Vapor lock issues on a 1972 Roadrunner 400 engine

[kcracegirl]

Our 1972 Roadrunner is still having a vapor lock issue. We have shorty headers on it which is causing the extra heat buildup. We installed a riser on the carb, a metal fuel line that runs right beside the engine block that we put a heat protector wrap on. We run electric fans too. I am thinking maybe the new fuel pump is getting to hot and not allowing it to pump the fuel correctly. It was 96 degrees on Monday and humid, we drove it 14 miles before it started cutting out. Today it is 68 and it went 32 miles before it started to hesitate. I need help on stopping the vapor lock from occuring. Any idea's? Any help is APPRECIATED!!!!

 

[twecomm]

Dont rule out that you may have a worn fuel pump push rod. I have a 66 Charger 383 4 bbl. I had symptoms that seemed just like vapor lock. It turned out that my push rod was won by about .3 inches. Just barely pushing enough gas to keep engine running. One day it would run fine, other day might crap out in minutes or hours.

 

[WP29440SE]

how do you know its actually vapor lock and not weak fuel pressure or flooding?

 

[Michael789]

Make sure you check all your rubber fuel lines and that they are not sucking together.I had a heck of a time with a plymouth and it turned out that it was a bad rubber line.

 

[khryslerkid]

Mid to late '60's Ford BBs were notorious for vapor locking on hot days. The car would stop, or when you went to restart there wasn't any gas in the carb. Just pulled back on the throttle looking at the squirters, and nothing!

Cure: wooden clothes pins on the fuel lines. From the pump to the carb. They dissipated the heat. Quick fix to tell if that's your problem. Just don't open the hood around anyone! Lol

 

[ws27]

Stupid question, but how old is the gas in your tank? Winter blend gas will always vapor lock in the summer. Lots of people fill their cars before putting away.

 

[Dennis H]

Sounds like you are doing the right things. I prefer a thermal fan with shroud. What is the engine temp? Ceramic coated headers reduced the overall underhood temps by 100 degrees for me.

 

[747mopar]

I had some of the same issues but a heat dissipater from Mr Gasket for the carb and some fuel lines insulation seems to have helped but need to get it out on a really hot day to be sure. Running a higher octane will help too.

 

[old_skool]

Run an electric fuel pump mounted back near the tank, push the fuel with electric not pull, no heat near the tank to worry about, bypassing mechanical pump. Make sure your fuel vent and return lines are clear and not blocked. And I bet your electric fans aren't pulling the air to the engine like the oem fans do, most people think they're just trying to cool whats in the radiator, that wasn't the whole design with Chrysler, a good sealed shroud and mechanical fan will beat those electric fans any day. Air needs to be pulled back to the engine.

 

[SlinktRR]

I need help on stopping the vapor lock from occuring. Any idea's? Any help is APPRECIATED!!!!

All great ideas here. Try a forum search, this was/is a common problem even in the days of leaded gas:



That's a screenshot from the 1969 Mopar Master Tech slide deck on "Performance Tuning for 1969", you can find them on youtube (mopar tv channel). I've been watching the whole series on Roku, found these old vids to be pretty informative.. especially on the 727.

A good first step is to control and monitor your fuel pressure using a regulator and a gauge so you know how the fuel system is performing. Flooding and vapor lock can give similar problems (stalling) and sometimes hard to tell between the two, but a gauge will help you. I struggled with this for a while but now have a great setup and learned a lot in the process. Good luck with your troubleshooting!

 

[coloradodave]

I love the "Performance Tuning for 1969" and the fuel pressure regulator with what looks to be 1/4" id (if that) fuel fittings. Counter-intuitive but interesting none the less.

 

[SlinktRR]

I love the "Performance Tuning for 1969" and the fuel pressure regulator with what looks to be 1/4" id (if that) fuel fittings. Counter-intuitive but interesting none the less.

You're right Dave I wasn't clear on that; the kind of problems kcracegirl describes is as old as the cars themselves and aren't necessarily vapor lock. The factory dealt with it by putting vapor separators and return lines on 6 pack and hemi cars, but offered these pressure regulators as an aftermarket way for the dealership techs to help with modified setups that were showing signs of flooding or vapor lock.

Based on the OP's description, things to consider:
1) If a new pump was installed with no return or regulator, the pressure could be too high and cause flooding.
2) If the fuel pump pushrod wasnt changed, it may be too worn to put out enough fuel pressure thus vapor lock. That would be one to measure for sure, there is a factory spec for pushrod length
3) Metal fuel lines cause less drag and pressure drop than rubber per unit length, but transmits more heat. If the fuel is dead headed against the carb needle at idle/cruise inside a metal line on a hot day, the pressure can increase and push past the needle. Might consider putting in a return line or a "fuel log" between two carb fuel inlets with a return. There are a lot of custom ways to do this.
4) Check the engine oil for fuel in the crankcase. Extreme flooding will wash down the cylinder walls and get into the sump, and a ruptured or leaking fuel pump diaphragm will put gas right into the oil. New pump doesn't mean good pump.
5) Clogged fuel filter

These are all guesses, but if you have a gauge on the line you will be able to understand whats happening in the fuel system and then come up with a solution. Too high, too low, needle oscillating, a gauge will help a lot, or at least rule things out. To keep from throwing parts at these cars its good to have a fuel pressure gauge, a vac gauge for the carb, and a multimeter for ignition.

 

[coloradodave]

It's all good. My remark was solely about: Performance verses 1/4" fuel line. Those two things don't go together unless you're working on a VW. Also, since vapor lock occurs when liquid changes to vapor, due to heat, I am confused as to how a pressure regulator like the one above can help. Flooding from too much pressure, sure. A regulator with a return line, sure. Below is from Wikipedia. They actually got it right! Easy to check if that is the problem, as somebody said. When motor quits, take air filter off and work the throttle. If no gas shoots in from the squirters, you may have vapor lock! BTW, MOTOR OFF...


From Wikipedia, the free encyclopedia:

"Vapor lock is a problem that mostly affects gasoline-fueled internal combustion engines.

It occurs when the liquid fuel changes state from liquid to gas while still in the fuel delivery system. This disrupts the operation of the fuel pump, causing loss of feed pressure to the carburetor or fuel injection system, resulting in transient loss of power or complete stalling. Restarting the engine from this state may be difficult.

The fuel can vaporize due to being heated by the engine, by the local climate or due to a lower boiling point at high altitude. In regions where higher volatility fuels are used during the winter to improve the starting of the engine, the use of "winter" fuels during the summer can cause vapor lock to occur more readily."

 

[snakeoil24]

The idea of having a regulator without a return only works if it has constant pressure. When gas is pressurized it raises the point where it will vaporize. the easiest way to resolve this is add a small pusher electric pump by the tank with a return line. I would use a 1/4 inch return line and a must is a return line regulator. Only turn the pump on when starting, off once the carb or carbs prime. if you do this you avoid breaking a diaphram in mechanical pump and flood ing crank case with Mr.. Fuel Bomb!

 

[SlinktRR]

It's all good. My remark was solely about: Performance verses 1/4" fuel line. Those two things don't go together unless you're working on a VW. Also, since vapor lock occurs when liquid changes to vapor, due to heat, I am confused as to how a pressure regulator like the one above can help. Flooding from too much pressure, sure. A regulator with a return line, sure. Below is from Wikipedia. They actually got it right! Easy to check if that is the problem, as somebody said. When motor quits, take air filter off and work the throttle. If no gas shoots in from the squirters, you may have vapor lock! BTW, MOTOR OFF...

Gotcha. Yeah I'm not saying to use that old mopar regulator, just a reminder that flooding and vapor lock can give similar problems. From what I've experienced a regulator in a deadheaded system is to prevent flooding.

If it's definitely vapor lock, the most direct way to deal with it is to get circulation of the fuel using a return line while maintaining suitable pressure at the carb. Insulating the fuel from the heat and rerouting lines helps especially if you have headers, but when all else fails, circulating the fuel or vapor back to the tank with a return system is the best way I have found no matter if its an electric or a mechanical (I have both on my car). I use the late 70's chrysler 3 port filter from Napa with a restricted return, fuel circulates so no more vapor lock and pressure is right for my carb so no flooding.

 

[coloradodave]

What happened to kcracegirl?

 

[WileE]

What happened to kcracegirl?

Got stranded from vapor lock somewhere ?

 

[Ron 73]

Im having the same problem on my 73, its a 318 that's been slightly built and don't know how much as that's the way i got it but ive had one problem after another on it and ive just made up my mind to get a larger radiator for it and talking now to a fellow member.. I consider it a DD as long as there's no muddy roads where im at and i had the gasoline filter and line going to the carb taped up higher which is jerry rigging at the least i guess but it happened again so im not fighting it anymore, when whoever did whatever they did they should have put a larger then one core radiator in it! You have a BB.... you might consider the same option... Mine's a SB but i don't need the headackes, gots nought of em...

 

[khryslerkid]

What happened to kcracegirl?

Did you ever catch yourself talking and realize there's nobody around?

 

[kcracegirl]

[we pressure tested the fuel and we are getting 7-8 psi consistently. Thanks for the idea though. We do appreciate it.

QUOTE=WP29440SE;910098389]how do you know its actually vapor lock and not weak fuel pressure or flooding?[/QUOTE]

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It is fresh fuel and we are also using the no ethanol gas.

Stupid question, but how old is the gas in your tank? Winter blend gas will always vapor lock in the summer. Lots of people fill their cars before putting away.

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It appears we are getting fuel in the carb - we did worked the throttle with the engine off and it is getting fuel. So here is our thought: could it be the distributor causing an electrical issue that just gives the appearance of a vapor lock issue? The current distributor is a mopar performance and was put on when we rebuilt the engine 3 years ago. Should we consider an MSD or would we be wasting 450.00?

QUOTE=coloradodave;910098855]It's all good. My remark was solely about: Performance verses 1/4" fuel line. Those two things don't go together unless you're working on a VW. Also, since vapor lock occurs when liquid changes to vapor, due to heat, I am confused as to how a pressure regulator like the one above can help. Flooding from too much pressure, sure. A regulator with a return line, sure. Below is from Wikipedia. They actually got it right! Easy to check if that is the problem, as somebody said. When motor quits, take air filter off and work the throttle. If no gas shoots in from the squirters, you may have vapor lock! BTW, MOTOR OFF...


From Wikipedia, the free encyclopedia:

"Vapor lock is a problem that mostly affects gasoline-fueled internal combustion engines.

It occurs when the liquid fuel changes state from liquid to gas while still in the fuel delivery system. This disrupts the operation of the fuel pump, causing loss of feed pressure to the carburetor or fuel injection system, resulting in transient loss of power or complete stalling. Restarting the engine from this state may be difficult.

The fuel can vaporize due to being heated by the engine, by the local climate or due to a lower boiling point at high altitude. In regions where higher volatility fuels are used during the winter to improve the starting of the engine, the use of "winter" fuels during the summer can cause vapor lock to occur more readily."[/QUOTE]

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Been busy trying all the suggestions you guys have posted.

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Well since you are asking, we did get stranded in the Wally World parking lot... But eventually made it back.