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1999 olds silhouette fluctuating heater gauge

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Feb 19, 2017, 11:52 AM

Post #1 of 3 (864 views)
1999 olds silhouette fluctuating heater gauge Sign In

Hello! Hope someone can help. I am at my wits end here.
I have a 1999 Olds silhouette.
The van was sitting for 5 years before I took possession. And it was leaking from the water pump gasket. So I changed the water pump.
After a 200 mile trip, with no warning, the heat gauge shot up in to the red. I immediately pulled over and opened the hood and steam started pouring out. I immediately thought stuck thermostat as the upper rad hose was stiff, etc. So replaced thermostat.
Shortly after, the coolant light started flashing even though the coolant was full. Otherwise it ran fine for a while and had heat.
Then the heat gauge started going crazy up and down. Didn't matter if it was in idle, driving through city or highway. It didn't overheat again, but the gauge would go over the halfway mark and then drop far below normal and continue this behaviour.
The heater would only work sometimes and only when the temp gauge was low.
I assumed this was vapor lock so I bled the crap out of it. Parked on a hill, opened bleeders, opened rad cap, installed bleeder on hose that runs to upper intake manifold and bled through that, etc.
It didn't help much so I flushed the system like crazy and replaced the rad cap. Didn't help.
What did help some was pinching off the hoses to the rear heater and then, later, not using the heater at all.
I have been able to drive it by bleeding when the temp reaches the halfway mark then it goes back down and I drive on.
I did a block test and the solution did NOT change colors however when running the engine with the rad cap off the antifreeze would spill over.
I changed the thermostat 2 more times to make sure it was not faulty, but it did the same thing when I drove it without the thermostat in it.
The oil (and other fluids) look fine. Even if the antifreeze does look a bit dirty. No white smoke. No visible leaks. I can't tellfor sure about an internal leak because of how many times I have bled it and had to top off from losing antifreeze that way. I put a piece of cardboard underneath it and could not detect any leaks.
Another thing I noticed was antifreeze would overflow when in idle with rad cap off, but would be sucked back into the radiator when revved.
I took it to a mechanic and they said i needed a new water pump and thermostat, which is obviously not the case.
Any ideas?

Tom Greenleaf
Ultimate Carjunky / Moderator
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Feb 19, 2017, 12:59 PM

Post #2 of 3 (859 views)
Re: 1999 olds silhouette fluctuating heater gauge Sign In

Now just of note that it sat for 5 years as it's been run at least those 200 miles:

With that and now know this has a rear heater too it's going to be hard to know coolant is really full and if it overheated of course a head gasket is now high on the list. Block testers aren't necessarily the most accurate way to know.

Coolant if low alone would make gauge reading erratic and heater(s) run warm, hot or stone cold even while overheating. Flushing if you noticed lots of junk come out probably has lots more still moving around too.

The system must be able to hold proper pressure via the radiator cap which is new - fine alone but can also pressure up from combustion gasses displacing liquid coolant for vapor and be doing this. Need to test more that a head gasket isn't now the problem and think a vacuum fill machine of cooling system may be the only way you are going to know it's truly filled without air/vapors still inside to finish diagnosis of blown gaskets,


Hammer Time
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Feb 19, 2017, 1:23 PM

Post #3 of 3 (852 views)
Re: 1999 olds silhouette fluctuating heater gauge Sign In

You can't run the engine with the cap off. The cap causes pressure in the cooling system which increases the boiling point. These engines are designed to run as high as 228 degrees at idle and you can't do that with the cap off. It will just boil.

These engines are notorious for air pockets in the back of the engine and will blow a head gasket quickly if run low.

We generally use a vacuum fill system to get all the air out.


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