Lazy Hacker Checks Fuel System For Leaks, The Easy Way

Old cars are great. They represent a different time, reflecting the state of society at the point of their design and manufacture, and can charm and delight while also providing useful transport. Except, well… old cars are great, except when they’re not.

With my Volvo 740 hitting its thirtieth birthday and cresting over 200,000 miles, to say its a little worse for wear is an understatement. The turbo dadwagon has suffered transmission issues, and cold starting woes… but most frustrating is the sudden spike in fuel use. After some work, my humble daily driver had slid from using an acceptable 21 miles per gallon, to getting just 15. Add on the fact that the turbocharged engine demands premium fuel, and you can understand my consternation.

Now that I was haemorrhaging cash on a gargantuan weekly fuel bill, I had plenty of motivation to track down the problem. Busy, and eager for a quick solution, I deferred to a mechanic recommended as the local expert in all things Volvo. Sadly, the results were inconclusive — initial appearances were that all the engine’s electronic controls were functioning to specifications, and I was told that it was “probably a bad batch of fuel”.

Unfortunately, several expensive tanks later, sourced from all over town, revealed that the problem was in fact real. With a supposedly reliable report that the fuel mixture was correct, thus ruling out culprits like the oxygen sensor, I began to wonder, was I simply pouring fuel out the tank?

I’d come to this worrying suspicion due to an amateur mistake I’d made when changing the transmission filter weeks prior. After the job was done, I began to jack up the car in order to remove the axle stands holding it up. Upon letting the car down, there was a resounding crunch.

I generally leave tyres, bricks, or in this case, ramps under the car as a backup in case a stand fails. I’d stupidly forgotten to remove them this time, and the car’s full weight was now on the ramp I’d left sitting directly under the fuel tank. I hastily jacked the car back up and removed the ramp, inspecting for damage. There was nothing obvious, but in the following weeks, my fuel economy had become terrible.

This led me to believe I’d perhaps damaged a fuel line or other tank fitting, and that I was pouring petrol out of the tank when going around corners. This needed to be checked, but how? Dropping the tank for inspection promised to be a big job, and ideally would be done when the tank was empty to avoid being crushed under the weight of 50 liters of fuel. There had to be a better solution.

An accomplished colleague of mine asked if I’d considered pressure testing the system. Of course! It was so simple. All I had to do was find a way to pressurize the fuel system with air, and see if it held pressure, or if I could hear leaks. Easy!

I scrounged around the house and put a plan together. First, I popped the hood of the car. Taking a look at the fuel rail, I determined it would be easiest to remove the return line from the fuel pressure regulator, which was held on with a simple hose clamp. This will vary a lot from car to car; I was lucky that this was fairly accessible. I then grabbed my trusty bike pump, and removed the valve adapter from the end, leaving just the air hose. As luck would have it, this fit snugly within the fuel return line. With the fuel delivery line still in place and any flow blocked by the regulator, it was now possible to pump air into the tank.

The aim here isn’t to put the fuel system under a great deal of pressure; a couple of psi above ambient is fine. Unfortunately, the gauge on my K-mart pump isn’t accurate below 20psi, but I was able to start pressurizing the tank and lines. After a few pumps, I heard a sound that I can only describe as a disgruntled moan. This was the fuel cap releasing the excess pressure. After a few more pumps (and awkward sounds), I looked under the car to check for any fuel leaking, and thankfully, none could be found. Just out of interest, I undid the fuel cap, which released the last of the pressure with a hiss. If this isn’t done, the return hose will tend to spew fuel all over the engine bay when the air hose is removed which can be a bit of a hassle.

In the end, it appeared that there was no leak at all. My hunt for a fuel economy problem goes on. But the great thing about this technique is that it’s readily achievable and requires no special tools – just the ingenuity required to mate up a few hoses. It can readily help you determine if your fuel system has a serious leak or not. If you’d like to see it done on video, we’ve got you covered — albeit, I confused the return and delivery lines. If, however, you’ve got a slower leak, like a dead fuel injector, or you’re trying to diagnose a failed check valve in a fuel pump, you may struggle to find it with this technique. In that situation, you’ll want to lay your hands on “the right tool” which is a fuel pressure test kit, and undertake a deeper investigation. [EricTheCarGuy] has a great guide on how to do just that, though it does require some more specialized equipment.

Overall, it’s a simple check that’s accessible by the shadetree mechanic that might help you spot a problem before it gets out of hand. We’d love to cover more garage hacking. How do you approach diagnosing fuel economy problems on your fuel injected rides? What are the other automotive hacks you’ve taken on over the years?