very rough idle

[Bryan Hartman]

HELP! once in awhile my 94 idles very rough right after I start it up. The whole vehicle shakes until you hold the gas down for a little or start to drive for a few seconds. It acts like it is flooded or something. Fuel pressure reg. ? It has 70000 miles on it . I am going to change the plugs and wires soon I think they are original.

 

[Alec]

This seems like a very minor problem. I think what's happening is this: when the engine is very cold, it needs to idle high for a few minutes to warm up. Yours is probably trying to idle at the normal level before adequately started. That's why giving it a little gas makes it better. My suggestion is to reset the computer and let it relearn a good idle. Here's how: disconnect the negative battery lead for about 15 minutes. Then reconnect it and let the truck idle on a flat slope with no accessories in P or N for about 10-15 minutes. This will let the computer relearn it's idle. After you do this, drive the truck a little harder than normal for a few days. Also, whenever the truck is really really cold, try not to drive it or drive it hard for about 5 minutes after starting.

 

[Bryan Hartman]

Thanks I will try it. I forgot to mention though that it does this when it is warm to. Like on a warm day driving to the store shutting it off and then getting back in it and it will shake and idle very roughly.

 

[JoeC5]

Check your vacuum lines. Mine ran extremely rough like you said, I found that a few of my vacuum lines were off. Purrs like a kitten now

 

[Waterboy]

Bryan, I have a 94 Explorer that did the same thing. What solved my problem was one can of fuel injector cleaner. You can try the Amsoil P.I. or Chevron Techtron available at Walmart and Pep Boys (which is the only fuel injector cleaner that Mercedes recommends). What is happening, you may have an injector that is sticking open and flooding one cylinder with a few ounces of raw fuel. It takes about one minute of running hot or cold to clear it up. When the varnish or carbon buildup is removed, the injector should be able to close properly. Now I run the injector cleaner every 3 months.

 

[Bryan Hartman]

THANKS ! I will try this .

 

[TPLYNCH]

Bryan, your IAC (Idle Air Control) might need to be cleaned/replaced. This sensor controls the idle speed. If you turn on your A/C, do you notice your rpms dipping and not coming back up? If so, then it is more than likely the IAC.

 

[SteveVB]

Vacuum leak

Sounds like vacuum leak to me, check the vacuum tee at the rear of the upper intake, the caps get brittle and can blow off, or the hoses can split or crack.

good luck.

 

[Ryan1]

dude dont mess changing you plugs
they should be ok

My 94 does the same thing hot or cold.
it Idles like it has a miss in it ..... but after a few seconds or minutes of plaing with the gas its fine

Do you ever get a sweet gas smell under the drivers side of the hood?

Im guessing (from reading some posts) that it is something to do with the Carcole canister. But It wont help if you change the canister.. there is something attached to it or inline to it that you can replace and it fixes it.

I will look for it again and let you know

Ryan