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1997 Chevy Silverado Coolant Problem

Hello All,

My son has a 1997 Silverado C1500 with a 305 and automatic with 140k miles. It had a problem with heat in the winter, so I looked and found the radiator was not full. I put 1 bottle of Bars Leaks and 1 gallon of antifreeze in it with the engine running and slightly warm. I looked for the leak and it appears to be either the water pump shaft seal or the gaskets between the block and water pump. I checked the dipstick for water in the oil and it was only showing oil on it, no signs of water or anitfreeze in the oil. I did not wipe and re-insert the dipstick to check levels, I just wanted to see if it was showing signs of water. It is winter and I figured we would wait for a warmer stretch of weather to replace the water pump.

Just today we checked the fluids again and first we found the antifreeze was slightly lower than I had left it before. No problem, I figured, since the last time I filled it, it was -2F outside! I filled it and found that there were black blobs in the antifreeze. The antifreeze was also an orange-ish, rusty color, with the black blobs in it. He drove the truck around a little and we checked it again. This time, it was warm and had pressure built in the system. We releived the pressure with the cap and took it off completely. It appears now that the cooling system is filled with black motor oil. It also smells like oil. We checked the motor oil and it was 2.5 quarts low. I am not sure the last time he checked it but it was at least 1 month.

So, it appears the cooling system has a leak which allows motor oil in but not coolant into the oil system. How can this be? I could understand the coolant getting into the oil system from a manifold gasket leak but not the other way around. Has anyone seen a condition like this?

I do believe that inside the radiator is a separate oil cooler radiator for engine oil as well as a transmission oil cooler radiator. Those are the two pipes going into the side of the radiator on either side. One set for oil and one set for trans. At any rate I’d suspect you have a breech in the oil cooler allowing oil into the coolant. So new radiator time, plus a flush to get the stop leak out, plus do the water pump at the same time. Water pumps can go pretty fast so the minute a leak is detected, its time to change it out.

I will need to confirm the oil cooler. If it has one, and the non-oem manual I have for that generation truck says they are very common, it makes lots of sense!! Thanks Bing.

Concur that the oil is the radiator is most likely from the transmission, not the engine. But it is possible for engine oil to move into the cooling system, that’s something a failing head gasket could cause. There’s both coolant and oil passages in the head gasket, so a gasket failure could allow oil into the coolant. Which way it goes, oil into coolant, or coolant into oil would depend on the relative pressures.

Still, it seems you got some work ahead. Coolant leak, low engine oil, some kind oil in the coolant.

I’m not 100% sure a 1997 1/2 ton Chevy truck actually has an engine oil cooler inside the radiator. I’m saying that because in our fleet it seems that many of the 1/2 ton GM trucks and suvs do not have an engine oil cooler. Apparently not all of the trucks were so equipped. Those that do not have the oil cooler have the base model radiator and a block off plate on the engine.

Only OP could confirm whether the truck has an engine oil cooler or not

And if OP sees cooler lines going to the radiator, might want to make sure they’re oil cooler lines, and not transmission cooler lines

Moving on . . .

I’m fairly certain OP’s engine uses those plastic intake gaskets that tend to leak and let coolant into the crankcase, and vice versa. I’ve replaced a few. There are a couple of ways that these can fail. The plastic can crumble away over time. The rubber seals themselves can split. I’ve seen both scenarios.

when the intake gaskets leak coolant externally, it often results in coolant trickling down the front or rear of the engine . . . or both. I suppose that could be interpreted as a water pump leak.

It’s also quite possible the water pump actually is leaking, in addition to the other problems. Wouldn’t be at all surprising on a nearly 20 year old truck, especially if OP doesn’t know when the water pump was last replaced, if ever.

I questioned the oil cooler too but looked at the Rockauto radiator. The picture shows two sets of cooler ports. One on each side. Plus the op said the engine oil was quite a bit low.

Well, we just got done changing the water pump and radiator and it is 12:29AM! All seems well but time and coolant system inspections will tell. The radiator does have an oil cooler in it on the driver’s side. The passenger side has trans cooler. Hopefully this cures things.

db4690: interesting about the intake gaskets, that was a concern I had at first since we had coolant leaking from the front of the engine. I thought it was the water pump gaskets but they looked kind of ok when we removed the pump. Maybe it is the intake gaskets…

We lost two quarts of oil into the cooling system in about 25 miles. Hopefully this new radiator is the cure!!!

@“part throttle”

Hopefully you’ll be okay

If you do need to replace the intake gaskets, it’s not that big of a deal. Getting the distributor lined up correctly is probably the only trick part