loud knock when AT downshifts under pressure - Benzworld.org

scrapper · 05-21-2008, 07:09 AM

Hi,

My 98' C200 T AT has the following problem:
When driving at low rpm such as 1200-1500 and then hitting the gas hard(or kickdown) the following things happen:
1) Engine chokes
2) while choking: Loud knock from the engine
3) AT shifts down to a lower gear
4) Car accelerates flawlessly

These things happen in 1-2 secs.
I have just replaced the MAF which got rid of most of the choking, and no choking/knocking whatsoever occurs at 2000+ rpm.

When the MAF was faulty, I had trouble starting the car since the idle was rough and it would stall.
However, it would also cause frequent loud knocks from under the hood and subsequent rpm drops when the car was parked and warming up.

Can someone suggest what to check? Its really scary!

Thanks for reading/helping!

cjshade · 05-21-2008, 03:48 PM

Try cleaning your Throttle Body. A dirty T.B can do what you are describing so will a bad MAF as you found out. You may be looking at a double whammy!

Good Luck!

C as in 220

scrapper · 05-22-2008, 04:24 AM

thanks again cjshade!

Im such an engine dummy, does loud knock = misfire?

I searched here again, using other search words and noticed that many have had this problem.

Now the question is, is the throttle body cleaning an easy DIY job?
Should I take it appart, use some sprays or take it to my mechanic instead?

cjshade · 05-23-2008, 09:48 PM

I have not yet tackled mine but probably should. From what I have read you take off the intake tube and spray some carb cleaner on a dust free rag and wipe the inside of the tube removing the worst of it. then you can spray the rest clean. Look around and maybe some has posted some pictures. If anybody reads this that has the low-down on T.B. cleaning please chime in.

An engine Knock usually means pre-ignition.

My wife says it's time to get in the Spa. I never argue with that.

Good Luck!

C as in 220

xvvvz · 05-23-2008, 09:58 PM

CJ probably meant to write this, but you want to use a quality intake cleaner for fuel injected engines vs. a carb cleaner. Carb cleaners often have chemicals that are too harsh for some of the things it will come in contact with as it moves through the intake system. Quality intake cleaner usually has an integrated lubricant which helps protect the moving parts. I like the CRC products.

cjshade · 05-24-2008, 10:31 AM

Thanks xvvvz,

I will remember to use CRC cleaner when I do mine soon.

Good Luck!

C as in 220

scrapper · 11-26-2008, 09:10 AM

Hi Guys.

Some updates.

What I have done:
-changed spark plugs
-changed MAF again
-cleaned throttle body
-replaced air filter

Exactly the same problem as in the first post.

cjshade · 11-26-2008, 11:14 AM

What grade of Gas are you using. It sounds like you may be experiancing pre-ignition. Maybe an Octane Boost is in order.

Good Luck!

C as in 220

thepinoc · 11-26-2008, 11:15 AM

91 octane should be fine for our cars though, no?

cjshade · 11-26-2008, 11:22 AM

Yes 91 Octane is fine for where we live. At high altitudes in the U.S. they only go as high as 89. It is because of the thinner air.

Scrapper is in middle Europe somewhere so I have no idea what he has available.

Some people swear they can run 87 but I would never take that chance, it's in the Manual.

Good Luck!

C as in 220

http://en.wikipedia.org/wiki/Octane_rating