I have some more diagnosis results to share. I did a back pressure test and found that there is significant back pressure. At sensor-1 bank-2, I seem to get 3+ psi back pressure when I snap the throttle. Running the engine at a constant RPM, high or low, pressure seems to stay in the green on my pressure guage.

For comparison, I also read the back pressure of sensor-2 bank-2 just after the cat and all readings are normal; no pressure at all. So there's no obstruction in the muffler. It seems to be a partially clogged cat if I'm diagnosing this right. Any thoughts?

 

What is the pre cat back pressure at idle and again at 2,000 rpm steady state?

George

 

It remains in the green. The pressure seems to build up initially then falls back to 1 psi.

 

So, the pressure builds as I rev, but it falls back down holding steady.

 

If your pressure is 1 psi @ 2,000 rpm then you do not likely have an exhaust restriction.


George

 

Really? I'm quite puzzled at how to solve this rough idle, low acceleration, poor mpg problem then.
(Not really rough idle, maybe subtle misfire. I can't tell.)

I recorded a video on the guage. I need to figure out a way to upload it here
Nevermind. The video doesn't really tell a different tale.
@George Is the 3+ psi when initially giving it throttle really normal? I thought any pressure above 1 is bad.

 

I don’t think that the transients during a snap throttle tell you anything. The steady state value at 2000 rpm is your main data point. If you want to explore it further, take it for a test drive with the O2 sensor still removed and see if it runs better.

There are many possibilities to explain your symptoms. We can explore them and get to the bottom of your problem if the back pressure thing doesn’t pan out.


George

 

Here is the factory service manual procedure:




George

 

Thanks so much for the service manual page, [@George](member.php?u=9429). I did my back pressure test again following those instructions. Pressure guage connected to bank 2 sensor 1 (before the cat). Pressure started out at 1.25 PSI at idle. Rev'd her up to 2,000 RPM, Pressure jumped to 2.5 PSI instantly then fell back down. But as I held the RPM at 2,000 the pressure slowly climbed up to 3 PSI.

That was the first test I ran for about a minute.

The second test told a very different story. After cooling off for a bit, I started her back up and this time I slowly rev'd up to 2,000 RPM and held fot about another minute. Pressure stayed at 1.3 PSI the whole time.

I thought this was weird, so I let her cool off a bit before a third and final test.

On this test I started her up, snapped the throttle quickly and held at around 2,000 RPM. Pressure stayed at less than 1 PSI; almost 0. I checked the rubber hose on the guage incase a hole melted through it. But no, the hose is fine. In fact every looked perfect. There was hardly any back pressure at all on this run.

So why am I seeing different results on each test? If this behavior is predictable, then back pressure numbers seem to improve as the exhaust system heats up. I'm starting to wonder if the catalyst and etc has to be at certain operating temperature in order for the diagnosis on back pressure to be correct. The first test I did was on a cold engine and exhaust. Didn't move her or anything this morning. I just connected the pressure guage and started her up for the first time today. I did this on purpose for the sake of keeping everything cool as a cucumber so I wouldn't burn my arms working around a 400 degree exhaust pipe.

I understand that everything has a desired operating temperature, but (given my inexperience on exhaust systems) I'm scratching my head on whether or not that 2+ PSI back pressure on a cold exhaust is normal.

 

I don’t have any experience with back pressure testing on cold vs warm systems you’ll have to do your own research there. Leave the O2 sensor hole open and go for a drive. Does it run better, then you’ll have your answer. You’ll also **** off your neighbors but hey it’s engineering which can be messy sometimes. Lol.

George