rattling and hissing when engine turned off

[Racer_Z_]

Most of the replies are what I was thinking. The real problem with trying to help diagnose this problem is not enough information.

If the timing is advanced, or if the idle is to high, or if there is gobs of carbon build-up, the engine can diesel. Diesel is the engine trying to run, usually backwards, after the key has been turned off. This can cause the carbs to catch fire. Modern fuels have additives in them to prevent carbon build up, so I don't expect it to be carbon build up in the combustion chamber. But it could be.

The noise could also be the water in the radiator/engine collecting heat and starting to boil. If the cooling system is full of water/anti-freeze and in proper working condition this won't happen.

A third possibility lies in the carbs. There is nothing wrong the Flat-Top carb...you all just need to get beyond that myth. Like the cooling system, the carburettors start collecting heat just after the engine has been shut off. The fuel starts to boil. In this scenario, your only real option is to make sure the factory heat shields are installed. Switching to a Round-Top or any other after-market carb won't fix the problem. This is quite common with ALL carburetted engines, regardless of design. With the L24 design, it's a bit worse and more noticeable because the carbs are just above the hot exhaust manifold.

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I would suggest you open the hood before shutting off the engine. Then shut it down and look/listen/feel what is going on. Try to get an idea of where the noise is coming from.

What do you hear?

Where does the sound(s) seem to be coming from?

What do you see? Do you see something moving or escaping? Is the fan and pulley trying to turn backwards?

What do you smell? Do you smell gasoline or anti-freeze?

What do you feel? Do the carbs feel hot? Do they feel like they are gurgling inside?

Be careful when you touch things though. Some things are hot enough to blister your fingers.

[sblake01]

What he said^^^^^^all of it!

[Bruce Palmer]

While you are waiting to identify a solution, when you shut the car off leave it in gear with one foot on the brake while letting out the clutch. This will stop the engine from contiuing to turn over.

Extended and unattended dieseling will break ring lands on the pistons resulting "diesel" owner being one each unhappy camper.....

[yoshi_w]

Thank you so much everybody! I'll try all of those tomorrow if I have time after classes! This is definitely something I need to fix asap. Diseazed, how will I know if I loosened it too much or too little on the distributor, or is it something that I will just be able to tell? And, I really doubt its the flattops, and I plan to keep those, but I already have a rebuild kit so that may be the next thing if the timing doesn't work. If the timing is off, this could cause the engine to overheat, correct? And if there is carbon build-up, what ways can I clean it up without a full engine rebuild?

[Reverend]

I did open up the hood in my hissing Z. I even removed the air filter cover and when i turned the car off, white puff of smoke came from carbs. Carbon or what?

[Diseazd]

Loosen 1/2 to 3/4 of a turn.......you"ll figure it out......go ahead....get your hands dirty. That"s how you learn. You" ll be able to hear the engine smooth out as you twist the distributor. That would be the easiest thing to check first.

[yoshi_w]

I like to get my hands dirty, no worries about that! But I'll try that and let you know how it works out. thanks! And I forgot to mention this until Koalia mentioned the air filter, not the same thing as his, but I removed the old air filter that the previous owner had in it, and it was the filter in metal cage type thing, it was covered in a very oily substance and even had some gunk just melted onto one part, is this an indication of something?

[FastWoman]

Yoshi, when your engine reverses (as it's dieseling), it will blow exhaust out your carbs. That will deposit all sorts of stuff on your air filters.

Although it's possible to do a rough timing by ear and by feel (if you know what you're doing), I think it would be far better to buy a basic inductive timing light from your local auto parts store and set your timing the correct way. (It should cost you $40 or so.) The bottom-most pulley on the front of your engine is the crank pulley. It's got a shallow notch in it somewhere, and that lines up on a scale that hovers over the top righthand side (passenger side) of the pulley. You'll hook up the timing light to the battery and the front-most (#1) spark plug wire and run your engine. The light will flash everytime the #1 cylinder fires. When you point the light at the crank pulley, it will look like the notch is "frozen" in place beneath the scale. Then you twist the distributor to line up the mark at the correct advance, approximately 10 degrees on the scale. You'll want to pick up some general sort of "how-to" book to explain this better.

Timing will also have a lot to do with the adjustment of your points, so you would check/replace/adjust those first. You'll need a set of feeler gauges for that. Again, that would be in a general how-to book.

For the specifics of how your engine is put together and how to service and adjust it, I can't recommend a Nissan Factory Service Manual enough. Get one for the year of your car. The L28 under your hood is very much like your original L24. You can find them used on Ebay and Amazon for roughly $80, which is worth every penny.

[yoshi_w]

I have the l28 and 240z fsm in pdf form already, so I'll be sure to check that. As for adjusting the points, I have a electronic conversion, so Ill probably adjust the timing until I am able to install it. If timing turns out to be the problem, you guys are truly my heroes!

[Racer_Z_]

I did open up the hood in my hissing Z. I even removed the air filter cover and when i turned the car off, white puff of smoke came from carbs. Carbon or what?

No it's not "carbon". Carbon would be black deposits. If all it is, is one puff, that can be a normal part of the engine finding it's resting spot. It sometimes happens, on occasion, but not often. If it happens all the time it's an indication of a bad valve, valves out of adjustment or a need of tune-up.

I like to get my hands dirty, no worries about that! But I'll try that and let you know how it works out. thanks! And I forgot to mention this until Koalia mentioned the air filter, not the same thing as his, but I removed the old air filter that the previous owner had in it, and it was the filter in metal cage type thing, it was covered in a very oily substance and even had some gunk just melted onto one part, is this an indication of something?

That would be a sign of "dieseling". When the engine tries to run backwards, the hot exhaust shoots out the carburettors and melts the plastic part of the air filter. The black deposits are also part of this backwards exhaust. You're lucky it didn't catch on fire.

Yes, bad timing can cause an engine to run hot. So far what you're describing sounds like a car in bad need of a tune-up. How long has it been since it was serviced? ... Oil changed, new plugs, points, ect....??? For sure you need a new air filter.

[yoshi_w]

Already replaced the air filter, just bought a new fuel filter, need a fuel filter for the pump, have the electronic ignition conversion to replace the points, oil was brand new when I bought the car, needs new plugs though, have a carb rebuild kit, just a matter of putting them on. I wanted to save the z from the previous owner who really took poor care of the car, so wish me luck, timing and fuel filter are going to go in within this week or next depending on classes!