor lock on the output side of the fuel pump where all the heat is, or on the input side of the pump where there is little heat but which I had understood to be the only place vapor lock could occur?

Perhaps a fuel system upgrade is in order--replace the fuel pump with a recirculating pump, the kind that recirculates back to the fuel tank the fuel that doesn't go to the carburetor so that there is always cool fuel in the lines. But this is complicated by the dual tank setup; I'd have to choose one tank or the other as the default recipient of the recirculated fuel. This could work very badly if both tanks were full and I was taking fuel out of the non-recirculated tank.

Hmmm... this gets me to thinking. How about a simplification of the whole system, like the attached? OK, it's not a simplification, is it? I get rid of one component (switching valve) and add a bunch of other things, but as a scatter-gun approach to solving the problem it ought to be sure-fire. I like the fuel pump redundancy idea, too. If one pump fails, I can continue on the other one. Center-off position on the SPDT switch also gives me theft protection, because God knows, the car thieves are out in force looking for 1971 school buses!

Are the check valves necessary, or are they built into the pump(s)? I don't want fuel recirculating to any tank other than the one that fuel is coming out of in order to prevent overflow in a full tank.

One problem I foresee is finding someone to do the work. Most mechanics are reluctant to work on fuel tanks, particularly if welding is involved, and I think there would be welding required to install the return line fittings to the tank. Are there any magic fuel-resistant epoxy adhesives that would work? I've used a fuel tank leak repair epoxy that worked for years after the repair. Could I drill a hole in the tank, thread a fitting into the hole and then gloop the hell out of it with epoxy? [Gloop = mechanic's technical term]

tanstaafl.

I like what Larry818 said about the two filters.
Do a quick test- take the filters out of the circuit and drive around in the conditions that cause the problem.
If your tanks are super clean, you've got (not much) to worry about (so far as a temporary lack of filtration is concerned).

I think you're still at the stage of diagnosing the problem- a bit early for proposing solutions.

The fuel pump will be fairly forgiving of dirt and there (should be) a screen in the pickup tube to protect the pump from large crap. Just take one filter and throw it away, and move the other to after the pump.

What's happening here (most likely) is that the fuel is boiling in a vacuum, not due to heat. There's only atmospheric pressure, on the suction side of the pump, to push the fuel through the filters. Each filter will have a pressure drop that increases with flow. Under high load conditions the flow will be the greatest. The vapor pressure of gasoline is somewhere between 6.5 to 8.5 psi. Note that 1 atmosphere is 14.5 psi. So, your pump is probably pulling the absolute pressure in the fuel line below this vapor pressure, boiling the fuel. Removing the filters will give you the most pressure at the pump suction.

If you wanna read up on this, search for the term "npsh".

I went out to the storage yard and climbed under my school bus to be sure that the fuel flow path was as I remembered. It almost was. I've attached a modified diagram to show the differences. Essentially, the two fuel filters are between each tank and the selector valve, rather than in series after the selector. That will alleviate some of the problem - the fuel on the intake side of the pump only goes through one filter instead of two. I also found a filter on the output side of the pump, and there is a fuel strainer attached to the intake of the pump.

So, while the fuel only goes through one filter on the way to the pump, there is another aspect to the fuel path that will make things worse. Maybe. The fuel pickup is (obviously) in the bottom of the tank. But the fuel exits from the top of the tank, so there is about a two foot elevation gain. From there the fuel line loops back down to about a foot below the bottom of the tank to the fuel filter, then goes back up about three feet to the selector valve before coming back down to the fuel pump. Hey - don't blame me, I didn't design this! In the normal course of events, these elevation changes wouldn't matter, there is a net elevation loss between the fuel pickup and the pump so fuel would flow to the pump through siphon action even if the pump were not developing suction. But in our case, where the vacuum in the lines may drop the line pressure to the point that the gasoline boils, the siphon may break. I'm not sure about this. Maybe the ups and downs are irrelevant. I've attached a drawing more or less to scale showing the vertical travel of the fuel on the way to the engine.

There are a few other data points that support the idea that the fuel is boiling on the intake side, not from temperature but from vacuum.

(1) The previous owner swears he never ever, not even once, had a vapor lock problem. After I purchased the bus the filters were added on the intake side of the fuel pump because the fuel tanks were filthy inside with the remnants of 20-year-old gasoline. (That's how long the bus had sat parked, and why it cost me so much to get it going). The shop that did the work no doubt thought they were doing the right thing.

(2) There is virtually no engine or exhaust heat anywhere near the fuel tanks and fuel lines until after the fuel pump, yet the problem is absolutely hot-weather related. I'm not talking about weather hot enough to boil gasoline on its own, but warm enough to help it boil in a reduced pressure situation.

(3) Every time I had vapor lock occur, it was apparently on the input side of the pump because it was going "thocka-thocka-thocka" just as it does when one of the fuel tanks runs out (there is a fuel gauge on only one tank, so I run the other tank to empty before switching to the "gauged" tank).

The shop that did the work on the bus did a good job of cleaning the tanks. The fuel filters have clear plastic shells, and after 4000 miles one filter is still completely clear and the other one has just a trace of sediment on the bottom. Both filters are simple in-line filters and all I need is a screwdriver and a couple of feet of 3/8" fuel line hose to take them out of the system.

When I do that, I think I should replace re-route and insulate the fuel line between the pump and the carburetor to get it further away from the exhaust headers.

This has been an interesting and informative thread for me. I consider myself quite knowledgeable about internal combustion engines, but this was new territory for me.

I would, however, like a definitive answer as to whether vapor lock can occur on the output side of the pump, that is, between the pump and the carburetor.

tanstaafl.

I'm thinking the elevation changes are no big deal, it's only the length of line that will cause friction, and gas in a carb setup doesn't move fast, so essentially no friction loss.

You mentioned clear plastic filters. Are these tiny? The ones that were clear that I remember are. A good fuel filter should be about 2" dia x 4" long (just swag numbers here). Can you take a pic of them?

That fuel strainer is all you need on the suction side.

Filters are maybe 1.5" diameter, 2.5" long. Not tiny, not what I would call full-size either. Pictures are difficult as the bus is parked in storage a good bit away from me.

What you are hypothesizing about the line pressure dropping below the vapor pressure of the gasoline makes sense, but it is hard to reconcile with some of the observed behavior. Specifically, if it were caused by high fuel demand lowering the input line pressure, this problem would be self-correcting the moment the engine stopped running, yet the only remedy I have found is to park the bus and open the hood for 5--10 minutes and let the heat dissipate. It is clearly heat related, both from the point of view that it only happens on warm days, and only happens when the engine is generating extra heat under heavy load. And yet the entire intake side of the fuel system is well away from heat sources (exhaust, engine) while nonetheless behavior points towards vapor lock on the input side of the pump (pump runs as though it is not receiving fuel).

I guess I'll just have to pull those filters out and the next hot day take the bus out for a nice hilly drive.

What's your best guess as to whether the output (pressurized) line can vapor lock? Might it be dependent on carburetor design, that is, how does the carburetor handle vapor instead of liquid entering the float bowl? It seems likely that the float bowl would be vented to accommodate such an occurrence.

tanstaafl.

So you're saying you wait ages for a fuel filter in your bus, and then three come at once?

I'll get my coat.

Peter

I don't think you can get vapor lock on the discharge side on this car. The carb is probably a Carter (or Holley if it's been replaced) and are just vented.

The metal fuel line on my '66 Chevy is anchored to one of the exhaust manifold bolts, and it has no fuel problems. The alternator is bolted to the other exhaust manifold. GM is such a wonderful company...

I'm sure it's your filters. They all use the same filter medium, so the smaller the filter, the higher the pressure drop. Gasoline also changes it's vapor pressure by a couple of psi from cold to hot, so it's reasonable that, if it's marginal on cold days, it'll lock on hot ones.

I'll have to ponder why the lock persists for so long. I have a '92 Volvo that vapor locked on me recently. It has a totally unnecessary in-tank pump and a much larger pump near the tank under the car. The in-tank pump died and became a massive pressure drop in the line. The main pump vapor locked (it's lower than the tank) and it did take about five minutes to clear.

Well, I don't think it'll just snap back into the liquid phase in one go. It's a while since I last did any thermodynamics, but isn't it the case that boiling the petrol by lowering the pressure will (as for any liquid e.g. CO2 fire extinguisher) cause a drop in temperature to below ambient, so heat will then flow into the fuel line and vapour from the surroundings, resulting in vapour that's not in equilibrium with the liquid phase, and so even when the pressure returns to normal (which heats the vapour even more) it needs to cool down again before it recondenses?

Peter