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Engine block drain plugs?

10-25-2004, 03:19 PM
I'm attempting to do a flush and refill of anti-freeze in our '95 Blazer. I found the petcock for the radiator and drained the radiator. For those that don't know, you have to remove the air filter housing to get to it.

Now I'm trying to find the two (that's right, two!) drain plugs on the engine. The Haynes manual says that they are located just below the exhaust manifolds. One plug is below one row of three cylinders, the other plug is below the other row (I guess there is a drain plug on the left and right sides of the engine). I can't figure out where these plugs are and how to get to them.

Anyone have any advice?

-- recurve

10-25-2004, 06:09 PM
Drivers side is just above the oil pan in the middle of the engine, 9/16" socket for this. Might have to punch the hole out with a screwdriver to break through the built up scale, normal. Other side probably has the knock sensor in this hole, just forward of the starter. Unplug wire and use a big socket to remove. Thinking 22 mm but could be a 7/8". Maybe too late to help now but next time you'll find them.

10-25-2004, 11:10 PM
Thanks for the clarification Tom,

Two people have told me offline that it's better to leave those two bolts alone. They claim I should just run the engine with the petcock valve open on the radiator and just keep adding water from a garden hose. They say this will circulate throughout the system flush it completely. Both of these people go on to claim that I might actually do more harm by removing the drain plugs on the engine block becuase it might destroy the seals.

But the Haynes book, Chilton's manual, and Tom all agree that I should pull those drain plugs.

Are there basically two "correct" schools of thought?

Slightly confused,
-- recurve

10-26-2004, 12:20 AM
The problem with not draining the engine block is that you won't get enough anti-freeze into only your radiator to give you a 50-50 mix for adequate protection. There are no seals to destroy because both bolts have tapered threads - which accomplishes the sealing.

I'm curious. I have a 98 Blazer and I just changed out the coolant. You should have the same 4.3-liter V6 engine as I do. My drain plugs were near the rear of the engine block, one on each side, and both bolts were the same size - 12mm I think (sorry can't remember). I could see both bolts when I was under the Blazer on the centerline, my head was about even with the rear of the engine, and I looked up and forward. In order to get the bolts out, I needed to use extensions and work from underneath the Blazer.

Chevy did supposedly "refine" the engine in 1996 like putting in a stiffer engine block. The refinements were to decrease noise and vibration levels. Maybe that's the difference.

10-26-2004, 08:11 AM
Thank you mw2 and Tom. So the main point in removing the drain plugs is to clear everything out of the cooling system that way you can guarantee that you have a 50/50 mix of new anti-freeze and distilled water. Got it, makes sense.

However, the following statement from someone else has got me a little freaked out:

"The engine block has plugs which are driven in after the sand used as a mold when casting the engine block is cleaned out during manufacturing. I would not remove those plugs unless there is a leak and unless I had help from an experienced mechanic or engine rebuilder."

He went on to say that you should only touch those plugs if they are leaking and causing trouble. Removing them could introduce a problem that wasn't there before. Conversely, in another thread in another forum, I saw someone who said the bolts have a tapered thread and seal up the engine block without any problems when you reattach them. My guess is that this all boils down to either under-tightening or over-tightening those bolts. But neither the Haynes nor Chilton's manual says what to set a torque wrench to on those bolts (or I overlooked it).

Here is a thought. See if there is something wrong with this logic. What if I leave the drain plugs alone but use distilled water to flush the system by pouring it into the radiator. I'd have the engine and the heater turned on while the flush was in process. Once the system started draining clear I would stop adding distilled water and let whatever fluids can drain continue to do so before locking the petcock on the radiator. At this point, there is some unknown amount of pure 100% distilled water in the system. Now I add 1.5 gallons of pure anti-freeze to the system through the radiator cap. At this point I have a richer dose of anti-freeze in the system, probably a 75% - 25% ratio. Finally, I use distilled water to top off the system, perhaps 0.5 gallons up to 1 gallon - probably can't put 1.5 gallons in because there was initially quite a bit of distilled water in the system. At this point, I should have a perfect 50% - 50% mix.

This seems like a good method without any disadvantages that I can tell. Comments, corrections are welcome! :-)

Thanks for being my sounding board,
-- recurve

10-26-2004, 09:51 AM
Whoa now. Couple things. You're talking about two different things, core plugs and drain plugs. The core plugs are about 1 1/2 inch diameter and driven in the block sides in a couple places each side. Leave them alone. The drain plugs are threaded pipe plugs, one each side. May be a knock sensor on the passenger side in that hole, may be an oil cooler line in the driver's side in that hole.
Also, draining the radiator and the block will not drain the whole system, you'd be surprised how much water stays behind. Your system holds about 17 quarts of coolant so when you get it all drained, put two gallons of pure antifreeze and top off with good water. You'll see it will hold maybe another gallon at most, the rest stayed in the system. Also for a much quicker flush, take the thermostat out and flush without it, circulates the coolant without waiting for the engine to warm. Good time to replace the thermostat and gasket at this time.

10-26-2004, 11:11 AM
Agree about core plugs.

You don't need a torque wrench for the drain plugs - unreliable anyway because of extensions, contortions to get to drain plugs, and lack of room. If you use a 3/8 drive, you shouldn't have problems about overtightening. If you use a 1/2 drive, just be careful. I am cuz I tend to overtorque. If you're concerned, use Permatex Threadlocker when installing the bolts. I used Blue although it looks like Chevy used Red.

Sorry, I don't understand your modified flush procedure. If fluid drains clear, there's no anti-freeze. Don't know how you come up with 75/25 mix.

You also don't talk about using a radiator flush product. Maybe you don't need to, but I use it to make sure that the engine block, heater core, and radiator are as clean as possible.

A couple of stats on my 98. The coolant system capacity is 11.7 quarts. The radiator by itself only has a 3-4 quart capacity. After I had the system completely flushed and drained, I put in 6 quarts of anti-freeze and topped it off with 4 quarts of water - this was into the engine block and radiator only. I then filled the recovery tank with 50/50 mix. I don't think your engine is that different from mine.

Couple of hints. The drain plug on the right side was over the starter motor. I disconnected the battery because the terminals on the starter motor are uninsulated and one of the terminals always has 12 volts on it. I also put a protective wrap over the motor to try to keep water out as the water was draining. I drained the left side first and then the right side - less water that way draining out of right side. At first, I thought I needed to buy a flex-socket. But I finally found straight shots to the plugs from underneath - the left side was the harder. Finally, when you get the drain plugs out, there might be nothing coming out. You need to poke a screwdriver into the hole to remove the crud buildup.

Good luck.

10-26-2004, 12:21 PM
Oops, my bad, thinking of a V8 system with the 17 quarts. Getting off track.

10-28-2004, 10:25 AM
Thanks to everyone :-) I just flushed and filled our '95 Blazer and am working on the '97 Blazer now :-)

Take care,
-- recurve

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