30 Car Mysteries Solved: Trailer Turn Signals

Q:

I just picked up a nice used camper trailer. The turn signals all light up, but they flash about three times per second instead of once every 2 seconds. The gentleman who sold me the trailer claims it worked just fine with his truck. My mechanic can't find anything wrong with my car--but I can't even tow the trailer over to him until I get the lights working.

A:

You need to upgrade your flasher relay to a heavy-duty flasher. Here's how those old-fashioned flashers work. There is a set of switch points attached to a bimetal strip, all in series with the turn-signal bulbs and the battery. When the turn signals are switched on, current flowing through the bimetal strip heats it up, and the strip starts to curl, breaking the circuit. A second or so later, the strip cools off, making the connection again, a sequence that repeats until the signals are canceled. Plug in the trailer, and you're adding two more filaments to draw more current, which makes the bimetal strip heat up faster, accelerating the blink rate. Go down to the auto parts store and get a flasher rated for towing. It costs around five bucks and plugs right in under the dash. Can't find it? Turn on the signals and follow the click. The heavy-duty flasher either uses a circuit that isolates the bimetal strip from the load, or just does the whole thing electronically.

I sometimes get letters from readers who can't hear the flasher relay at all, regardless of how fast it's blinking. A lifetime of rock-and-roll, high-caliber handguns, motorcycles and race cars is starting to affect my hearing, too.

Fortunately, you have an option. They make special, louder flasher relays that are also dirt cheap. Just substitute one for the old flasher.

Here's the bad news. Many modern vehicles have integrated the flasher-relay function into a fancy-dan lighting-control module, or whatever they call it, and you're stuck with what's built into it. Most of these vehicles will handle a trailer, but the volume of the flasher is fixed. I bet a decent mechanic or one of those shops that installs car stereos could splice in a chime or a buzzer or something, probably for a lot more than the cost of a new relay.

Best (or at least cheapest) solution I've seen was a fellow who epoxied the lid from a tuna can to the standard flasher relay, making a sounding board to increase the volume substantially.

Altitude Correction

Q:

My home and shop are at an elevation of 8000 feet. How does this change my readings when doing a compression test on my car?

A:

Compression readings are done by removing all the spark plugs, attaching a compression gauge to an individual cylinder and cranking the engine to measure the maximum pressure built up in the cylinders.

Expect the readings to be lower at altitude than at sea level. Normally, I'd expect to see readings somewhere between 120 and 160 psi at sea level, but the number will vary, determined by your car's compression ratio and camshaft timing. Depending on the altitude, the barometer reading on any given day and how robust your battery is, the actual readings you get on a compression test can vary 10 to 15 percent up or down. What's far more important is the consistency between the cylinders. If they're all a few pounds low, it might be that your battery is just a little discharged, or the barometer is low, or the throttle valve isn't propped open far enough while cranking the engine. If only one is weak, the condition might be indicative of a burned valve, leaky ring or scored cylinder wall. If two adjacent cylinders are very low, I immediately suspect a blown head gasket between them.

Counterintuitively, high-performance engines often show lower compression readings because the hot camshafts leave the valves open well past bottom dead center, lowering the effective compression ratio.

A number of late-model cars can replicate a compression test without removing the spark plugs. Under the command of a technician armed with a scan tool, the computer disables the cylinders one at a time in some sort of random sequence. After a few minutes, it will return a "Cylinder Balance" result. The cylinder(s) with low compression will affect idle speed less than the others when switched off, pinpointing any problems. I still like to do a compression test or leak-down test, because it gives me a chance to look at the plugs.

Locked Out

Q:

I own a 1969 Ford with a 428 Cobra Jet engine and the car constantly stalls. It has a mechanical fuel pump and "hard" fuel lines from the pump to the carb. I checked the fuel pump pressure and it reads 7 pounds. The car drives anywhere from 1 mile to 20 miles before it stalls. My next fix is to replace the fuel pump, but I am wondering if the car is suffering from vapor lock. It is a big block with long tube headers, so it certainly gets hot under the hood. The fuel line does not touch the block or manifold at any point. The car had a rubber fuel line from the pump to the carb when I purchased it, but it was not installed any farther away from the block or manifold than the hard line. If it is vapor lock, would changing back to a rubber fuel line solve the problem?

A:

I agree, it's probably vapor lock, caused by liquid fuel boiling in the carb or fuel lines, preventing sufficient liquid fuel from reaching the jets. The vapor pressure of gasoline is much higher than it was in 1969, since we use a lot more of the crude oil in the barrel, including a higher percentage of more volatile fractions that used to be flared off. And it's those fractions that boil at lower temperatures than the rest of the fuel. Fortunately, fuel-injected cars aren't prone to vapor lock because the fuel in the engine compartment is usually between 30 and 60 psi, not the seven or less used by carbureted vehicles.

Changing back to a rubber line and moving it away from the headers might help reduce the temperature of the fuel inside the lines. Just insulating the hard line might be enough to prevent boiling. The fuel is probably actually boiling in the float bowl, however, not in the line. They make insulated base gaskets for those carbs, and some cars with carbs used sheet-metal heat shields to keep heat from the headers from radiating into the carburetor.

Toward the end of the carburetor era in the '80s, some manufacturers resorted to recycling some of the fuel from the fuel pump back to the tank. This was done with a tee fitting that had a small restriction in one leg of the tee, allowing a small trickle of fuel to go back to the tank through a separate line. The cool fuel from the tank, moving continuously through the line, kept the fuel from boiling. This required a fuel tank with an additional fitting for the return line. You might check if your tank could be modified, or replaced with a later model tank with the fitting. Come to think of it, the fitting was often in the fuel sender plate, so all you might have to change is the fuel sender for one with the extra fitting, and then run an additional line from the engine compartment back to the tank.

If you have air conditioning, you might try my favorite fix: Wrap the fuel line around the cold a/c refrigerant line as the refrigerant line heads back to the compressor from the evaporator. Strip off the insulation, wrap the hose around the cool line, and rewrap with fresh insulation. I'll bet that if the engine is hot enough to vapor lock, you're hot enough to need the air conditioning on.