Corner Wrench: Don’t get fuelled again

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It never fails; when a tech (whether a seasoned and licensed pro, or a veteran DIYer) is faced with a head-scratcher of an engine performance problem, one of the last things they check is fuel quality and flow. They can spend hours scanning for trouble codes, running wiring harness tests, activating and exercising various components, checking injectors (or carburetors); all of which pass every test and sometimes even two tests for good measure. Then finally, someone suggests checking fuel quality and/or pressure and flow.

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We’ve talked about gasoline degradation before and how quickly even the best-quality fuel can go bad. It can cause rough idles, stalling, poor acceleration, jerky operation, and no-start conditions. But even bad fuel has to get to the engine first, and fortunately carbed and injected engines can exhibit the same symptom when fuel is actually getting into the engine’s intake; it creates an unmistakable odour. With carbed or throttle-body injected engines, you should actually be able to see liquid fuel spray when cranking the engine with the air intake hose or carb-mounted air cleaner removed. Taking a fuel sample doesn’t have to be an onerous job, but it is extremely necessary to check fuel quality. An easy way to siphon out a sample from the tank is with a hose of course. But if you don’t want to risk a taste of aged unleaded, cut a small hole in the side of the hose near the output end and use a blow-gun air tool to create enough air flow to get the fuel moving. If it’s immediately darker than normal (with or without an unusually bad smell), you don’t have to wait to let it settle to know that the fuel is bad and that a tank and lines cleaning is needed.

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If it looks and smells normal enough, let it settle in a clear container to see how much water collects at the bottom. Any amount that’s visible in a small sample (such as a mason jar) is too much.

There’s an easy test to see if an in-tank electric fuel pump is the problem. If you can’t hear the momentary hum of the pump when flipping the ignition key to run without cranking the engine (the priming phase of the pump), try slapping the bottom of the fuel tank while a helper is trying to start the engine. Either use the flat of your hand or a broom for easier reach. If the engine roars to life, you’re likely dealing with defective pump motor brushes. An alternative source of fuel, (such as quick-start or ether spray) can be great for short tests, but due to its explosive nature, use it sparingly and only in a well-ventilated area.

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If you’re trying to diagnose a fuel delivery problem on an injected engine, a fuel pressure gauge is definitely helpful, and there are some parts stores that will rent out such tools if you don’t want to buy one. If you discover poor pressure, don’t jump straight to replacement pump. Remember that a weak or failing electric pump may be a symptom of a debris-clogged tank screen or filter, or plugged lines. Check the current draw — a higher-than-specified reading may point to this type of strain. If a pump has been running dry or under strain for a while, it may be worth replacing altogether.

One more thing: if your ride is carbed and uses an engine-mounted mechanical pump, remember that they don’t do well with fuel blended with alcohol. It tends to weaken the diaphragm to the point of failure, sometimes within a few hours of running time.