Piston Slap: A Stovebolt starving from a lack of fuel?

[Hagerty]

I have a 1938 Chevy 1/2-ton pickup with an inline six-cylinder engine and manual transmission. The truck has an erratic problem. At times it starts, runs, and drives perfectly for months at a time. Then it begins to stall while driving and will not re-start until I prime the carburetor.
https://www.hagerty.com/media/maintenance-and-tech/piston-slap-a-stovebolt-starving-from-a-lack-of-f...

[tczarnina]

It sounds a lot like vapor lock. From the picture you can see that long expanse of stainless steel fuel line going into the carburetor. I would start by replacing that with a non-metal hose or try wrapping the metal parts of the fuel system with insulating tape. 

[CK]

Is the fuel tank and line leading to the pump clean? Debris can temporarily restrict fuel flow under suction. A fuel line restricted with tar or rust can lead to vapor lock. 

[DMcC]

Something adrift in the fuel tank was my first thought too.

[Larry2035]

Thanks CK, I have replaced the fuel pump, fuel lines and fuel filter. I've pulled the float and looked inside the tank which shows no rust. I have an air compressor for inflating tires, how could I hook that up to blow out the fuel line to see if it is blocked?

[ctaarman]

I had the same issue with a 1940 International D-2 pickup but lucky for me my problem got worse. It turned out to be a cracked feeder tube coming out of the fuel tank, which caused periodic loss of suction. ( I removed the tank and found the issue.)

While putting some air pressure on the tank, I also found two small areas that "weeped". So I had the irreplaceable tank sealed and thereby avoided another potential issue down the road.

[Larry2035]

Thanks for your reply. I have an air compressor for inflating tires. How could I hook that up to my blow out my fuel line?

[ede2357914]

I'm not sure if that has a filter on the pickup tube on the float but I have other cars that the filter on the tube would collapse and the car would stall like it was running out of gas. Then would run fine at times.

[XJ6]

In my experience a vapor lock happens when you turn it off and try to restart, not while driving down the road. I had a '49 stovebolt 6 for 40 years and never had problems with modern fuel. I would check the fuel pump, tank pickup etc as mentioned in the other posts. The multiple priming you did sounds like the float bowl is empty.

[Larry2035]

Thanks for your reply. I started the old truck a couple of days ago, it ran for a minute or two and then suddenly died. Wouldn't restart. I don't think it's a vapor lock. I've replaced the fuel pump, fuel lines, fuel filter, and installed a new carburetor. Nothing seems to help. Any ideas?

[Speedyz68]

I have a '52 3100 with the 216 in it. Take your fuel line off the tank side of the pump and blow back through it with a compressor set for around 20 psi or so. If this clears up your problem, the tank will need to be removed and cleaned out.

[Larry2035]

Thanks for your reply. I'll certainly try blowing out the fuel line...Larry

[62]

speedyz68 is absolutely correct with the problem. you have all sediments of dirt and rust over the years. while the fuel pump tries to siphons the gas thru the fuel line from the tank it draws all the sediment to that location until it completely blocks the opening. that's why it so random.

[Hagerty Fan]

I’m pretty familiar with early 50’s Chevrolet 6 cylinders and agree with those suspecting vapor lock…especially if the problem is made more frequent in hotter weather. But I do disagree with some of the suggestions…
I just don’t like the idea of sacrificing a section of metal line in that position above the engine to rubber. And IMHO an electric fuel pump increases leak issues and fire risks in the event of a leak . I’d keep both the stock pump and also keep the metal line as-is and insulate. Then I’d look to reduce radiant heat to it. You can do that with a shield but that would look odd. I’d rather have the exhaust manifold cerma-coated/heat coated. Your manifold looks nice but suspect it’s just header paint. Heat-coating looks very similar but does what the name implies. Then I would absolutely source only non-ethanol fuel. That alone will probable do more than anything else. And ethanol is an oxidizer and tends not to play well with some of the older rubber compounds, gaskets and seals anyway. Also, in my experience while not as common as when the car is stationary and/or just idling, vapor-lock CAN and WILL occur while driving.
A simple way to confirm the issue next time it happens…and to get it home, is to carry a bottle of water and an old rag. Soak the rag in the cool water and lay or wrap it around the section of fuel line just upstream of the carburetor. Wait a couple minutes and then try to start the engine. If it starts, it’s almost certainly a vapor-lock issue.
The only other possibility that comes to mind for your issue would be an intermittent sticking timing advance maybe???

[Larry2035]

Thanks Jim, The problem I'm encountering is really puzzling. I tried starting the old truck up a couple of days ago and it died the first minute or so. Just like someone had cut off the gas supply. Wouldn't re-start. Doesn't seem like a vapor lock. I'm at a loss to figure out what's wrong.

[Swamibob]

Hello Sajeev et al;
I believe this is a time for testing, testing, testing, rather than just throwing a bunch of stuff at the possible problem. I agree it's probably vapor lock, but it could be somethgin else. I like the idea of of a pressure gauge, in the fuel line, and it would give excellent data. But, I don't think we need to do that just yet. So, as Jim R pointed out, lets get the truck out and running and keep a rag and a really cold bottle of water (a bottle of water in a cooler with ice, so it's really cool would be best) and when the problem occurs, soak the rag in COLD water and wrap it around the fuel lines. If that fixes the problem, then the final solution is up to the end user on how to fix it. The fuel line looks like it's riding right on top of the valve cover, so that would impart a lot of heat into the fuel line.
Looking at the carb; I see it's sitting on some sort of adapter? I've not seen whatever that is previously. I don't think that's causing the problem, I'm just curious what it is?
We really need more info here; especially the circumstances of when the problem occurs. Full tank, partial tank, ambient temp, is the fuel tank vented? A replacement gas cap won't be vented, like the original cap, and that can cause problems with fuel flow under certain circumstances.
A compromised tank pick up, or screen, could be a problem. A rusty tank where material covers the screen sometimes, but not others could also be a problem. Fuel can always be a problem particularly if the fuel has a lot of ethanol in it. Modern fuel doesn't have any of the anti-boil additives like it did when all the vehicles were carbureted. So, fuel can boil really quickly when you park for a short period of time and the carb or other parts, get heat soaked.

[Larry2035]

Thanks so much for your reply. As Sajeev suggested, I have ordered a Holly fuel pressure regulator for the old truck. I'm hoping that will solve the problem. It is really frustrating. I started it up a couple of days ago, it ran for maybe a minute and then suddenly went dead. Wouldn't restart. Doesn't seem like a vapor lock. I've tried running it with the gas cap off, no change. Problem occurs no matter how much gas is in the tank. I've pulled the float from the tank and the interior of the tank is not rusty. Any ideas?

[Jnick]

My Camaro stopped dead once from a pinhole leak in the fuel suction hose. The car would run fine for a while then shut down at the wrong times. Check the garage floor closely after it has sat overnight for a small telltale drip.
My biggest success story however in terms of fuel if originality is not paramount is my 1939 Dodge truck which was converted to propane many years ago. No more fuel pumps an Iveco regulator which runs an engine heated water hose through it to prevent icing; very clean burning and though I still have other problems fuel is not one of them.

[Larry2035]

Thanks so much for your reply. As Sajeev suggested I have ordered a Holly fuel pressure regulator which I hope will help. I have replaced the fuel lines on the truck. New carburetor, fuel pump and fuel filter, replaced spark plug wires. Any ideas?

[Hagerty Fan]

@Swamibob 

I saw that too. A wild guess is that it might be a coolant manifold of sorts intended to warm up the carb before the days of an auto-choke? Looks like maybe the ports have been capped. Just a guess, but it looks like some kind of sender also goes into it.  Curious, b/c under my 52 carb there’s a bake-a-lite insulator…just the opposite purpose. I’ve never personally seen anything like that on post-war cars. 
Edit: I happened to visit my 93 year old dad this evening and showed him a close up but he didn’t remember seeing anything like that either.  Maybe the OP or someone else can shed some light?

[Larry2035]

Thank you all for your comments and suggestions. I am thinking of trying the fuel pressure regulator, hoping that will cure the stalling problem. I'm going to also insulate the fuel line to help against vapor lock. I tried starting it up yesterday. It ran for a couple of minutes and then shut down suddenly and refused to re-start. Doesn't sound like vapor lock. I've removed the float from the tank and cannot see any trash in the tank. I've had the truck a long time and have never been able to enjoy driving it. Out of frustration, I'm thinking of selling it.....Again, thank you all for your help.....Larry

[rickrichards1]

I am surprised someone hasn't suggested the older way of beating back vapor lock ,use a sheet of aluminum crinkled and wrapped around the fuel line . simple fix ,not much for looks. good luck. got to wonder if the pickup tube is cracked too.

[GRP_Photo]

In many old cars, the fuel pump was driven by a cam on the camshaft. If that's worn, the pump won't work well. My books are unavailable at the moment, but the old test for pumping problems is to disconnect the fuel line at the carb and connect it to a hose run to a jar. Disconnect the coil wire and crank the engine for a measured minute. I can't be sure of my memory, but there's a certain amount of fuel that the pump should deliver in one minute. If you have that much in your jar, everything from the tank to the carb is good.

[GRP_Photo]

Back with my books. Make the setup as I described here, but instead of pumping for a minute, cranks until you get 1 pint of gas in the jar and time that. You should get 1 pint in 60 seconds or less. You should also measure the pressure by hooking a gauge to the line. Normal pressure is 3-5 psi.