Low intake vacuum = bad brakes on start-up?

[ølaf]

Hi all -

I'm still trying to hunt down the source for my hard pedal on cold starts, particularly at high elevation. This morning, at 10,000ft, I measured the vacuum at the intake plenum where the brake booster gets its vacuum from. I believe you re supposed to get 16 - 20 in here.

Instead, at cold start-up, I had 11 (!!) and after warm-up, it crawled up to 13-14. I understand that at higher elevation you will get lower readings, but could this be the cause of my weak/hard brakes at start-up (for the first 45 minutes, give or take)?

Any one know what their cold-start vacuum is coming from the intake by chance?

And secondly, besides overall engine age/low compression, any other thoughts on what could cause the low vacuum?

Idle is buttery smooth, and I cleaned the Throttle Body, IAC Valve and intake plenums a few months ago when I replaced a file injector.

Thanks for any ideas!

 

[gamefreakgc]

I believe that on the brake booster there's a check valve that's known to freeze up or go bad. Hopefully someone can chime in more that has experience there, or try searching the forums. I have a supercharger on mine which changes the vacuum routing significantly or I'd just go in the garage and find it.

 

[ølaf]

Thanks gamefreak.

I really wish it was the check valve. Swapped booster and check valve for OEM, and issue persists.

 

[19963.4lsr5]

What is your idle RPM?

On my 97 it was the check valve in the brake booster that caused the hard pedal for a little. Usually after the first braking it freed up and gave a normal pedal.

You installed a new check valve or a used one?


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[brillo_76]

I am curious what your engine vacuum is a low altitude. 10k ft is getting up there with air starting to be thinner. You changed just about everything it could be. Yet your issue persists. This is definitely interesting.

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[ølaf]

Thanks guys.

I changed the check valve and grommet as well. No change.
Idle sits at 700 once warmed up.
Vacuum on the warm engine in Denver, at 5280 ft reads 17 in.

Apparently you loose 1 inch of pressure for every 1k in elevation, which puts both vacuum readings, from 5000 ft and 10000 ft, in the right context. Adjusted for sea level, 17 inch @ 5000 ft would be 22 inches, and 13 inches at 10,000 ft would be 23 inches at sea level.

I'd love to hear from someone else in Colorado to see if they have any similar experiences.

- ø