engine rattle?

[Hillbillybuddha]

The only time I've heard denotion (spelling? Engine knock) was in an old American V8 engine and it sounded like a knock (hence is name) This doesn't sound like that old car. My 84 720 z24 has a very loud rattle (more of a tap then a knock) at high speeds when I press the gas peddle. When i take my foot off the gas the sound stops.

 

Any ideas?

[datzenmike]

Then it's engine load related.

 

Cracked exhaust manifold. Exhaust gets louder as you 'step into it'.

 

Have a look at the wiring going to your distributor. Are there three wires in a harness and a single white wire with it's own plug?

Like this one?...

 

[Hillbillybuddha]

I'll look when i get home. I wish i wasn't, but i am driving it right now. I pulled over to ask the original question and as i was pulling away i noticed that the oil pressure was low so i went to a gas station and picked up some oil. I was only a little low (my oil light didn't light up) noise still happening though.

 

About the exhaust leak? Wouldn't i still hear the tap noise in neutral? I don't hear it at neutral. But it does sound like an exhaust leak now that you mention it.

[datzenmike]

Exhaust gets louder when you step on it. A small crack or bad gasket might be quiet at idle. Just something simple to eliminate first is all. Check your distributor and get back also.

[Hillbillybuddha]

Distributor: i changed the cap last month. But yeah, 3 wires; one blue, one black and one red with a single white wire coming out by itself.

[datzenmike]

Well first check that your timing is correct at 3 degrees BTDC (or whatever the emissions sticker says.

 

 

 

OK on those Z24 engines there is a vacuum switch that senses low vacuum from full throttle highway driving or passing. This switch forces a ground condition on the white wire. This in turn tells the distributor to turn off the exhaust side plugs. The distributor automatically advances the timing to compensate for the increased burn time. All this, according to the FSM, is to reduce engine noise under heavy load. Now if that switch is unplugged or the vacuum line off or leaking the distributor can't turn the exhaust plugs off... and it may get noisy.

 

The vacuum switch in mounted on the pass side inner fender. Unfortunately there are two of them, round (ish) about the size of a shot glass with a vacuum line connecting them and to the intake, and two electrical wires to each. For now check both are plugged in and the hose intact.