aeschen

I have had this for a while and seems to be getting a bit worse. Basically when the car is cold the car hesitates and makes a thud type sound. Imagine droping a baseball onto a wood floor from waise high level. ANy ideas on where to start looking? It seems fine after about 5 min of warm up.

thanks in advance,
Aaron

Mark Houghton

How about a little more information? Is this "thud" happening while sitting in the driveway at idle or when blipping the throttle, or is it happening while under motion in any or all gears?

aeschen

oh right, sorry. It only happens when accelerating in gear I don't think it happens just reving the motor. Basically, after I leave my garage it will make the "thud" sound and buck a bit everytime I try to accelerate.....

57_300SL

Do you have the original puck style clutch in the car? It could be the rubber puck going away.

Mark Houghton

That rubber centered stock clutch disc is a good possibility. This could be your clue to stop driving the car until fixed, or risk being stranded. Also, check your CV joints on the axles. Make sure the allen headed bolts are tight and there is no side-to-side play in the joint(s).

Mark Houghton

Oh....and don't forget to check the obvious stuff like transmission and motor mounts. Could be one loose or broken, allowing the whole assembly to torque and thunk when hitting the gas. Good luck!

JFairman

I think you may be hearing small backfires or coughs from the intake when trying to give it gas while it's cold.
The "thud" may be the sound of the air sensor plate being blown upwards and hitting the rubber stop.
That would coincide with the buck you mention.

Richening up the control pressure/warm up regulator just a little bit might make it better.
The whole cold start system could probably use a tune up.
If you can adjust it yourself you may be able to fix it or if you replace the Warm up regulator and auxilluary air valve it will be expensive.

aeschen

Thanks for input guys. Jfairman I think you are on the money because I just checked and it does make the sound out of gear and the sound comes from around the sensor plate area. I hope I dont need a new wur! Could sombody humor me and explain the technical reasons for an intake backfire? I wish I understood the cis system better, quantum physics is more intuitive!

Thanks again guys

WERK-I

Congratulations! You are experiencing the classic CIS backfire!

Your CIS cold pressure settings are off creating a lean condition when the engine is cold. Have you recently changed anything on your engine? Headers? Different cams? Aftermarket low restriction muffler?

Mark Houghton

It's hard to diagnose across the internet, not actually hearing or experiencing your problem. But, here's one more source of mysterious thunking noises: There is what's called a diverter valve - basically a big spring loaded piston that moves in response to manifold vacuum - that diverts boost pressure back to atmospheric when you take your foot off the throttle (as in during a gear shift). This prevents all the boost pressure from trying to exit back through the turbo when you back off the throttle, thus letting the turbo contine to spin freely rather than being forced to rapidly slow down. That piston butts against a silicon gasket that over time hardens up...and you'll hear a clunking each time you rapidly get on-and-off the throttle if the gasket needs to be replaced.
I think everyone has pretty much diagnosed your issue as backfire related, and most likely that's just what it is. So just add this one to your memory banks of the mysterious workings of these motors.

JFairman

If you do your own work on it, a small hammer and 1/8" or 3/16" drift punch and just the right taps on the small round metal plug in the warm up regulator would probably fix the first couple few minutes of lean cold start stumbling you're experiencing..
There's alot of information on making them adjustable online.