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Last summer I put a new engine in my Z. Unfortunately, despite my best efforts, I was apparently unable to clean ALL of the rust out of the block. It was relatively unnoticable at first, but after sitting in a garage for the winter it looked pretty bad. I didnt want to have to pull all the water pump and externals off just to flush my engine so I came up with an alternate method that requires relatively little work. As most of you probably know, it is difficult to empty the block of coolant because of the thermostat, not to mention that the drain plug is directly above the front subframe, and very difficult to get at. What you have to do is use the engine's process of cycling coolant to your advantage. PREP:
Here's what you'll need for the process.
I bought my parts at Lowe's, I work there so I get a discount and I knew they had what I needed. Some parts I had to do a little extra to make it work, but it worked nevertheless. What you see here:
-1 1/4" Barb to 1 1/4" Female Pipe Thread (dark gray piece to the right)
-1 1/4" Male Pipe Thread to 3/4" Female Pipe Thread PVC (threaded white piece)
-3/4" Male Pipe Thread to Female Hose Thread (brass piece)
-Teflon Tape (blue and white)
-1 1/2" Rubber Plug with Clamp (black piece)
-1 1/4" PVC Coupling (white smooth piece, the OD of this piece is approximately 1.5 inches, so it works)
-1 1/2" to 1 1/4" Rubber Conversion (light gray piece)
-2 lengths of regular garden hose.
 You need to make a part to plug the inlet of the radiator which is about 1 1/4" OD (at least on the NA). To do this you put the rubber pieces on either end of the PVC Coupling and tighten the clamps. They are a little small for the OD of the coupling, but it will work. These are the pieces you will need for this unit followed by what it should look like afterward.
 You also need to make a drain unit for the coolant coming out of the engine. To do this you will wrap the male threads on the brass piece, and the PVC piece with Teflon tape. Dont worry about being liberal with this part, but make sure you wrap in the same direction as you will be threading these pieces into their female counterparts. Thread the PVC into the Barb to FPT (female pipe thread) and then the Brass piece into the FPT part of the PVC. You should be able to get these tight enough with a good set of strong hands. Here's the parts you need for this unit and what it should look like when you're done:
 THE FLUSHING PROCESS:
1) First, you'll want to start as if you are just draining the radiator. Obviously, let the engine cool if you have just been driving it, but remove the radiator cap and the breather valve (if you want it to drain quicker). If you need to jack the car up, or if you have a low clearance bucket you can just reach the lower petcock under the bumper with the car on the ground. Let the radiator drain completely. After it has drained, replace the lower petcock and upper breather valve but leave the radiator cap off. 2) After this, disconnect the upper rubber hose from the radiator, this is the coolant out line from the engine, the hot "used" coolant.
 3) Fit the 1 1/4" end of the rubber/PVC plug you assembled over the coolant inlet on the radiator and tighten the clamp so it's sealed. Next slide the barb end of the drain unit you made into the open end of the coolant out line and tighten the clamp on the out line so it seals on the barb. It should look something like this (In this picture I have not yet reinstalled the breather valve on the top of the radiator, dont forget to do this):
 4) Now, fill the radiator back up with coolant or water. You should use distilled water and not tap water, I plan on getting a Koyo Radiator and doing a complete coolant system overhaul this summer so I just used tap water. If you use distilled water you'll want a few gallons of water for the process. If you take the distilled water route you will only need one garden hose. 5) Thread the garden hose onto the brass piece in your drain unit. You shouldnt need Teflon tape here because the brass piece has a rubber seal inside it. After you have connected this hose, make sure the radiator is still full. If so, start the car and just let it idle. Here's what mine looked like just prior to me turning the key.
 6) The engine will run a few minutes to heat up. Turn the heater on as high as it can go, this opens the heater core up so you can clean it out too. Keep an eye on the temperature gauge in the cockpit as well as the coolant level in the radiator (the cap should stay off throughout this process). It would be helpful to have a friend who can sit in the cockpit and keep an eye on the gauge so you dont have to run back and forth. Once the engine gets to the right temperature the thermostat will open. This "used" coolant will be pushed out the drain unit you made and out the garden hose. It wont spray out with any force, it more or less just trickles, but thats fine. Once this happens the temp gauge should be about halfway. You will notice the coolant level in the radiator start to drop. At this point just fill the radiator back up and wait for the next cycle when the engine once again opens the thermostat and clears out the "used" coolant. From this point on its just a wash, rinse, repeat; as the coolant in the radiator goes down, just top it off. 7) After you are satisfied with the coolant coming out the waste line, you're done. Shut off the car. You might want to drain the radiator a bit so that when you take off the plug you made it doesnt leak water all over your engine. Remove the drain and plug pieces you assembled and reconnect the upper coolant hose with the inlet on the radiator. After this, refill the radiator with the coolant/water mix you desire. Also, top off the reservoir. Replace all the radiator and reservoir caps and take a quick drive to move the coolant around the engine. After this (And after the engine cools if its hot) take off the radiator cap and check the level of your coolant. You might need to add some more, you might not. For the next couple days keep an eye on your temp gauge and check your coolant levels in the morning or whenever, before you start your car. Refill the radiator as necessary.
ENDNOTE: I ran my car for about 30 minutes to try and clean out as much as I could, but you basically just need to run it as long as it takes to get clean water coming out the garden hose. This is the dirty water that drained out shortly after I started.
If you use jugs of distilled water, and you run out, shut off the engine immediately. Dont try to run it without having something to fill it back up with.
Realistically, you dont even need the drain unit that I made, but I didnt want coolant getting all over my engine bay. Some people may be concerned with the temperature of the draining coolant, and yes, it is hot, so dont go dance around in it or anything. However, it is not hot enough to melt anything. The brass pieces of the hose and conversion piece will get a little hot, but not that bad. The only signs of heat I found was on the barb piece going into the upper rubber coolant hose, but it was nowhere near melting, and I ran my car for quite a long time. Keep in mind that the fresh water you are pouring into the radiator is cold so it keeps the engine at a lower than normal temperature. If you are close to a 60 or 120K maintenance you could just flush the engine then when the water pump is off. This is basically a method that shortens the amount of labor required to flush the engine. For the first time I did it, it worked relatively well, and I did notice that my car stayed a bit cooler today despite the higher temperatures. Hope some of you find this useful and if you have any questions, don't just jump in headfirst. Overheating is the fastest way to kill your VG.
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DIY Coolant Flush Minus Extra Labor - 
