Questions and Answers

Mercedes Benz 300SEL Has No Power Q. Dear Mr. Ciulla, I am the proud owner of a 1988 Mercedes Benz 300SEL with 362,000 miles on it. I love this car. It has an automatic transmission, fuel Injection with full power everything. I have owned the car since 1990 and it has been a dream. There has been nothing major except the routine work one would expect.

Because of the mileage on the engine it burns about a quart of oil every 1,000 miles. The other day I went to pick my son up at school and barely made it home. There are a few steep grades on the freeway between his school and our home and the car would not accelerate up the grade. The more I pushed the gas the more it slowed down. My speed on the flat portions of the freeway was 65 to 70 mph but on the hills the car slowed to 30 mph. I did make it home.

The next morning I took it to a mechanic who specializes in Mercedes autos. My long time mechanic who has worked on the car for me for the past 14 years is gone. To reach this garage from my house there is a steep grade about a ½ mile long and I would guess about 10% incline. The car would not accelerate past 10mph until I reached the top and the flat portion of the road. I was able to gradually reach the posted speed of 50 mph.

The mechanic thought it could be a problem with the catalytic converter as it exhibited sizes of a plugged exhaust. Scoping the converter with an infrared heat gun indicated that the converter was not burning properly so he put a new one on. The pre-cats were in good condition. This made the car run a little better but did not solve the problem.

He began to diagnose the ignition system. Wires were fairly new and in good condition. He did change the spark plugs as they were a little worn. He put on a new distributor cap as it looked to have a hot spot. The ignition switching was fine. According to the computer read out of codes all systems were okay. The problem still was there.

He changed the fuel filter and then did a complete diagnostic on the fuel system. The injectors were working okay and fuel was being delivered as expected. He thought that maybe there was still something in the exhaust system that could be causing the problem so he completely disconnected the exhaust and ran the engine and it would still would not rev properly.

He checked cylinder pressure. Spark firing order, timing and almost everything he could think of and nothing seems to work. The only thing he tried that seemed to help was that as he pressed on the accelerator the pressure plate in the intake would move as expected and he sprayed in a little choke cleaner. This seemed to help the engine rev better.

I have a 25 year experienced Mercedes Benz mechanic that is stumped. Do you have any ideas? I really love this car and do not want to have to get a new one.

Steve
Vista, California

A. The 3.0 liter engine used in the 300SEL was prone to pulling intake valves through the valve seats. Removing the camshaft will determine if this has happened. The effected valves will be noticeably higher than the rest.

If this is the case a lot of expensive work would be required to repair the head, probably prohibitively expensive. And this is a case where a used head may not be as preferable solution as a new head would be.

If the valves look okay, then I would check the lifters for proper operation. If they check out, then I would suspect the injectors. Mercedes Benz injectors are notorious for being, how shall I say, garbage? I have gone through 50 or 60 new injectors before I found six that were good.

The injectors are mechanical and there is a special tester for them. There are calibrated springs inside that allow the injector to open at a specific pressure. In this case they should open at a minimum of 46 psi. And the spray patter should be a nice conical pattern without drips and blobs.

Fuel pressure and volume should be checked as well. Normal system operating pressure should be between 76 and 79 psi.Fuel volume should be no less than one liter, a little more than a quart, in 40 seconds.

It is also possible one of the two fuel pumps are bad. A special fuel pressure adapter is required to test the pressure for both fuel pumps.

Other than that, you will need an On-Off Ratio, or Lambda, tester to check the rest of the system out. This is a very specific piece of test equipment and only a Mercedes Benz dealer would have it.