Staying Trim

Trouble codes and driveability complaints stemming from poor fuel trim are among the more common issues seen in most repair shops.

It could be a P0171 in a Toyota that has become a pattern failure. It could be a troubling 18619 (P2187) code in a VW that otherwise appears to run fine. Or it could be a false P0420 caused by bad fuel trims that even a new catalytic convertor won’t fix.

Screen capture of a 2006 4.6 L Lincoln Town car with 256K on the engine. You can see the fuel trims for both banks. Add the values together and this vehicle is approaching what I think of as the limit.

Some of these codes can be quickly diagnosed… while others can be a real nuisance.

The main issue is to figure out if you have an actual fuel trim problem or a fuel control issue.

The first part of your diagnostic is to verify whether the vehicle has effective fuel control. The fuel control system includes all the parts of the engines that try to keep the exhaust gases flowing into the catalytic convertor at the proper stoichiometric ratio of 14.7:1. This is important because if the gases aren’t at that ratio, the resulting emissions are going to cause pollution. And that’s bad.

The two common fuel control methods used by manufacturers are: speed density and mass airflow. Both systems estimate the engine’s load to provide the correct fueling to maintain proper stoichiometric ratio. The speed density system uses manifold absolute pressure sensor (MAP), barometric pressure sensor (BARO), and engine speed (RPM) to calculate load. The mass airflow system uses the mass airflow sensor (MAF) in conjunction with RPM to perform the same task.

Volkswagen/Audi 2.0L PVC system. It’s a very complicated oil separator diaphragm style PVC on this turbo engine. Not only did this thing whistle like mad, but it set the fuel trims wrong and was causing misfires.

Both systems also involve temperature sensors, air/fuel (A/F) or oxygen sensors, and many other sensors. If issues are found in any of the base components for proper engine performance, these must be fixed first. They are the PCM’s fuel control inputs. The components all have to be working as designed or the proper fuel control can’t be attained.

In basic terms, the PCM is programmed to know how much fuel to inject under a given set of ideal circumstances in order to provide a perfect mix of exhaust gases for the catalytic to deal with. But we all know that this doesn’t always happen. Engines age, oil gets dirty, and things start to leak, so the PCM will have to add or subtract fuel to achieve and maintain the 14.7:1 air/fuel ratio.

How does the PCM do this? Well, it looks at the values that are being provided by the A/F or O2 sensors. If they show that the exhaust is rich (below 14.7:1), it will decrease the amount of time that the injectors are on. If the A/F or O2 sensors show that the exhaust is lean (greater than 14.7:1), the PCM will increase the injectors’ pulse width and keep them open longer.

The A/F sensor (left) and O2 sensor tell the PCM what is happening in the exhaust after the gasoline has been burned.

This process of constantly trying to reach the balanced value of 14.7:1 is what we’re seeing as fuel trim numbers.

The results are presented to us as short-term fuel trim (STFT) and long-term fuel trim (LTFT). There are many different names used in the industry. Remember that each bank that has its own O2 or A/F has a set of fuel trim values associated with it. (Yes, even rear O2s have some effect on certain vehicles.)

The LTFT on a good running vehicle should always be fairly close to zero and steady under most driving situations. The STFT will move quickly on an O2 system and will usually follow the output of the O2 sensor as it switches from rich to lean. The A/F sensor STFT is usually steadier and reacts much quicker and over a wider range due to the sensors’ ability to detect O2 levels faster and more accurately.

Looking at fuel trim numbers, there are a couple rules of thumb that I have used over the years with good success.

This purge solenoid valve was allowing fuel vapors into the engine when it was not expecting anything from the purge system. The result was a P0174 rich code.

The first rule is to add both the STFT and the LTFT together to get a single number. This is the total fuel trim that is going on in one set (or bank) of monitored cylinders. This will work on most vehicles, but not all. Nissan, for example, splits its four-cylinder engines into two banks.

So, let’s say you have a STFT of +4% and a LTFT of -3% adding them together you have a combined fuel trim of +1%. I like to use a +/- 5% on all newer vehicles. Older vehicles – late 1990s to mid-2000s – could do OK with a +/-10% most times, unless you’re dealing with a European vehicle, then go back to the +/-5% rule.

The other thing to note is that almost all of this fuel-trim information will be displayed in the OBD-II data, so generic scanners will usually do just fine. (I actually prefer OBD-II information over the factory scanner when looking at fuel trim issues because I don’t have a factory scanner for very make that comes in, and it’s OBD-II that’s turning on the light).

Close up of a MAF sensor. The red thing is the air temp resistor and the actual sensing assembly is under it. We had two of these come in the same day that had Chestnut Tree flowers stuck on them. I carefully removed the stuff and the fuel trims and running issues were solved.

So now that we’ve covered the difference between fuel control and fuel trim, let’s go a little deeper and see how this understanding will make some trouble codes and driveability diagnostics a little easier.

Fuel trim concerns can be triggered by a number of factors. When I diagnose fuel trim issues, I break the system down into specific areas to aid in my diagnostics.

THE BASICS

The engine has to be functioning correctly with proper compression and timing, and with no mechanical issues. The exhaust system has to be leak free and not restricted. A restricted exhaust will affect the way air flows through the engine and affect its volumetric efficiency. That’s a factor that is programmed into the PCM’s fueling calculations. Restricted exhaust system on GM pickups set lots of fuel trim codes before the lack of power complaints are raised.

PCM DATA

You need to be sure the data that the PCM receives from its input sensors is correct. MAF sensors get dirty and they do fail. MAP sensor hoses get cracked or become loose. Thermostats go bad. A/F sensors wear out. And O2 sensors slow down with time or when exposed to contaminants. There’s also the dreaded possibility of connection corrosion that doesn’t allow a sensor to perform in its full range.

FREEZE-FRAME DATA

Different styles of MAF sensor. I have a huge collection of these high-failure items. If the customer has over-oiled their fancy high-output air filter, oil will often cover the sensor assembly, causing a fuel trim issue.

Freeze-frame data provides valuable clues on when the issue occurred. Certain Toyota and Ford engines have trouble with unmetered air/vacuum leaks (air that is not measured by the MAF) only when cold. It rarely will show up with a lean running condition when you’re given the vehicle to work on. Look closely at the IAT and ECT for when the code was set. If it was when the engine was cold and it’s a high-kilometer vehicle, chances are good that you have a rock hard plenum gasket giving you grief. It also helps that these are huge pattern failure vehicles.

On BMWs, be aware that they are also having issues with V8 intake gaskets.

Be sure to look at what RPM the code set at. Vacuum leaks usually show up only at low RPM or idle. Bad MAF sensors traditionally underestimate the amount of air at higher engine speeds or cruise RPMs so this is when the fuel trim issue shows up.

LEAN CODES

MAF system lean codes are very prevalent. Unmetered air can be an issue, caused by vacuum leaks, torn intake ducting boots, leaking EGR valves, or leaking intake manifold gaskets. The PCM doing the fueling calculations is counting on the engine package performing as it was designed too; it’s assuming that the MAF is measuring all the air being used in combustion.

If you have a newer Chevy Malibu in for service and don’t screw on the oil cap, or if you leave the dipstick unseated, chances are that this vehicle will show up with a P0171 in a matter of hours.

A new air intake snorkel for a Mini Cooper. This is another place that will allow unmetered air to enter after the MAF. It used to be a huge issue but the manufacturers have changed the plastics/rubber compounds so they don’t crack as much anymore. Nevertheless, installation errors and loose clamps continue to cause issues.

These engines are very tight and the PCM can’t physically tell that you didn’t install something correctly but it can tell that there is a fuel-trim issue because air is being used that wasn’t measured by the MAF sensor.

Make sure to pay particular attention to the vacuum diagram under the hood. Most VW/Audi vehicles have the evap leak detection pump mounted in the rear fender area and that giant vacuum leak that you’re looking for may be a long way from the engine.

UPSTREAM/DOWNSTREAM

Exhaust leaks in front of the A/F or O2 sensor provide false air to that sensor that wasn’t measure by the MAP or MAF. By way of example, a 2007 Hyundai Elantra will set a P2095 lean code when the flex pipe has a crack upstream of the downstream O2 sensor.

FUEL SUPPLY ISSUES

The fuel pump has to deliver the correct pressure and volume of good fresh fuel to be burned. Plugged injectors, fuel filters and weak pumps can cause a lot of fuel trim issues.

OVER-FUELING

If the code shows a rich condition, over-fueling could be causing the engine to get more fuel than prescribed. Leaking canister purge valves, lack of maintenance, and contaminated engine oil are all potential issues. But there still could be a dripping or inaccurately functioning fuel injector.

WILD CARDS

A Ford fuel pump driver module that is seriously corroded. Proper fuel pressure and volume is often overlooked due to the difficulty of checking it.

There is no shortage of other issues to look for.

For one thing, check the PVC system. Many vehicles no longer have the traditional PVC valve. Most now have some form of oil separator and a complicated fixed orifice and diaphragm system. The extensive use of turbos has led to a huge increase in this style of PVC system and they’re starting to cause not only fuel-trim issues, but misfire codes, noises and driveability issues.

Using a vacuum gauge on the dipstick tube if it has one, or on the oil filler cap is the best way to check excessive crankcase vacuum problems. (As a rule of thumb, 1” Hg is about normal at idle).

The PCM may need to have a software update. If after all the above has been checked and nothing found, spend the time to look for software updates that address the fuel trim issue. Manufacturers and vehicle engineers don’t always get every possible scenario right the first time. They may update the software to solve the problem only after seeing a particular problem many times.

A bad air snorkel that was setting lean codes.

Anyone that has been repairing vehicles for some time will understand that I’ve only touched the high spots when it comes to effectively diagnosing a fuel trim issue, but the basic principles are going to be the same on almost all vehicles that we see.

Understanding how the vehicle is controlling fuel is the first step in diagnosing a fuel trim issue.

Then start covering all the basics and making sure the engine has all the correctly installed and properly functioning pieces and parts.

Only then can you perform the detective work necessary to uncover the reason the engine controller is making excessive changes in the fuel trim as it attempts to hit the magic ratio of 14.7:1 where the catalytic convertor performs most efficiently.

Jeff Taylor is senior tech at Eccles Auto Service in Dundas, Ont. You can reach him at jeff@ecclesautoservice.ca.