spawn_x

'03 ML500 with 107,000 miles, feels kind of slugging on lower RPMs, say going up a hill in low gear, not even talking highway speeds.

Engine sounds healthy, and picks up power after 3500+ RPMs (but maybe not enough?). Are there variable timing components that might need servicing at this age/miles? O2 sensors that might help? I'm on original plugs, not sure if that'll make a big difference? Air filter is new..

Thanks

pjw1967

Longshot, but maybe the cats have reached the end of their life? Once you get up to speed the effect might be lessened due throughput being increased along with temp. At lower speeds they might be causing back pressure. Plugs should certainly be replaced after 100k. Problem is without an accurate diagnosis, you could start changing parts and not solve the problem. http://www.ehow.com/how-does_5206525...ter-last_.html

pjw1967

Here are five signs that something may be wrong with your catalytic converter:

Your vehicle’s fuel efficiency suddenly drops.

Your vehicle doesn’t accelerate when you step on the gas pedal.

Your vehicle may refuse to start.

Your vehicle fails an emissions test.

The MIL or Check Engine light comes on.

The most common cause of failure in a catalytic converter in an older car is that it becomes so clogged that the exhaust gases can’t get through it to the muffler and out of the car. (If air can’t get out the rear of the vehicle, it can’t come in the front end, so the engine dies because no air is coming in to form the fuel/air mixture.)

Every car sold in the United States since 1996 has had an OBD (On-Board Diagnostic) II system that tests the catalytic converter (among many other things). If the unit allows too much pollution to escape from the tailpipe, it illuminates the MIL and produces a trouble code that can be read by a technician with what’s called a “scan tool.”

The catalytic converter is sensitive to changes in the contents and temperature of the exhaust gases, so another possible cause for failure is if the cylinder head gasket is damaged, allowing oil or coolant to get into the combustion chamber and be burned in the cylinders. Also, if your ignition system isn’t operating properly, unburned fuel in the exhaust gases can cause the catalytic converter to wear out or break down. This is another good reason for going for tune-ups at specified intervals!

spawn_x

I considered the cat converter, but the engine feels good at high RPM where it obviously moves a lot more exhaust. It does a better job going uphill at 85 than 25, MPG is pretty good, so I kind of ruled the cat out

There's no check engine light, it passes smog, I have not done a leakdown test to check for worn rings

I just bought the car a couple months ago and do not have the history on it, so I hope the engine is not neglected and worn out..

pjw1967

Second symptom above is sluggish acceleration which gave me the idea. Good luck no matter what.