Diverter valve vacuum motor not holding vacuum. - Hot Rod Forum : Hotrodders Bulletin Board
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Old 02-01-2013, 10:46 AM
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Diverter valve vacuum motor not holding vacuum.

Hello all.

Been gone for a while once the tranny in my 84 GMC K1500 died. I decided I needed a newer truck, so I picked up an 85 Chevy K10. Same basic truck including a working 700R4, a better running engine (both are LE9 5.0's), and very little rust on the body. It's a little cold-blooded and was idling really rough due to many small vacuum leaks. Most of them were in rotted/broken vacuum lines. I got it from 8" of vacuum to 15", which is still low. I think it's leaking either at the carb-to-manifold junction or at the primary throttle shaft in the baseplate.

But, if I disconnect the diverter valve (AIR system, not PCV/evap decel valve) and plug the line, I get another 2" of vacuum. The valve works, but it won't hold vacuum at all. I can pump it up to about 15" and it leaks down in about 2 seconds. I tried the same thing on my 84 (which is currently being scavenged for parts for this one) and it does the same thing, but it leaks down a little slower.

I don't remember back from when I was a mechanic (many, many, many years ago) if that behavior was by design because of the way that the diverter valve works.

Any ideas? And, please, no "Tear-the-emissions-stuff-off-of-there-its-not-doing-you-any-good-and-its-a-communist-conspiracy" answers. I have to have it for emissions testing here.

Matt

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Old 02-01-2013, 02:07 PM
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Originally Posted by morcey2 View Post
But, if I disconnect the diverter valve (AIR system, not PCV/evap decel valve) and plug the line, I get another 2" of vacuum. The valve works, but it won't hold vacuum at all. I can pump it up to about 15" and it leaks down in about 2 seconds. I tried the same thing on my 84 (which is currently being scavenged for parts for this one) and it does the same thing, but it leaks down a little slower.

I don't remember back from when I was a mechanic (many, many, many years ago) if that behavior was by design because of the way that the diverter valve works.

Any ideas? And, please, no "Tear-the-emissions-stuff-off-of-there-its-not-doing-you-any-good-and-its-a-communist-conspiracy" answers. I have to have it for emissions testing here.

Matt
I had a similar problem where the check valve in the exhaust manifold tubes had gone bad, allowing hot gas pulses to reach the diverter valve. Naturally, this caused the diaphragm in the valve to fail. You might want to replace it (and the check valve). Unfortunately, these diverter valves don't seem to be cheap.
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Old 02-01-2013, 02:27 PM
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Originally Posted by joe_padavano View Post
I had a similar problem where the check valve in the exhaust manifold tubes had gone bad, allowing hot gas pulses to reach the diverter valve. Naturally, this caused the diaphragm in the valve to fail. You might want to replace it (and the check valve). Unfortunately, these diverter valves don't seem to be cheap.
Yeah, they're not cheap. NAPA has them for about $130, which is about $80 cheaper than anywhere else I've seen. The one off the 84 is just different enough to not fit. I may go in to napa or some other parts store and see if they'll let me test a couple of "similar" ones with my vacuum tester to see if they hold vacuum.

Matt
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