Got gas? Get ready for fuel delivery diagnostics

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Got gas? Get ready for fuel delivery diagnostics

Truglia is the owner of Car Clinic, a state-of-the-art repair facility in Mahopac, N.Y. He is ASE certified with a M.A. from Columbia University. In the automotive world he has been trained by Technicians Service Training and Automotive Technician Training Services. Car Clinic’s facility is fully equipped with factory-level equipment and services American, European and Asian vehicles, including diesels and hybrids.

The present-day is an interesting time for the automotive technician. Perhaps now more than ever, the vehicles that are on the road resist breakdown, and with the help of a few repairs can routinely last about 200,000 miles.

This is a far cry from the day when vehicles had carburetors or early versions of fuel injection. While it appears that fuel-saving technologies such as direct fuel injection and the increased use of turbochargers may also break down quicker in the future, many of these vehicles are so new they have not yet reached the point where they have breakdown-related issues.

So, the question service technicians need to ask themselves is whether or not they are prepared to deal with a fuel-related no-start or misfire.

Because the truth is, it has been about 20 years since OBD II technology hit the market and aside from General Motors vehicles, it is not very common to run into one of these issues.

Tip Number One: Give up on current ramping, or don’t even start if you have heard about it.

Back in the day, when 90% of vehicles were made by three American companies, there was not a lot of diversity in what fuel pumps these vehicles had. Not coincidentally, when the amperage of these pumps was ascertained using an amp clamp and a labscope, they produced neat looking waveforms, such as the one seen in Figure 1.

Many technicians think that all fuel pumps can be diagnosed this way. However, if they actually tried the technique on vehicles made after the year 2000, they would see that the fuel pump waveforms don’t lend themselves to such easy interpretation.

Take a look at Figures 2 and 3. Does this vehicle have a bad fuel pump?

Many current ramping gurus would say “most definitely.” The problem is, the actual answer is no! This fuel pump is fine and this is not even a new vehicle, it is a 1998 model year GM. Further, as one can see in Figure 4, the vehicle’s fuel trim is not highly positive, compensating for a bad fuel pump. It is well within normal range. Quite frankly, the fuel pump is fine.

Further, to effectively current ramp fuel pumps the technician is required to undergo the impossible task of taking specs of every new car he finds. And it is not always easy to access fuel pump relays. Rents and taxes are too high and labor is too expensive to waste time doing this, especially when, quite frankly, it does not work anyway.


Tip Number Two: Checking fuel volume is the only way to test for a bad fuel pump or restricted fuel filter.

Many technicians have a fuel pressure gauge and the vehicle usually has a fuel pressure sensor on the rail. Both the gauge and the scan tool can help us test for fuel-pump drain back and an underperforming pump, so they can be quite useful.

However, it should be known that just because a vehicle has low fuel pressure, it does not mean that the fuel pump is bad. For example, a vehicle with a bad fuel filter may have barely any fuel pressure, especially if it is causing such a restriction that the vehicle will not start. Yet, the fuel pump can still be good.

So, if the shop can spring for the bucks, a fuel volume gauge is the way to go. It is simply placed in line, preferably before the fuel filter, and it tells us both fuel pressure and fuel volume in gallons-per-minute.

Better yet, there is no need to memorize fancy specifications. Every single vehicle that’s a four cylinder will flow at least 0.4 gallons per minute (gpm) and any other vehicle with a turbo or larger engine will flow at least 0.5 gpm. To be honest, do not be surprised to see even a known-good Honda four cylinder flowing at least 0.6 gpm. If a fuel pump is good, it will generally flow even above these numbers, but when it is bad it will flow below these numbers.

Just remember, if you flow after the fuel filter (even though most filters are in the tank anyway these days), a bad fuel filter can cut both your pressure and volume.

How can you measure fuel volume without the gauge? If you can command on the fuel pump by jumping the connectors where the relay goes and disconnect the fuel line, you can take an empty windshield washer fluid bottle and see how much fuel you get in a minute. If about half of that bottle gets filled up, your fuel pump is fine.

However, sometimes this is too messy and inaccurate. There is one other way to get the job done without purchasing a fuel volume gauge.

If you are performing fuel system cleanings in your shop, chances are you have a tool that connects to the fuel rail in order to flush the injectors out with a cleaning solvent. If this is the case, you can test fuel volume indirectly.

Simply connect the tool to the fuel rail and fill it up with gasoline instead of solvent. Then, put the tool at about 40 psi (which may be risky so be sure to wear glasses or a face shield) and start up the engine. If the engine runs normally, then what you have essentially done is proved that there was a fuel volume issue.

Why? Because if the vehicle runs normally hooked up in this fashion when it did not before, the only thing that changed was the volume of fuel that reached the fuel rail. So, the problem has to be something fuel volume related farther down the fuel system, likely the pump or filter.

John Thompson of Thompson Automotive Labs has been known to say, “Use a new fuel filter as a $10 tool.” If it fixes the problem, then you saved a lot of diagnostic time. If it doesn’t, a nice and profitable fuel pump job is in order.

Tip Number Three: Be on the lookout for obvious stuff.

Here, on this 1998 Honda Civic, the customer had a bad fuel tank (see Figure 8). He put a junkyard fuel tank on and, not surprisingly, a few months later, the engine starts running funny.

After testing at a fuel volume of 0.15 gpm the technician ascertained the junkyard tank must have had rust in it and clogged the pump. Lo and behold, it did! (See Figure 9).

Just remember these three tips, because even if you are a little “rusty” with fuel-delivery problems, they will help make what can be a much more complicated diagnosis into a simpler one!   ‚óŹ

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