A month or two ago I replaced exhaust system with ebay special headers and pipes down stream. I don't know proper words for it. Reused item is the original cat converter. It was less expensive to go this route than go with OE style down pipe and I wanted to try headers of any kind anyways. On this header, the O2 sensor bung is located on #3 pipe and another one located near the cat.

At this time, the sensor is installed on #3 pipe, taking reading from this cylinder only. The engine has 244k miles and lately #3 is showing less compression than the rest. The sensor is on somewhat less efficient cylinder, it may be adjusting F/A ratio wrongly because of that. OE header (cast iron) has sensor port on the section where all 4 ex gas mix.

I have started thinking last week that what would happen to engine operation, fuel economy and such, if I moved the sensor to mount onto another bung where all of ex gas go through.

I wonder if anyone can give me some idea on this.

 

The original location was on the Exhaust manifold, so I would say to put the sensor back to that location if there is a bung in that "special header".
You will get the correct reading the system was set up for at that point.

 

Physically the location of the sensor on the header is where OE exhaust manifold had it. The header goes from 4 into 2 at the front of the engine and then 2 into 1 near the cat. Bung is located before the first collector.

I did the oil change just now and pulled the spark plugs, too. #3 is sooty and the rest is looking good. I will do compression check now.


(5 minutes later)

I did the compression check and this is what I got. #1= 165, #2= 166, #3=90 amd #4=167. And I was wrong about the bung location. It is located on #2 cylinder.

So, I think I will keep the sonsor at this location (#2) for now.

Thank you for time.

 

Do the compression test again just incase something strange happened.

Then pour a spoonful of oil into #3 & test again. If it goes way up, it's rings. If it goes up only a little, it's valves.

 

The last time I did compression check was on June 10 this year. I did it cold. And the numbers were, #1=160, #2=158, #3=90 and #4=160. I put in "Engine Restorer" additive and the vibration I was feeling got better somewhat. Didn't do oil thing the last time because I was fearing it will be the rings/cylinder that need mechanical attention.

This time, after I posted those numbers, I did oil check and the number went up from 90 to like 135. This kind of seals the deal that the cylinder has major issue in ring/barrel department.

What do you think?

Thanks