FREE ASE PREP: Fuel Supply Problems A8/L1
If you are taking the smog course to renew your Smog Repair License here is another FREE ASE / Specific Diagnostic Course information Page.
There are several reasons for fuel supply problems either the vehicle is getting too much fuel causing a rich condition or to little fuel causing a lean condition.
When necessary when you are having a Fuel Supply Problems conduct a fuel pressure check against all data specifications or Mitchell on Demand and perform a fuel volume test.
This is the best way to check the fuel supply system and fuel pump!
A noid light checks for a injector trigger
The fuel volume test is conducted by cranking the engine with the ignition disable for 30 seconds, the fuel supply circuit should be able to fill a pint in 30 seconds.
The heart of any computer control circuit for fuel balance is the O2 sensor, the sensor helps balance the air/fuel mixture.
The O2 sensor is able to tell you if you have a vehicle running lean “low voltage reading” or rich “high voltage reading”.
A balance O2 sensor will move up and down above and below 450mv evenly indicating a good air / fuel mixture.
But in some cases especially when we talk about newer cars even a normal looking reading may not be reliable alone.
FREE ASE PREP: Fuel Supply Problems – fuel pressure:
Clogged fuel filter
defective pump relay
improper gauge connection
Bad fuel pump
Once in awhile I get a student who tries to be the Instructor and this particular student said aggressively, a clogged injector would cause no fuel pressure also. I tried to explain to him, this is impossible, then I told him after a hour back and fourth to set his ass down or leave.
Fuel System Doesn’t Hold Fuel Pressure:
After the key is turned off the fuel system is suppose to hold fuel pressure for a minimum of 15 -20 minutes depending on the manufacturer.
When you turn the engine off and your fuel pressure gauge indicates a loss of fuel pressure immediately, here are some of the systems you should be looking at.
External Fuel Leak
Fuel Injector leaking
Fuel pump check valve
Fuel pressure regulator
FREE ASE PREP: Fuel Supply Problems this an example of a Fuel Supply Problem:
If you have Short Term Fuel trim at cruise at +7% and the long term is +33%, that’s a total correction of 40%.
That’s a 40% fuel trim , which means that the PCM is adding 40% more fuel from base performance in order to get the O2 sensor to cycle.
So looking at the O2 sensor by itself will fool you, it has gotten me a few times, so if you see a high or low fuel trim and a varying O2 sensor the vehicle stills need further diagnosing for a Fuel Supply Problem.
FREE ASE PREP: Fuel Supply Problems – Problems that can shift the O2 sensor and fuel trim:
A mis-calibrated Ho2s “heated o2 sensor”
Air Injection faults
Upstream Air leaks
Fuel pressure regulator has a intake manifold vacuum leak,
Injector not pulsating – a easy check is with a noid light on some systems
A misfire on one or more cylinders that would cause the “PCM” power train control module to think the vehicle is lean resulting in a higher fuel trim than normal.
FREE ASE PREP: Fuel Supply Problems – Trim Interpretation:
A vehicle with a positive fuel trims at low and high speeds look for a vacuum leak or fuel delivery problem.
FREE ASE PREP: Fuel Supply Problems
Negative fuel trim at low speed and loads look for a leaky injector the injector pintle spring is no longer closing the injector.
leaky pressure regulator or an EGR stuck open if the vehicle has a MAP sensor.
If you have a vehicle with high Long Term Fuel Trim “LTFT” and high Carbon Monoxide “CO” this rich exhaust is being created by the fuel trim.
This could be to an O2 sensor ms-calibrated, an air leak upstream of the sensor an Air injection fault or a defective O2 sensor.
FREE ASE PREP: Fuel Supply Problems – O2 sensor:
The front O2 sensor should be able to switch from 200mv up to 800mv; a better range would be 100mv to 900mv without exceeding 1000mv.
If the front sensor voltage shows a low (lean)or non switching voltage it could indicate a problem with the sensor or a mechanical condition with the engine, for example, a vacuum leak, misfire or dead sensor.
If the front O2 sensor shows a high voltage (rich) or non switching voltage it could indicate a problem with the fuel delivery system also.
The front sensor should be able to switch from lean to rich or rise from 200mv up 800mv in less than 100ms.
Typically the rear sensor should produce a fairly steady signal, if the voltage is below 250mv check the activity by snapping the throttle or raising the rpm to 2,000 rpm’s, some movement should be noticed.
FREE ASE PREP: Fuel Supply Problems – What happens if you find a defective O2 sensor?:
As long as the switch time “front sensor” is less than 100 ms the sensor is technically good. However, if the upstream sensor is switching in 90ms with a downstream sensor switching in a better condition a false PO420 code may set for the catalytic converter.
A lazy front o2 sensor can and will set a false code “PO420” and turn on the Malfunction indicator light, “MIL”.
So it’s probably better to tell the customer to replace both sensors and if you’re like me, I’ll suggest to them to perform a tune-up for maintenance, before I change that expensive cat.
FREE ASE PREP: Fuel Supply Problems
The best method for checking a fuel pump is a performing a fuel pressure check, by watching the fuel pressure this can tell you if the vehicle is receiving too much fuel or not enough fuel under certain situations.
Remember the injectors pintles are close with spring pressure, the pulse width opens the injectors and fuel pressure delivers the volume of fuel through the nozzle.
Throttle body injection compared to multi-port injection are relatively low fuel pressure systems, there are some high fuel pressure systems, but most are low fuel pressure. Also the electric fuel pump pressure is independent of engine speed, meaning as speed increases ‘RPM” the pressure is independent.
Most fuel pressure regulators work from manifold vacuum, as vacuum drops fuel pressure should increase on most cars. If the fuel pressure leaks, the pressure will be higher than normal. A quick indication that the fuel pressure regulator is leaking is fuel in the vacuum hose. So pull the vacuum hose off from fuel regulator, and if you see fuel dripping from the hose the pressure regulator is bad.
If you ever have a fuel pump that doesn’t work when the ignition switch is turned on, but the fuel pump does operate when the priming terminal is energize, you should check the fuse first.
A noid light is another good tool, it’s a quick way to check injector windings if the windings have an open the light will not flicker. Also, if the light stays on and doesn’t flicker the Power Train Control Module “PCM” may not be triggering the injector.
A long crank time can be due to a defective fuel pump relay if the vehicle doesn’t start until the oil pressure light goes out check the relay. Also, leaky injectors can cause a long / hard start so in this case check the fuel pressure and watch for a drop.
Remember an injector can fail mechanically due to dirt in the injector nozzle for an example or a failed internal electrical problem such as open windings. Both conditions will cause a driveability problem or misfire code.