Fuel system diagnostics

Fuel delivery, fuel condition, and fuel volatility are an important part of today's vehicles.  Getting the fuel to the engine is a lot different today than it was back in the days of carburetors.  The only thing the fuel had to do then, was make it from the fuel tank to the carburetor by means of a mechanical pump.  Today, it's high pressure pumps, multiple volume controlled fuel pumps, and varying fuel pressures to accommodate engine loads.   With all of this going on the fuel has to be monitored constantly to insure of it's condition.  These days, the fuel delivery system  is monitored and the actual condition of
the fuel lines, the fuel tank itself, as well as the gas cap are
all a part of the evaluation conducted by the PCM.  Emission
systems have come a long way from the first attempts at
environmental controls.   The very first systems merely
pulled vapor out of the tank by way of a vacuum system and
fed the hydrocarbons into an activated charcoal canister
system for release into the atmosphere. Now, we not only
accomplish that task, but we verify the quality of the job the
system is doing and the integrity of the components in the
emission system.

It's almost impossible to talk about fuel systems without getting into EVAP systems as well.  EVAP codes are a very common occurrence.   P0455  is one of the most common.  “Gross” meaning “large” leak can be an indication of a leak or more than likely... operator error... (forgetting to put the gas cap back on correctly.)  P0457 is the more direct code for the fuel cap not installed correctly but either code really means the same thing... a large leak.  
Using this example; a 2003 Mazda Tribute,  if a P0455 is set the EVAP monitoring system will shut down and not run its self test.  A secondary code is usually associated with it (DTC P1450 Unable to bleed-up fuel tank vacuum).  The EVAP running loss system monitor will abort and not continue with the leak check portion of its test. If the leak is taking care of (IE.. put the gas cap back on) and the target vacuum is obtained the change in the fuel tank vacuum (bleed-up) will be calculated for a calibrated period of time.  The calculated change in fuel tank vacuum is compared to a calibrated value preset in the PCM for a leak as small as .016 mm (.040 inch) in the EVAP system.  If it is less than that the EVAP will pass the test.  If it fails the PCM will attempt to run the monitor at  least three times before setting the code again.  DTC P0442 (small leak)   These small leaks can be hard to find.  A smoke machine works well in these cases.  I prefer a smoke that uses a UV dye, but a good smoke machine with just plain smoke will do the trick too.

Here is a list of some of the common EVAP codes
     P0440 -    general EVAP failure
P0441 –     incorrect or un-commanded purge flow (leaking purge valve)
P0442 -    small to medium leak detected
P0443 –    Purge solenoid electrical fault
P0446 –    Blocked canister vent (high system vacuum)
P0449 –    canister vent solenoid electrical fault
P0452 –    tank pressure sensor voltage low
P0453 -    tank pressure sensor voltage high
P0454 -    tank pressure sensor voltage erratic
P0455 –    large leak detected
P0456 –    small leak detected
P0457 -    gas cap is not sealing

Most vehicles today will have some sort of fuel tank pressure sensor,  or in some cases it will be called an EVAP pressure/vacuum sensor.  The FTP (fuel tank pressure) sensor is used to measure the pressure during the EVAP monitor test.  When the PCM receives a signal from the FTP that there is excessive tank pressure or vacuum the PCM will then send a signal to the EVAP purge solenoid to correct the fuel tank pressure level. 

On Toyota's the FTP sensor should read around 3.3 volts with no vacuum or pressure applied to the sensor.  The sensor is a three wire sensor with one wire as a ground, one as the incoming reference voltage (5 volts) and the sensor wire to the PCM.  Codes for tank pressure are generated by the results from the FTP sensor voltage which the PCM then interprets into the corresponding code. (service code).  When the PCM requests the EVAP solenoid to run it's monitor of the tank pressure it uses the FTP sensor as the determining factor of the actual tank pressure.

Running these EVAP monitors are accomplished two different ways.  Ford, GM, and some Asian imports use vacuum to run the monitor, while Chrysler, most European and a few Asian products use pressure to run the monitors. There are a lot of factors that have to be correct in order for the PCM to carry out the EVAP monitor.  The ambient air temperature needs to be between 40 and 100 degrees (for most vehicles) and the fuel level needs to be near a half of a tank full.  (The air temperature in the tank will effect the fuel vapors.) The barometric pressure must fall between 22 in. Hg on Fords and 19 in. Hg on most GM vehicles.  (8,000 to 10,000 feet above see level).  Most of the systems are set up to only run the test with the vehicle under 35 mph  so that tank slosh is to a minimum. The last but most important thing is there cannot be any other fault codes stored in the PCM.  Since the job of the EVAP system is to reduce emissions it stands to reason if there are any emission related codes stored in the PCM the eventual outcome of the EVAP test would be inaccurate.

On vehicles such as Chrysler that use pressure to perform the EVAP monitor test a small vacuum operated air pressure pump is used.  The LDP (leak detection pump) pump uses vacuum to pull a small diaphragm up and down, then releases the vacuum to make its opposing chamber draw in ambient air and then force that air into the fuel tank system.  A small reed valve allows the air only to go in one direction which allows the internal pressure to rise in the fuel tank system.  These systems are called “return-less systems” (no fuel travels back to the tank) and are widely used on most of today's modern cars.   Generally it's a small amount of pressure that is applied (.25 psi or more) but it's enough to run the EVAP monitor.  The PCM has a preset calculation for the amount of bleed down time it will look for to determine the actual fuel tank pressure. 

One other important part of the EVAP system is the
charcoal canister.  This is where the fuel vapors are stored.
Charcoal is made by heating wood to around a 1000 degrees
in a sealed container.  With no oxygen present during the
burning of the wood, it really doesn't burn but all the
organic compounds are driven off  leaving nothing but the
carbon itself.  To make it into “activated charcoal” 
(the kind used in a charcoal canister) the “carbon” is put
under pressure with oxygen present.  This expands the
pores of the carbon molecules creating “activated charcoal”. 
Gasoline vapors adhere to the charcoal granules like static
electricity clings to a balloon.  A light breeze across the charcoal
pulls the fuel vapors (hydrocarbons) off and cleans the activated charcoal for the next volume of fuel vapors.

Fuel temperature sensors are quite popular now too.  A fuel temperature sensor does as the name implies .  This is another reading the PCM will use to determine if and when it is appropriate to run the EVAP monitor.    On some models you'll even find a FRP sensor (fuel rail pressure sensor) (found on some newer Ford models) this sensor is actually reading fuel rail pressure just as it would be with an external gauge. You can read the fuel pressure without having to hook up a fuel pressure gauge to the line.  Installing a fuel pressure gauge to the line and comparing it to the reading on the scanner is a good way of insuring the FRP sensor is reading correctly.

A lot of times the fuel system is blamed for no start, hard to start, and even rough running when in fact it can actually be related to other components in regards to engine operation.  One very common problem tends to be the MAF sensor.  I've had several vehicles where the car seemed to run just fine when you first started it up but soon after would cough, lose idle, and stall.  Fuel pressure is good, spark is good, and the fuel filter is clean.  My favorite test for this problem is to use the tools I've got handy.  A multimeter with a “min/max” recordable feature is my choice.  (There are other ways to accomplish the same task but this is the one I prefer because it works on every type of MAF sensor ever made.)  Out of the 4 wires that run to the MAF, one is ground, one is battery voltage, one is 5 volt reference, and the last wire is the sensing wire.  Hook the meter onto sensing wire and ground the other lead.   Record the voltage spike signal as you take the car from a standing idle to full throttle with a quick snap of the throttle.  Once from idle straight to full throttle is all it takes.  The record feature on the meter will record the voltage increase as a graph on the screen showing you the actual voltage.  Some Toyota's (V6 and 4 cyls.) will only make it up to 3.8 volts... this is considered a  good MAF sensor but only on these engine types.  If the sense voltage doesn't reach 4 volts the MAF sensor is bad.  Simple, but effective test.  If you run across a MAF that has 5 wires, the extra wire is usually the air intake temp sensor.  If your unsure which one it is try the snap test anyway,  obviously if you snap the throttle open and the voltage didn't increase it's probably the wrong wire to be checking. (air temp sensor voltage won't change from idle to full throttle.)

One other part of the fuel system that can be a little tricky to understand is how some of the manufacturers have used the dual fuel tank systems that are found on full size trucks.  Instead of two separate tanks these tanks have now become combined together.  On a 98 GMC  for example, – A short to ground in the sender or wiring will provide a fully empty reading.  An open in the circuit will produce a full reading on the dash gauge. 

These dual tank vehicles use a fuel pump balance module that measures the resistance of both fuel tank sending units. The module will average the resistances and produce an equivalent resistance reflecting the average of both tanks. Which is what you will see as a gauge reading on the instrument cluster.  

The balance module effectively equalizes the level in the two tanks by way of a pump balance relay.  At any time the rear tank has about 2 more gallons than the front tank the module will turn on the relay and send fuel to the front tank until the front tank is within a 2 gallon range.  (If there is around 2 gallons in the rear tank and the front is empty (or near empty) this process effectively will empty the rear tank first.  (I've had customer tell me they filled the rear tank but the gauge still shows ½ tank.  Because their front tank was completely empty.)  You get a lot of puzzled looks from the customer when you tell them, “That's OK, it's actually correct.”

More and more you'll find the fuel tank readings, fuel line pressure, and the EVAP sensor values  on a scanner making the whole process of evaluating fuel delivery problems easier to understand.  Diagnosing the systems takes a bit of background in understanding the way each individual system actually operates per manufacturer.  Before attempting any repair on a fuel system always make sure to follow the guide lines from the manufacturer.  In parts of the country where high levels of salt are used for snow removal be aware that a lot of vehicles mount the charcoal canister, EVAP purge valves and other components under the vehicle and can become susceptible to failure due to corrosion from the salt or even damage from impacted snow under the vehicle. 
It's all about having the right information.  Learning the way these systems work will greatly enhance your understanding of how to repair them.   In the business of automotive repair... you can never know too much, and there's always something new to learn. 

Fuel Tank Pressure Sensor  
                       typical charcoal canister