BACKYARD MECHANIC: Fuel gauge resistor board at fault

Hey there, time traveller!
This article was published 02/11/2012 (5063 days ago), so information in it may no longer be current.

QUESTION: I am having a problem with my 2003 Pontiac Montana van. The fuel gauge suddenly started acting up — the reading is not always right on the gauge. Is there an easy fix that I can do myself? I have checked the connectors underneath just below the driver’s seat and they seem OK. Any advice you can give me to save some money would be appreciated. Thank you.

ANSWER: Usually an incorrect fuel gauge reading is due to poor electrical connections at the wiring connectors but in your van’s case, the most likely cause is a fault with the sender unit in the fuel tank. The fuel sender unit has a small resistor board on it with a wiper needle that slides across the resistors to change the resistance of the unit. The wiper needle is connected to a float so different fuel levels are indicated as different electrical resistances. The problem is the resistor board has become contaminated.

There were a few years where the fuel we were using contained contaminants picked up in the pipeline as the fuel was transported. In most locations, these contaminants were filtered out as the bulk delivery points but in some locations the fuel came directly from the pipeline. The resistor boards that GM was using in its fuel sending units would be coated with these contaminants and it would cause erratic resistances which showed up as erratic fuel levels on the fuel gauge.

There’s nothing you can do about the fuel, so the fix is to replace the fuel gauge resistor board in the fuel sending unit. In some vehicles, the resistor board is only available with the complete fuel sending and fuel pump unit, but GM did make the resistor board available separately for many vehicles. The fuel tank will have to be removed and the sending unit taken out to replace the resistor board but it is still cheaper than replacing the complete fuel pump unit.

QUESTION: I have a 1984 class A motor home with a 454 c.i.d. engine.

Last summer I was on the highway towing a car on a dolly when I decided to stop and check on the car. The engine was idling and the temperature outside in mid-30s. When I got back in and started to leave, the engine died. I tried to start it several times but no go. The temperature gauge showed the needle past the middle. I looked down the carb and there was no gas so I grabbed a container of water, crawled underneath and started splashing the fuel pump. As you know, it’s under the engine, but still nothing. I waited over an hour.

The engine cooled down and I was able to start it and had no problem after that. I suspect it was an “air lock” in the fuel pump. I have never had that problem before. Fuel filters, fuel hoses and pump were replaced three to four years ago. What do you think happened?

ANSWER: You have diagnosed the problem accurately. The engine had what is referred to as a “Vapour lock.” The engine heat caused the fuel in the lines and pump to boil into a vapour state. Because the fuel pump can only pump liquid, the engine wouldn’t start. After cool down, the fuel returned to the liquid state, the engine restarted and ran fine.

Pouring cold water on the fuel pump helped correct the problem, but there was still too much heat from the engine to allow the fuel to return to a liquid state. In the future you could insulate the fuel lines and fuel pump in the engine compartment and turn off the air conditioning when the vehicle is idling. More airflow through the engine compartment would help prevent the fuel from boiling but this isn’t always easily done.

Fuel-injected vehicles seldom experience vapour lock because the fuel systems are under a much higher pressure. Carburetor systems operate at five to six PSI, while the lowest pressure fuel injection systems operate at 11 to 13 PSI and most operate in the 40 to 60 PSI range. The higher pressures prevent the fuel from boiling. In your motorhome, you have experienced vapour lock once under specific conditions. With luck, you will never have it again, but if you do, let it cool down and you are on your way again.

Jim Kerr is an experienced mechanic, instructor of automotive technology, freelance journalist and member of the Automobile Journalists’ Association of Canada.

kerr.jim@sasktel.net