You owe me an apology Bill

There were many times as a young technician I was responsible for diagnosing a problem on a vehicle that turned out to be pretty complicated. It often happened that the car developed a problem, but it wasn't real bad so the owner kept driving it. Then another one developed on top of the first, and still the owner kept driving it, and then another, and yet another. Eventually it got so bad the owner had no choice but to try and repair it, or have it repaired. But which problem finally convinced the owner to bring the vehicle in to be fixed? Worse, how can a tech see every problem that the car may have when one (or some) are very obvious and confirmed? That was the case with a Ford Ranger that another shop dropped off to me back in December. They had already removed and replaced the left cylinder head reporting that it was cracked, but they didn't do anything with the right hand head because it had good compression. After performing that repair, the truck ran but had no power and they noted the right hand bank of cylinders weren't firing. They tried all kinds of things to solve the problem, (replaced spark plugs and wires, swapped injectors etc) but eventually ran out of ideas so they brought it to me.

The testing went pretty much in line with a standard routine. I confirmed spark, and fuel injector pulse and compression and like they said the RH bank wasn't firing. Today we test compression with a pressure transducer and the oscilloscope. This gives us a look at every phase of the engines operation.

Here is cylinder number 2 cranking and showing 160 psi of compression. Next its time to look at the cylnder running, and then when the throttle is snapped.


Here is number 2 running with the exhaust stroke
of the cylinder marked with the cursors.
The rounding of the exhaust pocket isn't normal.







Next is TDC at the end of the exhaust stroke and beginning of the intake stroke. Again the rounding of the wave form is a problem.

The next test, snapped throttle tells the whole story, and here it is. The tall peaks are the compression strokes of the cylinder, the shorter ones that ramp up are the exhaust stroke. Note the value shown by the cursor is 37psi. That's how much back pressure is in the exhaust and that's a mechanical issue. The exhaust is restricted.

The shop that sent the truck to us didn't want to go any further with this. Basically if they didn't want to fix it there wasn't much of a reason to go any further. They picked it up and it wasn't until today that I heard back from them and now Bill is trying to tell me that I got the diagnosis wrong. After they punched out the cats, instead of replacing them the truck still ran bad. So they sent it off to another guy who allegedly had it solved in twenty minutes. The claim is that he tried to adjust a screw with the throttle position sensor, and when he did that, the computer then generated a code, so he substituted a computer and the truck ran perfect. Well not perfect but supposedly much better that he was then able to get another computer and now its fine. The part that irritates me about this is no doubt Bill had no idea the level of testing that was done when the truck was with me in December, and he darn sure didn't realize that everything we test gets documented and can be pulled back up exactly as you see here. Did the truck still have a drive- ability issue after the cats were punched out? I'll take his word that it probably did, after all here is the mass air flow sensor waveform from a snap throttle from back in December. 

The trained eye will see that either this engine isn't breathing right, or else the sensor can't report the airflow correctly. Don't worry about the drop outs, that was a scope connection issue.

With the exhaust restricted, the MAF sensor  cannot be condemned yet, but it couldn't be confirmed to be good either.

So why does Bill owe me an apology? Easy, had I not have had such documentation of the truck's problem back in December, I wouldn't have a leg to stand on today. With a confirmed mechanical issue, there was nothing else for me to do, the engine has to be able to pump air correctly. Even though I was correct, he is trying to tell me that I failed, yet he was the one who didn't want to spend any money to fix this, "because its an old truck and not worth anything. No doubt he has done this same thing to other techs that were in his employ through the years and gotten away with it. That kind of "bullying" demoralizes, and quite often embarrasses the technician, and without the kind of proof that I have here they might even doubt themselves. I know all about that, I've been there in the past. But not this time, you blew it Bill, I can't help it if you haven't been to training and don't know how the system works, which is why your explanation about the throttle position sensor doesn't hold water. And no, I can't make you look at the email that I sent with the capture of the snap throttle which shows the exhaust back pressure. It isn't the 70's anymore Bill, real techs don't have to guess, they test and produce verifiable results.