STAR TECHNICAL Q&A

The following questions were received by mail, email, or on the forum. The answers were composed by Richard Simonds, Senior Technical Advisor, Star Magazine

Vapor lock on carburetor cars

Q: I’m having a recurring  problem with my 1961 190 Ponton. If I’ve been driving it for a short time and shut it off, I have trouble getting it to restart. I’ve been told that the problem is with vapor lock that’s preventing fuel from getting to the carburetors.

A: The problem you have is not uncommon with the early postwar cars with carburetors. The general opinion on the Internet boards is that an electric fuel pump and retrofitted fuel return valve and line are a very effective way to solve this problem. The combination of electric fuel pump and fuel-return line can get cooler fuel into the carburetors and make starting easier.

There is a section in the factory Service Manual for the early Pontons describing the procedure to retrofit a fuel-return line, Solex fuel-return valve and modified carburetor linkage with additional arm extension. This option was installed as OEM equipment on later 220S cars and included as standard equipment on all W111 Finback 220S models and subsequent W108 models. Clearly, the factory was aware of this issue, and in keeping with its devout policy of constantly improving models during production runs, moved to correct this on subsequent 220S units and later models.

With this retrofit, the only issue remaining unsolved is that of the fuel boiling into the carburetor fuel bowls when you shut off the engine after a long run. This makes hot-engine restarts difficult and puts a lot of wear on the starter mechanism, ring gear and battery.

An aluminum heat shield combined with insulated fuel lines work well. McMaster-Carr’s heat insulation contributes to the control of vapor lock while driving but does not fully address the issue of a hot restart. The most versatile fuel line heat insulation I have found is made by Thermo-Tec, which also sells a number of products to insulate exhaust manifolds, wiring and so forth.

These solutions might be especially attractive to owners of collector models who are interested in maintaining their vehicle’s original appearance as much as possible while enabling their cars to be driven confidently in hot weather.
 
New car battery maintenance

Q: I live in the Northeast and my 2013 SL550 often sits for many months at a time during the winter. How long will my battery keep my car’s electronics active if I don’t drive it? Is a battery cutoff/disconnect an option? I do not currently have access to electrical power where I store my car.

A: The following statements are a very brief summation found in the official MBUSA Policy #S-B-54, 10/264a: If your Mercedes-Benz is going sit unused for more than six weeks, a battery tender should be used to maintain the battery charge of yur automobile. If this is not possible, the battery will need to be recharged and the onboard electrical and electronic systems will need to be checked by a Mercedes-Benz technician using proper equipment.

Devices that cut off the battery connections are not advised as electronics are very sensitive to voltage variations/spikes and a complete restoration or replacement would be prohibitively expensive. None of these are covered under warranty.

It is likely that the cost to run 110- or 120-volt power to your storage space will be substantially less than replacing your car’s battery and electronics and will return the cost of installation by saving the cost of only a few dealer service calls.

If you absolutely must jumpstart your recent-model Mercedes-Benz, there is a positive-terminal blade under a sliding red cover and a negative-terminal brass post in the engine compartment to which jumper cables can be attached. On the most recent models, direct access to batteries is not feasible. The owner’s manual includes several serious warnings: If the instrument lights do not illuminate at very low temperatures, the battery may be frozen and it should not be charged or jump-started until it has thawed. Do not use a rapid-charging device to jumpstart your car: Use only another 12-volt battery to avoid voltage higher than 12.2 volts. Your Operator’s Manual (if accessible to you) has a number of checkpoints under “Jump-starting” to help you avoid risks of damaging your car. Always refer to the Operator’s Manual – it’s your best resource.
 
Wheels & Tires for Cool Driving

Q: I want to get another set of wheels and tires to use for winter driving. What are the basics of selecting wheels and tires to be safe and maintain car control? Also, during the rest of the year I want to make my Mercedes-Benz stand out from the crowd with special wheels and tires such as those advertised in The Star. What do I need to know?

A: Interestingly enough, the answers to both of these questions are quite similar: the issues are, first, lug bolt compatibility with non-original wheels; second, wheel offset that has an affect on handling, bearing wear, and braking; and third, ride quality.

When mounting winter wheels and tires, it is common practice to substitute common steel wheels for more expensive alloy wheels. Generally with these two types of wheels, the lug bolts that secure the wheels to the hubs have different lengths and are shaped differently at the point the lug bolt seats with the wheel. If the lug bolts are not properly matched with the wheel, they can easily work loose while driving, causing a wheel to come off the car. It is not an experience you will either enjoy or forget to see your wheel and tire suddenly appear rolling down the road beside you while you are driving! Two important tips: Be sure to keep the lug bolts for the winter wheels separate from the lug bolts for the alloy wheels and do not forget to have a set of the correct lug bolts for the spare wheel and tire. Of course, always torque the bolts to the correct setting using a high-quality torque wrench.

When customizing wheels and tires it is important to have the correct lug bolts for the new wheels (both length and shape). If you plan to keep the original wheels and tires, be sure to keep the original lug bolts as well.  Also, be sure that your custom wheels have the same offset as the original wheels. Wheel offset on custom wheels and tire size can affect handling, bearing wear, braking, and tires rubbing on the fenders and wheel wells. Tire size (because of aspect ratio/profile dimensions) can seriously affect ride quality as well.

Your Operator’s Manual lists the sizes of wheels and tires that are approved and recommended for your specific year and model of vehicle. Anything other than these can affect your warranty and vehicle safety, so if you do decide to switch wheels, for winter safety or just to be cool the rest of the year, it’s best to work with your dealer, or a good wheel and tire dealer that knows Mercedes-Benz automobiles.

If you have a technical question, you may submit it to Richard Simonds, [email protected]. He will answer your question, if it can be answered, by email, and the interesting questions and answers will be published in the next issue of The Star. We're sorry but questions can not be answered by telephone or mail.