Knocking in the Engine

I recently bought a 90zx. It has a small knock in the engine at start up. Once its on the road and warm it isn't there, what could be the problem?

I don't know, but I sure as HELL wouldn't drive it like that at ALL...

how bad is the knock? that is weird that it does go away.

i have a crazy idea, it might have to do something with your cylinders because with the pistons in modern engines, they are oval in shape (not enough to be noticeable, but they aren't completely circular) when it is cold, as the engine warms up, the pistons expand and become circular.

i don't know, maybe it has to do something with that. that is all i can think of.

Probably not that cuz that wouldn't cause knock. Knock is detonation...the uncontrolled burning of fuel before the ignition spark. Quite possibly the WORST thing that can happen to an engine. Besides...if it had something to do with the pistons, all Zs would do it.

Try running the highest octane fuel you can find in it...see if it goes away. It SHOULD...if not...fuck if I know...kinda odd problem.

oh man, i completely forgot about detonation. i just shared the piston shape because it is a weird fact that i learned in auto. tech. make sure you are putting 91 or higher octane in your z, she doesn't like it when there is anything lower.

Maybe your EGR valve needs to be replaced.

There are a few factors that could promote knock in the good-old VG...

With my first project ZedX... 1990 Non-Turbo. There were a few main problems.

-Timing was off significantly.
-Valves were bent from striking the pistons
-EGR was clogged EXTREMELY bad (EGR is that bronze polished valve on the passenger side of the upper plenum, on the back of it, it has arms that snake under each side of the plenum, if they are clogged, this could promote a knock)
-PCVs were clogged
-EFI Harness was corroding

I fixed all of these... It was one thing after another I found.

Goodluck, VG's are tough to work on.

-Wes

Just as a note to anyone else trying to get help with a problem, telling us there is a "knock" is too vague.


Thus from that descripition it could be anything. It could be something as truly evil as a loose connecting rod nut or something as simple as your car is low on oil.

Thats actually my first thought, low oil level that allows certain parts to knock around til the oil gets warmed up enough to flow to those areas. Try topping off or changing it. No sense tearing the engine down to find out that it all could have been fixed in about five minutes.

i think that he was talking about detonation when he said "knock"

RX, is your 91 an NA or TT. I'm guessin its a NA you put a TT front fascia on but I just thought I would ask.

its an NA. the previous owner put the TT front fascia on it. I just bought it like that

when i was looking at a z for me i looked at rx's. the exterior is in great condition, the interior can be worked on. if i knew how rare slick tops were, i would've thought of getting it. is there still a little knock in the engine rx?

That knock in the engine was beacuse the EGR wasnt working. I just had to fix a broken wire under the hood that was part of the EGR system

Yeah I didnt know anything about slicktops until after I bought it. I met up with a bunch of Z32s from the local Nissan club and they were all like "I''ve never actually seen a slicktop before" lol. I was like slick-who?