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Greasy residue on radiator cap


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Astral Projection
I am merely driving my van
I am merely driving my van
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How many wheels are on an Astro van? (4): 4

Greasy residue on radiator cap

Post by Astral Projection » Wed Jul 26, 2017 8:42 am

I bought a 2001 Astro AWD 4 months ago, then joined this forum. Thanks for taking the time to read my post and I appreciate any responses. This my first Astro/Safari and I'm still getting to know it. I'm no mechanic, but have remodeled houses professionally for over 10 years, so I can understand how things work and occasionally do some small repairs/maintenance on my vehicle.

I bought my astro at auction. Previous owner was the city of Redmond, WA. It had only 45,000 miles on it, looked and ran great. I don't really know anything about how it was maintained - but generally government fleets follow a maintenance schedule. I didn't do a thing to the van, then I put 5,000 miles on it commuting over the past 4 months with no problems.

Recently I noticed that the oil level had gone down slightly but was still well within the OK range and looked pretty clean. Also the coolant level in the recovery tank was low. But no overheating problems. Then when I remove the radiator cap I find a greasy brown residue on the cap and tube from the radiator. Coolant is slightly low in the radiator, and looks kinda opaque, not quite right. Oil still looks normal to me, not like chocolate milk or anything.

So, any ideas why that greasy stuff is in the radiator?

Can't be good. Bad head gasket, intake manifold gasket maybe ... there are many stories about Dex Cool and failed gaskets, and the coolant in mine is orange. Also I guess there's a transmission cooler in the radiator, maybe an oil cooler too? Anything other than oil or trans fluid getting in there that would make the cap look like this? Ways to tell if it's trans fluid or if it's oil? I think trans fluid just means you have to replace the radiator, but oil likely indicates much more costly repairs.

I'm really hoping this is something that I can afford to fix, I like the van and was hoping to drive it around for years to come.


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Re: Greasy residue on radiator cap

Post by Mmusicman » Wed Jul 26, 2017 9:35 pm

I don't know anything about Dexcool, which could be related to your issue... but I am aware it can be trouble.

I do know however, that it is possible to get combustion into the coolant, which could cause the same or similar issue. It is typically introduced one way under the pressure of combustion, and can seep through even the tiniest crack, either in block, head, or head gasket (more likely). This is not typically detected by normal cylinder compression tests, or pressure tests of the coolant system. They do however, make a chemical tester that shows if there is combustion in your coolant. If the leak is severe, then the other tests may show it too.

The leak can be so minor that it doesn't warrant an expensive repair. But it can also grow worse over time, to the point where it might start pushing out coolant while driving, possibly starting to cause overheating issues. If the coolant recovery is working correctly, it can "mask" the trouble... allowing the coolant to be replenished in the system when the engine cools. You may not even show any loss of coolant, but combustion can still be an issue.

There is always the opportunity for the leak to go both ways... which might start to become evident by some moisture out the tail-pipe when you first start the vehicle.

All this assuming the leak is not to/from the oil chambers... which would be evident in the oil.

Rust in the system can also produce some goo at the cap.
I would at least fully flush the system and start fresh... and take it from there.
2000 Chevy Astro LS AWD | 31" LT265/70R17 Tires | 8" LIFT | 10" TOTAL
92 Chevy Astro Shorty | V8-350

I am merely driving my van
I am merely driving my van
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How many wheels are on an Astro van? (4): 4

Re: Greasy residue on radiator cap

Post by interplanetarybarry » Fri Nov 03, 2017 5:42 pm

That stuff looks like a powder type stop-leak product. The grains are supposed to get stuck in the gaps where coolant flows out, then they swell up and seal the problem (until it gets worse!) maybe there’s a product out there that can clean it out during a flush?

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