Vapor Lock Problems With Your Imperial's Carburetor


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Question from Patrick (1964):

I am having problems with my 64 Crown Coupe. Sometimes driving it isnt getting enough fuel, at least I think that is the problem. I have one of those clear glass filters on the fuel line that has a **** micron filter I can see that it isnt filling up with fuel while running it just sort of flows in there sort of like a trickle. One thing I do see is alot of air space. Im assuming there should be nothing in that line but fuel, solid with no air. Am I wrong?

My engine at times has been stalling out or at least coming close, I restart and it takes reving the engine as its it sputtering to keep it going.
At first I though it was something with the distributor since I put too much lubricant on the shaft. I have since used electric motor cleaner to resolve that, sanded the points a little and regapped them.

With the difficulty in starting and this new unreliability issue Im a bit concerned since tomorrow I have to drive 100 or so miles to get a rooster and a hen for my new hen house.

Any ideas on this fuel thing, Is it vapor lock or something like it?


From John:

I would suspect one of several things. The first is a vented gas cap is required on the '64 if you have the proper fuel tank in it. Next would be a lot of debris in the tank, but that should show up in the filter. The 3rd would be the fuel pump is heading out. I had a problem with my '60 stalling & not wanting to restart after the car had sat unused for about a year. Dick B. suggested removing the carb needles & spraying carb cleaner through the openings, followed be a blast of air. A lot of junk came out & I've had no further problems since.

From Bob:

Sounds like the fuel pump to me!

From Steve:

Could bes:

Could be the fuel pump on its way out.

Could be the rubber lines from the tank to the fuel pump are failing

Could be the fuel pump is full of poo and the sock filter is stopping up.

Personally I vote #3. I fought this forever on a lesser Chrysler product. Car would just die randomly. Once you pushed it off the road it would start right back up and run like there was never a problem. Turned out to be a big mess in the tank.

Question from Jaon (1968):

Yesterday I was out driving my new '68 hardtop for about an hour and got it good and hot, after which I parked it for about a half hour on the street while running some errands. When I came back, the car would crank, but wouldn't catch for the life of me, sounding like it wasn't getting any gas. Figuring I had lost the original fuel pump to the mtbe in the gas here, I had the car towed home. Upon investigating, I found the pump to be fine and the new filter to be unrestricted. I just tuned the car up this weekend, and verified the ignition system is in top form, so I'm pretty sure this was a fuel problem. What's strange is that I went to go start the car cold this afternoon, and she kicked right over and ran just
fine, leading me to believe this is an issue with vapor lock.

I'm thinking my next step is probably to stick a phenolic spacer under the carb, insulate all of the exposed rubber lines, and put an inline electric pump in the fuel line under the front passenger door where the previous owner seems to have spliced the original line with rubber.


From Chris:

Did it seem to crank a little bit slowly, or at least seem to run the battery down more rapidly than usual?

I say this because your starter might be on its way out. I do not know how many miles you have on yours, but when the starter begins to wear, it becomes increasingly susceptible to hot-start problems, mostly because the whole thing expands, making cranking harder on the starter itself.

I thought for years I had a fuel problem when my '67 was hot (and it's probably almost as hot under the hood after 30 minutes sitting than it was when you turned it off), when it was nothing more than a 35-year-old starter with 123,000 miles on it gently begging mercy.

I'd have your starter draw checked first, then go after any possible fuel delivery issues.

From Roger:

Put an amp meter on the stater line and see if it draws more when engine is hot then cold. Oil leaks in valve covers can cause starter problems. If you have a 440 they were notorious for this. Exhaust manifold would cook the out side of the cork gasket. They make a heat proof valve cover gasket for the 440 now. I had to do this with my winni-baggle. I fixed the gasket and rebuilt the starter and didn't have any more problems with it.

11 volts pull on a volt meter is a lot for any engine amp most pull 10-15 depeding on conditions.

From Bob:

I had a problem like this in Chrysler, younger than yours. They provided a factory fix after several thousand complaints. We have all experienced this problem for many years with a lot of cars. the problem is not the carb mounting nor is it the fuel pump. It is the gas supply from the tank to the fuel pump. Apparantly, the fuel pump cannot pump gas vapor; it tends to go to a vapor on the way up to the pump. The factory fix was the addition of a small electric pump installed in the tank and the problem went away. These pumps were made by GM, but are quite simple and still available in many places. Another fix might be to insulate the fuel line from tank to fuel pump. Also, make sure that the fuel tank vent is functioning and its vent line is working. I don't believe your '68 has either of these things on it, but just for the heck of it, make some checks.

From Michael:

Hot soak is a multiple whammy. As Chris pointed out, the underhood temp on a hot day after the engine has been shut off for 15 or 20 minutes can be considerably higher than the temperature was when running and underway. This has several effects: the engine internal clearances are reduced, increasing friction; the starter motor itself tightens from the heat and can't spin as fast; the increased load and draw from the starter combine to reduce the voltage available to the ignition system; and to top it all off you're trying to start an engine whose carburetor has boiled fuel out of the bowls, upsetting the mixture greatly.

Making sure the starter and battery are up to snuff will help, but the 440's in particular can benefit greatly from insulating the carb from manifold heat. You can get a number of different kits for this, they are avialable in the Jeg's or Summit catalogs. I like the one which uses interleaved layers of aluminum plate and fiber gasket. You can get 1/2 inch or 1 inch high stacks. I wouldn't use the 1 inch because it's not needed and could cause hood clearance issues. You may have to trim the aluminum plates to clear linkages or other obstructions, as they are larger than the base plate, like radiating "fins". The insulator kit will make a big difference on your 68, and will make a phenomenal improvement on the later 440's with the ThermoQuad carb. Back when those cars were current models we
used to fix hot-soak hard start gripes by lowering the float level slightly, but that's like wearing gloves to write with a leaky pen in a way, and besides it can cause slight leanness under normal operating conditions. Better to eliminate the root cause, excessive heat transfer to the carb.

From Demetrios:

I sometimes have a hard time starting it, but I have never been stranded yet due to this.

If it happens again, have a little bottle of gasoline. Open the air cleaner and pour some in the carb. This will hopefully sustain the engine long enough for your fuel pump to bring fresh gas. If not, the second or third timemight do it. For your convinience, leave the ignition on, and crank the engine by shorting the starter relay with a screw driver, while pouring small amounts of gas in the carb.

From Bob:

You're not going to believe this, but put some wooden clothes pins on the fuel line where it might be getting hot.

This is the cheapest and easiest way to solve vapor lock in gasoline engines in the heat that I know. They apparently help to dissipate the heat.

I know this sounds like something from Bewitched. but ...believe it or not, it works!

Of course, take the clothes pins off before you have your car judged at a show!

From Rich:

Another little tip is to try to keep the metal fuel line running upward towards the carb. If it is higher at the front it may trap an air pocket.

From Joe:

Here is my cure for vapor lock. As some of you know, I live in the hottest enviornment to own an older car not designed for the 125 plus Summers. I do not get vapor lock anymore and the cure was so simple. Since you want the metal lines found on the Imperial from the factory to stay, then what I do is wrap them in rubber hoses and tie wrap them on all of the metal lines. No matter where they are, but mainly in the engine department. I use either 3/8 or 7/16 ID hose. I slit it length wise, slip them over the metal line without disconnecting them. I tie wrap them at intervals where it will not come off. I turn the rubber so it faces the heat sources and the open slit areas away from the heat source. I add more ties where it curves so it will follow the curve of the metal lines. I have NEVER had a vapor lock in the last 16 years of desert living.

Question from Craig (1971):

I have a 1971 New Yorker 440. When I drive around for any extended period of time and stop. My car has major vapor lock. I have to let the car cool before it will start. Any ideas?


From Marc:

You need to check two things 1. Do you have a heat spacer between your carb and your intake manifold - I used a Morrosso 3/4" phenolic spacer under my 4160 Holley on my '67 which stopped the vapor locking. 2. The other problem I had was the fuel filter elements in the cheaper filters collapsing and clogging.

From Michael:

First of all I will assume that you have a Holley 4160 series carburetor on your '71. If this is so, and you have never rebuilt the carburetor, or the rebuild is at LEAST 3 tears old, and it was done with ORIGINAL replacement parts, chances are your Float setting is to high.WHY? The original Needle and Seat that allows the gasoline to flow into your carburetor is made out of a rubber compound that was called "VITON", and after a few years of usage and/or sitting, the rubber loses its sealing ability, and compresses a small amount. The compression allows the float to rise a little higher in the bowl allowing a little more gas in than needed. The lack of a good seal does the same thing, making your fuel level rise enough to cause your vapor lock......When your engine is cold you have no problem, but after it has been running a while , enough to get the intake egg-frying hot, the fuel actually expands and will reach high enough to enter the idle circuit in the primary metering block, flow through the orifices and down into the throat of your intake. With the throttle plates naturally closed 90% of the way, when the gasoline hits the HOT intake, it VAPORIZES and stays there , ready to be drawn into the intake valves along with the mixture that is being drawn in during engine crankup. I'll wager that if you depress the throttle all the way to the floor and hold it there, it will eventually start.,OR if you let it cool down for about 1/2 an hour or more you can usually start it with minimal effort. I discovered this annomally one day 20+ years ago on one of my attemps to figure out what was causing the problem. I had the air cleaner off of my mother's '69 LeBaron, which I still own, and had just come back from a 5 mile drive on a hot and humid summers day. Standing on a couple of wheels so I could peer into the throat of the carburetor, I watched for approximately 2-3 minutes and THERE IT WAS!!! I not only HEARD the sound of gas hitting the intake and being instantly turned to a steamlike mist, I SAW itrising out of the carb. Hmmm I thought as I then went back to the drivers seat and attempted to start the engine...but NO LUCK...Vapor locked (or actually flooded out). My solution was to replace the needle and seat with new ones, which did help a little after I reset the floats to specs. BUT that only lasted a few weeks, until I finally happened upon a product that REPLACES the VITON needle and seat with a brass ball on ball setup. This setup actually FLOWS more gasoline than the old setup, and therefore you can set the float levels 1/16 to 3/32 lower than specs., keeping the fuel level low enough so that when it does get real hot , it will not rise far enough to leach into the idle circuit. I have never had a problem since, and I only use a normal sized carb to manifold gasket. I recently rebuilt the 4160 Holley on my 67 IMPERIAL CROWN and went on a 1100 mile trip with it and have had no problems, right through the hot, dry summer. I am in the process of contacting the company that makes these needle and seat replacements, to see if they are still available. If so, I will post another E-Mail tomorrow. These are produced not only for Holleys, but Stromberg, Carter, Rochester, etc., back to the 1920's. Be advised also that if yours is a holley, and you rebuild it, there is a gasket between the primary metering block and carb body that only goes on one way.

Question from Mark (1974):

Can I be experiencing vapor lock? My '74 Imperial starts right up in the morning...then runs fine and re-starts ok for 2-3 times.  Then all of a sudden it will not start again for several hours; then the whole process starts all over again.  Is this manual versus thermoquad or my car saying to me, "I am going to sit out this Tampa heat for a few hours"...any help?


From John:

This sounds like the carburetor. I had a 77 NYB & it did just what you describe. Replacing the carburetor solved the problem after also burning out the starter in the process.

Follow-up from Mark:

I just replaced the starter yesterday and here is another piece of the mystery.  The car will only re-start in the morning after it has been shot fully of carburetor cleaner the night before.  I am going to try a bottle of Gumout or equivalent on each new tankful and use Shell Premium on the next several tankfuls and see what happens.

From Jim:

Having grown up around many Chryslers including Imperials, I can tell you that your problem was common when they were new. That's one of the reasons they came up with the Thermoquad. Once the cooling system is not running (engine is shut off) the heat "soaks" out through the engine and "boils' the gas out of the carburetor and into the intake. It floods the engine. Yes, letting the car idle for several minutes will actually cool the engine down and reduce the "Heat Soak" that sets in if it is shut down without the cool down idle period. It does help. Something that I always hated was my dad opening the hood to let the engine cool off but that did help too. You just can't do that at the mall or you'll wind up with an empty engine compartment. I do like the earlier suggestion about lowering the float level as long as it doesn't lean out the mixture when the engine is running. Making sure the float bowl vent operates is a must as well.

From Julian:

Don't be so sure it is vapor lock if you have electronic ignition.  Your pick-up is probably bad.  Change the pick-up inside your gas tank and make sure your distributor and gap are set properly and you won't have any problems.

From Kerry:

I agree it's MOST likely the carb. Probably because it is flooding due to engine heat expanding the fuel in the bowl and letting it run down into the motor. The fix for this is relatively easy and is a cure for a lot of 'hard starting when hot' problems. Remove the top of the carb and holding the top upside down, adjust the float level to 1" from the 3/4 inch at which it is probably set. Reinstall and give it a try. What this does is 'lower' the fuel level in the bowl enough so that when it heat soaks from the engine it will not expand enough to overflow. This solution has had a magical impact on my 73 and 64 and several other IML owners reported similar results.

Follow-up from Mark:

Mine is a rebuilt TQ circa '97 issue and doesn't have the fast idle solenoid (mushroom looking part on the choke side with the electrical connection on it).  Where all the rebuilds like this? Could that explain why the idle doesn't increase with the a/c on?

Reply from Kerry:

Don't know Mark, my fast idle solenoid is vacuum operated (73). Since it's connected to the choke, it should not effect idle anyway except when the choke is on.

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