Diesel as an additive for vapor lock?

Dear Tom and Ray,

In a December 2003 answer to a question from Greg about adding a gallon of Diesel per tankful of gasoline to avoid vapor lock, Tom said “If it were 1961, and there were no emissions systems or fuel injection, the advice wouldn’t be so bad. But nowadays it’s lousy advice, and I hope most of his listeners ignored it.” Good advice for most people, but I am in fact stuck in the sweltering Kansas Summer of 1961 and I’m having what sounds like symptoms of vapor lock in my 1959 Chevy Apache truck (Original Inline 235, 1 Brl Carburetor) and am wondering if I would do any harm by trying this as a diagnostic tool?

Thanks for the show, Love it!


You will not get T and R on this forum; only a group of people who try to help others with car problems. And cars are always probelmatic.

Personally, I would skip the diesel treatment and go another route.

Buy or fabricate an insulator block for the carburetor; preferably at least 1/4" thick.
If you have a metal fuel line routed over the engine change the bulk of it to a rubber line. (While it’s pricier, fuel injection hose is much tougher and is the best bet)
Lower the carburetor float level just a tad.
This truck running a sediment bowl? If not, maybe the installation of one would help.

There are also things that can mimic vapor lock so don’t rule out ignition components; especially the coil and often overlooked condenser.

In addition to OK’s good advice, you can also add insulation to the fuel line anywhere it’s exposed in the engine compartment or where it runs near the exhaust pipe (you’d be amazed at the stupid places they used to run fuel lines). The insulation I’ve used is closed-cell insulation used by air conditioning contractors. Finally, make sure your fuel pump is putting out the appropriate pressure - too low and you’ll get more fuel boiling.

Just another big advantage of fuel injection.

Your real problem is todays high vapor-pressure gasoline…Modern vehicles with sealed fuel tanks and in-tank high-pressure fuel pumps have no problems dealing with it. But older carbureted vehicles can suffer on hot days. Adding diesel fuel will not help much…In severe cases, adding a electric boost pump mounted as close to the gas tank as possible usually cures it. Detroit has a pretty good cure…They added a 3-port fuel fuel filter with a return line back to the tank so the constantly flowing fuel kept the lines and fuel pump cool enough to avoid vapor lock…Av-Gas is a sure cure also, it’s very low vapor pressure (no propane or butane in it) but it’s usually to expensive and hard to find to be burning in an old truck…

My 84 S-15 had a major problem with Vapor locking…and GM knew about it. For that year alone they went through 10 different designs. They finally got it right.

Diesel no.
But I used a pint of transmission fluid in my 30 gal tank for the 80 Bronco.

However , the best long term fix for that truck was the three port fuel filter Caddyman suggested.
In the Bronco I put it in-line very near the carb and ran a return hose all the way back to the filler neck whre it was easiest to install the return hose nipple.
With the gas cap off you could see fuel coming in from the return line.

Worked wonders and completely solved the problem.

I am 55, been wrenching for cash 35+ years had more than a handful of poor running cars dispatched to me with the info. that the owner diagnosed “vapor lock” it just hasn’t turned out to be the case in my life. I drove one of those inline sixes from 75 to 86 here in the West and never had to deal with vapor lock. Guess I am just a lucky guy.

It is so easy for a suspicion to turn into fact as the years pass.

When using the old Chevy sixes on a farm, driving slowly with a heavy load and in the hot Mississippi sun, vapor lock would sometimes be a problem. Of course just opening the hood for a few minutes would allow the engine to restart. And pouring water over the fuel line where it wrapped around the valve cover at the front would quickly get it cooled. After it was wrapped with aluminum foil there was never a problem. I have seen all kinds of cures. Clothes pins, wet gauze, plumbing insulation, and all were sworn to be the best cure.

As for diesel, it would lower the octane considerably and result in heavy detonation problems.

I’ve actually seen vapor lock quite a bit. At one time many years ago I owned a 78 Ford Granada and it was prone to vapor lock on 100 degree days and there was no doubt vapor lock was the problem.

The last 5 years or so of carbureted Subarus was, for want of better words, Holy Hxxx as far as vapor lock problems. Nothing but one TSB after the other and after a year or so of wrestling this greasy pig even the factory service reps stated their brand new off the line Subarus were suffering this problem even though all had been modified on the production line.

One could run a car until it’s warmed up on a 100 degree day, shut it off, wait 5 minutes, and look underneath the air cleaner at the carburetor float bowl sight glass, and watch the gasoline boiling in the float bowl just like a coffee pot percolating away.
Ditto for carbureted Nissans. (used the same carbs as Subarus, etc.)

At one time even Toyota outfitted some of their carbureted models with a tiny fan that was mounted on the intake manifold. This small fan was temperature sensitive (just like a radiator cooling fan) and would blow air over the carburetor float bowl.

To all those with an interest, I have an update.
After replacing the fuel screen in the fuel tank the “vapor lock” problem went away completely, no matter the temperature outside.

Congrats, that’s a great looking truck!