Diagnosing automotive intoxication in a Chevy Avalanche

Jan. 26, 2017
A 2008 Chevy Avalanche (Figure 1), with a 5.3 liter automatic engine, was setting codes P0172 and P0175 (System rich, banks 1 and 2).

A 2008 Chevy Avalanche (Figure 1), with a 5.3 liter automatic engine, was setting codes P0172 and P0175 (System rich, banks 1 and 2). This vehicle, which had 94,000 on the odometer, had this problem for well over a year according to the vehicle owner and there were times the spark plugs would foul to a point where the engine would experience an engine misfire. The fix at that point was to pull the spark plugs, replace them, clear the codes and head down the road for another few months. The shop had already replaced the MAF sensor and a suspected #1 injector thinking the rich condition was being caused from a shift in MAF calculations or a leaky injector on the #1 cylinder.  This however did not resolve the customer's ongoing issue.

Figure 1

The owner of the vehicle was very anxious to get the car resolved especially with an overdue inspection sticker on the windshield. The truck was not running well, and he thought he now had cylinder head problems because of all the carbon buildup from the prolonged rich operation and add to the fact that many people had told him that these trucks were known for bad valve guides. All the avenues had been exhausted trying to  resolve his problem so he convinced the garage to perform a valve job on his truck. The garage was against it but the customer was persistent and willing to foot the bill so the cylinder heads were pulled to have a valve job done. The heads were visually examined and the shop was able to see the carbon buildup on the #1 cylinder but the other 3 cylinders on the same head did not seem bad (Figure 2). The spark plugs that came out of the engine did have black carbon on them but the #1 cylinder showed heavier deposits. The shop sent the cylinder heads out for repair and by later in the week the truck was reassembled and released to the customer.

Figure 2

Well, that didn’t work.

The valve job did not resolve the issue and the customer was soon back within a week with the CEL lamp on again. At this point I was now thrown into the mix to unravel the shop's dilemma. When I arrived at the shop I hooked up my scan tool to check the fuel trims with the engine fully warmed up. This would give me an indication of what the ECM was actually doing to control the fuel. The engine was not running smooth and it had a slight rough idle. The Long Term Fuel Trims were down around negative 26 and yet both the upper O2 sensors were still seeing a rich operation (Figure 3). The ECM was having a hard time in acheiving O2 switching.  The whole concept of fuel control is to achieve a constant high to low switching state above and below 500 mv. The Short Term Fuel Trim will constantly move inversely l to the O2 sensor. When the O2 goes low the Short Term will go positive. When the O2 goes high the Short Term will go negative. The Short Term will eventually drive the Long Term Fuel Trim slowly to a final acceptable value of plus/minus 5 percent to 10 percent. Once the Long Term trims exceeds plus/minus 15 percent the ECM will usually set a code on most systems.  I pulled the brake booster line to see if the O2 sensors would respond to a major lean condition to rule out O2 sensor failure and their readings did go below 500 millivolts. At this point I was convinced that excess fuel was coming from somewhere or the truck had a sensor that was out of range.

Figure 3
Figure 4

I next viewed some basic raw data with the engine running such as coolant and air temperature, Mass Air Flow, Throttle Position, Accelerator Position, Manifold and Barometric Pressures to see if everything was reading within reason (Figure 4). At idle, I typically expect 1 gram per liter of engine displacement per second for the MAF reading. This was a 5.3L  engine, so the 7 g/sec at idle was not too far off and the other sensors all seemed to be within spec.  I was looking for something that could really add some fuel such as leaky injectors, high fuel pressure or a leaky purge valve. So I first disconnected the purge valve line at the left side of the engine just above the ignition coils (Figure 5) but this did not have any effect on the rich operation. I even placed a fuel pressure gauge on the rail and measured the pressure, finding it within the spec of 60P.S.I. and when I turned the engine off the fuel pressure held without any signs of leakdown.

Figure 5

I was puzzled to how this engine was getting more fuel then it could handle. I had to start thinking out of the box to find something out of the ordinary.  I did run across a vehicle a while back where a shop replaced a 5.3L ECM with a used 6.0L ECM. Both ECMs had the same hardware numbers but different software and the engine ran but was being over fueled. I pulled up the ECM info on the scan tool (Figure 6) but the VIN and software for this vehicle was correct. My next thought was to try and reset the Fuel Trims back to zero just in case the ECM was correcting for an old problem that was no longer there. Simply clearing codes or disconnecting a battery does no always guarantee resetting the fuel trims. Many learned adaptations can be stored in a Non-Volatile Memory than can only be reset using a scan tool function. I proceeded to the Fuel Trim Reset function in the scan tool and reset the learned fuel trims back to zero. I ran the vehicle and the upper O2 sensors were still seeing a rich condition so the ECM began to ratchet down the trim values again. This car had something unusual going on and it all pointed to a possible Engine Control Module issue but I wasn't too quick to just go ahead and try one.

Figure 6

What was I facing?

I had to venture out on the internet hoping that someone out there had a piece of the puzzle to this head scratching issue. My diagnostic paths of approach kept hitting dead ends without any reasonable clues to grow on. We live in a cyber world where information is now at our fingertips so I opted to Google my problem. I came across an issue about a Flex Fuel problem where you had to go in and reset the Flex Fuel alcohol counter if the ECM calculation was flawed somehow. I was unaware that this truck was Flex Fuel so it was not something I would have even given thought in my diagnostic process. I went back into the scan tool to dig for the feature and there it was - "Fuel Compensation Reset." I proceeded to the function screen and was in shock to see a calculated Fuel Alcohol Content of 73 percent! How was it that this vehicle was calculating 73 percent alcohol mixture? How was this abnormally high reading possible if the owner never used anything but straight up gas? Did he accidentally pull up to an E85 station and fill up with the wrong stuff? Was this some type of driverless vehicle that went out on the town for for a night of drinking? There are many unanswered questions here but by simply resetting the alcohol content back to 3 percent default (Figure 9) and resetting the fuel trims again the vehicle was back in fuel control and ran like a champ (Figure 10)! Now I needed answers as to how this counter had gotten so high in the first place.

Figure 9
Figure 10

The Flex Fuel vehicles have been around since the late 90s and are designed to handle the 85 percent alcohol to 15 percent gas mixture. The alcohol used is an ethanol made primarily from corn but it can also be made from a variety of starch or sugary crops including potatoes, sugar cane, beets or even organic cabbage. Vehicles using E85 usually have specially lined gas tanks and have fuel system components that can handle the alcohol. The claim is you get a little more torque and horsepower and higher Octane ratings of up near 105. The alcohol does burn cooler and cleaner than gasoline. The downside is that you will need more of a gas mixture in colder climate that can vary from 25 percent to 30 percent mix of gas to aid in cold weather starting. Also alcohol has a lower BTU rating than gasoline so the fuel air ratio has to be bumped down from the norm of 14.7-1 to a low of 9.7-1 thus the reason why an onboard controller has to run a richer mixture and as a result your gas mileage is lower. Most cars since 1995 without Flex Fuel can handle 10 percent alcohol in their system because they are built to handle it but put that amount of alcohol in an older car and you’re asking for trouble that can lead to corroded components, damaged seals or severe fuel pump/engine damage. Today's non Flex Fuel cars can handle up to 15 percent alcohol. This gives the gas companies more reason to "Stretch the Soup" just to meet their needs. Pulling up to a gas station today be sure to read the warning of alcohol content in the gas you buy (Figure 11). It would be nice if the fueling station would just offer you an option at the pumps to have a shot of alcohol to drink or just have you directly place it in your tank. There are some countries such as Brazil that will use a full concentration E100 alcohol for their specially designed vehicles where cold starting issues are not a factor. They will usually denature the mix with 2 percent of gas or Iso-propyl to keep people from trying to drink it!

Figure 11

This Avalanche does not use a direct Flex Fuel sensor to measure the alcohol content but rather Fuel Trim algorithms to adjust the alcohol calculation. If this truck were ever to experience a lean condition the ECM could easily mistake the lean condition for an increase of alcohol in the gas and miscalculate the mixture. When filling the tank with E85 you are supposed to do it on a warmed up engine ONLY and after a fill up you are supposed to drive the vehicle a minimum of seven miles to recalculate the mixture. If you fill the tank and immediately park the car and let it sit for too long the ECM with calculate the mixture on warmup the next key cycle and cause a higher than normal alcohol mixture content value. I can tell you that the owner had this problem since the day he purchased the vehicle from a lot in Pennsylvania and he lives in New Jersey. I did a Google search and there are about 2,758 E85 filling stations in the US and most are concentrated in the middle of the U.S. and there are only a few in Jersey.

The prior owner did leave a couple of clues behind, though. He left a ski rack on the roof of the truck and never deleted his trips in the navigation system that revealed some extensive driving in Colorado, North Dakota, Minnesota, Michigan and Nova Scotia and he had access to many E85 stations because his travels were centralized in the "Corn Belt" states. It would be hard for me to believe that an ECM could calculate 73 percent alcohol from bad driving habits or prior lean conditions. I'm thinking the original owner was very fond of his Flex Fuel vehicle on his ski journeys across the US and possibly was the culprit to adapting his truck to be a compulsive alcoholic. It's just beyond me how the vehicle was unable to adapt back to non E85 gas. Just when you think your game plan is set in stone to resolve a simple rich condition that you may have conquered many times in the past along comes a curveball of events that just is mind boggling. This is why I love this business. You just never stop learning. Hope this story hits home with you fellow technicians out there.

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