The Stovebolt Site Forums The Shop Area Engine Shop Vapor lock

It's back.... '56 GMC 270 motor.

I rerouted the fuel line over the top of the motor and covered with rubber fuel line to insulate. Still happens.

I'm using 87 octane ethanol fuel.

Anyone have suggestions?

Thanks in advance.

I think the rubber fuel line would only make the matters worse. Is your gas cap vented? Had one that wound stall a few years ago and it would up being the gas cap vent was plugged creating a vacuum on the fuel tank.

Thanks Roy,

The cap is vented. The rubber is insulation for the metal line.

Any other suggestions? I did a search and found nothing for vapor lock.

If your gas cap is OK how old is your fuel pump? Vapor lock is often indicative of a weak pump and a precursor to pump failure.

Tiny, I'm not sure of the age of the pump.

The motor has never stalled from vapor lock. It happens after being shut off for a short period of time, then is difficult to restart.

you start off as if everyone has been following your daily life - we're not the NSA, elaborate on the symptoms and situations and the vehicle

why call it "vapor lock"? do you know that the liquid fuel in the line is vaporizing before it gets to the carb? vapor locking needs heat sinking, not insulation, and rubber tubing isn't either - if it's vapor lock, then squirting a bit of gas into the carb should start it, how about if it's a carb problem and the engine is flooding when shut off?

Bill

Come on Bill, I know you have nothing better to do than follow my foibles and antics as I try to keep this old farm truck on the road.

I've never tried to add gas to the carb when it happens. I'll need to get a can of starting fluid and see if that works.

I developed a vapor lock problem during the heat of the Phoenix summer last year. Turned out to be a faulty fuel pump.

Matt

Have you been driving your truck in the winter with no problems?

Is is a summertime only problem?

Yes, I drive it all year. Problem when it gets hot like today 93 degrees. It stalled on me today and was hard to start. With full choke it started. Had to feather the gas until it ran smooth again.

What thermostat do you run?

180. It stays dead on no matter how fast I drive. Will raise up to 190 after coming off the Interstate at a stoplight.Then drops back to 180 when moving.

I'm not familiar with your engine

Ill post my experience with a 1950. 1954 chevy 235. Carter carb.

I run 180 stat in the winter.
I was experiencing same issue you are in hot weather.
After I would drive a bit in the afternoon and stop. It would not want to restart right away.

This happened about 4 times. I switched to a 160 stat and the problem seems to have gone away.

Again. Not your truck or engine, but might be worth a try.

carb is boiling dry when shut off - do you have the proper thick insulator under the carb?

Bill

Bill, I have the phenolic spacer that was on the motor when I got it. spacer

I am looking at your pictures and noticed how the fuel lines are routed on your engine. The routing is vastly different that what I have seen on say, an AD. I can see from your pictures that your truck is earlier than an AD (assuming your fender pictures go with the engine we are discussing). I am wondering if your current routing is contributing to your problem. BTW, your ability to straiten out totally trashed fenders makes me sick.

Carl, I routed the fuel line totally away from the exhaust to try and keep heat from it.

As far as the fenders...no one told me they were too far gone so don't waste my time.

Wayne, my question about the fuel lines was not where they clear the manifold, but where they go down between the water pump and the timing cover. Understand that I am not familiar with your vintage of truck, and the way you have the lines routed may very well be correct for your truck, but I was just observing that the way you have your lines routed is quite different than what is on an AD, and I was just throwing it out there as a possibility. I'm still envious of your fender banging skills, BTW.
Carl

Carl, The lines were already in place when I bought this GMC 270 motor. I now have them routed over the top of the valve cover. It didn't fix the problem.

I am going to change my thermostat to 160 degrees and see if that will work. If not I'll spring for a new fuel pump.

Try this link:

http://www.thecarburetorshop.com/Troubleshooting.htm#Hardstarthot

Jon.

Thanks Jon. I'll use this info next time it happens.

Good Link.

This just happened to me. Ran several errands. Stopping the engine once for less than 1 minute. It fired right back up not problem.

Then went to wal-mart and shopped for 20 minutes. Came back out. Temp was showing probably 160. Would not start. Flooded. Put accelerator on the floor and it would try to start but not quite. Didn't want to run battery dead. Went back in the store for 20 minutes. came out. Put accelerator on the floor with the left foot and hit the starter with the right and it fired up.

I'm running NO thermostat.
I don't have the insulator piece under the carb but will get one. Won't raising the carb require me to rework the throttle linkage?
My gas cap is origial with origial rubber that has gotten hard. It does not seal pressure or vacuum.

Any other things to check for?

Rob, adding the insulator shouldn't require more than threading the rod out a bit, it's not thick enough to change much .... and you really should put in a thermostat, at least a 160, the thermostat sets the minimum temp the engine runs at, which is good for longevity, running it cold all the time is not good

Bill

I replaced my thermostat today with a 160 degree. I still have hard starting issues when warm.

Also I had it sputter several times when merging onto the Interstate. It cleared up after spitting and stuttering for about 5 seconds, all while driving about 50 mph.

Next I going to order a new fuel pump.

Anyone have other ideas?

You could try taking the gas cap off and see if that makes a difference, that would confirm that your tank is venting.
If it were me I would replace the fuel pump.

Has the carb been rebuilt recently? If so maybe the float is closing the fuel off too early.(Set too low)

The routing of the fuel lines in the photo is exactly as my 270 (a 43 by engine SN) so I don't think moving them or insulating them is an issue.

I read the link from the carb shop and its basically saying to do what you would do for a flooded engine (well, its in my manuals).

My manuals procedure for a flooded engine:

Keep the ignition OFF
Pull the throttle (my truck has a manual throttle cable, I think it was a common item in the 40s) full out
Push the starter foot pedal for 10 seconds.
Shut the throttle cable.
Start engine normally.

This basically would open the throttle fully and allow just air into the intake to try and evaporate the fuel in the intake (the air under the hood would still be quite warm and with no gas in the air it would not drop in temp). Since you are not using the accelerator pedal the accelerator pump would not actuate (the linkages were designed that way).

One other thing I was told for 'hard starting' was after you stopped to pull the throttle cable out, opening the carb fully to air and allowing the carb/intake to 'cool'. Not sure if I buy that.

One thing I would check not mentioned would be if your heat riser was opening/closing properly. If its stuck 'shut' it may be heating the intake all the time.. making it to hot when at idle. Easy to check the spring and the if the shaft is free.

Update: I changed the fuel pump out for a new one.

I drove it today stop and go with no problems starting or driving.

Apparently the old fuel pump was failing!

Thanks for all the advice!

I had a similar issue, a vented cap solved it until the fuel pump finally failed on a road trip. Luckily I had a spare.

I'm glad to hear the fuel pump seems to have solved the issue.

A thermostat allows quicker warm-up, and CONTROLS the temperature of the engine. It is not an off-on valve, but varies continuously with coolant temperature. A working thermostat will in fact FULLY open 15-20 degrees above its rated temperature. Running without a thermostat can cause sludge and poor fuel economy.