2004 Thor Infinity W22 8.1 gas fuel trim

[Patsslab]

Greeting all,
While driving my motorhome today I checked my fuel trim levels. I really don’t know what to make of the numbers so I thought I’d post them and get some thoughts
Idle
LFT 21.9 %
SFT 0.0 to -0.0 to 1.6

2500 rpm
LFT 5.5 to 7.8
SFT 0.8 to 4.4

Driving down the road the sum of the two stayed below 12.
Thoughts?

[Isaac-1]

High fuel trim at idle is often an unmetered air leak

[Arch Hoagland]

Have you ever cleaned your MAF?

[Patsslab]

I sprayed brake cleaner around the intake a couple weeks ago with no increase in RPM indicating I don’t have leakage around the intake.
I replaced the MAF in the last 300 miles.

[Ultra RV Products]

High fuel trims on both banks at idle that come down under load is almost always a vacuum/intake leak. I find the most common source is at the back of the intake manifold, there is usually an unused vacuum port that should have a rubber plug but over time it could have deteriorated and is now cracked or completely missing. Let us know what you find.

[David 70]

Which bank were these on? Bank 1 or bank2?

[Patsslab]

I didn’t see an option to check the banks individually. I’ll have to dig deeper into the functions of my sensor functions.
I will take a look at that rubber cap on the back of the engine.
I was having issues with the bank 2 knock sensor. I replaced that sensor just before I took these readings. From what I read yesterday it’s possible these readings are out of whack due to the bank sensor issue and will level out over the next 100 miles or so. Does that sound correct?
Thank you for the input.

[yeloduster]

Probably not. High LTFT at idle is almost always a vacuum leak. It probably will stay until you find the leak and seal it.

What scanner are you using to read LTFT?

[Patsslab]

Originally Posted by yeloduster

Probably not. High LTFT at idle is almost always a vacuum leak. It probably will stay until you find the leak and seal it.

What scanner are you using to read LTFT?

I am using a Blue Driver OBD II reader.

[David 70]

A thorough understanding of fuel trims is needed in order to diagnose the problem. There are a lot of different videos out there that explain it. I found one a year or more ago that was a real good one, but I did not save it. I don't know well enough to try to explain it on here, so I won't try.

[MoCoTom]

Originally Posted by Ultra RV Products

High fuel trims on both banks at idle that come down under load is almost always a vacuum/intake leak. I find the most common source is at the back of the intake manifold, there is usually an unused vacuum port that should have a rubber plug but over time it could have deteriorated and is now cracked or completely missing. Let us know what you find.

The fitting on the back of the manifold that Jon is referring to was the problem with mine. My fitting has two outlet nipples and the rubber cap on one was severely cracked so replaced and that brought the LTFT back to respectful levels. Further investigation I found the vacuum hose on the other nipple was dried and had cracked about six inches upstream. Once that was fixed I'm back to 3s-4s at idle.
Tom

[Ultra RV Products]

Here's a write up I did for the Workhorse Club newsletter a few years back that some may find helpful, I can also email in PDF format to anyone that would like:

Did you know that your 8.1L Vortec engine could be running dangerously lean or rich without setting a check engine light? Scary, I know! The DTC (Diagnostic Trouble Code) monitors that are present in a light duty vehicle that would normally set a check engine light don’t all apply to the medium duty chassis that your RV is riding on. The only way to check this is by viewing live engine data with a scan tool, such as a ScanGauge2 with the properly programmed X-Gauge commands. The most important parameters that indicate the status of the engine’s fuel & air system health are the fuel trims. A basic understanding of fuel trims can act as an alert to problems that may be otherwise undetected. It will also lead you to a quicker diagnosis as to what is causing the issue.

To understand fuel trims, you must first know a couple important automotive fundamentals. Your engine needs the correct mixture of air and fuel to run properly and meet emission standards. This is referred to as the stoichiometric air-fuel ratio, which in modern gasoline engines is 14.7 parts of air to 1 part of fuel. This is where terms such as “rich” and “lean” regarding the fuel mixture come into play. A mixture that is too rich has a lower ratio (ie 12:1) which means less air and more fuel. A rich mixture is less efficient but will generally produce more power and burn cooler. A mixture that is too lean has a higher ratio (ie 16:1) which means more air and less fuel. A lean mixture is more efficient but can also cause higher cylinder temperatures. A proper fuel mixture balance is achieved by a combination of engine computer settings and inputs from engine sensors.

The engine management system is comprised of many components but the most important to know about for our purposes is the engine control module, mass air flow sensor, manifold pressure sensor and the oxygen sensors. These sensors are designed to work in concert to correct for variations in conditions to maintain the stoichiometric air-fuel ratio. In order to maintain this air-fuel ratio, engine sensors monitor and send signals to the engine control module hundreds of times each second. The engine control module has “base-line” settings that it operates from, which are then adjusted and fine-tuned by the various engine sensors as the engine operates.

Now that we have a basic understanding of how the fuel system operates, we can move onto the fun stuff. There are two different forms of fuel trims, short-term fuel trims and long-term fuel trims. For our purposes, we will focus on long-term fuel trims as that is the most useful to monitor. Fuel trims are displayed as a percentage, ranging from negative 25% to positive 25%. A negative value indicates a rich condition and a positive value indicates a lean condition. In a perfect world, the fuel trims would always be 0%, however, this is not usual and there is a tolerance of +/- 10% that is accepted as normal. The 8.1L is a V8 engine, meaning it has two rows of cylinders, referred to as bank 1 and bank 2. Bank 1 is the driver’s side and bank 2 is the passenger’s side which directly correlates to the exhaust pipes coming off each side. When referencing bank 1 oxygen sensor, that is the driver’s side oxygen sensor. Since there are two banks, fuel trims are measured on both sides and displayed as bank 1 and bank 2 fuel trims.

Under normal conditions with a healthy running engine, the long-term fuel trims will oscillate constantly as the engine runs but should remain consistent within that window of positive or negative 10%. When the long-term fuel trims begin to creep into the double-digits, it’s time to investigate what is causing this. An entire article could be devoted to troubleshooting excessive fuel trims, so in the next Workhorse Club newsletter, we will explore different real-world scenarios, from detection of issue to resolution.

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In the previous Workhorse newsletter, I provided a basic overview of the engine fuel management system, how it adjusts in the form of fuel trim percentages and why they are an important indicator of your engine’s health. The main thing to remember from this article, if nothing else, is that the long-term fuel trims need to stay within + or – 10% of zero. This window of operation is ideal and means that the engine is typically healthy and running efficiently. When we fall outside this range, we must start looking for possible causes and their solutions.

A negative fuel trim value indicates that the engine is running rich and the fuel management system is “cutting back” the time the fuel injectors stay open to compensate and maintain the stoichiometric air-fuel ratio, which is 14.7 parts of air to 1 part of fuel. Inversely, a high positive fuel trim indicates that engine is running lean and the fuel management is keeping the injectors open longer to compensate. It is also important to note that the maximum compensation value is positive or negative 25%. In most cars and light duty vehicles, when this limit is reached, the check engine light will illuminate, and a diagnostic trouble code will be set in the computer’s memory for the fuel trim being at its maximum limit. Unfortunately for Workhorse owners, this diagnostic trouble code is not enabled in the engine control module (ECM) programming, so it will never set a check engine light or store a code alerting you that there is anything wrong. This is where monitoring the fuel trims with a live data monitor such as a ScanGauge2 becomes paramount in ensuring you are within the normal range of fuel trim operation.

There are many things that can cause an engine to run too rich or too lean but for practicality purposes we will focus on a common scenario of an engine running too lean. Bob has a 2003 Workhorse W22 chassis with 39,000 miles on the clock and he decides to purchase the UltraPower Custom Tuned ECM upgrade for his coach to get more power and improved shifting. When he receives his new ECM he notices that there is a separate device in the package called a CRecorder, and instructions for its use. The CRecorder plugs into the OBD2 port underneath the dash and records engine data as the vehicle is driven. We include the CRecorder with all our UltraPower ECM’s to ensure that the engine is running optimally and so that the customer realizes the full benefits from the upgrade. After Bob has run his coach on the first trip with the CRecorder in place, he e-mails the recorded data to us for analyzing. Upon review of the data, we find that Bob’s long-term fuel trims are at maximum +25% at idle. As you will recall, a high positive fuel trim indicates a lean condition. The oxygen sensors monitor the exhaust coming from the engine and are measuring too much oxygen and not enough fuel in the exhaust. The likely reason for this, is that the engine is receiving unmetered air, that is oxygen that is present in the engine yet did not get measured by the mass air flow sensor upon entry to the engine. The mass air flow sensor reports to the ECM how much oxygen is entering the engine and in turn the ECM commands the fuel system to deliver the matching amount of fuel to achieve the proper air fuel ratio of 14.7 to 1. From this information, we can deduce that the likely source of unmetered air is either a crack in the air induction system or a vacuum leak. We report our findings to Bob and let him know that he most likely has a vacuum leak as his fuel trims reduce to within specification once the engine is put under load. We instruct him to remove the engine cover and inspect his vacuum lines for any signs of leaks. Bob immediately spots an unused vacuum port on the rear of the intake manifold that has a rubber plug which has deteriorated over time and rotted away. A new rubber plug is installed, and a new recording is made with the CRecorder to be sent off for review. The new data looks much better and Bob’s fuel trims are now hovering between 2-4% at idle, just as they should be. Had Bob never been told that he had a vacuum leak, he would have continued to drive it this way and over time may have experienced an engine failure!

[TimLoon]

Jon, Thank you for the inciteful information.
I too have the high fuel trims at idle and had to cap off the vacuum ports recently. My LFT still runs in the teens on both banks at idle but settles down on the move.
I have not found any other vacuum leaks and not sure where to look next. I did note that when starting the unit the other day in the cold, I had quite a bit of exhaust coming out of the joints at the crossover pipe. Can that also cause this condition with fuel trims?

[Ultra RV Products]

Yes, it can. Any un-metered air entering the system between the MAF sensor and the oxygen sensors will effect the fuel trims. An exhaust leak right before the O2 sensors will present similar to a vacuum leak at idle.