Truck occasionally loses power when put in gear

I have a 2004 GMC Envoy XUV, V6 with 65,000 miles. For about the last six to nine months it seems to lose power about every 20 starts. It starts, no problem, but when it’s time to get in gear and move, it makes a sound that is hard to describe and seems not towant tomove forward or backward. This lasts for a bit, and then the sound disappears and it drives just fine until the next time it happens. The sound is similar to cold engine idle in the winter. Turning off the engine and restarting makes it disappear. My mechanic has test driven it to labor the transmission and has checked the engine codes as well. Nothing! Any ideas? I dread the dealer taking it for hours at $160 per. Other than this seemingly stupid problem, this truck is a dream. It’s is in the last two months of an aftermarket warranty, so I am pretty desperate to find an answer so it doesn’t cost an arm and a leg later to fix it. — Arlene O.

This is a tough one to understand and figure out! In a follow-up communication Arlene confirmed the tachometer reading was rather high as the fault occurred. She had also seen an article/reference about “reduced engine power” and wondered if this might be her concern.

Based on Arlene’s description I’m wondering if the transmission may be slipping, possibly due to low fluid level or another fault. Using a mobile phone to take a narrated movie of sounds, conditions and the instrument panel display as the fleeting fault occurs may be her best bet to demonstrate the symptoms to her service tech. Since it happens in forward or reverse, it doesn’t appear to be a fails-to-upshift issue. Her “reduced engine power” reference was likely speaking to a possible electronic throttle control symptom. Should a fault in this system occur, engine power will be reduced, either about half or almost completely, depending on fault severity, for safety reasons, along with an illuminated check engine light and stored diagnostic code. If this should occur, there would be little/less engine noise/RPM, not more. You mentioned once in your newspaper column that modern ultra-low emission vehicles have catalytic converters that activate faster than traditional cats, so the exhaust is clean almost from startup. How do they achieve this? Using a different catalyst? Exhaust heaters? — Ben W.

Cold-start emissions are the holy grail of emission control efforts! Modern engines run so well they can tolerate a leaner/cleaner air-fuel mixture at start-up, and efforts to achieve rapid light-off of the catalytic converter are strong. Moving the converter closer to the engine, adding a small/low-mass pre-converter, and adding an electric heating element are current strategies, along with new catalysts that can begin functioning at less than the typical 800 degrees F.

Here’s an informative article by the Association for Emission Control by Catalyst: https://www.aecc.eu/technology/catalysts/

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at bradbergholdt@gmail.com; he cannot make personal replies.