The Stovebolt Site Forums The Shop Area Engine Shop Vapor lock?

After running my 47 6cyl for a while and it gets very hot under the hood, the engine gets starved for gas and dies out. In most cases, I have an auxiliary electric fuel pump I can switch on and it seems to smooth out and will run okay 90% of the time. On hot days like today (90 degrees) it is not enough to keep the truck running and the engine dies. In most cases after sitting for a while and cooling down it will start right up and I can continue on my way. I know "vapor lock" gets blamed on a lot of things but in this case, I am not sure what else it could be. The rubber fuel line is hot to the touch under the hood and so is the carburetor. Short drives I typically don't have any problems but if I travel any distance this happens. Could the gas be getting too hot in the carb bowl.

I have a new gas tank. New mechanical fuel pump. New copper/nickel fuel line. Just the last one foot going to the carburetor is rubber. Newly rebuilt carburetor.

Any ideas?

Try mounting a small electric fan or a squirrel cage blower under the hood with a toggle switch in the cab, aimed at the carburetor. It's possible that the air coming from the fan is carrying enough radiator heat to heat, not cool the carburetor. This one is 12V, but there's probably something similar available in 6V if you're running an original electrical system.

www.ebay.com/itm/334921886428?

Jerry

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for Car&Auto Wire Loom Brake Cable (1/2" IDX10', BLACK)

A problem with insulation on a fuel line is that eventually the insulation itself heats up and then the fuel line can't cool down, even if it wanted to.

Strange that the insulation is black.

thats not right. if there is that much heat. its loose fitting

There are thousands of old Stovebolt engines running around in 90° heat with no issues. I’m not debating if you have a vapor lock or not, but I can see clearly, you may have other issues.

The first clue is you “new” fuel pumps needs an electric backup at times to continue your journey. The quality of these pumps as others on this forum can attest to is “crap”. They have a very high failure rate within the first year. Find an old pump American made pump and either rebuild it or have it rebuilt.

There seems to be an abundance of heat going to your carburetor. Do you have an insulator block between the carb and the manifold? Your fuel line may be routed too close to the block and exhaust manifold. It should pass in front of the block so the fan can help keep it cool before going to the carb.

Do you have a fuel filter in line? Check to see if it is plugged.

What thermostat are you running? You could try running a 160° and see if that helps.

Also check the float level in your carb as it may be too low.

Is your fuel tank vented correctly?

It's really hard to picture a electric fuel pump not over coming a vapor lock issue, but noo air getting into the tank will cause your issues.

My Carter W-1's get so hot you can't touch them after a hot drive, it has never vapor locked, even setting in a drive thru line on a 100 degree degree day with A/C on and a crappy flex fan.

Dave - One other possibility for the excessive heat is to check your exhaust manifold heat riser valve. It could be stuck or limited in range to cause excessive/continuous heat on the intake manifold and carb.

Gas tank is vented. I had a clear filter in front of carburetor and I could see there was no liquid gas in line. I turned on electric fuel pump and it filled fuel filter bulb. Started right up. Farther down road with electric pump running same issue and would not start. I tried removing filter and even though engine and pump was off and no visible fuel, it was under high pressure when I started loosing filter connection. I installed a heat shield this morning and fuel line insulation tomorrow and then try it again.

I`d try the truck with one fix at a time so you know what fixed the situation.

I drive Ol' Roy in 90-100 weather in rush hour traffic in downtown Atlanta with no vapor lock. Rebuilt glass bowl fuel pump (after two new ones failed), Carter YF carburetor with phenolic spacer, properly working heat riser flap, original radiator, 160 T-Stat. Engine temp stays at 180 and may creep to 190 if stopped in traffic.

First do a fuel pump test to check output volume and pressure. Make sure your heat riser flapper is working. Pick up an infrared thermometer and start shooting temperatures all over the engine, radiator, carb and fuel lines. Make sure water is flowing and engine is cooling properly.

I also believe you have other problems, none of which are vapor lock. Or, if vapor lock is present, your engine is not cooling itself properly.

If your auxiliary electric fuel pump does not correct the hot start problem, my guess is that your problem is not fuel related, let alone a vapor lock problem.
When coils begin to go bad, that fail when it gets hot and when the under-hood temperature is hot.
Once that sickly coil has a chance to cool down after a short while, it will go back to functioning somewhat properly.

You said no fuel in the fuel filter, so start at the fuel pump. The electric may not be able to push fuel through the mechanical pump when it heats up, stuck valve maybe???
Bypass the mechanical pump for a few test drives and run off the electric pump.

Fuel should be flowing through the filter even if the carb is overheating. If the line is over heating, it must really be hot to vaporize fuel with two pumps running.

This sounds like a real possibility......

Heat shield installed, fuel line rerouted away from the manifold and insulated. Took it out for a drive. Same issue. First time it started to stall I turned on the electric fuel pump and it smoothed out and ran okay. farther into the drive with the electric fuel pump running it died. It keep trying to start but wouldn't. I let it cool down for 15 minutes and it started up and got me home.

Next is the coil and fuel pressure test. I have done this in the past but I will check again on the gas tank vent.

Pull the fuel sending unit/tank float and check the strainer for clogging and/or collapsing.

Is your TEMP gauge registering HOT?

Drive it again without a fuel cap, or atleast a loose cap. With two pumps running, it really hard to imagine your mechanical pump isn't flowing enough. Sounds more like vent issue or hot ignitionl. Take a cold bottle of water along, if it dies out with the tank cap loose, dump the water over the coil or wrap a towel around it with ice to cool it. If it takes right off, buy a new coil. If neither fix it, look at the pickup tube BD mentioned above.

Even with a hot coil and no running, it will still have fuel in the carb.
No fuel is caused by a no Vent issue, clogged pickup issue, or in your case, two bad pumps.

Next time the engine dies on you and won't start, check to see if you have spark. Also look down the throat of the carburetor while goosing the throttle lever, looking for a squirt of gas. Those two tests will narrow down the possibilities as to what's wrong.
Heck, you could have a defective coil that craps out when it heats up internally but works fine once it cools off, AND 2 bad fuel pumps at the same time.

I have had an issue with a coil in the past and don't doubt it's a possibility but I am not sure it it was the coil why it would run okay for a while when I turn on the electric fuel pump. I will definitely try all the suggestions on the next test drive.

Don't assume anything.

I once had a car towed home and spent all weekend tracking down the problem. It was out of gas.

Another more recent issue was a clogged fuel line at the intake pipe of the gas tank. It was intermittent. The blockage would move around and then get sucked back in.

I initially thought that was the issue. I had a gas tank leak. I ordered a new one when I replaced the tank I discovered it had the original cork float that was deteriorating. I was relieved. I thought exactly what you described was happening and I solved my problem. New tank, new fuel line and new sending unit. Nothing changed.

I ordered and will be installing an inline fuel pressure gauge in front of the carburetor. If I do have vapor lock, what should happen to my pressure reading. Would it drop or go up. I am thinking if the gas is vaporizing pressure would go up, but wanted to get feedback from the experts.

I installed the pressure gauge. With the mechanical pump I have 4 1/2 PSI. When I turn on the auxilary electric pump the PSI jumps to 5. This is with the engine RPMs at 1500-1600. I let the engine run for a while until the engine was hot. No electric fuel pump. I had it idleing at 600 RPM. The engine had not stalled but I checked the pressure gauge and it was reading zero. I turned on the electric fuel pump and the pressure, after about 10-15 seconds jumped up to 4 PSI. This is what happens when I am driving. When the engine starts to cut out, I turn on the pump and after a few seconds it smooths out and continues until I guess it gets too hot for the electric fuel pump to keep up. I need to see if I can figure out why the engine is running at 190 degress. I have a 180 thermostat. I think the manifold flapper thingy is stuck. I will try that first and also confirm my fluid is circulating through the engine.

I have the gauge right in front of the carburator.

According to the FSM, max fuel pressure should be 4psi. 2psi to 4psi is the acceptable range. The needle valve in the Carter YF can be overpowered and the fuel bowl will overflow.

My truck runs on 2 1/2 psi.

I did some more test. See edited version of my latest test in my previous post.

Several things here:

1. 4psi is the maximum psi you should have.

2. Zero psi with the mechanical pump means it is defective and should be rebuilt or replaced.

3. 190 is not overheating (my truck stays at 180 most of the time but will creep to 190 or so in stopped rush hour traffic).

4. The FSM recommends a 160 thermostat but 180 isn't going to cause your engine to overheat.

5. What makes you think your heat riser flapper is stuck? Move it manually (when cold) to see if it is moving freely.

6. Is the spring intact on your flapper?


The T-stat does not govern the maximum operating temperature of your engine. It only allows the engine to warm up from cold faster by staying shut and keeping hot water in the engine. Once the engine reaches the temp stamped on the T stat, it opens up and water flows into the radiator. The engine will operate at whatever temperature is normal for it to operate. Modern cars usually run between 200 and 220 degrees.

A thermostat sets the MINIMUM operating temperature of the engine. If the coolant temp rises much above the thermostat setting in normal operation (not pulling heavy loads uphill, or other high stress situations) you either don't have enough radiator, or the radiator and/or the engine water jacket is dirty. Last weekend, I towed a heavily loaded trailer over several mountain passes- - - - -the temp gauge on my tow vehicle never moved above the bottom third of the gauge range. No, it wasn't a stovebolt, but it has a clean, properly designed cooling system.
Jerry

I did one more round of observations on the engine and fuel pressure. I started the truck with just the manual fuel pump. the fuel pressure was 4 - 4 1/2 PSI ideling at 600 RPM. I continued to allow the truck to run and get up to operating tempreture. Over a 20 minute period the fuel pressure slowly dropped from the starting 4 PSI to less than 1/2 PSI. I used a digital thermometer to double check some tempretures around the enging. The block was too hot to register with the thermometer. I think the limit it will read is 200 degrees. The goose neck on the engine at the radiator hose was 184 degrees. I peeled back the insulation on the fuel ine about 20 inches from the carbruetor and the metal fuel line was about 125 degrees. The metal fitting at the carbruetor where the fuel line attaches was about the same. 125 degrees.

Can this slow steady drop in fuel pressure be contributed to a failing fuel pump? If based on this latest information it still points to a fuel pump issue I think I will go with an electric fuel pump replacement. Is 125 degrees too hot for the fule line. should I switch to a rubber fuel line or is the metal one better. I am using Nickle/copper line for my fuel line.

The truck continued to idle and run okay with just the 1/2 PSI but I assume if I had waited longer it would have dropped to nothing and died.

Your fuel pump is shot. If it's an original pump, rebuild it. If it isn't, buy an original pump and rebuild that one. New pumps are not worth installing at any price.

Make sure your fuel pressure doesn't exceed 4psi.

My truck has all steel fuel lines and I live in Atlanta, GA where it's 90+ all summer long. The fuel doesn't get too hot. I use regular 87 deathanol. I also use only a mechanical fuel pump with no electric pump installed.

What did you discover about your manifold flapper? Is it working or not? If it isn't, your engine will never operate correctly.

It's hot under the hood of a car. 125 on a fuel line doesn't seem that bad.

That does not seem abnormally hot. Both fuel pumps bad?
A fuel pump would just die and not work sometimes when HOT??
With all he has changed I`d suspect something moving in front of the pickup tube in the tank.
But that does not explain why it dies when hot. I`d almost try running fuel out of a separate clean container
and do the same test.

I have replaced the tank since this issue started. New tank, new sending unit.

The diaphragm in the pump is probably stretched or something. Deathanol did that to the original 1956 fuel pump in my Cadillac when it was mandated in metro Atlanta by the Feds. The pump would only put out half the required amount of fuel.

Ethanol proof rebuild on the pump did the trick.

ANyway, it doesn't matter why your pump is faulty, replace it with a rebuilt original. Send your original to Then & Now Automotive for a rebuild. Top notch work and reasonably priced.

Then and Now Automotive

447 Washington St
Weymouth, Ma 02188
Ph: 781.335.8860
Fax: 781.335.1925
https://www.then-now-auto.com/

Has the tank vent been verified? Cap or vent pipe not plugged?

A slow pressure drop is strange, I don't see heat doing anything to the fuel lines or carburetor that would drop the pressure setting still at idle.
The heat riser would cause running issues if stuck open, not a fuel pressure issue.
You are down to the pump or the tank vent. The valves in the pump could be binding up as it heats up with the engine.
The next test should be bypassing the mechanical pump.

Any chance the electric pump is slowing the fuel flow when not running?

I will duplicate the test. Once the pressure drops i will remove the cap to confirm venting is not issue. If not. I will definitely move on to the mechanical pump. Current pump is a carterafter market. I will search for original to rebuild.

Two Thumbs Up From Siskel and Ebert!