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Old 09-13-2014, 03:39 PM   #1
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vapor locking issues. Please help!

Hi All

My 1979 Dodge Tioga Class C which I Bought back in July drives really good. In fact in 3 months I have probably but close to 3000 miles on it. However it has issues with vapor locking. I have done a lot of car work on automobiles of all kinds. I gave the motorhome a complete tuneup shortly after I bought, changed the oil, and took it on the interstate to really clean out the motor. It passed the safety and smog tests. It's a smooth ride.

Motor and tranny are in great shape. But I cannot seem to resolve the vapor lock issue.
This is the carbourated Dodge 440. Big block. It's like an older HEMI engine. Only has 91 k miles on it which is very reasonable for this year. It acts up when it's cold and when it's hot. Does not cause drivability issues just hard starts. I finally resorted to carrying a can of starting fluid around. Spraying a couple of puffs into the intake of the carburetor whenever I go to start the motor. I also give it health dose of carburetor cleaner when I have the motor running in idle. Seems to have fixed the issue for now. But if I run out of starting fluid then I'm back to square one. So I need to find a more permanent fix to this problem. Your input may shed some light on an idea I might have missed. So please help if you can. Thank
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Old 09-13-2014, 03:46 PM   #2
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Vapor lock is caused by fuel boiling in the carbuetor. Rebuild the carb and be sure to install an insulating spacre between it and the manifold. If that does not cure the problem it is time for a fuel pressure check. Possibly have a bad fuel pump.
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Old 09-13-2014, 03:50 PM   #3
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Install a high volume electric fuel pump isolated from heat and control radiant heat around the carb. But honestly if the symptom occur during cold temps and starting, I doubt it is a vapor lock issue.
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Old 09-13-2014, 04:01 PM   #4
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Just a ponder....Is the choke working?? Otherwise I would check the fuel pump pressure.
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Old 09-13-2014, 04:05 PM   #5
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Just a ponder....Is the choke working??
Yes the choke is working. I pulled the dog house off while the motor was at operating temperature. The choke was wide open.
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Old 09-13-2014, 04:06 PM   #6
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Is the 1979 production date for the motor home portion of your RV.
Or the chassis build date ?
1978 was the last year for the big block, 400 LB /440RB , installed in Chrysler products, but a huge inventory pile up lead to 79 and even some 80, MHs being built on these chassis.
Anyhow , that's all beside the point.
Questions .
Is your engine equipped with a Holley 4bbl carb; or a Carter Thermoquad?
The Holley carbs tended to be worse for the after boil problems, and there was a 3/8" thick bake lite spacer , released to try and deal with the issue, by insulating the carb from the manifold heat.
I've seen wooden clothes pegs on the fuel lines , from the pump to the carb , used a home remedy for the problem, but that's really not a true fix.

The 426 Hemi , was a derivative of the 413/426 max wedge engines; and is not the old Hemi, that started production many years earlier under the names , Fire Dome & Red Ram. 284>392 CU.IN.
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Old 09-13-2014, 04:13 PM   #7
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Install a high volume electric fuel pump isolated from heat and control radiant heat around the carb. But honestly if the symptom occur during cold temps and starting, I doubt it is a vapor lock issue.

The rv has a mechanical fuel pump on motor and a factory installed electric pump in the tank with an on/off switch by the steering wheel. This gives me a thought though. Because the carb is inverted back under the dog house, I wonder if it's not getting enough air?
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Old 09-13-2014, 04:23 PM   #8
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The factory never installed an electric pump in these units, and if it's controlled by a switch on the dash , it's a definite , add on; probably to attempt to fix the issue you're having.
Do the air cleaner housing snorkels , have duct work that extends over the top of the rad to pick up cool air from behind the grill? ( in front of the rad.)
That may have be a mid 80's factory addition to try and deal with the after boil issue, that continued till throttle body fuel injection was introduced.
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Old 09-13-2014, 04:26 PM   #9
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I had a vehicle with a 454 years back that I thought was vapor locking, what was actually happening was the carburetor bowl was draining back to fuel tank, wish I could remember more details, but something to check anyway. Good luck


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Old 09-13-2014, 04:26 PM   #10
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Quote:
Originally Posted by Skip426 View Post
Is the 1979 production date for the motor home portion of your RV.
Or the chassis build date ?
1978 was the last year for the big block, 400 LB /440RB , installed in Chrysler products, but a huge inventory pile up lead to 79 and even some 80, MHs being built on these chassis.
Anyhow , that's all beside the point.
Questions .
Is your engine equipped with a Holley 4bbl carb; or a Carter Thermoquad?
The Holley carbs tended to be worse for the after boil problems, and there was a 3/8" thick bake lite spacer , released to try and deal with the issue, by insulating the carb from the manifold heat.
I've seen wooden clothes pegs on the fuel lines , from the pump to the carb , used a home remedy for the problem, but that's really not a true fix.

The 426 Hemi , was a derivative of the 413/426 max wedge engines; and is not the old Hemi, that started production many years earlier under the names , Fire Dome & Red Ram. 284>392 CU.IN.
. The production date is on the chassis. As I open the drivers side door. It's the dog tag style tag, mounted on the top of the wheel hub cover inside the vehicle. To the Best of my knowledge, this is a Holly 4 barrel carb This is the 440 7.2 liter v8
. no this is not the Fire Dome or Red Ram. The engine has the regular Chrysler insignia on the tags, stating these are original Chrysler Factory parts.
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Old 09-13-2014, 04:28 PM   #11
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Quote:
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Yes the choke is working. I pulled the dog house off while the motor was at operating temperature. The choke was wide open.
My question was more towards the cold start issue. Still would suggest putting a fuel pressure guage on the line to the carb and making sure both pumps are working. JMHO
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Old 09-13-2014, 04:29 PM   #12
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Quote:
Originally Posted by Annierufus View Post
I had a vehicle with a 454 years back that I thought was vapor locking, what was actually happening was the carburetor bowl was draining back to fuel tank, wish I could remember more details, but something to check anyway. Good luck


Joe 1998 Itasca Sunflyer Ford 460

I'm just curious. What did you do to solve the problem?
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Old 09-13-2014, 04:34 PM   #13
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My question was more towards the cold start issue. Still would suggest putting a fuel pressure guage on the line to the carb and making sure both pumps are working. JMHO

The motor runs even if the electric pump is off. And I can hear the motor to electric pump when I flip the switch. So yes both are working. But I will give it a fuel pressure test to see if maybe one of the pumps are under performing or if I have a vacuum leak.
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Old 09-13-2014, 04:45 PM   #14
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Might check that the fuel line that comes out of the fuel pump and goes to the carburetor is not close to any thing that is a big heat producer.
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