Fuel Trim: Finding a Vacuum Leak and Performance Issues Using Fuel Trim Data

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Fuel Trim: Finding a Vacuum Leak and Performance Issues Using Fuel Trim Data

Fuel control driveability complaints and the various issues that accompany them are a frequent concern in most repair shops. Are you faced with a customer’s vehicle performance concern? Or a trouble code and an illuminated SES light? Or is there a combination of these factors? Verifying that a vehicle is in fuel control is important and looking at the fuel trim values can be a valuable diagnostic tool to finding and solving your customers’ driveability issues. Here’s what you need to understand fuel control.

A customer doesn’t generally care about whether their vehicle is in fuel control, but they certainly care if it’s not running correctly or the service engine soon (SES) light is illuminated. Accurate fuel control is needed to maintain the correct air-fuel ratio (AFR) that is supplied to the engine’s combustion chambers for ignition.

Without proper fuel control engine drivability and emission control can suffer.

Fuel control provides easily understandable data that can be a valuable tool in diagnosing a drivability issue or concern. But fuel control is often misunderstood and not used properly.

Do the math

When gasoline is the fuel being used, the optimum AFR or stoichiometric AFR is 14.7:1, which means 14.7 parts of air to 1 part of gasoline. This AFR ratio will change with the fuel type: E85 fuel has a stoichiometric AFR of 9.75:1, if the gasoline contains 10% ethanol the stoichiometric AFR is 14.04:1.

Today’s powertrain control module (PCM) is going to try to maintain the AFR as close to stoichiometric as possible to keep the catalytic converter at optimal efficiency, limiting emissions, and provide the best drivability to the driver.

Catalytic converters function most efficiently when the exhaust gases coming from the engine are within about 4% of stoichiometry.

But the perfect AFR isn’t always best for proper engine performance and operation. Acceleration requires a richer mixture (12.5:1 to 14:1 AFR) for maximum power and the PCM may also request a richer mixture to prevent detonation under certain conditions.

To maintain the perfect AFR, the PCM will use a set of factory engineer-created fuel maps or values stored in its data base that will detail how much fuel should be injected for a given load, rpm, temperature, etc.

The PCM will operate in two distinct fuel control modes: open or closed loop. During open loop, the PCM has limited feedback to achieve the proper AFR, but when in closed loop the PCM will work continuously to stay as close to the proper AFR as possible.

The PCM uses sensors in the exhaust that will report whether the desired AFR was attained (this is fuel control) when it is in closed loop. If the desired AFR wasn’t achieved the PCM will decide what corrections are needed to correct the AFR (this is fuel trim). The goal of the PCM is to maintain the appropriate AFR, and by using its fuel maps as a starting point, it will calculate the specific amount of fuel to inject. But engine wear, faults and parts issues can occur throughout its operational life, and when this happens it will affect these calculations, so adjustments need to take place.

Fuel trim is the PCM’s adaptive strategy that allows it to deal with these issues by adjusting the amount of fuel that is injected into the engine, but only once it is in closed loop operation.

Short- and long-term fuel trim

Fuel trim’s adaptive strategy consists of two components: short-term fuel trim (STFT) and long-term fuel trim (LTFT). In the European world, these terms are known as additive for STFT and multiplicative for LTFT. On V configuration engines there will be two complete sets of STFT and LTFT data that will pertain to each individual bank of cylinders, even some four-cylinder engines will have two sets of data separating the cylinders into pairs. Doing this provides better fuel control and also aids in diagnostics.

STFT is the direct result of the feedback that the oxygen or air/fuel ratio sensors are providing to the PCM. This current, real-time information is used by the PCM to adjust the amount of fuel that is being injected into the engine. STFT adjustments happen almost immediately, but these changes are just temporary.

LTFT is the accumulated information that is recorded over time. This historical information gathered from the STFT information is stored in a data file system known as fuel trim cells. These cells hold fuel trim information over a wide range of rpm’s and engine loads and are used by the PCM to make quick decisions when the engine load changes. LTFT is a slower reacting PCM-based calculation that has no direct relationship to an exhaust sensor.

Fuel trim numbers are displayed as a positive (+) value or a (-) value. These values reflect the amount of change that the PCM is applying to the amount of fuel the engine is receiving as the PCM attempts to maintain the stoichiometric value of 14.7:1.

Fuel trim information is of enormous value when it comes to many diagnostic decisions. OBD-ll fuel trim data can provide us with a starting point to diagnose issues and allow the correction of them, if we properly interpret the data and understand how fuel trim reacts to different situations.

Lean codes and the drivability issues that accompany a lean running condition would be reflected with positive fuel trim numbers: an indication the PCM is adding fuel. In this situation we need to concentrate our efforts on why the lean condition exists: vacuum leaks, fuel pressure, false air, skewed sensor data, for example, should be looked at.

Rich codes and the drivability issues that accompany a rich running condition would be reflected with negative fuel trim numbers: an indication the PCM is subtracting fuel. Leaking injectors, broken fuel pressure diagrams, improper fuel pressure and skewed sensor readings are a few examples of issues that could result in a rich running condition.

When observing the fuel trim data of a vehicle for diagnostic purposes, the total fuel trim of the engine must be calculated. Total fuel trim is an additive value: STFT+LTFT=Total Fuel Trim. On V style engines there will be a total fuel trim for each operating bank of the engine. Total fuel trim spread should also be looked at and should not exceed 10% (-5% STFT plus +5% LTFT= fuel spread of 10%).

For many years the rule of thumb was +/-10% total fuel trim for most North American vehicles (European vehicles with traditional O2 sensors were lower at +/-5%). This value has changed with the predominant use of air-fuel ratio sensors that provide faster more accurate results on exhaust gas oxygen levels even during acceleration and deceleration. Today a better rule of thumb is tighter: +/-5% total fuel trim.

NOTE: Many manufactures are also using rear fuel trim to further fine-tune their fueling strategy. This information may show up in scan tool data and can affect overall fuel trims, but the base STFT and LTFT need to be addressed first.

Fuel trim diagnostics

Here are two examples where fuel trim assisted in the diagnostic direction and repair.

2011 Chevrolet Cruze 1.8 LUW, 102,000 miles

The complaint was a rough idle, stalling at a stop and the SES light on. The car ran OK on the highway, but the customer wasn’t happy about the idle and stalling issues.

A scanner was installed and OBD-ll data observed; it showed only a DTC P0171. The freeze frame fuel trim data showed that the engine had been running lean when the code was set. The STFT was at +22% and the LTFT at +20%, and these trims were occurring at idle on a warmed-up engine. Adding STFT +LTFT = +42% of total fuel trim when the code was set; it was obvious the PCM was adding fuel to compensate for an issue.

A good visual inspection was performed; there were no vacuum lines that were cracked, broken or missing and the engine’s intake snorkel was attached and not cracked leading from the MAF sensor to the throttle body.

These engines are sensitive to un-metered air, loose oil filler caps and even a dip stick that isn’t installed properly or has a torn O-ring is enough to cause a fuel trim and MAF codes on these engines, but my visual inspection had turned up nothing.

At idle with the engine running in closed loop, the STFT and LTFT were both in the +20% area and the engine was missing, bucking and running rough to the point the SES light was flashing (although it hadn’t set any misfire codes).

But both the STFT and LTFT returned to almost normal values when driving and accelerating, just as the customer had described.

The information that I had gathered so far was leading me to believe that I had an issue with un-metered air in the form of a vacuum leak.

Why? Because the fuel trims data is pointing me in that direction... but it depends on the type of induction system the engine uses.

How does a vacuum leak affect a non-turbo engine’s fuel trim values?

During idle when the throttle plate is closed, the vacuum in the intake manifold is high and very little air flow is entering the engine, so even a small amount of un-metered air will have an effect on fuel trim. This forces the PCM to react to the lean condition with positive valued fuel trim readings. The fuel trims will return to almost normal during cruise and wide-open throttle when the throttle plate is open and there is little vacuum in the intake but lots of air flow into the engine. The small amount of un-metered air creating an idle vacuum leak is hidden or masked when the engine’s airflow increases, so the effect on fuel trim is less evident.

How does a vacuum leak affect a turbo-charged engine’s fuel trim values?

The use of a turbocharger changes the way an intake leak will affect fuel trim. If a turbo-charged engine has a vacuum leak after the throttle plate, the fuel trim numbers will be positive at idle (like a non-turbo engine), when there is no boost and a vacuum in the intake. But when the engine is in boost mode and positive pressure is provided in the intake, the vacuum leak will allow boost air to escape. This causes the fuel trim numbers to become negative during a boost situation. This happens because the PCM is supplying fuel for the amount of air the MAF sensor measured. But because all that air isn’t being supplied into the cylinders, the PCM is supplying too much fuel, creating a rich mixture. The fuel trims will react with a negative value.

Suspicious of an intake leak, I installed my smoke machine, but the intake was not the issue, neither was the brake booster or the hose. I tried to remove the dip stick to check the integral positive crankcase ventilation (PCV) valve in the cam cover for an issue and the cause of the problem was uncovered.

The diaphragm in the PCV oil separator assembly had ruptured, allowing full manifold vacuum to enter the engine (some manufactures will supply a crankcase pressure or vacuum readings for diagnostics, although GM doesn’t for this vehicle).

The PCV is integrated into the valve cover and is not serviceable, so a replacement valve cover was ordered and installed. I reset the fuel trims (I used a factory scan tool to do this, but disconnecting the battery will do the same) and road tested the car to verify. The result: The issue was resolved.

2001 Dodge Grand Caravan 3.3L, 246,000 miles

This minivan had an illuminated SES light, and the customer complained that the van sometimes ran rough when first started with the engine cold. The code retrieved from the PCM was a P0420, a catalytic converter efficiency code.

A look at the freeze frame data showed a STFT of -10% and a LTFT of +6% when the code was set. This shows a -4% total fuel trim, an acceptable value, but the spread between the STFT and LTFT is 16% and this indicates that the PCM is struggling to maintain overall fuel control.

Further inspection showed that fuel pressure was fine, temperature was OK, it entered into closed loop and that there were no vacuum leaks present. The only other thing that had to be verified before the installation of a new converter was the rough running issue when cold.

The van was parked outside overnight to allow a cold start. When started cold in the morning, the van cranked a long time and missed badly when it started. The miss didn’t last long, and the van ran OK after a few moments, but this condition had to be examined before a new converter was installed.

I shut off the engine and installed a fuel pressure gauge to investigate the hard start and rough running, being suspicious of a leaking fuel injector (the STFT was a clue to this as well indicating rich).

The fuel pressure check indicated a slow decay in residual fuel pressure and again this was another clue. In combination with the rough running cold dead miss, I was thinking about a bad fuel injector. I removed the plenum and fuel rail to inspect for a leaking injector and even without applying full fuel pressure the leaking injector was obvious. It was a slight drip, enough to drain the fuel pressure overnight, and cause the long crank and start-up dead miss.

But this leaking injector wasn’t bad enough to cause a continuous dead miss, or even set a misfire code once warmed up, but it did skew the fuel trims to the rich side, but again not enough to cause a rich code. This malfunctioning injector was continuously allowing raw fuel to enter the converter. This condition can cause the converter to become super-heated, beyond the normal operating conditions that a converter was designed for, resulting in damage to the internal substrate.

A new injector was installed along with a new converter and the memory in the PCM reset. The van started and ran smoothly, and the fuel trims were checked; STFT was +3% and LTFT was +2% for a total of +5% just fine for this vehicle, and providing a good exhaust gas for the new converter to function.

Fuel control drivability complaints and the issues that accompany them are a frequent concern in most shops. The complaint may be accompanied by a trouble code and an illuminated SES light, a performance concern or a combination of both.

Lean vs. rich

I have experienced far more lean running conditions than rich conditions recently. False air from internal valve cover PCV systems, especially on European cars, has become a frequent issue. I am starting to see purge solenoids that will leak vacuum after they have passed the actual OBD-ll purge leak test, and will be closed when you are testing them.

Verifying that a vehicle is in fuel control is important and looking at the fuel trim values can be a valuable diagnostic tool to find drivability issues. Using total fuel trim and fuel spread can point you in the proper direction for a successful repair that will restore the vehicle’s drivability and its emission controls.    ■

Jeff Taylor boasts a 32-year career in the automotive industry with Eccles Auto Service in Dundas, Ontario, as a fully licensed professional lead technician. While continuing to be “on the bench” every day, Jeff is also heavily involved in government focus groups, serves as an accomplished technical writer and has competed in international diagnostic competitions as well as providing his expertise as an automotive technical instructor for a major aftermarket parts retailer.

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