Vapor Lock

I drive a 2003 Mazda 6. In the summer of 2005 and 2007 I was driving my car at an altitude higher than my normal Kansas altitude and experienced vapor lock. (just outside Limon, CO and Cheyenne, WY) The guy who towed my car in 2005 recommended a little diesel (1 cup per tank)to remedy the problem. My husband suggests also using premium gasoline. I have the oil changed regularly and never use ethanol blends. I’m about to head out again this summer to Wyoming. Have you heard of vapor lock in a new car like this? And have you heard of this solution? THe premium gas and diesel did seem to resolve the problem.

Is your car fuel injected or does it have a carburetor. I don’t believe vapor lock is a problem with fuel injected engines, but you learn something new every day.

Vapor lock was much more common in cars of the 40’s and 50’s and seems so rare today that I wonder if you really experienced vapor lock. Old cars with mechanical fuel pumps only develope about 4-5 lbs of pressure. The high heat in the engine compartment combined with high ambient temperatures could get so high that the liquid fuel would vaporize in the fuel line(s) leading to the carburator. The carburator is set to vaporize liquid fuel and couldn’t handle fuel already vaporized and the car would stop. You’d open the hood and wait for the motor to cool or for a quicker fix carry water in the car to dump on the fuel lines to cool them and then fire up the car and be off.

Fuel at higher pressures does not vaporize in the fuel lines and modern cars with fuel injection all use high pressure systems. To get vapor lock in your car you’d need a problem such as a weak fuel pump that is not producing the high pressures it is supposed to generate.

Instead of vapor lock what is more common in today’s cars is the electrical components overheat and heat kills electronic things pretty fast.

I’m not saying you didn’t experience vapor lock, but if you did it is very unusual in a car such as yours. I don’t know about the cupful of diesel, and I don’t think the differences in vaporizing of regular or premium fuel is much different. Perhaps a pressure test of your fuel pump would be advisable?

I’ve never heard of this, but my guess was that he was suggesting the diesel fuel as an additive to reduce the octane and effectively enhance the detonation characteristics of the fuel. Lower octane fuels detonate more readily. Interestingly, your husband’s suggestion would have the opposite effect.

I’ll have to ponder this before agreeing or disagreeing with it. It’s new to me.

Vapor lock is highly unusual in a fuel injected vehicle. we’ve had long dissertations on the subject. Basically, the entire fuel system is under high pressure from the pump to the injector, so the opportunity for fluid boiling just is not there.

I don’t know why your vehicle stopped. It may have been because the system was unable to compensate for the sparse air. But I find the post interesting and anxiously await the inputs of others more knowledgable than I.

'03 Mazda 6’s were multiport injected.

I doubt it was vapor lock, unless you have a defective fuel injector. The fuel is held at high pressure until injected into the engine, altitude should have no effect (otherwise you’d see lots of cars at the side of the road in high altitude areas). I also recommend against the diesel solution. Premium sure won’t hurt, but I don’t know why it would help. Both would increase the octane (make the fuel harder to burn). If was me, and assuming no codes were created in the computer (did no lights go on?), I might run the max dose of Techron fuel treatment through it (get it at Walmart, read the label) and run premium, just in case. But that’s all just based on hoping it cures a problem I can’t explain…

I seem to recall from other episodes you actually can use a lower octane gas in the mountains. Fill your car up with the minimum octane requirement would be my suggestion. Is premium suggested in your manual? That would tend to go with adding diesel as the net effect is a lowering of octane, I think.

Vapor lock can and does occur on fuel injected vehicles.

A fuel injected system is a closed loop system. That means whatever fuel isn’t used at the engine is returned back to the fuel tank. As this cycle continues, the fuel gets heated at the fuel rail(s) and starts heating up the fuel in the tank. This gets worse when the fuel level in the tank drops. This fuel in the tank becomes more volotile the hotter it gets, until it returns back to the hot fuel rail where it starts boiling and you have vapor lock. E10 gasoline also adds to the volotility of gasoline.

So on those hot days on the road, keep your gas tank a half full or more, and try to avoid E10 gasoline.


My husband suggests also using premium gasoline.

No, if anything you need lower octane in the mountains. Note: That does not mean you NEED lower octane, only that your car will get along fine with lower octane.

I’m not a fan of the vapor lock theory.
Your car was towed so what were the circumstances and symptoms to cause it to be towed?

Quit instantly on highway, stalled in traffic, sputtered and coughed before giving up the ghost, etc.?

I picked 5 “vapor lock” sites at random off the net,all said vapor lock on fuel injected cars “highly unlikely”.

This has NOTHING to do with octane…It has to do with vapor pressure, the boiling point of the fuel in question. Usually, the normal pressure maintained in the delivery system prevents vapor lock. But in the gas tank, with zero pressure, the fuel pump can become vapor locked on hot days, with the fuel level low. Altitude compounds this. Adding a little diesel does nothing. Replacing a weak fuel pump MIGHT help, but no guarantees…

Thank you to all those who’ve replied.

2005: on I-70 15 minutes west of Limon. Temps had been over 100 for about a week. I had been driving about 6 hours on yet another 100 degree day. Yes, cough and sputter was exactly what it did. It wasn’t immediate but it was choking and gagging and dragging to the point that I had to pull over to the side of the road. I turned the car off and it wouldn’t start again. It took 2 hours for Mazda/AAA to get a tow truck to me and then they towed me to CO Springs taking another 2 hours. (Mazda insisted there were no Mazda dealerships in Denver.) (As I waited 2 other perhaps older cars sputtered by along the side of the road.) We arrived at the dealership just before closing at 6 pm. THey were going to make me wait until Monday to look at it. The Tow driver suggested he put the car down off the truck and I see if it would start. It started just fine. He told me to go to the nearest gas station and put a cup of diesel in it. Putting diesel in for the next week of driving in the Rockies and back to Kansas it did just fine.
In 2007, I was 10 minutes from Cheyenne, WY. I pulled over at the first sputter and stopped (I had forgotten to do anything special at my last fill up. It wasn’t as hot (90s) so I wasn’t thinking about it). It would not start so I waited a few minutes and started again. We made it to the exit ramp. It stopped again. I waited a bit and then got it started. Cruised into a nearby gas station, got some diesel mixed with my gas and let the car cool off a bit. It was fine the rest of the way to the Tetons and back a week later.

It seems like the problem comes from altitude and from heat, and that diesel helped. I am not very clear on how the electronic controls modify the air fuel mix in injected cars, but it seems to me that your car is not adjusting itself properly to heat and oxygen content in the air. Certainly carburated cars stumbled and ran badly at altitudes, and sometimes it was just about impossible to get them started because of insufficient oxygen. If you are under 80,000 miles you should still be covered by the mandatory warranty on the emissions system, and you could get Mazda to scan the system for you, under warranty.

What part of the “emissions system” is covered by a 80,000 mile warranty?

I’ve vapor locked twice in my '95 Nissan Sentra, both times at high altitude (> 8000MSL), after having driven at freeway speeds to get there. The lock was at startup; engnie would crank normally but not fire, then run rough ~15 sec after catching.

One time I simply cranked it until it caught (letting starter cool as I went); the other time, I had the advantage of being atop a hill, so I roll-started it when it wouldn’t start normally.

I don’t think Diesel has anything to do with octane here; I think the higher-carbon Diesel reduces the overall volatility of the fuel. Also, I was under the impression ethanol had a lower vapor pressure than gasoilne.

I know the (fuel-injected) A36 Bonanza vapor-locked frequently; there was even a “vapor-locked startup” checklist, so I don’t know why it couldn’t happen in a FI car.

Thanks for the added info. How full was the gas tank at each incident? I think the problem is altitude-related, with a little heat added. Heat by itself is not the cause, It’s 104+ in Dallas for days, no cars sputtering to a stop by the side of the road. I think you may have a weak fuel pump, this can be checked by a good shop, also check the fuel injectors. Get it checked, you don’t want to be stranded in the mountains.

Driving back 140 miles from picking up a boat a friend bought in a F150 on a 98 degree day. At 120 miles the engine started sputtering and jerking and lost power so we pull over on the side of the freeway. I look over and see there’s only a quarter of a tank of gas in the second tank which we were running on. I said vapor lock. He says no! It’s fuel injected! I grab a partial bottle of windshield washer fluid from behind the seat, open the hood, and pour the washer fluid on the fuel rails. He starts the engine, and we go to the next gas station and fill up. We get back with the boat with no problem.


During the winter months, refiners add both butane and propane to their gasoline to increase vapor pressure and volatility which aids fast cold weather starting. As temperatures warm, these components are reduced and vapor pressure lowered. Gasoline stored in underground tanks in mountainous areas can be very cold. On a warm spring or early summer day, this cold gasoline will act like club soda in a hot vehicle fuel tank. Back in the age of carburetors, this could cause MAJOR problems, with HUNDREDS of cars disabled. I suppose under the right conditions, it could happen today…

Use the low octane gas that they sell there and you should be alright. Do not use leak causing diesel fuel.