Service Repair

Solved: GMC Yukon with low compression

Dec. 2, 2024

A 2015 GMC Yukon was misfiring under all operating conditions, even though the engine seemed to crank over well.

The broken exhaust valve spring is the cause of the driveability fault. Keep in mind that a broken spring can present intermittent faults or faults that only occur under certain operating conditions. It’s important to realize that a broken spring can still allow a perfectly passing compression test (or relative compression test) but fail a running test miserably.

Scott Jett was in Ooltewah, Tenn., was faced with a 2015 GMC Yukon with a misfire under all operating conditions. The engine seemed to crank over well, and Jett experimented with easy-to-perform tests to guide him toward more pinpointed tests.

Get caught up on this vehicle test case details.

What would you recommend doing next, given the data bullet points in last month’s challenge? 

Replace the piston/rings/recondition block
Replace the intake valve spring
Machine the cylinder head gasket surface
Replace the exhaust valve spring

For those of you who chose answer No. 4, congratulations!

Answer No. 1 is incorrect. Although the data shows that the fault is only present during the duration of the intake valve opening, this fault could not be in the low-end of the engine as the fault would be present and showing symptoms whether or not the engine was cranking.

Answer No. 2 is incorrect. Replacing the intake valve spring wouldn’t make sense as a fault with the intake valve or spring would not exhibit during intake valve opening duration. Answer No. 3 is also incorrect. A cylinder head gasket sealing surface fault isn’t likely to present only during intake valve duration. Furthermore, measurement would show if the cylinder head required machine work to true-up a sealing surface.

The only logical answer is No. 4. After careful inspection, the exhaust valve spring was found to be broken. During cranking conditions there was enough spring tension to return the valve to the rested/sealed position. Under running conditions, the engine is rotating four times as fast and the spring cannot keep the exhaust valve closed. The spring was replaced and driveability was restored.

Be sure to read the next Motor Age issue for the answer to this month’s challenge and what was discovered!

About the Author

Brandon Steckler | Technical Editor | Motor Age

Brandon began his career in Northampton County Community College in Bethlehem, Pennsylvania, where he was a student of GM’s Automotive Service Educational program. In 2001, he graduated top of his class and earned the GM Leadership award for his efforts. He later began working as a technician at a Saturn dealership in Reading, Pennsylvania, where he quickly attained Master Technician status. He later transitioned to working with Hondas, where he aggressively worked to attain another Master Technician status.

Always having a passion for a full understanding of system/component functionality, he rapidly earned a reputation for deciphering strange failures at an efficient pace and became known as an information specialist among the staff and peers at the dealership. In search of new challenges, he transitioned away from the dealership and to the independent world, where he specialized in diagnostics and driveability.

Today, he is an instructor with both Carquest Technical Institute and Worldpac Training Institute. Along with beta testing for Automotive Test Solutions, he develops curriculum/submits case studies for educational purposes. Through Steckler Automotive Technical Services, LLC., Brandon also provides telephone and live technical support, as well as private training, for technicians all across the world.

Brandon holds ASE certifications A1-A9 as well as C1 (Service Consultant). He is certified as an Advanced Level Specialist in L1 (Advanced Engine Performance), L2 (Advanced Diesel Engine Performance), L3 (Hybrid/EV Specialist), L4 (ADAS) and xEV-Level 2 (Technician electrical safety).

He contributes weekly to Facebook automotive chat groups, has authored several books and classes, and truly enjoys traveling across the globe to help other technicians attain a level of understanding that will serve them well throughout their careers.