Everything was Replaced?
This is a case study that was originally written by my friend and one of our peers Pete Rudloff with the information collected on a vehicle I diagnosed and repaired. Not sure how this post is going to turn out but wanted to contribute something rather than just be a spectator, an observer and just comment on other post.
The late great Dave DeCoursey taught me years ago at a TST Big Event during a misfire diagnostic class to have a systematic approach and stay with the system that works for you. Thanks Dave, that class changed my career for ever.
This truck was towed to me from another shop about 30 miles away, I'll save you the conversation I had with the shop owner over the phone, it was humorous and unprofessional on my part.
Back story on this everyday typical Ford pickup is that the truck came to the shop with a misfire on cylinder #3. The shop performs a complete ignition tune-up, plugs, boot kits and a new coil for cyinder #3. When that didn't correct the problem they swapped injectors around to try to move the problem I assume. Next they figured to call in a favor to a Ford master tech from another shop, his suggestion was the PCM, so the truck was taken to a local Ford dealership to have a PCM installed and programmed. Problem still present. Now this truck is taken to another reputable shop in their area and they diagnose the engine to be a valve issue of some sort, so with about 150k on the odometer the original shop sells and brand spanking new Ford engine, possibly re-manufactured. Again, no change in the running condition. What was left the shop figured was the engine harness, needless to say after replacing with one from a parts recycling plant, aka junk yard, no change. This next part is when I lost my professionalism with the shop owner. They decide to take the truck to a 3rd shop, this shop is totally convinced this new engine has a valve issue and the cylinder head needs to be removed. Wait for it............The original shop removes the cylinder head and installed another head.
After 2 years of tinkering and having to purchase the truck from the owner my name gets dropped and that's when I got involved.
Parts replaced: Spark plugs and boots, #3 coil, PCM, Engine, engine harness and Bank 1 cylinder head.
My approach to any conventional, non-hybrid, internal combustion engine misfire is 1, some sort of relative compression test 2, test ignition 3, test fuel delivery and 4, go in-cylinder to look at breathing and valve events. That works for me and has not let me down since learning how to narrow problems down by eliminating funnels.
My relative compression test was performed with my high current amp clamp around the starter cable.
Primary testing and current ramping the #3 ignition coil gave me no reason to suspect an ignition problem.
Primary testing and current ramping gave me no indication of an injector issue, if this truck didn't already have injectors moved around I may have hit it with propane as another quick fuel test.
Finally, I get to play with the pressure transducer. Bam, there it is, pressure building on the exhaust stroke. Valve openings are pretty obvious, even for a Snap-on scope. Maybe a Cat converter?
Drop the transducer in another cylinder on the same bank. No pressure on the exhaust stroke tells me the clog is isolated to just cylinder #3.
After 35 minutes of quick basic testing and 15 minutes to remove the manifold that was already removed a couple times when the engine work was done, there it was, a bad casting in the manifold.
Turns out that the shop put that manifold on the truck a year or two before the customer started complaining about the slight misfire.
I wanted to teach the shop how I came to the diagnosis but he didn't want to learn, I suppose a little embarrassed.
Nice find ! Testing not guessing . With hard facts to back it up
Good job and approach David.
Yes, having a proven logical organized approach is absolutely key to successful diagnostics for any issue. The simple fact that it must be understood is that all engines are essentially just air pumps and any irregularities in the ability of the engine to move air into and out of the cylinders will be reflected in a variety of ways. How we interpret the clues, is down to us. Maybe it is a cranking cadence, abnormal by feel or sound, visual or smell. The senses are every bit as important to me in the application of my diagnostic approach, as is using simple or sophisticated tooling to effect a diagnosis.
As a technician working flat rate, most of the time I used the "KISS" approach to diagnostics, including using the simplest tooling that would net the desired results whenever possible. It was simply about diagnosing and fixing vehicles as efficiently as possible and moving on to the next vehicle without getting overly attached or intimate with a diagnosis, unless it was particularly interesting and worthy of a case study. I'd drag a scope or other diagnostic equipment out sometimes just to support a diagnosis that may already have been completed with simple tools, since a "picture is worth a thousand words."
One thing that I did do as a technician (and I often still do in my current role), was for every new model that entered the shop, I'd spend 2-4 hours scoping and scanning and base lining the numbers from known good vehicles. A lot of technicians are unwilling to invest in their own education by doing a little research on their own time to learn or better understand system functions. I have always figured that being an active and willing participant in my own skills development can be of direct benefit to me. I have always treated this work seriously and dedicated time to enhance my skillset.
Back to mechanical pumping issues which are the focus of your post. An example of a mechanical malady might be the classic Chevy small block flat exhaust cam lobe of old that many of us experienced, where air in the cylinder could not leave via the exhaust. So, it exited the only way that it could, via the intake manifold and carburetor with associated "barking" sounds in the manifold, often with flame back in the carburetor and air cleaner.
Pressure testing has been around for many years now, but has matured with better sensors and DSOs to display the cylinder events nicely. In earlier times we didn't have such luxuries as provided by current tooling. However, I do recall from my time as a Mazda technician in the '70s, that graph testing for compression testing of rotary engines was the norm. So, while the concept of creating waveforms to measure events is not new, the quality of the waveforms is well beyond anything we dreamed of in the "good old" days.
I recall an instance from many years ago while working in a GM dealership, The problem child was an early '80s Malibu 305 ci V8 that had been to two other shops before arriving at our doorstep. The condition was a random misfire condition that was not present when the vehicle was brought in. How hard could it be to locate a fault that another shop had missed? My co-worker Ron the shop foreman, happened to get this ticket and went for a road test with the customer, who unfortunately could not duplicate his concern at the time. Undaunted, Ron reviewed the history of the vehicle, which was now just beyond warranty. Every conceivable "tune-up" component had been replaced more than once by a couple of shops in an attempt to fix the concern, without any resolve.
Ron ran the vehicle connected to the "big box" ignition scope, monitored engine vacuum and more. There was nothing abnormal experienced during this or a subsequent return visit and no parts were replaced. Ron had been pretty thorough, even considering the possibility of a broken valve spring that might be rotating back and forth with an occasional coil bind causing a misfire. He'd pulled the valve covers, but found nothing abnormal. It was discussed with the customer and arranged that he would monitor the "what, why and when" conditions and drive directly to the shop, if the condition occurred during business hours and we would drop everything and investigate it immediately.
So, I got involved when the car arrived for a third visit. Unfortunately, while the reported misfire had been present initially, the engine had started running normally just a few blocks away from the dealership. This owner was a very patient and understanding person, a very willing participant in the process as so few really are these days. Not wanting to be "bitten" by something others had missed, I completed compression testing, leak down, vacuum testing scoping the ignition and a raft of tests that confirmed basic mechanical integrity.
Now, knowing that customers like visiting repair shops as much as dentist and lawyers, I was pretty sure that this customer wasn't one of those rare few who enjoy visiting shops with "phantom" concerns for the heck of it. All cylinders were contributing equally on a physical power balance and the engine ran fine until it suddenly began to misfire when I decided to do a stall test in the bay. Returning to idle, there was a dead miss and the vacuum gauge was pulsing rythmically, clearly showing a single cylinder issue. The scope identified a cylinder on the left bank, as misfiring. A quick check of the spark plug and ignition system verified electrical integrity, so it appeared that we were clearly faced with a mechanical issue of some kind.
Since carburetors are not selective fuel delivery devices, that could be ruled out. I'd gotten the shop foreman involved by this point, both as an assistant and interested party. Keeping in mind the concept that the engine is just an air pump, we decided to see what happened when using the cylinder leak down tester, while slowly rotating the crank. We also removed the left valve cover to monitor valve lift while hand cranking the engine.
There was air exiting from the tailpipe when the exhaust valve opened, but when the intake valve opened there was no rush of air into the intake manifold and carburetor. Almost excitedly, the intake manifold was removed and the root cause of the randomly occurring misfire was clearly evident. A perfectly shape piece of casting flash filled the entire intake port! when touched, it would rotate just like a throttle plate "butterfly", cutting off air/fuel from entering the cylinder. We soon had one happy customer for life and an unusual tale to tell, that we will never forget!
Looking at this account of diagnosing an "air pump" issue on a very simple engine having no on-board diagnostic support, it used to take a number of physical mechanical tests to verify mechanical integrity. Now that the use of pressure transducers has become an option for the modern tool box, the same diagnosis could have been determined with less physical testing, once the condition presented.
Things that I like to know when performing any diagnosis and especially with an ongoing concern, is the vehicle history and what has been replaced. I never ASSUME what the previous technician did was done correctly, so always start a diagnosis as if nothing has been done to the vehicle. Freshly replaced components will be inspected if my diagnostic approach takes me in that direction, although a visual inspection is always performed to catch any errors during prior work.
On thing is for sure, that doing the same over and over again and expecting different results.... Well, we know that doesn't usually work out!
After all of this, the engine is still just an air pump and everything drivability wise is relative to the air flowing through the cylinders.
Again David, good job. It's too bad that "hero" status lasts only until the next job arrives in the service bay, but there is self-satisfaction from performing a proficient and efficient diagnosis!
Dave it looks like you had a good diagnostic game plan. When a drivability problem rolls in it important to have that plan. If you don't have a plan you either won't find the problem or it will take forever finding it.
Glad our buddy Dave DeCoursey (may he rest in peace) was able to put you as he did with many on the right track at one of our … Big Events.
Hey Dave... you the Joe's , Ron and crew did a great job with Super Saturday. I am glad you guys had me teach so I could be part of such a high powered traing event.
Brandon was just talking about this case study this past Saturday at Super Saturday Event. In pressure waveform acquisition and analysis Class he was teaching. Which by the way was so an awesome class To sit in all day.
Thanks to everyone for all that you‘s do for this industry
Yep, Brandon shared this and it was great. Great approach. I saw the TST video on YouTube of Dave Decourcey's misfire strategies, that video alone drove me to grab a transducer. Amazing instructor. Thanks for sharing David, it made for great class material!
Good morning Dave , thanks for remembering and mentioning Dave in your post
Dave had a great way of making the complex look easy
when I’m stuck with a tough one I alway stop and say “ what would Dave do “ what would be his next move
his guidance still keeps me focused when getting lost in the automotive forest
Great post and awesome find thanks for sharing
Just when we think we have seen all the possible causes of a fault we find something like this. I find this to be great detective work. Without the in cylinder pressure testing this would have required blind luck to find. Great job in diagnostic approach. Finding something to effect your fault is normally a good way to find a related cause, but being able to measure and analyze is key.
AWESOME example of checking all of the BASICS first, which put you in the correct FUNNEL ( "G" General Test ) then driving down the FUNNEL with ("P" Pinpoint Test). Nice Diagnostics, My Friend.
I told you this the other day, completely impressed! The whole time Brandon was giving this presentation on Saturday in the back of my head I kept thinking... no way! John Thornton did a case study on something so close to this a few years back on a Jeep 4.0. I never in a million years thought I would see a fault like this again. I love how systematically done your approach was to this particular failure. Well done my friend, well done!
Thanks Andrew, that was my thought when the truck came into the shop. As soon as the problem was narrowed down into the correct funnel I could see where the problem had to be. Honestly I was thinking a rag or something was stuck originally. I remember John's case study and had trouble thinking that was possible.