Antifreeze in my chevy s-10 4.3 L engine oil

I found during an oil change that there is coolant in my oil, I had suspicions of this because of having to add coolant more than I should of had to. There is probably either a crack in my block, head gasket or intake manifold, my question is how can I tell which. I figure that if I preform a compression check it will tell me that I have a cracked block and or a cracked head gasket, what will tell me if I breeched my intake manifold. Any help would be gratefully appreciated.

I think you need a good independent mechanic to check this out for you. I had a problem just like yours with a Jeep Cherokee that I owned. It turned out to be a ruptured radiator tank that was letting coolant in through the transmission cooler tank. That’s always a possibility as well as a cracked block or blown cylinder head gasket. The mechanic should find the problem rather quickly.

I would suspect the intake manifold gasket first–GM V6 motors are known for this. Not sure about your year.

Thanks, for tell me about your jeep I hadn’t thought about that possibility, I am going to consult a mechanic eventually, however I’m trying to narrow down the list of possible malfunctions.

Knowing the year and which engine it is would help.

I can tell you that the oil does not share any of its cooling with the radiator, as missileman’s automatic tranny does, so mixing there isn’t possible.

I can also tell you that (and I say this not knowing what engine) the intake manifold is unlikely to contain any oil passages, so while that can allow coolant to get drawn into the engine in some cases it’s not a potential mixing location.

The only repairable possibility is a breech in a headgasket. The 4.3 liter engines are V style engines, so what you’ll wan to do is do a pressure leakdown test to find out which cylinder(s) is(are) involved. From there you’ll know which head needs to be pulled…or if they both need to be pulled.

If you’re having the work done, don’t try to tell the shop what to do. Give them a complete description of the problem and let them do the diagnosis. I’d suggest having them diagnose and provide a repair estimate before moving forward. Then, based on the year and condition of the truck overall, you can decide whether to proceed. You’ll have to pay for a few hours shop time, but it’s worth it.

Post back with the results.

I’m sorry about not putting a year with this vehicle, it’s a 1997 chevy s-10 4.3, 2wheel drive and I also 2 years ago replaced a fuel injector which required me to remove the intake manifold.

The engine has an upper and a lower intake manifold. The upper intake manifold is removed in order to replace the spider assembly.

The coolant leak would occur at the gaskets for the lower intake manifold.


I would try the easy stuff first and check the intake manifold. When you open it you will be able to tell if there is a leak. Hopefully its that. If its something else then it will be an expensive fix.

That’s okay. There are no oil passages in the intake manifold. I’m afraid you’ll have t pull a head… or two. Your only repairable possibility is a headgasket breech. A cracked head might be considered repairable, but in all honesty that’d be a real rarity. IMHO your odds are 99:1 that it’s a headgasket.

A leaky intake manifold gasket will let coolant into the oil. I had to have the intake manifold gasket replaced on my 2000 Blazer due to coolant getting into the oil. Once the gasket was replaced I drove it for 9 more years with no further issues (at least with the intake manifold gasket).

Ed B.

What engine?
I could be wrong, but I do not believe the PO’s engine has any coolant sealed by the intake manifold gaskets.

Guys? Anybody have any in-depth experience on these engines? Can you confirm or correct my belief?

A 2000 Blazer with the 4.3 has coolant passages through the intake manifold. The lower intake gaskets are a composite flat with o-ring seals around the intake ports and the coolant passages. As the composite material degrades with age it no longer provides a solid mating surface for the o-rings to seal against, and coolant begins to seep into the lifter valley under the intake, and sometimes leak externally as well. Fel-Pro offers replacements made of steel rather than composite material to prevent repeat failure.

There is also a possibility that there is a radiator failure, as most of these have an engine oil cooler in the left tank of the radiator and a transmission oil cooler in the right side of the radiator.

Head gasket? Not very likely without an overheating episode or excessive rust or corrosion in the cooling system.

However, the question with the intake gaskets is not if they need to be replaced but when. I’m surprised they’ve lasted this long. Several years ago I’d see a few of these every month. Now, I think they’ve all been repaired or junked.

Ase, thank you sincerely for your comments. I’ve learned something, and that’s a good thing.

Apologies to the OP for having misled him.


I hate to say it . . . you’re wrong

We have tons of GM trucks with the 4.3 V6 in our fleet

I know from personal experience that failed lower intake gaskets WILL let coolant into the crankcase.

Here’s another interesting thing. All that coolant in the crankcase isn’t great for the lifters either.
They sometimes start making noise, because they prefer engine oil, not coolant.

The good news is that the new gaskets are an improved design

Mea Culpa, bd.
Asemaster has already straightened me out on this. Now, if I could take a moment to remove my feet from my mouth… I’ll prepare for my next embarrassment… :- /

Oh come on mountainbike, it’s not that bad. This is an internet forum populated by many DIYer and handman types. Only a professional would/should know the particulars that we sometimes speak of.

Well, still, I walked around my mouth pretty thoroughly. And I sincerely appreciate your stepping in and setting me straight. I try to learn something new every day, and you’ve made it a good day. I just hate it when I mislead someone that badly. I shouldn’t do that. My heart was in the right place, but my foot was firmly planted in my mouth.

And I know I have a terrible memory for details. I should know better.

You guys are a great and understanding bunch. Life is truly good.

you guys are all a big help to me, I’m glad to hear your thoughts on my problem and appreciate your input. I’m thinking of looking into the radiator as being at least part of the problem, when I found that my coolant level dropped a considerable amount I also had to remove my overflow container to remove about a handful of brown goop from the area around the intake from the radiator hose , I didn’t know why it formed there, but if there is a n oil cooler in the radiator then maybe it corroded through. that would be the easiest fix I could hope for or at least a good place to start.

One last thing I’m goin’ ask.

Has this cooling system had Dexcool all it’s life?


Dexcool is what… the GM type antifreeze, I think, if it is then I’m not sure, Iv’e never used it myself in the 4 years I’ve owned it, I bought the truck from my sister in law after my brother passed away and am not sure what he used. Dexcool is a problem for intake manifold gaskets though isn’t it. If it isn’t the oil cooler inside the radiator then I would assume the intake manifold gasket.