Basic No-Start Guide: Part 2 – Fuel Pressure

Alright, so your car is not starting and you’ve verified you have spark by checking out the things outlined in last week’s [Basic No-Start Guide: Part 1 – Spark], and you’re on the right path to getting your machine back up and running.  Next up, fuel.  Fuel Pressure is the vital second step in the equation, so here we go!

Is The Fuel Pump Activating?

First thing you want to do is more of a common practice than a rule.  When you click your ignition on, listen for the fuel pump to prime.  It’s important to note that not all cars prime their fuel pump when starting, and in some cases the fuel pump will only be activated when cranking.  If that’s the case, you’ll skip to the next step anyway.

So, with the car completely off, switch the ignition to the on position.  You should hear a hum or a thrum as pump pressurizes the fuel system.  It’ll shortly stop after it does that.  If you have a fuel pump access hatch under your rear seat, it’s great to open it up to get the closest you can to the pump.  If not, have a friend duck their head under the tank and listen for the pump to prime.

If you hear nothing and the pump is completely silent, you have two options.  The first is to open your hood and access your Fuel Pump Relay and get out your cheap hardware store multi-meter, set to volts.

Check For 12 Volts At The Relay

The Fuel Pump Relay is a heavy duty electrical switch that handles the power going to the fuel pump, so the computer doesn’t have to.  If you are getting power to the relay, listen to see if it clicks on at all.  If it doesn’t click when you turn the key or crank, you might have a bad relay.  You can jump the relay with a jumper, a procedure which I won’t cover here.  There is ample information online about this.  If you are not getting any power, or are getting a low voltage, you will need to start trouble shooting continuity through the relay, wiring, and grounds.  If you have power and it clicks, access your fuel pump power connector.

Check For 12 Volts At The Pump

When you click the key to the ‘on’ position, you should be getting 12v to the fuel pump.  If you are, and the pump is not priming, the fuel pump is most likely your issue.  Additionally you can use a car battery to directly put 12v onto the fuel pump connection and see if the fuel pump activates.  If not, it’s time to buy a fuel pump. Fuel pumps generally last between 100-150k miles, and are a very common reason for break downs.  Personally whenever I buy a used car I replace the fuel pump preemptively, after one time I broke down in the bad part of a city.  It’s easy peace of mind.

Fuel Pressure At The Rail

With the key to the ‘on’ position, crack the banjo bolt ever so slightly. Pressurized fuel should spray out.  If fuel lazily drips out of the rail with the engine cranking, then you will need to start troubleshooting low fuel pressure.  Modern cars come with Schraeder valves built right in to the fuel rail, which makes testing fuel pressure at the fuel rail much easier.  You should find a valve that resembles a tire valve stem.  Use a fuel pressure gauge to see what kind of pressure you are actually getting.  We have fuel pressure testers available for sale at eEuroparts, and one of these is an essential item to any tool kit.

You could theoretically bypass everything said above just by plugging this into the fuel rail and cranking the engine.  On cars without a schrader valve, you can work out any number of adapters to attach one at the banjo connection.

Simply speaking, if you put 12v on the fuel pump (you can do this from the Fuel Pump Relay output without dropping the tank, consult your service manual for a wiring diagram) you should have full fuel pressure.

Contributors to low fuel pressure include an underperforming fuel pump, a clogged fuel filter, and a bad fuel pressure regulator.  However these things generally will not impede a car from starting, but will make the car run terribly and probably stall.  If you have correct fuel pressure at the fuel rail, and you have spark, your car should at least start (even with a bad fuel injector).  If it’s not starting, then it’s time to move on to the dreaded step 3.  Next week, we finish this basic 3 part guide by addressing the third thing your engine absolutely needs to start, COMPRESSION.  See you then!

Pulse Width Modulated Fuel Pumps

Special note: Many newer cars equipped with direct injection will not be able to use the conventional diagnosis steps above.  PWM Fuel Pumps do not have fuel pressure regulators or return lines, because a special fuel pump control module is utilized in conjunction with a pressure sensor in order to allow the pump to supply fuel based on demand.  The fuel pressure is monitored via fuel pressure sensor built into the fuel delivery system, and the fuel rail will not feature a Scrader valve.  Instead of supplying a constant 12v to the fuel pump, the computer will actively vary the amount of voltage based on required duty cycle.  That way your pump isn’t running at 100% all the time like conventional systems.

DO NOT UNDER ANY CIRCUMSTANCE attempt to loosen fasteners in the fuel system of a direct injection car, as these systems operate at thousands of PSI.  Doing so could be potentially catastrophic.  PWM fuel systems on direct injection cars can only adequately be diagnosed using a dealer scan tool because of the variables built in.



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