Knocking on Cold start or when turned off?

[Wschnitz]

My 1988 325 is making a shaking sound whenever you start the vehicle, or when I turn the car off. I Don't believe its a Rod Knock or a Piston slap, as it only happens right when you start the vehicle or turn it off, and only for a second. It also happens when the RPMs are really low, as in like 300-500 (as in if it were about to die from letting clutch out to fast, low gear, etc).

It basically sounds like what a car sounds like when it wants to die and not idle, but only happens right away, my first suspect is a sensor has gone bad or is not working correctly, and ignition or fuel is off at low rpms.

someone help please

[jlevie]

Check the motor and transmission mounts.

[dnguyen1963]

rpm dropping to 300 is suggesting vacuum leaks. Check for leaks then mounts.

[Wschnitz]

rpm dropping to 300 is suggesting vacuum leaks. Check for leaks then mounts.

Its doesnt drop on its own its not that, I will probably check the mounts thought thats a good idea.

[dnguyen1963]

so...how does the rpm drop down to 300? My idle speed never drops below 650. If you have no vacuum leak then your fuel supply must be a problem for the rpm to drop like that.

[Wschnitz]

The idle dropping isnt a problem, im saying when the idle drops below 500 such as going up a hill in too high a gear or letting the clutch out to fast causes the knocking noise. again, the idle is not the problem, it idles at a steady 650-700 all day long.

let me make it even more clear, this is only when cold start or shutting the car off.

[dnguyen1963]

Remember the Chrysler K-cars? Every time someone tried to shut the engine off, the whole thing knocked and bucked for a few minutes before shutting down. Have you checked your ignition switch? A faulty one can cause these symptoms.

[Wschnitz]

Remember the Chrysler K-cars? Every time someone tried to shut the engine off, the whole thing knocked and bucked for a few minutes before shutting down. Have you checked your ignition switch? A faulty one can cause these symptoms.

It is similar to this, but it's like 2 seconds of knocking noise after you cut the engine off, almost like you turn off the car but the cylinders are still pumping for a few seconds.

[dnguyen1963]

Check cylinders compression?

[Wschnitz]

It's definitely not that, ill take a video soon, car runs and drives fine the sound is just worrying on start up and kill.

I mean I can go get a pressure gauge but the numbers will run fine.

[sitosnilloc]

Did you ever fix this issue? does it do it when the clutch is depressed and turned off?

[e30_sloth]

It could be carbon buildup on the piston face, mine does the same thing on shut off.

[Andy.B]

Mine does that too, but only at shut down. Strong running 230k m20b27 with good compression and oil pressure.

I'd love to know the cause.