bucking/jerking

[timmyj51]

My '95 C220 has been intermittently
bucking and jerking at idle, not real hard
but a real annoyance. Feels like a
misfire but check engine lights not on and
had the plugs changed less than a year
ago. Does it mostly in D/R, rarely in N. Otherwise
car runs fine.
Started doing it right after my crank
position sensor was changed. Did the
mechanic screw something up when he
put it in?

[jnenad16]

he might have pulled a vacum line. check that all the vacum lines are plugged in in the back of the engine.

[timmyj51]

Originally posted by jnenad16
he might have pulled a vacum line. check that all the vacum lines are plugged in in the back of the engine.



Kinda doubt its a vacuum leak. Beginning to think it has
something to do with the throttle. When the car comes to
a stop the rpms intermittendly start to drop so the computer's probably compensating
by giving more gas. The car's bucking because you've got
your foot on the brake while the computer's giving it gas.
ONLY does this in D/R, never in N. Also does this intermittendly
when I start; rpms start to go down, then the computer shoots
the rpms up for a few seconds then it idles perfectly. Is there some throttle relay/sensor that controls this?

[strictlyspeakin]

I'm not sure that you should rule Nenad's idea out. It is a valid suggestion.

I have also seen this behavior with bad fuel. Fuel treatment and new Filter cleared the problem.

[timmyj51]

Originally posted by strictlyspeakin
I'm not sure that you should rule Nenad's idea out. It is a valid suggestion.

I have also seen this behavior with bad fuel. Fuel treatment and new Filter cleared the problem.




Have used nothing but 93 octane and had the fuel filter
replaced less than a year ago.

[strictlyspeakin]

I do as well.
That didn't stop it from happening though.

[jnenad16]

if the crank sensor has been replaced, chances are that the vacuum lines in the back of the engine, that run above the sensor have been either pulled out or broken. I would definitely start with that.

[mbsickness]

Originally posted by jnenad16
he might have pulled a vacum line. check that all the vacum lines are plugged in in the back of the engine.

I would seriously check this out as I have a 94 C280 and one time it exhibited the exact same behavior as your car right after I changed the oil. Sure enough a vacuum line was disconnected. The line in question was right at the back of the engine and runs almost on top of the oil filter housing.

[timmyj51]

Originally posted by jnenad16
if the crank sensor has been replaced, chances are that the vacuum lines in the back of the engine, that run above the sensor have been either pulled out or broken. I would definitely start with that.



Can you give me an idea of where to look or what hose to look for? Took
a peek back by the firewall but nothing was
immediately obvious.

[timmyj51]

OK, update. Looked under the hood and noticed the short, stubby, "S" shaped hose that connects the air duct with the
crankcase seemed to be pushed forward. Pushed it back until I heard a "pop". Since then no bucking or jerking, so seems the
problem was a vacuum leak on this hose.

However, when I start the car it still intermittently stumbles, then revs up to 2500 rpms for a second or two, before settling into the normal idle speed. Doesn't do it all the time, sometimes the car
starts normally. Cleaned and lubed the throttle linkage
thinking maybe something was sticking but no improvement.
Anyone have any ideas?