Service Repair

A tight squeeze in a Ford Escape

Dec. 2, 2024

Sometimes seeing misfires reflected on a scan tool is the result of an ignition system fault. Typical testing for these mechanical faults can usually be discovered by simple relative compression testing. But, if there is no leak (or not a very significant leak), where do you go from there?

Brandon Steckler

Figure 1: A correlation of the CKP sensor signal to that of the CMP sensor(s) signals can be a great way of inferring camshaft timing. Keeping in mind the limitations of the test, it only truly reflects the timing of the reluctors. If they have lost their reference from their respective shafts, the timing will not be on.

Sometimes seeing misfires reflected on a scan tool is the result of an ignition system fault. Other times it’s the fault of the fuel injection system. But then again, engine mechanical faults can and do arise as well. Typical testing for these mechanical faults can usually be discovered by simple relative compression testing. But, if there is no leak (or not a very significant leak), where do you go from there?

Today’s Subject Vehicle

I was providing technical support to a shop facing a 2018 Ford Escape with a turbo-charged, four-cylinder engine. The vehicle suffered from a poor idle concern, intermittent cylinder No. 3 misfire at idle, but no DTCs stored. After preliminary evaluation of scan data (from the driver’s seat), the technician discovered that this speed-density system yielded a total fuel trim value of nearly -20% at idle. Of course, the technician evaluated the MAP values (as this is the main speed-density input responsible for engine load/fuel demand) and found them to indicate really weak intake manifold vacuum of only 8”Hg however, this condition improved with rpm and load. This was also confirmed with a mechanical vacuum gauge.

Figure 2: The results of the relative compression test show that each cylinder loads the starter motor similarly, inferring the compression is similar from cylinder to cylinder. This also supports the cause of the cylinder No. 3 misfire to not be caused by compression loss.

Concerned with a timing fault, the technician investigated with a lab scope capture of both the CKP senor and CMP sensors for correlation (Figure 1). They appeared to be properly timed according to a known good capture. This placed a timing fault at the bottom of the list. Considering the results of the relative compression test did not indicate a loss of compression for the suspect cylinder, the remaining list of possible faults to test for was shrinking (Figure 2).

Pinpointed Testing

The easy-to-perform tests guided the technician to evaluate the No. 3 cylinder more stringently. After all, there is little else that could cause the combination of weak vacuum and significantly negative fuel trims. An in-cylinder compression test was taken for cylinder No. 3 and it was evaluated (Figure 3).

Figure 3: Zooming in on this cylinder No. 3 in-cylinder running compression test shows a few details pertaining to the health of the cylinder. The minor pressure differential between the exhaust and intake pocket shows no significant compression loss (backing up the data from the previous RC test). However, the rounded exhaust pocket infers an inability for the exhaust valve to seat properly or even tight exhaust valve lash. If this condition were present, excessive valve overlap could be the cause of the misfire. And since a similar capture was obtained from other cylinders, this could lead to low overall engine manifold vacuum and negated fuel trims, as this engine is experiencing.

Per the horizontal cursors, the capture shows the weak manifold vacuum but only a minimal amount of pocket differential (no cylinder leakage). However, the exhaust pocket appears to be significantly rounded and immediately caught my attention as cause for concern. A second similar capture was acquired from another cylinder and the results were similar.

The Data Doesn’t Lie

With all the information in front of us, and the desired information not yet obtained, we are faced with deciding how to proceed. Here are some bullet points of what we know to be factual, and I will ask all of you, diligent readers, for your input on what they mean to you, collectively: 

Weak overall engine manifold vacuum

Camshafts appear to be in time

Misfires counting for cylinder No. 3

Suspect-Cylinder (No. 3) shows rounded exhaust pocket

Given this information, what would you do next?

Inspect valve lash.
Decarbonize induction system.
Treat crankcase for stuck piston rings.
Replace camshafts for decoupled CMP reluctors.

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.