Service Repair

Solved: A Jeep Cylinder Misfire

May 29, 2025

A Jeep Wrangler was experiencing intermittent misfires, and our go-to testing was only showing what was functioning properly, instead of indicating what was wrong.

Brandon Steckler

The cylinder head was removed, and the valve train was inspected. Cylinder No. 3 exhaust valve had excessive stem-to-guide clearance. This allowed the No. 3 exhaust valve to rock along the seat and created an intermittent sealing issue. The cylinder head requires reconditioning and/or replacement. These cylinder heads are a known issue so replacement is prudent.

A 2013 Jeep Wrangler was experiencing intermittent misfires. Test after test and the results showed us what was functioning correctly, but didn't seem to point to what was wrong.

We had a DTC for P0303 "Cylinder No. 3 misfire." The link will allow you to get reacquainted with the case details.

Given this information, what would you do next? 

Adjust the valve clearances
Replace the lifters only
Replace/recondition cylinder #3 exhaust valve
Replace/recondition cylinder #3 intake valve

For those of you who chose answer No. 3, congratulations! The relative compression test confirmed the suspect cylinder’s cause for misfire was of a mechanical nature. However, the other pressure waveform captures proved that during the loss of compression, the intake pressures did not increase, but in fact decreased. A leaking intake valve would not allow that to occur. This not only eliminated the intake valve as suspect but simultaneously condemned the exhaust valve for not sealing properly (See photo). Answer No. 4 is incorrect.

Although tight valve clearance can cause an issue like this, these valves were non-adjustable. That is determined by reading service information, so answer No. 1 is incorrect. Pumped-up lifers can cause valves to hang open but that characteristic would be visible in the pressure waveforms. Answer No. 2 is incorrect.

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.