Advice:
Loose gas caps and "check engine" lights
by Phil Bailey

Ever since computers were first introduced into cars, that dastardly little dashboard light that says “check engine” or “power loss” or “service engine soon” has been the bane of every driver's existence at one time or another. A lot of people panic when they see this light come on and even those, few in number, who read what it says in the owner's manual, still are disturbed when they read that “Your car should be serviced at your Dealership as soon as possible”.

In fact, if the engine continues to feel normal to you as you drive, there is no real urgency to the matter and you can take an appointment, not necessarily at the dealership if your warranty has expired and whenever it suits you. Of course, if the car begins to act up in strange ways, then a visit to someone with the necessary analytical equipment is a must.

Just lately, we’ve been inundated with calls from people whose engine light has come on, usually just after fueling up the car. Readers should be aware that, on late model cars, if you don’t tighten up your gas cap by 5 clicks, at least, the computer knows and will chastise you. And if you foolishly attempt to refuel with the engine running, that will also give your CPU a bad case of grumpiness. A tightly sealed gas tank is now part of the emissions system and that pesky computer knows if the cap isn’t on tight enough or if you didn’t turn off your engine.

In fact, we blew up a gas tank this week. No, no one got hurt or had to rush for a fire extinguisher. We literally blew it up, like a party balloon. Fuel injection systems are return systems. That is, gas is pumped up to the injectors at fairly high pressure and then returned to the fuel tank. Ordinarily, there is a pressure relief valve, attached to the canister purge system that keeps the tank pressure equalized. If this valve is blocked or fails to function, then the tank comes under tremendous internal pressure and this one of ours, being a cheap and rusty galvanized steel example, puffed itself up like a zeppelin and then started to leak at the seams.

When you undo your gas cap, you will feel some pressure relieve itself, but this is known as vapour pressure and is quite normal. A volatile fluid, such as gasoline, when shaken about in a container such as a gas tank, will evaporate and generate its own pressure. Conversely, if that ubiquitous little gas cap ever blocks and allows no air to enter the tank, you may find your car stalling frequently as the poor old gas pump tries to suck gas against a vacuum.

Personally, I believe that the stalling problem currently plaguing late model Nissan Altimas will turn out to be something similar to this, in that there is a fault in the vacuum relief system somewhere. Nissan's current advice is to keep the tank full, which of-course immediately provides vacuum relief. I’m not sure of this diagnosis, but I wouldn’t be surprised.

So even something apparently as simple as a gas cap is now, nowhere as straightforward as it seems. At one time my VW Rabbit would start and run just fine, thank you, until I turned off the hot engine. At which point, it wouldn’t restart. In playing around, for some reason, I pulled one spark plug and then turned the engine over on the starter motor. The engine promptly fired on three cylinders, with whooshing noises coming from the open plug hole! Put the plug back in and the engine started and ran - until the next time.

In those days, not being in the car repair business, I continued to pull the plug and start the engine for some time before I discovered the problem. It seems that over pressurisation at the injectors can cause them to develop a bad case of post nasal drip. The car starts fine from cold, is driven somewhere until the engine is hot and is then shut down. Ten minutes later, the car refuses to start until it has stood and the engine has cooled down, then it starts and runs just fine. What’s happening is that while you’re in the coffee shop, or the cinema, or whatever, an injector or injectors is happily dripping gasoline down into the engine. After ten minutes of this procedure, the engine is hopelessly flooded. When, in my case, I removed a spark plug, of-course, I instantly provided more combustion air and the engine cleared itself of excessive fuel.

In most cases, removing a spark plug is not necessary, because when the engine cools down, it needs more fuel to fire up and part of the overload has evaporated. Once the engine starts, it is not over charged with fuel to any large degree and runs just fine... until the next time.

However, the check engine light will come on as the computer detects a rich mixture and confirms that, in this case, at least, there is something amiss.