Suction from valve cover grommets
October 21st, 2014, 06:31 PM
#1
Suction from valve cover grommets
I've been having no luck diagnosing a miss in my vista cruiser with a rebuilt 350, but that's another story altogether.
Now, the 307 in my 85 Custom Cruiser has been running rough for a while. One of the tests that my brother-in-law got me to do on the Vista, as recommended by his friends dad is to see if there is any suction from the valve cover grommets. If so, that is a tell-tale sign of an intake manifold gasket leak/failure.
Well, the 350 had not such suction, but while searching for any indication of a vacuum leak in the 307 (the system was pulling a steady 17in), I noticed that the driver's side valve cover where the snorkel hooks up to would suck in my thumb if I let it!
Has anyone ever heard of that test? And even though I'm pretty sure the answer is yes, would a leaky intake manifold gasket cause this engine to run rough and just over-all crappy with random surges? There are no other signs of a leaky gasket that I know to look for. No coolant in the oil, but it does tend to smoke when I start it up, but then again, what 30 year old car doesnt?
Now, the 307 in my 85 Custom Cruiser has been running rough for a while. One of the tests that my brother-in-law got me to do on the Vista, as recommended by his friends dad is to see if there is any suction from the valve cover grommets. If so, that is a tell-tale sign of an intake manifold gasket leak/failure.
Well, the 350 had not such suction, but while searching for any indication of a vacuum leak in the 307 (the system was pulling a steady 17in), I noticed that the driver's side valve cover where the snorkel hooks up to would suck in my thumb if I let it!
Has anyone ever heard of that test? And even though I'm pretty sure the answer is yes, would a leaky intake manifold gasket cause this engine to run rough and just over-all crappy with random surges? There are no other signs of a leaky gasket that I know to look for. No coolant in the oil, but it does tend to smoke when I start it up, but then again, what 30 year old car doesnt?
October 31st, 2014, 07:32 AM
#4
72Cutlass S
Join Date: Jul 2011
Location: Bakersfield, CA
Posts: 1,034
The correct way to do the test is to pull the pcv valve out of the valve cover. Then plug the holes on both valve covers with your thumbs. If vacuum instead of pressure builds up under your thumbs you have a leaking manifold gasket on the bottom side of the intake. And yes this will cause surging and rough running along with increased oil consumption.
October 31st, 2014, 07:41 AM
#5
GS72 the surging, roughness and oil consumption IS what I'm getting at the moment.
The side with the pcv pushes exhaust. When connected to the hose to the carb, the pcv pulls serious vacuum itself. The grommet where the pcv goes pushes exhaust.
Now, the side where the crankcase breather goes is the side that sucks air. It is rather strong. Not as strong a vacuum as the carb pulls, but it will suck my thumb in.
I only did this text one thumb at a time, though. I will perform it with both valve covers and see what comes up. Thanks!
Buz
The side with the pcv pushes exhaust. When connected to the hose to the carb, the pcv pulls serious vacuum itself. The grommet where the pcv goes pushes exhaust.
Now, the side where the crankcase breather goes is the side that sucks air. It is rather strong. Not as strong a vacuum as the carb pulls, but it will suck my thumb in.
I only did this text one thumb at a time, though. I will perform it with both valve covers and see what comes up. Thanks!
Buz
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