Needing advice on vapor lock problem.

[Mayor West]

Greetings,

The truck:
1983 K5 350 HEI with Holley SA 670cfm , TH350 trans, and 4WD

I get good pressure (at initial idle) and good vacuum.

I am looking to put an end to the vapor lock problem I have been running up against all summer and would like some friendly advice on some Ideas I have. Generally leading with the less invasive cost effective measures first.

One main thing is what are people’s thoughts on wrapping stainless headers? I did not wrap mine because I understood it to be unnecessary. However, they create an immense amount of heat! So if wrapping would help and lessen my vapor lock woes, I am all for it. Advice?

Secondly, I am installing a Holley Mighty Mite (12-427) electrical fuel pump to slightly pressurize the fuel before the mechanical. Also, as a potential fix is to fill up with non-ethanol gas. By adding slight pressure to non-ethanol fuel I may be able keep the fuel from vaporizing altogether. One can hope.

What makes for a good fuel line (or fuel line wrap) to battle heat transfer? Any recommendations? If headder wrap can be recommended, which is the right one to go with?

A much later project will be adding an in-tank EFP. I would like to upgrade to a 31 gallon tank and may as well do it at the same time.

Let me know if I left out any pertinent info, and thanks for all the help!

[geezer#99]

What’s the symptoms?
Motor quitting, bogging, surging?
What’s your fuel pressure like?
How hot does your motor get?

Hold off on the electric pump. You likely don’t need extra pressure.

[Bandit130]

Welcome

[Dead Parrot]

If the headers are bare metal stainless, they will be just as hot as normal bare metal headers. The ceramic coated ones will run cooler.

Does the exhaust pretty much run next to the frame rail with the fuel tubing? If so, you could be getting heating all along the fuel system and not just in the engine compartment.

What kind of fan setup do you have? Hard to beat a 7 blade OEM fan on a properly working fan clutch for getting good air movement in an engine compartment.

[Mayor West]

Thanks all!

Originally Posted by geezer#99

WhatÂ’s the symptoms?
Motor quitting, bogging, surging?
WhatÂ’s your fuel pressure like?
How hot does your motor get?

Motor will do fine driving highway speeds into town, but as soon as I hit the stop and go of traffic lights the idle wants to drop off. Shifting into neutral and holding the gas peddle prevents it from dying early on, however it does eventually die.

One garage put in a new alternator (and rewired the leads to it as the old ones were undersized and smoked) and mechanical fuel pump, and another garage tested for vacuum and fuel pressure and reports those were all good. I was going to get a pressure gauge and record actual numbers.

The engine heats up nicely and the thermostat opens at 180. ItÂ’s a stock fan setup. The shroud does look incorrect and probably could be removed if thatÂ’s any problem. Temp stays fine at 180 until it comes to idling at lights or slow moving traffic where the engine temperature then climbs up to ~210.

When it dies it will not restart until it cools down after an hour or so. The motor will turn over but will not fire. The ambient air temperature is typically ~90 degrees.

Thanks again for the help!

[Mayor West]

Originally Posted by Dead Parrot

If the headers are bare metal stainless, they will be just as hot as normal bare metal headers. The ceramic coated ones will run cooler.

Does the exhaust pretty much run next to the frame rail with the fuel tubing? If so, you could be getting heating all along the fuel system and not just in the engine compartment.
...

Thanks for asking! They're bare metal stainless and they replaced a set of regular steel ones that had become warped and rusty. Also, the exhaust does pretty much follow along the rails with the fuel line (steel) and unfortunately they get closest near the header collector. The truck is lifted 3 inches and the exhaust has nice distance except in the 'pinch' spots of near the collector and along the gas tank where it exits off the side behind the tire.

[geezer#99]

210 isn’t that hot. Should keep running.
Lean carb and retarded timing can cause the heat.
What’s your timing like?
Do you have a heat shield between your carb and intake?

There’s a possibility it’s your carb. Here’s a big thread on hotrodders about the 670.
https://www.hotrodders.com/forum/hol...ix-507033.html

Are your fan blades half way inside the shroud?

[Ziegelsteinfaust]

Vapor has happened to me when the fuel pump gets old, and the internal bypass "gets weaker" as rpm goes up. For lack of a better term. Besides proximity heat issues.

Like others have mentioned verify you fuel line is not getting unduly hot from proximity to the exhaust.

Also is the fuel line from the pump to the carb touching the motor? They often get buried, and are the easiest way for heat to vapor lock your car.

If you lived in Vegas or some other represively hot location running a higher psi mechanical or electric if you have too. Into a bypass regulator. This keeps fresh cool fuel practically at the carb. I like mine on the manifold if visuals do not matter.

My friend had a hot 454 in a rcsb C10. It would always vapor lock on 100* days. We put a bypass regulator practically on the Holley 750, and it never did it again. We even took it to Vegas, and it was 115-120*. No issues.

[Mayor West]

The timing probably could do with some tuning. I'll be able to check what it is set at later this evening and report back.

No heat shield, just a riser.

That forum thread was FANTASTIC! So much information and could possibly be my same issue. My Holley is new and I thought by going with a known good unit I could eliminate other problems I was having at the time!

The blades for the fan are mostly within the shroud. It is a single piece aftermarket unit that seems too close for my liking. I have a two-piece shroud on my other Blazer and it fits nicely.

edit: looking tonight I did see that the fuel line from pump to carb is tucked between the block and alternator with little room to wiggle. Tomorrow I will remove the alternator and route that better, and I’ll also check what it is currently set on timing. Thanks again, everyone!

[Mayor West]

I’ve been busy trying the different suggestions and will report back what worked out the best. As you all know that working on carburetors can be fiddly since there is a lot of making single adjustments then waiting for a cool down to try something else.

- Working on timing currently. Looks to need slight adjustments.
- Rerouted fuel line from pump to carburetor. Also sleeved it in a fibre heat wrap.
- Adjusted the electrical choke cap.
- Filled tank with No-Ethanol 91 gas.

It is still acting up, but these changes have had an overall positive cumulative impact. The article on the Holley blocks had the fix that I am pretty sure will do the trick ultimately. I‘ll wait on doing that when I have some more free time to remove the carb.

I’ll keep reporting back and let everyone know what the final conclusion is. Thanks again!

[cadillac_al]

It sounds like the return line isn't doing its job; or the fuel pump is just weak as already mentioned.

[truckin 79]

Tell me if I am wrong here but sounds like you have a fixed fan blade on tuck. I would definitely go with a thermal fan clutch fan blade. These were made to help keep things cool when sitting at idle and lower speeds. The gauge may read 210 but with headers under hood I bet there is a lot of heat being generated under that hood. Just a thought

[Bandit130]

Mine has the metal line from pump run in front of block up to intake then rubber line to carb. I put pipe insulation from pump to top of intake on metal line in hopes to eliminate vapor lock. So far, 4 years and no vapor lock at all and it didn't do that before since purchase in 2015. You can get insulation at hardware store.

[Mayor West]

Originally Posted by cadillac_al

It sounds like the return line isn't doing its job; or the fuel pump is just weak as already mentioned.

Are there particular fail spots to check on the return line, or just give it a good once-over? The mechanical fuel pump was replaced a month ago (in an attempt to fix this problem) with a new AC Delco, I hope that hasn’t crapped-out already.

Originally Posted by truckin_79

Tell me if I am wrong here but sounds like you have a fixed fan blade on tuck....

D’oh! It’s fixed. (EDIT: Just double checked. It is clutch style but NOT a thermal!! Still...D’OH!) That’s a great idea to upgrade! Any opinions on a fan shroud? If someone can provide a decent photo of how it should look properly installed that would be fantastic.

Originally Posted by Bandit130

Mine has the metal line from pump run in front of block up to intake then rubber line to carb. I put pipe insulation from pump to top of intake on metal line in hopes to eliminate vapor lock.

Mine was similarly situated. Before I replaced the fuel pump it was super close to the block, sandwiched between it and the alternator bracket. I wrapped it with insulation and now it runs more freely around the alternator. Good call.

[Dead Parrot]

Sounds like you are making progress.

In my experience, a standard duty thermal fan clutch is what you want. The heavy duty ones engage at a lower temperature and can sound a bit like a jet engine at slow speeds. I went with a standard thermal clutch and don't have any overheat or vapor lock problems even in summer heat while running the AC. Have ceramic coated headers.

5 blade or 7 blade fan?

Might put some fuel line wrap at your 'pinch point' by the header collectors. If the return line is there as well, might wrap it just in case the fuel in it is boiling and limiting your return flow.

Carb spacer or a thick thermal carb gasket?

[Mayor West]

Originally Posted by Dead Parrot

Sounds like you are making progress.

In my experience, a standard duty thermal fan clutch is what you want. The heavy duty ones engage at a lower temperature and can sound a bit like a jet engine at slow speeds. I went with a standard thermal clutch and don't have any overheat or vapor lock problems even in summer heat while running the AC. Have ceramic coated headers.

5 blade or 7 blade fan?

Might put some fuel line wrap at your 'pinch point' by the header collectors. If the return line is there as well, might wrap it just in case the fuel in it is boiling and limiting your return flow.

Carb spacer or a thick thermal carb gasket?

Thank you! When I looked at the fan blade and clutch I was surprised that the clutch wasn’t a thermally regulated type. Other than saving a few bucks and maybe quieter running I can’t think of why that type was installed?

I’ll get to wrapping, too! At the end of the season I can look into getting the headers properly cleaned and coated. Live and learn.

There is a ~1“ phenolic spacer installed now.

Today I’m putzing with carburetor tuning. Yesterday while out, as long as I shifted into neutral at stops and kept the idle up, it didn’t die...it wanted to, though! So somethings are getting better.

[Bandit130]

Found this and thought I would share.

https://www.youtube.com/watch?time_c...&v=-t6geNGbEXM

[Mayor West]

Thanks again, everyone! I have been laid out with a nasty cold this past week, but will report back once I am recovered.