JuMo

I have ’90 300ZX 2+2 non-turbo with a problem. Engine seems to runs fine. I am experiencing black smoke from the driver side exhaust.
No engine Codes.
Engine not overheating. No coolant loss.

Plug on cylinder closest to the driver (is it 3 or 6 ?) has oil on it. Other plugs are fine. Did a compression test on all 6 cylinders, all show about 170 psi EXCEPT the one with the oily plug. It shows 100 psi.

My mechanic buddy who looked at it concluded that it is a cylinder problem stopped at this point.

Anyone experienced this. Any suggestions ? Any recommendations for a mechanic in the Minneapolis area ? Any help will be appreciated.

KasbeKZ

pretty typical case of worn out rings. you need a new (used) motor or a complete rebuild by an experienced nissan mechanic. not any old machinist can rebuild this engine, and there are no short cuts like dropping the pan, pulling the crank and replacing just the rings. get one of those $1000 jdm engines off ebay or find someone to rebuild yours.

btw, how many miles on the engine? and has it been abused that you know of? just to satisfy my own curiosity.

ZLover4Life

What? An engine's an engine, they all have the same basic requirements (torque settings, ring positioning, etc)... with the exception of Wankel rotaries, any competent machine shop can rebuild any engine. The hard part is getting to the engine... so if he gets it out, he can have it honed and rebuilt just about anywhere.

You're right that there are no shortcuts like some domestics have - the heads and pan need to come off to do any piston work - but to say that "not any machinist can rebuild the VG30" is kind of absurd. A machine shop that can't put pistons in a block and torque everything down is not a machine shop at all.

JuMo

Car has about 20K miles on the Engine (odometer shows 90K). I am the 3rd owner. Engine was rebuilt 5 years ago by the 2nd owner (his dad was a Nissan Master Mechanic), when the 2nd owner purchased the car from the original owner dirt cheap because the timming belt broke and the engine needed to be rebuilt.

Any way to tell if it's valve issue versus ring ?

If was going to go the JDM engine route. Any recomendations for that ? I've horror stories about those as well.

ZLover4Life

The horror stories about JDM engines usually concern retards who just drop the engine in and expect it to work. ANY purchased used VG30 should get a new oil pump, water pump, timing belt, etc (basically 120k kit + preventative items) before going in. But compression on them is usually ok.

As for testing rings vs valves, do a leakdown test.

KasbeKZ

i've read a lot about how the z engines have tolerences that are different than most engines.

firstly the wrist pins take about a forth of the tolerence that is standard on most. that's one mistake a machinist will make on these.

secondly, the crank can't be turned unless it is re treated the same way nissan treated it. i know 95% of the back woods machinists around my area would turn the crank and throw it right back in, and it wouldn't last.

thirdly, nissan's main bearing clearances are tighter as well then most american machinists would be used to. and nissan checks the size of each bearing separately, so if you're using a turned crank shaft, you have to measure them all separately and buy 6 or whatever different bushings. if you are using a crank that hasn't been turned, it's stamped right there, so no problem.

if you want to take it to a machinist and say do it, go ahead i guess. zlover knows what he's talking about. i guess what i'm saying is here are the things that you have to make sure that the machinist knows. there are several tt.net threads to this tune.

snwbrderphat540

could be a valve, rings or HG. a easy way (thats not 100%) to tell if its a ring is to put a small amount of oil directly into the cylinder through the spark plug hole and quikly do a compression test. (by small amount i mean an oil cap full, and not the big mobil oil caps) and if the compression raises from that its most likely the rings/piston. if its stays the same its a valve or possibly the HG.

ZLover4Life

All Japanese engines have tighter tolerances than American engines... that's why they last longer. For example, the old Mustang 5.0 engine... they manufactured 3 slightly different size pistons. They measured the cylinders and installed whichever piston fit best in a given cylinder. Japanese companies don't work so haphazardly, and I'd expect any machine shop to know this. Of course it never hurts to remind them, but they don't just throw an engine together like the high school graduates at the Ford factory in 1987 did.

But regardless... I've always been an advocate of do it yourself. If you're patient, there's nothing a "professional" can do that you can't. I'll be doing the machining on my engine, in addition to its assembly.

KasbeKZ

that's awesome. please keep me posted on that process. i've looked and looked and can't find a very comprehensive document on doing the assembling of an engine of this era. i did most of the block on a midget engine, but it's way different. a pushrod with a timing chain...

what will you be doing for a crank? machining one and re-treating it, or using a low mileage one?

ZLover4Life

I already have a fresh crank from my old built VG30.

And all of the required specifications are in the FSM.

rtmorgan

http://www.twinturbo.net/nissan/300z...d/rebuild.aspx

here ya go kas hope it helps

KasbeKZ

haha i forgot about that write up. good stuff. thanks.