Odd engine noise...

[surfwax95]

OK, this is rather difficult to explain...but...here I go.

First off: 1995 240sx NON-SE, 5spd, AEM C.A.Intake, 300zx fuel filter, advanced timing to ~25*, running 93 octane gas.

When I shift from 1>2 or 3>4 at low (2500) RPMs and give it throttle the engine makes a quick *kachunk* noise. Very metallic, very brief. It sounds very very similar to a diesel for a split second, then it's normal. The noise doesn't occur everytime this happens, but when it's rather warm outside, and I give it a lot of throttle after a ~2500RPM shift, it makes the noise.

Please help me figure this one out!What is it, and how should I go about fixing it?Thanks.

[surfwax95]

I'm bumping this to see if anyone missed it.

[Koopa Troopa]

Does the engine sound like a diesel engine and become sluggish or what? If that's the case check your fuel system but I need more info and the fact that I am unexperiance in transmission problems doesn't help either.

[lee.]

Run a can of Seafoam thru your engine then change the oil. Castrol Synthetic is what I use. Seafoam took away almost all the top end noise, car sounds new.

[:: orion ::]

If it sounds like a diesel, that's pinging, or knock, or detonation.

25* base timing is too much...there are dimished returns after ~22 BTDC or so.

I'l bet if you knock it back a few degrees, you'll notice the car is no slower, but the knock is gone.

- Brian

[SHIFT_240sx]

i had that same sound come out of my car a couple of hours before the engine blew, a friend of mine told me that one of my piston rods had come loose and that it was knocking around in the block, maybe thats just me though

[Hijacker]

rod knock is a very definable sound. What he's describing, and seeing his alterations sounds like valve rattling, or also known as detonation. And given the circumstances it happens under, it has to be that ignition timing.

[surfwax95]

Thanks for all the replies, but I seriously doubt it's detonation. I took the timing light to the crank and it looks to be closer to 20* than 25*. That's beside the point though, I didn't have the timing bumped up when the noise started.

It ONLY happens when I shift at LOW rpms and give it a lot of throttle. It makes the noise ONCE.

1st gear>2500rpms>2nd gear>throttle>noise>normal

Hard to describe, and very hard to duplicate, unlike detonation which should happen everytime, right?

Point: I've only had the timing bumped up to ~23* for a few weeks, and the noise has been here ever since I got the car. Also, I used to put 87 oct. fuel in, the noise has reduced (in volume and prevelence) since I've began putting 93 oct. fuel in.

Thanks guys.