rickg56

Symptoms:
Idles a bit high, A/C and heat won't stay blowing thru dash vents at anything much above an idle, and under HARD acceleration, especially if in a slight turn, I'd hit about 4K or 5K RPM's, and the engine would be like someone turned the key off. As soon as the car would settle down a bit, it'd come back to life, start revving hard, then shut off again. REALLY weird!
Took 'er to a shop I use when I get too stumped on something. They couldn't find a thing. Ran fine for them, except for a quick stumble once, and nothing on the computer scan. But one of the other techs said he had a C4 several years ago that did a similar thing. Somehow he narrowed it down to a bad ECU, guess it was sending a random spike from somewhere, thinking it had a extreme lean condition, and would shut off the engine momentarily. Great. just what I need.
Anywho, in the mean time, decided to track down a vacuum leak to try and fix the first 2 issues. Finally found a rotted out 2-way check valve on the passenger side of the intake plenum. The nipples on either side LOOKED like the hoses were connected, but when I wiggled it, there wasn't anything left connected. AHA! Bad vacuum leak. Auto parts store had one on the rack, costs $4.25. Put 'er on, and now it idles a bit lower, and the A/C blows out the dash vents!
AND! The weird engine-shut-down-under-acceration-thingee has gone away! I'm gonna guess an extreme lean condition when the engine torqued and tugged on the hoses, or something like that.

hcbph

Good to know, thanks for sharing.

70ZZ3 96LT4

Rick did your cruise control work before you changed the valve? I read somewhere that the cruise control will not work if this vacuum valve is broken.

Dt86

Yup that's pretty common for those. They dry out and break. Mine looked good but under the vacuum hose it had holes which was my vacuum leak. That was fun to find!

JrRifleCoach

NOTE to Noobs.
Go to the store, buy one of these and install it.
Or throw one in your spares box. Thats the left hand cargo compartment.

Joe C

is that check valve pretty much the same on all C4's? I'm redoing all the vacuum lines on my 85, and come to find out the line coming off my check valve, going to the cruise control is pretty much non-existent. probably why the cruise control didn't work - ??? the other side of that thing seems to be going thru the firewall, and I'm thinking the HVAC. I'm not sure that was working either! i'll need to procure one fairly soon, and I'm guessing it's available at most any parts supplier.

cuengineer

Curious as to whether the part was Corvette specific at the auto parts store or whether you just found one that fit? And what kind of parts store in case I need one.

rickg56

Sorry, don't get on the forums I belong to as often as I should.
Anyway......
My cruise control DID work before I fixed this leak, for whatever odd reason. Maybe there was just enough suckage left.
When i showed my old rotted out fitting to the guy at the parts store, and he went over and grabbed this one from the rack, I had him double check the application, just to make sure. He looked it up, and it showed something like "Chevrolet - 1985-2000something". Not exactly sure the year range now, but it was quite a wide spread. So, yeah, must be the same thing used on ANY Chevy for those years.

rickg56

...oh, and as for the engine-shut-down-under-acceloration thingee, I think I have re-diagnosed that issue. I was low on gas. Found my gauge was WAAAAAY off! I ran out on the way home a couple weeks ago, but the gauge still showed a quarter tank when I left work. And I only live like 15 miles from work. Yuppers, I had the corroded tank float rheostat issue. But thanks to this forum, I learned how to fix that
So, hopefully that one is behind me too!