Engine throwing out smoke and runs rough.

"5 gallons gone after I run it around of a building twice." So the "smoke" is unburnt gas ? that's o2 sensor and MAF or timing or bad spark or lots of things …
1. get a manual ( they are online pdf files )
2. check the codes on the ecu

also what year/model is the Z ? turbo ? non turbo ?

What color smoke, black like gas blue like oil or white like water?

I got new plugs in, the smoke is white when I start it and when I let it run for a while.
The car is a 1987 N/A

White smoke, I'm guessing it's oil being mixed in with the gas. But I wouldn't know how the oil would mix into the gas in the first place

Check your oil. If it’s starting to look milky it’s a head gasket. Still may be a head gasket if it’s not. Also when it’s cold, take the radiator cap off and check for bubbles as it’s runninf and look for like in the radiator fluid. Assuming the smoke is white.

Both head gaskets were bad on my car and that is exactly how it ran.

Do a compression check. It'll tell you which, if any cylinders are bad.

White smoke = burning coolant. Unfortunately, Z31s are known to have headgasket failure problems. A compression test won't always show bad headgaskets, you really need to do a leak-down test. If you don't have access to a compression tester or leak-down tester, you can try a block tester. You can get the kit from an autoparts store for fairly cheap. A block test tests your coolant for exhaust gases, which shouldn't be there if your headgaskets are healthy. Do you know the history of this car from the previous owner?

I don't think Z31s are prone to blown headgaskets on a whim like cough* subarus or early production last gen Civic Type R's*.

Mistreating any car will lead to blown head gaskets. Overheating in particular.

I agree, but I also know of countless Z31s that have had blown headgaskets… just look on Craigslist and Facebook Marketplace, a good deal of them for sale list blown headgaskets. Now, 90% of the reason for this is owner error, that is true. But, being that this is a USED car, it isn't too far of a stretch to think that the previous owner did the no-no of overheating it.

So update on the 300.
- The car still smokes like crazy
- But it runs strong
- White smoke smells like burning oil
- Idle is still bad on the car, needle tends to fall when the motor get revved up
-Lastly, the car is staying cool, no overheating issues


Thank you to everybody that has answered on this post, I hope the people who's helped me diagonoised the problems stay until the car is finished.

-Update
I have given up on the car, I don't have anytime for it, the last thing I did on the car was prime it. Instead of keeping the existing motor, I thought of switching it for a used vg30.
Now I'm looking to sell the car, along with an 88 N/A GL that runs.
I'm about to buy an 88T GL a friend of mine has in the two weeks or so.



Thank you everybody for the help and anyone is interested in two N/A Z31's one that has a lot of rust, but runs with smoke and one with rust and a motor that runs strong, PM me.

Use a metal wire brush and scrape off the contacts of the distributer cap. You might want a new one and rotor eventually but that is a good test

Did you ever do a compression test? Change of fluids? It sounds like you had a bad head gasket.