Good comments above. I can add a few observations is all. I doubt your compression readings are accurate. That is if you feel the engine is otherwise in pretty good condition, hasn’t been abused, has always been given the routine maintenance per the owner’s manual on schedule, never overheated or run out of oil, etc. Getting accurate compression readings isn’t a simple thing. In general you have to remove all the spark plugs , disable the fuel supply, disable the ignition system, prop open the throttle valve to max open position, and have a properly warmed up engine and a 100% perfectly charged battery. And a gauge you know is calibrated properly. You have the screw-in type gauge, right? The cheap-o type where you try to hold the gauge against the spark plug hole is almost impossible to get an accurate compression reading. With the screw-in type you have to coat the gauge’s gasket with a little oil each time too. When you do the test watch the gauge carefully as it rises on each compression stroke. It usually takes 5 or 6 compression strokes before it settles out, and that’s your reading for that cylinder. Did you do anything differently?
If you have new NGK spark plugs compatible with the engine that are gapped correctly, I doubt this is a spark plug problem. Suggest you surf to rockauto’s website and confirm you have a spark plug they list as compatible for this engine, just to be sure. Sometimes the parts stores make a mistake when they hand out spark plugs.
If all that checks out, next place I’d look would be an intake manifold vacuum reading, coil resistances, crank position sensor function, fuel rail pressure, and clean engine/head ground straps to chassis ground.
This all presumes all the routine engine maintenance items like engine air filter, battery, etc are up to date.
No way to know. You’re going to do another test anyway, to get a wet reading, so do another dry test first, this time with the throttle blocked wide open.
All the plugs out, throttle wide open, battery fully charged and crank the starter until the gauge levels out and stops rising. Then repeat with the oil in the cylinders.
Note: For the wet test put oil in ONE cylinder, test it, then put oil in the next cylinder, test that one, and repeat until you’ve done all the cylinders. Don’t just squirt oil in all the cylinders at once and then start testing.
WHY do you doubt the compression readings are accurate . . . ?
As I said earlier, I’ve seen more than a few tired old engines run on all cylinders, and I know for a fact they had low compression, because I’m the one who measured. And they were running reasonably well, clearly less get up and go, versus a high compression new engine, but they still operated, and vehicle was able to be driven in a normal manner
GeorgeSanJose, thanks for the help with the process. I don’t recall if I had the truck warmed up first, and I definitely did not think to oil the gasket. I turned the engine exactly 7 times per cylinder, but I’ll watch it this time and keep cranking until pressure stops increasing.
The engine has not always been given proper maintenance. I am not the only person who has owned or driven it. It was originally a fleet vehicle belonging to the state of NC, and I have no idea how well they care for their vehicles except that it ran well when it was purchased. The old employer I bought it from rarely changed the oil in his massive payment truck, so I doubt he did anything to his “other little truck”. I was only permitted to change the oil and replace the air filter once in the two years he owned it, and I’m certain he didn’t take it to a shop. I also loaned it to a retired friend of mine for a couple of years while I was in training and then deployed in the Army, and his idea of maintenance was checking fluid levels. If he ever did anything to it, it was probably topping up the high mileage synthetic oil I use with Dollar store detergent conventional oil. All said, I am only certain it had proper maintenance for about 50% of it’s life.
I have verified that the plugs are compatible, once in the past and once today for verification purposes.
I don’t think it’s a coil issue, since it only uses one coil. I’m pretty sure the same issue would happen on all 6 cylinders, or at least not discriminate to just the odd numbered cylinders. The crank position sensor though… that sounds very promising. I recently replaced a broken flexplate, and it would have been easy to damage the wires while removing or re-installing the transmission. Beyond visibly damaged wires, is there a way to check it other than replacing it?
crank position sensor: is it one for engine or do you have some per-bank ones?
the reason why I ask: you have one bank consistently misfiring, so trouble is likely connected to that one bank, so if it is one sensor, it is much less likely to be the problem as it would not be so much biased against one bank only… besides, broken single crank position sensor would likely result in engine not even starting
while on this topic: if this is one sensor and this sensor reads “notches” on some rotating “thing” (“tone wheel”?)…
imagine you have one of notches to be somehow damaged or have a piece of something stuck to it: it would make readings to skip and potentially might throw timing off for one bank only.
I know it is a far stretch… I’m trying to throw ideas at the wall and see if anything sticks
. A misfire occurs when an insufficient amount of fuel is burning in a cylinder. … A misfire from one or more cylinders can be caused by many reasons from a faulty ignition system, fuel system or internal engine failure.
I know this is an old post, but I wanted to say thank you and give an update on my issue.
In my R-Class the catalytic converter was faulty. I took my car to a local muffler shop and they did a quick test. The cat is clogged. So $750 for a new cat fixed the issue. Thank you all for the advice provided. This and another article I read on youcanic Misfires on one side of engine helped me narrow down the issue. I would have spent a lot more money with diagnostics otherwise.
According to my mechanic this is a common problem with this particular model. Car runs perfectly now. MPG is where it should be, so I don’t expect any fuel issues.
The cat wasn’t completely clogged, but it was restricting flow.
I can’t speak for your mechanic . . . but I’ve been at it for a long time
And generally speaking, cats don’t clog to the point of causing misfires on one bank for no reason
With one “sort of” exception, in my opinion . . . the monolith has broken free of the shell in such a way that it is restricting flow, as you said. But . . . and this is a big but . . . you as the driver would hear the monolith rattling around. You would not be unaware of the noise and/or rattle
And I said “sort of” because what I described wasn’t exactly a clogged cat. Although the symptoms might seem similar to the driver
