Question about 1999 Pontiac Bonneville

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I am looking at a 99 Bonneville that is using anti-freeze. It's not leaking into the oil but must be burning it. I replaced the gasket on the top of the intake manifold under the plastic plentum, but it didn't do the trick. Seems to be running a bit rough but not bad.Can you tell me what to look for next?

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  • chuckobar Sep 04, 2010

    No, sorry much more in depth than that. It's not leaking it, or blowing it out. I found excess anti- freeze inside the plentum when I took it off, looks like it could be the intake itself. I see there have been problems with the coolant passage deteriorating. Does this seem legit?

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For there to be a loss of coolant, there has to be a leak in the pressure system.

Borrow a pressure tester from a place like Autozone.
Pump it up slightly greater than the pressure the cap was rated for.
Wait 5 minutes to see if it holds pressure.
If so, that is good but there is one more extreme test to do.
Start the engine and see if there is any oscillation from 1 cylinder leaking into the coolant system.
If so, or if pressure did not hold, then you need a new head gasket.
But also check under the water pump, because that is the most common place for a leak only when the engine is running.

Posted on Sep 05, 2010

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  • Master
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Radiator cap. The radiator cap is rated for different pressures. I have a 1999 Cadillac I was having the exact same problem. When the cap gets old, the spring in the cap looses tension. And when the car warms up to temp the weak radiator cap lets the pressure off and leaks water.

Posted on Sep 04, 2010

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