Questions and Answers

Mercedes Benz 300D Lacks Power Q. Mr. Ciulla, I found your web site and looked through your answered questions section, hoping to find some help. I saw you are a big fan of these turbo diesel Mercedes. Lucky me. I've been having problems lately with power fall off, and I'm hoping you can confirm some suspicions for me.

Within the last couple weeks, I took my 1984 Mercedes Benz 300D in to a Mercedes specialist in the Seattle area. I've heard he knows his stuff, so I felt fairly safe. I was having some serious smoking issues, and he was able to fix that problem with a number of small adjustments. Apparently the valves were too tight. He adjusted them so the engine would not have to work so hard opening and closing them. He also leaned out the fuel/air mixture as much as possible.

I plan on increasing the ratio myself, little by little, until I find the right balance with the smoking. He also found and replaced some fuel lines to one of the cylinders. Really, the engine sounds great and seems to run as it should. Driving around town, I hardly notice any problems.

However, when I get out on the highway, the car just won't accelerate. I can floor the pedal, make the car downshift and crank up the rpm's, but it will literally take a few minutes to speed the car up by 5 mph, even on level road. It used to be really peppy, so that I could step on the pedal and just the initial boost from the turbo would make the speed jump 5 mph immediately.

It becomes really obvious when the car will lose 15 to 20 mph on the highway going up hills (even some hills with very small inclines). I just went to a neighboring town this weekend, which is down in a valley with a 7% grade up the hill.

Speed limit is 65, and I used to be able to maintain 70 mph without making the engine too hot, usually a little under 100°. This weekend, I struggled to keep 50 mph and the temp rose above 105°, maybe 110°, I can't remember exactly. Then it cooled back down to 90° when it leveled out again.

So, I'm wondering if the muffler became too clogged with soot back when it was smoking so badly a month ago. If it's clogged, then there's excessive back pressure which could affect both the engine and turbo performance. Especially at highway speeds, when the throttle is fully open, then there will be more exhaust being produced faster.

This would cause the back pressure to rise even more. It would also explain why driving around town is not as much of a problem, because there is less exhaust produced at lower speeds, hence less problems cause by back pressure.

My main question is would it be worth the time and money to get the exhaust system/muffler inspected and possibly replaced? Or is it possible that there are other problems that would be affecting the power at high speeds?

Thanks for your time to help me out.
Kevin

A. There is a Continuous Trap Oxidizer, the diesel version of a catalytic converter, that may be partially clogged. You can drop the exhaust pipe at the manifold and see if the power returns. if it does, then you know the cause is an exhaust restriction.

I had a similar problem when I was going through the mountains of Pennsylvania. Even on flat Interstate with my foot to the floor, my 1983 Mercedes Benz 300D was still slowing down. I pulled into a rest stop, replaced both fuel filters and the problem went away.

I never go anywhere without spare fuel filters and the tools to replace them because diesel fuel is not as refined as gasoline.