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Does diesel fuel help prevent vaporlock?


Fr Mike
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Has any one had experience adding diesel fuel to the gas in an old car to help prevent vapor lock? I have been advised that this reduces the vaporizing of that RFG stuff we get at the pump, as well as does a little lubricating.

I don't want to re-open the whole can of worms about vapor lock, as it is already on the Forum and has been discussed almost <span style="font-style: italic">ad nauseam</span>. But this is one question I need <span style="text-decoration: underline">specific</span> Forum feedback on.

Thanks.

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<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Fr Mike</div><div class="ubbcode-body">Has any one had experience adding diesel fuel to the gas in an old car to help prevent vapor lock? I have been advised that this reduces the vaporizing of that RFG stuff we get at the pump, as well as does a little lubricating.

I don't want to re-open the whole can of worms about vapor lock, as it is already on the Forum and has been discussed almost <span style="font-style: italic">ad nauseam</span>. But this is one question I need <span style="text-decoration: underline">specific</span> Forum feedback on.

Thanks. </div></div>

Sure, it will prevent vaporlock.

You can't have vaporlock if the car won't start or run, now can you?

How much would you add? An ounce? A gallon? Add enough and you'll be draining the tank and purging your fuel system.

I would never add diesel fuel to a gasoline powered vehicle.

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I've run quite a bit of diesel mixed with gas, in gas cars. If you keep the diesel to less than 25% it'll run. Apparently diesel has low octane and you will get pinging during acceleration. I turn the distributor to slow timing to solve this. Diesel increases vapor lock in my opinion.

I have some vehicles I let sit for years at a time. I mix in a few gallons of diesel in the tanks to prevent the gas from turning to tar. It seems to work for me.

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I wouldn't try it in a 10:1 compression ratio engine. In a car of the early 20's or before you might well approach the sort of fuel the car ran with originally. I used the trick in hot weather when I had vapour lock on an AS160 Inter tip truck we used on the farm. I used a fairly strong mix and did not have to0 retard the spark. Power kerosine or jet fuel would be an alternative to raise the boiling point of your petrol, and would burn better with less smoke. But the diesel does work well.

Ivan Saxton

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Though I've never experienced vapor lock, I once tried adding diesel in a 34 Packard Eight in preparation for a LONG, HOT July 4th parade. I added 1 gallon diesel to a full tank (25 gallons) of regular unleaded. No, I didn't get vapor lock but might not have gotten it anyway, but the car sure SMOKED. And that was only at about 4% by volume added. I can only guess what 25% would look like - it would make the EPA cheer a coal-burning locomotive!

But if you think about it, and this requires a bit more physical chemistry than I have, adding diesel doesn't eliminate the low-boiling (most volatile) fractions in the gasoline, it only decreases them by virtue of dilution.

Seems to me there are so many other established and proven ways to handle vapor lock that I'd think adding diesel would be dead last on a long list. In a carbureted car with a mechanical fuel pump at the engine, the last place you should get vapor lock is the line between the pump and the carburetor because that fuel is under pressure which raises the boiling point (always makes me laugh to see this line insulated, with clothes pins, etc). However once it dumps into the carburetor is is back to atmospheric pressure and could volatilize much more easily. A bit of diesel wouldn't eliminate the volatile fractions but should raise the average boiling point a bit. Insulating blocks and heat shields between the carburetor and intake manifold should help if that's the problem. Vapor lock is most prone to occur, all other things being equal, where the fuel in under negative pressure and that's between the tank and the pump. Using a pusher (electric) pump at the rear is perhaps the best solution if you have these problems; shielding/rerouting lines are other options.

The folks who I guess have it the worst are those with vacuum tank systems as the line between the tank and the carburetor only has the (almost insignificant) pressure of the head height of the fuel line.

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No, no; it does affect the boiling point; and it always will as long as you have a single phase, it evaporates very much in proportion. If you get a copy of Rickardo's classic on "The High Speed Internal Combustion Engine", it will tell you a lot about fuels as well as just about every other relevant topic. Now one of the best anti-knock agents is water, (and it is also one that is non-toxic.) This can be applied two ways, either through a small orifice at a high gas velocity area of the intake; or equally effective dissolved in the petrol by the addition of acetone. This was of particular use for ultra-high performance aircraft in battle. It would have been little use if it had not evaporated with the fuel. I have used acetone to dissolve water into the petrol in the past when I have copped a dose of it in service station fuel. If you want to try diesel you need to start with a very small proportion, and increase by small amounts till it works. If there is too much of a camouflage trail it is an unacceptable method. In 1968 when I was at The Pastoral Reasearch Station in western Victoria, one of the outdoor staff refueled a Falcn Ute with diesel by mistake. It did run on about 75% diesel, which I thought was a miracle, but it did not like it at all.

Ivan Saxton

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Bill, thanks for yoir reply. Since I am requesting feedback based specifically on experience, I assume your strongly expressed opinion is based on your own experience or that of others you know. What are the reasons you advise not to mix in the diesel fuel? Thanks.

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I don't have any experience with mixing diesel fuel with gasoline as regards vapor locking. However, about 40 years ago I worked a job somewhere in the New Jersey/Delaware area. One cold morning on the way to work I could tell the gas in my car was frozen. To be more accurate, there was water in my fuel tank that was freezing.

A truck driver told me to mix one gallon of diesel fuel in my gas the next time I filled the tank. He said the diesel fuel would mix with the water and get it out of the fuel system. I had not at that time heard of mixing alcohol with gas to get the same effect.

So the next time I filled up I also put in a gallon of diesel fuel. The gas never again froze and neither did the car, a 1946 Plymouth, experience any running or starting problems. It did not smoke. The gas tank held 16 gallons.

I can't promise anyone that they would get the same results. All I know is it worked for me and the engine ran fine w/o smoking.

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First off- How are you sure you have a fuel problem??? Very often the problem goes away with fixing the electrical system.

If it is really the fuel system consider this.

I would be more likely to replace the parts of the fuel system with new known parts.

We had a 39 Ford that experienced a verified vapor lock. It was a 4th of july parade and we lost fuel to the carb. My father had put in a modern fuel filter between the pump and carb that compounded the problem.

The problem had to do with the fuel pump not being able to pump through the expanded air space in the filter. The old rubber was not sealing well in the fuel pump.

A new set of rubber and we never had the problem again.

Look at what you have and were there is opportunity for a bubble to occur. Put modern new rubbers kits in where there are seals and diaphragms. If you have old rubber than the pump may not be able to pump through the problem. The little leak back at the inlet or outlet seal will not allow the pump to move enough fuel.

Unless you have a gravity fuel system. Then you need to fix the clog or the electrical problem.

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Fr Mike if I recall has an early 30's Buick with a "built in" vapor lock device. It has an updraft carb at the base of a long intake that has ex gas circulating around it to heat the fuel for better vaporizing. the fuel pump sits 12-14 inches behind the carb and the ex pipe sits about the same distance in front of the carb on that big straight 8.

I have used diesel fuel myself to stop vaporlock on a early 80's chebby 454 powered 1 ton that liked to vapor lock on heavy load on hot days. It was easier than sitting in the middle of the road for 1-2 hours waiting for an engine to cool enough to restart.

Modern fuel and old engines are not always too compatible.

Bill

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<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Owen_Dyneto</div><div class="ubbcode-body">...In a carbureted car with a mechanical fuel pump at the engine, the last place you should get vapor lock is the line between the pump and the carburetor because that fuel is under pressure which raises the boiling point... However once it dumps into the carburetor is is back to atmospheric pressure and could volatilize much more easily.</div></div> I agree. Vapor lock acts just like cavitation. Negative pressure and heat make gasoline boil. In high mountains on a hot day, watch out... but wait a minute, one other condition looks JUST like vapor lock.

Gasoline is 600 times heavier than air. If your fuel line has a tiny pin hole or loose connection, it's much easier for your pump to suck air through than heavy fuel. You would swear all those bubbles are coming from vapor lock.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Owen_Dyneto</div><div class="ubbcode-body">Though I've never experienced vapor lock...</div></div> I haven't either. It gets to 100*F here in Detroit, but I've never had vapor lock affect my cars.

I don't have experience running diesel with gasoline. I would be tempted to try it in a lawnmower first. If you are looking for lubricity, they make 2-cycle oil for that purpose. 40:1 (which is proper for my snowblower) turns out to be 2-1/2% oil. Remember, that concentration successfully lubricates the whole engine. If you are adding diesel to your gas, try half that concentration (80:1 = 1qt/20gal). Mix it well.

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<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Fr Mike</div><div class="ubbcode-body">Bill, thanks for yoir reply. Since I am requesting feedback based specifically on experience, I assume your strongly expressed opinion is based on your own experience or that of others you know. What are the reasons you advise not to mix in the diesel fuel? Thanks. </div></div>

No, I have never tried it nor do I know anyone that has, but I base my comments on a couple of simple facts:

1. Gasoline is designed to ignite by using a spark.

2. Diesel is designed to ignite under pressure.

A gasoline engine does not have a high enough compression ratio to cause diesel fuel to ignite. Particularly an older engine with low compression. So what happens when the diesel fuel is run through the fuel system and doesn't ignite? Raw diesel fuel goes into the cylinder, potentially fouling the spark plug. It throws the fuel/air ratio way off making the gas even harder to ignite. It then gets dumped out of the exhaust. Common sense tells me that none of these things are desireable from an engine performance standpoint, the potential for damage to the engine exists, and raw diesel dumping out the exhaust could not be good for the environment.

As I said in my earlier post, how do you decide how much diesel to put in the gas? Too little and nothing happens. Too much and nothing happens - the engine fails to run.

I could be wrong (frequently am), but this just does not seem to be a smart thing to do.

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<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Fr Mike</div><div class="ubbcode-body">I got what I was asking for. Thanks to sll who have shared their experience with adding diesel fuel to gasoline to help prevent vapor lock. Best wishes to all! </div></div>

Yes, but was it what you wanted to hear? grin.gif

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Bill, the Forum feedback confirmed my own (negative) thoughts about adding diesel. I simply wanted to hear about actual pro and con experience with using diesel fuel to help prevent vapor lock.

So far, I have insulated and shielded the fuel lines, fuel pump,and carburetor, which has helped some. Now also, with confirmation from the experience of a number of others, I've ruled out the diesel fuel additive route.

All this information is helpful in my narrowing the field of alternative methods of dealing with the vapor lock problem. I guess you could call it reseach.

A lot of other discussion about vapor lock is already on the forum and can be accessed. Like I said, I got what I wanted. Thanks again to all who replied and best wishes!

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<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Fr Mike</div><div class="ubbcode-body">Bill, the Forum feedback confirmed my own (negative) thoughts about adding diesel. I simply wanted to hear about actual pro and con experience with using diesel fuel to help prevent vapor lock.

So far, I have insulated and shielded the fuel lines, fuel pump,and carburetor, which has helped some. Now also, with confirmation from the experience of a number of others, I've ruled out the diesel fuel additive route.

All this information is helpful in my narrowing the field of alternative methods of dealing with the vapor lock problem. I guess you could call it reseach.

A lot of other discussion about vapor lock is already on the forum and can be accessed. Like I said, I got what I wanted. Thanks again to all who replied and best wishes!

</div></div>

That is good, then. grin.gif

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