98 m3 running rough (shaking)

My sons m3 will all of a sudden start shaking. If he turns it off and waits 10 seconds or so he can start it and it will run normal. If he turns it off and instantly starts it it will still shake. It almost sounds to me like its misfiring. It doesn't happen when he first starts the car. It will just randomly start shaking and he will have to do the turn off/restart thing to get it back to normal. Any ideas?
-Robert

Is the car showing a check engine light?
First thing to check is to pull the codes on it.
Shaking is hard to trouble shoot with out a lot more details.
It helps to provide us the year and as much information about the problem as possible like if the car is stock or not, what work has been done to it, etc...

Try turning the key on and off a couple times and prime the engine before cranking it, this will point you towards the fuel system if it seems to fix it, could be as easy fuel filter or as expensive as a fuel pump

How does turning the key on an off prime the engine?

Sends fuel to the rail ie primes it for start...To the OP On for a few seconds then off then on again then start it

If you turn on the key, the fuel pump runs and sends fuel to the rail. No need to turn on and off and on. Just turn the key on, wait a bit (10 - 20 seconds), and crank.

Originally Posted by Pinecone

If you turn on the key, the fuel pump runs and sends fuel to the rail. No need to turn on and off and on. Just turn the key on, wait a bit (10 - 20 seconds), and crank.

Depending on the situation, blocked fuel filter,faulty pump ect. A couple priming blasts of fuel would be better than one, yes if all is okay you can crank it up right after you turn the key
Edit:
Also cold weather (really cold weather) has proven to cause multi key turn ons to prime the engine correctly, might have to do with slow current draw to the pump or to just push the fuel thru the super cold/frozen lines... I've never had this problem (I live in TX) but just recently I recommended this procedure to a couple fest members with similar symptoms and it fixed their issue albeit temporary as there's prob a bigger underlying problem..

To the OP also check your intake boot for any small tears and clean up your MAF and Idle valve really well if they are Gummed up this could be the issue

Once the pump turns on, it generates the proper pressure. Just waiting a bit will allow the fuel to get where it needs to go.

Multi turns of the key doesn't blast anything.

And BTW, the fuel rail pressure should stay up for a while, as there are check valves to do so.