The Stovebolt Site Forums The Shop Area Engine Shop 235 manifold heat valve spring

I've experienced a problem with the heat-sensitive spring not opening enough. With the introduction of our winter-blend gasoline here in Colorado, vapor lock got me the other day at an outside temperature of 65 degrees! I installed a new spring and tested it with an electric heat gun with the manifold both off and on the engine. It worked fine then but in the real world not so much. I installed it with 1/2 turn of the spring to the anchor pin as illustrated in the shop manual. It seems that the engine doesn't generate enough heat to fully relax the spring to open the valve in the manifold with the engine running. Since I've completely unanchored the spring, my vapor lock symptoms (easily viewed by the percolating gas in the glass-bowl inline filter) have greatly diminished. The engine runs a little rough when cold but much better when warm. So, is there a happy medium to be found in modifying either the spring or the anchoring point? This is on the 48 suburban. My other trucks are also prone to the percolating gas routine but not as badly on summer gas. I live at 7600' elevation. Does gas boil easier at this altitude? Were these springs unreliable back in the day? Any thoughts welcome. Thanks!

I have the same trouble with a 2000 Jeep Cherokee. Winter gas and a warm day and you start it when warm it idles rough for a bit.

John

The spring will relax at idle but will not open all the way until throttle is applied, and exhaust gas velocity increases

When I manually rev-up this fully heated engine, the valve doesn't fully open. There doesn't seem to be any restriction in valve movement when moved by hand.

When it's fully heated is there still tension on the spring?

With the spring I talked about here it's completely away from the pin.

Yes, there's still tension in the spring. When I tested the spring with the heat gun before putting it into actual use, it fully released the valve with no residual tension. I don't know how much hotter the heat gun is than the engine under real world conditions. Your spring concoction looks ideal.

Like I mentioned in my topic I had one of these springs and unhooked it in the summer because I wasn't sure if it closed correctly or not. When I would give it gas after it was warmed up I would see it try to close but I don't think it ever did.

Yes, that's the same experience that I had. That's why I asked if these springs were maybe not the most reliable thing that GM ever designed even when they were brand new technology. I'm "glad" that someone else had the same experience and that I didn't hallucinate the whole thing.

I think there is a problem with the aftermarket spring design being different from the original. Or to say it differently, "Hing chow kwang dung chew qui chang cain."
Carl

Is that Mandarin Carl?

I put an original GM spring back on and went for a spin at 40 degree temperature with the spring hooked up this morning. I didn't get stopped by vapor lock, but the gas still percolates a bit. Maybe the gas comes from China too.

I don't know. I gleaned it from several fortune cookies and the Kung fu TV series. They do make excellent oranges though.

Try building a small sheet metal shield for the spring to keep the heat close. Adding a carb spacer may help also.

Joe

I'm not sure what you mean, Denny, about the high altitude that I live at relative to most of the country and fuel available at sea level. The fuel I buy at the local gas station is formulated for sale in Colorado. I think it's lower octane. The winter blend gasoline vaporizes at a lower temperature to facilitate cold temperature starting. That's about the full extent of my understanding of petroleum refining. I haven't rerouted the fuel line from factory configuration.
I have the factory carburetor heat insulator, Joe. The black one that appears to be made from Bakelite or some similar material along with a couple of gaskets. I think the enclosed sheet metal oven idea sounds plausible.

Is the carb hot to the touch? is it getting so hot from the manifold that it's boiling fuel out?

You sure the fuel line is not to close to the thermostat housing or close to the exhaust manifold?

It's hard to believe the fuel can boil at the filter from heat at the exhaust manifold, but I guess it can since you are seeing it. I wouldn't have though you can get that much heat from the riser.

Joe

Denny, that link is very helpful and describes my situation. Thank you. Joe, the carb is not hot to the touch. The fuel line does pass by the thermostat housing on the factory-mounted clip that routes it between the housing and the valve cover but the fuel line is not near the exhaust manifold.

To put an air of conclusion to this, I intend to retrofit all my trucks with chadkiser's 70 Gremlin spring. They all percolate to one degree or another. Spring, summer, fall, summer gas, winter gas, they all do it. I'm ready to believe that some small amount of the heat riser spring not opening all the way has no effect for the vast geographic expanse of the USA at lower elevations, but I need all the cooling of my fuel that I can get. Thanks to all of you for your help! Dave