I have a 1985 Ford Bronco 351 w 4bbl HO.
The AC was converted over to 134a and it blows cold as can be. Except when you accelerate and the rpms go over 3000 then it goes to defrost.
To me this suggest a vaccum leak but I did a smoke test and found nothing and I am perplexed.
Any suggestions as what to look for would be greatly appreciated.
Tim
You don't necessarily have a leak, what you have is a bad check valve in your canister. It's not holding the vacuum on the heater controls when you lose it in the engine. Look for the canister that looks like a #10 tin can and replace it, I suspect your problem will go away.
If I am correct I don't have a cannister that resembles a tin can. Do have a plastic one that is an emissions can and a round ball looking one on the firewall.
I will look again.
Tim
does the 'ball' have one hose going to it, or two ?
if two, replace it
if only one, there is a check valve somewhere between it and the intake manifold (and the CV is bad)
Clearly it's loss of vacuum. It might be a leak, or it might be your check valve is not holding. You need a check valve to hold vacuum on those dashpots when your manifold vacuum drops.
But the important thing is, the doors move when you lose your manifold vacuum, so there's no thing to be perplexed about.
A leaky check valve, cracked hose or cracked reservoir. If you have the "tin can" reservoir, they crack all the time. You can test all this with a hand vacuum pump. Discount auto parts chains sell them for around $20.
Al
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