Service Repair

Ram 1500: Driveability can be exhausting

Nov. 21, 2024

Figure 1- A great first step in any driveability analysis, the relative compression test shows how all cylinders harness and squeeze their contents, relative to each other. This capture shows all cylinders performing similarly.

Welcome back to another edition of “The data doesn’t lie,” a regular feature in which I pose a puzzling case study.

So frequently I’m called upon to view a lab scope capture or a scan tool data capture to offer an opinion for what I think may be wrong with the vehicle. Many times, it’s something simple and easy to see when you know what you’re looking at. But occasionally, I’m faced with data that offers a significant challenge. What I mean is, sometimes I really must stare hard at multiple pieces of data to draw a conclusion/diagnosis that I’m comfortable standing behind.

Buried in the data

Such is the case from my good buddy Tony Mossuto, in Jacksonville, Fla. Tony reached out to me about a 2020 Ram 1500 pickup with a Hemi 5.7L that was giving him trouble — a pesky P0303 (Cylinder No. 3 misfire). Suspecting an engine mechanical fault these engines are notorious for, Tony reached out on several Facebook groups and posted some lab scope captures. He hoped for some usable input but couldn’t seem to locate any that corroborated his suspicions. Tony shared this info with me and I will be the first to admit that I had to do a bit of soul-searching to pinpoint a suspect fault.

Starting with the basics, Tony wisely captured a relative compression test (Figure 1). With the blue trace representing ignition event for cylinder No. 3, and the red trace representing starter current for all eight cylinders, it’s easy to see that no significant compression loss is occurring for any of the cylinders, not even for suspect cylinder No. 3.

Figure 2- The results of this cranking intake vacuum test show a few characteristics. An engine mechanical fault is present, and the suspect cylinder is not leaking at the intake valve/seat.

I then viewed a captured including absolute pressure values from the intake manifold while cranking the engine over repeatedly. According to the piston chart, when suspect cylinder No. 3 is under compression the vacuum in the intake manifold intensifies (Figure 2). This proves a compression loss (although not significant) is present and the loss is not to the intake manifold. Now we are getting somewhere!

Figure 3- This capture of tailpipe pressure while cranking the engine not only corroborates the data obtained from the previous capture, but it also provides the nail in the coffin. It appears that the exhaust valve/seat is leaking for suspect cylinder No. 3. The fact that pressure doesn’t build during compression suggests that the valve can seal but may not seat properly while running. This can allude to a broken valve spring, however, the valve, guide, spring, seat and cylinder must be checked in case overheating/cylinder head distortion was the root cause.

I then instructed Tony to send me a capture reflecting pressure changes in the exhaust stream while cranking, this can be seen in blue (Figure 3). As No. 3 piston descends on the expansion-stroke the effect on the exhaust stream can be seen. The failure of the correlating event can be seen as a peak of lower amplitude (compared to the others). A similar event can be seen at the point where the No. 3 intake valve opens. This helps point me in the right direction but also begs a few questions.

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:  

No. 3 misfire, spark plugs and coils were swapped with no change
Relative compression test shows no significant loss in any cylinder
Intake vacuum intensifies under suspect cylinder No. 3 compression
Corroborating evidence seen in tailpipe pressure trace

Given this information, what would you do next?  

Replace the camshaft due to worn lobes.
Have machine shop inspect intake valve seat.
Replace head gasket due to compression in cooling system.
Have machine shop inspect exhaust valve seat.

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.