This is another in a series of articles that I am writing to help enthusiasts track the health of their engines. My experience is directly with the 2006-2008 Cayman S but frankly this discussion can be applied to the Cayman, Boxster, and 911. Previous articles in the series can be found here and here.
Today I’m going to talk about fuel flow. More specifically I will discuss the mundane (fuel economy) and the more exotic subject of Fuel Trims. First let’s talk about fuel economy.
FUEL ECONOMY
I would suggest that a simple indicator of engine health is your fuel economy. One of the digital readout options on the dash is average fuel mileage. Click the lower left stalk behind the steering wheel to select fuel mileage. If you have never “reset” this value, it will represent the cumulative fuel mileage since the beginning (or since you last disconnected the battery). You can reset this value at any time you want. The same stalk controls the process. If you haven’t done it before, follow the instructions in your Owners Manual. When reset, it will jump to some crazy value but as you drive a few miles it will settle in to a new value. My advice is to find a stretch of road where you can run 5 miles or so without stopping, reset the value, and see what it settles into. The number will of course be dependent on some variables but for my car, it settles in around 25 mpg at highway speeds. Gathering this baseline data doesn’t take any special equipment and can be done multiple times. In Tech Talk below, I will get into variations that might indicate a problem.
FUEL TRIM
Checking Fuel Trim requires a diagnostic device. For instance, it is readily available on my Durametric device. I am going to link a video by Tony Callas below, where he goes into some nice detail. Durametric reports various fuel trim values, depending on the generation of your car. The video liked below goes into quite a bit of detail explaining the various terms used over a range of model years. Interestingly, my 2006 car lists RCAT and Fuel Trim Mean Value, whereas my 2007 car lists RCAT, FRAU, and FRAO. In the video Tony describes expected values. From a standpoint of baseline values, you should just denote the values being reported by Durametric and save them where you can refer to them later. These values are stored in the engine management software. Pulling them down with Durametric or other software does not even require starting the car. You’ll just plug into the OBD port, turn the ignition on, and read them from Durametric Actual Values. I would note that the values can be erased if the battery is disconnected. If this has happened, they will be regenerated in fairly short order. This does not affect the operation of the car
TECH TALK
Tony’s video gets a little bit long winded but my number one take-away is that the most important indicator of engine health is the Long Term Fuel Trim at Idle, RCAT. It is a percent value that can vary between +/- 6%. Tony indicates that he would be concerned if it is greater than +/- 3%. I am not going to go into problem solving in these articles, as that can get to be quite complicated. I am going to add just a little bit more technical background, derived mainly from Porsche technical training documents.
Since the 1980’s Porsche engines have used electronic fuel injection. Very simply, the amount of fuel delivered by each injector is controlled by an Engine Control Unit (ECU), also known as a Digital Management Unit (DME). Things have come a long ways from the 80’s until now. Looking at the Cayman engine from 2006 to 2009, it uses Bosch DME 7.8_40. Inputs are measured such as engine speed, engine load, air temperature, coolant temperature, and throttle position. From these, a “map” is entered that selects the best fuel injector duration for the given conditions. The maps are developed on test engines by Porsche, where the best injector fuel flow can be selected for a wide range of operating conditions. Similar maps are developed for spark control, variable valve timing, etc. These maps are very good but as stated in the Porsche literature, engine hardware systems can degrade with use and age. To compensate, “adaptation” software is included in the DME. One primary adaption is Fuel Trim. Basically, Porsche adjusts the map values to accomodate aging. As discussed by Tony, this may be a “additive” value at idle or a “multipler” value at higher engine speeds. I found the following in the Porsche literature.
RKAT – Mixture adaptation close to idle speed range,
mass air flow of approx. 12 to 32 kg/h, new systems: 0.0
+/– 6.9 %; older systems: 0.0 +/– 4.5 %
FRAU – Mixture adaptation in lower load/engine speed
range or partial load, mass air flow of approx. 32 to 220
kg/h, control range: 0.70 to 1.30, mean value: 1.00
FRAO – Mixture adaptation in upper load/engine speed
range, mass air flow of more than 220 kg/h; control
range: 0.70 to 1.30, mean value: 1.00
If the lambda controller must correct a value that is significantly
different from 1.00, the corresponding mixture
adaptation range will be corrected during the next mixture
adaptation until the lambda controller moves back to
around the mean value 1.00. Mixture adaptation in newer
systems is divided into the three ranges RKAT, FRAU and
FRAO depending on the intake air mass.
For instance , on my 2007 car, RKAT is showing as 1.172% on Bank 1 and 1.734% on Bank 2, which appears to fall well within the normal operating range. Apparantly, if the ECU reaches a RCAT value of 6%, a check engine light will be turned on. But again, from an engine health standpoint, if your RCAT is within +/- 3%, you don’t have any significant issues with your fuel flow.
I said I wasn’t going to get into diagnosis but I will point out that Tony said that when the RCAT is out of range, he suspects system vacuum leaks and when the FRAU or FRAO values are out of range, he suspects fuel delivery issues, such as fuel pumps and injectors. Fortunately, my FRAU and FRAO values were very close to 1.
I hope this all helps. In my next article, I hope to take a look at using the Durametric to look at the health of your Variocam +.
New Hill Garage 
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