Technically Speaking
- Author: Wayne Colonna
- Subject Matter: Smooth engagement
- Issue: E85 flex fuel is culprit
Technical Training
Engine torque reduction is a computer strategy that has been used in vehicles for many years now. When the selector lever is used to place the transmission into reverse or drive from either park or neutral, a command is given to reduce engine torque. This assists in providing a smooth engagement into gear. Likewise, torque reduction is commanded to take place during each upshift for smooth shifting. This strategy can also be used in vehicles equipped with adaptive cruise control or when traction control becomes activated.
The strategic use of engine torque reduction varies depending on the vehicle involved. This also affects how certain codes are set reporting problems with its operation. For example, with some vehicles, if the transmission is having converter clutch issues, the traction control module (antilock braking system or ABS) may set a torque-signal code. It will do this because it’s being prohibited from requesting an engine torque reduction due to the malfunctioning transmission. Although the code is set in the ABS/traction control module, it doesn’t mean the problem is in the ABS.
When faced with engine torque reduction-error codes, a good initial diagnostic approach would be to conduct a complete health check of the vehicle. The types of codes set in other modules may reveal where the source of the problem is. If no other codes are set other than an engine torque reduction-error code, concentrate your initial focus on the engine. Look for misfire issues, bad wires, spark plugs, dirty air filter, dirty throttle body, vacuum leaks, cracks or tears in the air-intake duct work, dirty or faulty MAF sensor or TPS problems. Engine-related issues such as these can produce an engine torque-reduction error code with no other codes present making you work a little harder to find the problem. Mode 6 may also be of some help as well under these conditions.
Another item that can be added to the list that can cause an engine torque-reduction error code to set is fuel. A 2007 Pontiac Solstice with a 2.4L engine powering a 5L40E transmission came into Hardcore Automotive Inc. in Stuart, Fla., with a harsh shift complaint from another shop. The only code stored in the system was a P2637 for an invalid engine torque signal being sent to the TCM from the ECM. Harsh shifts are typically a byproduct of an engine torque-reduction error code being set.
The engine seemed to be running fine and the ECM and TCM had been replaced with no change. After clearing the code, within one mile of driving it would return. The diagnostic steps GM provides for this P2637 code is to first perform a diagnostic system check. This is the complete health check of the entire system. If any codes are set, record any freeze-frame data and record all codes. Then clear the codes drive it and see what codes set. If there are any ECM-related codes set among others, they need to be addressed first.
With only P2637 setting and no other code, along with a new ECM and TCM, it was time to focus on engine management. During this process, it was learned from the other shop that the vehicle had several codes in it before coming to Hardcore Automotive. One of them was a P0171. This is a fuel system lean code that indicates a performance problem in the fuel system.
Although this code was no longer setting, it was decided to take a look at the fuel system. Tom Johnson from Hardcore Automotive Inc. decided to check fuel pressure. In doing so he noticed that the fuel had an unusual odor and feel to it. He decided to hook into the fuel rail with an auxiliary bottle of fuel and give it a test drive. He had an immediate fix to the problem. Code P2637 did not return and the transmission shifted normally. The fuel that was in this vehicle causing the adverse condition was E85 flex fuel, a high-level ethanol-gasoline blend which contains 51%-83% ethanol. This fuel was drained and the tank was filled with regular gasoline and the problem was solved.
One last interesting piece of information, in reviewing their scan data prior to the fix, they noticed that long-term trim was running at -20. This is a big number that shows that it’s compensating for something. Apparently it had something to do with the combustion of the fuel that in turn affected engine torque management resulting in a hard-shifting transmission.
