I'm planning to perform a motor swap on a Z32 chassis (1990-1996) 300zx twin turbo. Low mileage japanese motors are plentiful and cost a bit less than motors sourced from US cars. The specs and engine codes appear identical, and I've verified that they bolt right in and work with existing US wiring. Are there any other precautions or caveats that I should consider before buying one of these?
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If you are able to get your hands on it be sure to do a compression test on it. If the compression is bad you will need to basically do a rebuild which will cost quite a bit more than what I am guessing you want to spend. Check the condition of the timing belt, good way to see if the engine was taken care of. That's all I can think of right now but of course there is more.
Make sure you doublecheck on the laws in your state before you buy any imported engines. I know in California you have basically zero chance of getting a Japanese engine smogged, even if the specs look the same as the US version.
I have only done this once, but I had incredibly good luck with it. In my case, it was a '86 Subaru GL-10, also turbo. I'm sure some will apply and some won't...
Check the sources warranties, some will say they're compression tested and what the different readings were, some will offer warranties, etc... The engine I got was in extremely good shape, and I had absolutely no problems with it for several years until I sold the car later. The story, as I was told it, is that emissions in Japan are extremely strict, and basically require a new engine to pass emissions after, say, 40K miles. I don't know that that's true, but that's the story I've heard as to why you can get good, low-mileage engines from Japan.
Know your responsibilities when receiving the engine. If they are shipping it, are they going to show up at your house with an engine in a crate and require you to unload it? Can you pick it up at the shipping company's freight yard where they have a forklift? What vehicle are you going to put it in? How are you going to get it out when you get it home?
My engine came with hoses cut and then taped off. It looked like a really fast job of removal. It had all the sensors, full induction system, turbo, and headers on it, even the belts, oil pan, etc... Mine did not come with the transmission.
I removed all the included sensors and induction/exhaust, and used what was originally on my engine. This way I knew they were 100% compatible with my computer.
At the same time, I replaced all the seals related to the parts I was replacing, and put new hoses on. I also replaced the clutch.
I was extremely happy with the engine.
In comparison, around a decade later I did a replacement engine for my VW Vanagon, and there weren't really engines from Japan available for it... I instead found a guy who was well respected in the VW community and bought a rebuilt engine from him, and shipped him my old engine after the swap (to rebuild for someone else). This guy knew the engine really well, and the cost was around the same as the used engine from Japan. The engine was basically brand new at that point, but on the other hand it didn't come with a spare turbo and all sorts of sensors, so... :-)
If I were in your situation, I'd probably be looking at having someone rebuild the engine, because then it has the opportunity to do some tweaks to bring the power further up. The VG30DETT is generally considered to be good for around 400HP before you need to start modifying the engine itself, so this is probably a good excuse to get some of those beefed up parts put in. But, that's going to push the price up (pretty much, the sky is the limit)...
I agree with ursa, if you can get hands on the motor before purchase then I'd pull the plugs and see how they look. Dip the oil in the dip stick and see how the oil looks, whether there is a lot of sludge or shavings or anything. Overall condition of the motor will tell you a lot about how it has been maintained. But the compression test is the best thing to do if you can, that will tell you a lot.