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Discussion Starter #1
I am considering performing this test at home once I buy or loan the special tools needed. I might have more backpressure when fully hot compared to the first 5 or 10 minutes of engine on... the guessing-theory is that my old cat might be swelling-up (inside) and causing more backpressure when it gets hot.

I have read that most professionals remove the upstream 02 sensor and insert a test hose & pressure gauge there. I could also do this, but I might have an easier method. My upstream O2 sensor is a little tuff to get out because of heat shield interference. I am not just looking for easy, there is a reason why I am considering an alternate location. I need your help and input on what you think.

My exhaust manifold is a mani-cat and this Dorman version is an exact copy of my Ford original, picture at link below->

http://www.amazon.com/Dorman-674-70...&qid=1408902824&sr=8-1&keywords=dorman+674702

A component of the mani-cat includes a metal tube that connects to my PZEV secondary air injection (SAI) system. This SAI tube is a perfect spot for me to disconnect the SAI rubber hose from the mani-cat and install a low pressure gauge. I "think" this is a good spot for measurement of backpressure.

The question is; what psi measurement would be considered ok, marginal, or bad on a Focus? I have the basic answer to this question thanks to the interwebs;->
http://www.aa1car.com/library/exhaust_backpressure.htm

"Backpressure readings at idle on most engines should generally be less than 1.5 psi (10 kPa). This will vary somewhat from one vehicle to another depending on the design of the exhaust system, the size of the pipes, how restrictive the converter, muffler and/or resonator is, and whether it is single or dual exhausts. We've seen some idle readings as high as 2.75 psi on a few vehicles, but for most 1.5 psi or less at idle is normal."

If you have done backpressure tests on a focus and remember the psi numbers, please post them. Thanks!
 

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Haven't, don't need to. Anything over five is bad unless big engine at higher rpm. Idle will be much lower. The phrase 'if it continues to build pressure with time' is the one to take note of. Idle can be fine and not show till higher engine speed, what most of them do. Idle problem will only show up after the higher engine speed problem does. It affects at higher speeds first.

Numbers given in that link make perfect sense to me. Lower is always better. RPM and load raise it and normal.

The cat does NOT swell inside from heat, at least not enough to affect flow through it. Either blocked partially or fully, or it's not. Now if ceramic brick matrix used inside, if broken the pieces can shift around to create more or less blockage.

If truly clogged, before engine dies if big enough it can run up to 50+ psi in pipe there, the engine is an air compressor.
 

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Discussion Starter #3
I ended up buying a cheap test set->
http://www.amazon.com/gp/product/B0002SQW9S/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

Did the test last night. Had to remove the upstream O2 sensor and thread-in the adapter for the pressure gauge; which on my car is not easy due to interference of the mani-cat & a heat shield that hides/recesses the O2 sensor bung.

My thought was maybe my exhaust back-pressure was increasing once the Cat was fully hot. I was wrong. No increase was detected.

While at Idle and at 2500 RPM, the pressure measured was 1 psi or less.

While at 2500 to 3000 rpm, the pressure would briefly spike to 2 or 3 psi when the throttle was released and while it was returning to Idle. -which seemed normal or ok to me.

Would be nice to get this new & cheapo gauge tested somewhere to verify that it works/is calibrated properly... maybe someday.

My Cat is failing the other tests; (1) Temperatures of Inlet verses Outlet (2) B1S2 O2 sensor performance. -indicates that Cat inefficiency DTC my set any day now. At least now I know that my cats/exhaust are not semi-plugged up.
 

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Doesn't sound plugged up, one possible problem checked off.

Blow in the gauge for a crude test, should respond nicely.

Some vacuum gauges also measure low pressures, tee another gauge with that one (spare odd vacuum hose, fitting) to see if they respond the same for a better test.

Sounds like you're still looking for a problem as I am currently.
 

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This might be redundant, but have you confirmed that the downstream O2 is actually working correctly? Also what were your inlet / outlet temps?
 

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Discussion Starter #6
^^ Hi. Yes, I confirmed the down-stream O2 is working + I saw it was working poorly <several times> which is often connected to a Cat that is working inefficiently. I really hope my PCM does not set a Cat inefficiently DTC before I get to emission testing due late 2015. -please!

Temps? Darn-it, not sure where I put them. Posted them in another thread, but I'm too lazy to hunt the temps recorded atm. End result was Hotter on the inlet-side than the outlet-side; which is opposite of what a healthy Cat does.
 

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Discussion Starter #7 (Edited)
... End result was Hotter on the inlet-side than the outlet-side; which is opposite of what a healthy Cat does.
Forgot to mention that;
A seemingly bad Cat might be misleading. Some bad Cats & their symptoms are a result of an engine problem that is causing the Cat to fail. So you gotta consider, maybe the engine exhaust gases are killing or are hurting the Cat and/or maybe the engine exhaust gasses are good & the O2 sensors (or Cat) are bad? Similar guesses can be said with temperature readings... Is it bad (rich?) engine exhaust that is causing the problem? -or a failed/failing cat?

Chicken or the egg?
 

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It's my understanding that very very rarely would a cat fail if engine-out gases are all good. There's always a reason for a cat failure and it has to be corrected or it will always result in another cat failure. That being said, you have to look back at repair history because the engine-out problem may have been corrected quite a while before the cat failure raised it's ugly head (e.g. previous owner).
 

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Discussion Starter #9
It's my understanding that very very rarely would a cat fail if engine-out gases are all good. There's always a reason for a cat failure and it has to be corrected or it will always result in another cat failure. That being said, you have to look back at repair history because the engine-out problem may have been corrected quite a while before the cat failure raised it's ugly head (e.g. previous owner).
I agree. Good post. Thanks!

I have an engine performance problem (different thread) and bad mpg; I believe I have bad/rich exhaust gasses now + a Cat that is barely hangin' in there. My next set of ideas are to remove and test the fuel injectors (tested by a pro with a real injector test bench -so not by me). Also, I have never done a cylinder compression test on this car.
 

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The question is; what psi measurement would be considered ok, marginal, or bad on a Focus? I have the basic answer to this question thanks to the interwebs;->
http://www.aa1car.com/library/exhaust_backpressure.htm

"Backpressure readings at idle on most engines should generally be less than 1.5 psi (10 kPa). This will vary somewhat from one vehicle to another depending on the design of the exhaust system, the size of the pipes, how restrictive the converter, muffler and/or resonator is, and whether it is single or dual exhausts. We've seen some idle readings as high as 2.75 psi on a few vehicles, but for most 1.5 psi or less at idle is normal."

If you have done backpressure tests on a focus and remember the psi numbers, please post them. Thanks!
While at Idle and at 2500 RPM, the pressure measured was 1 psi or less.

While at 2500 to 3000 rpm, the pressure would briefly spike to 2 or 3 psi when the throttle was released and while it was returning to Idle. -which seemed normal or ok to me.
Your numbers are definitely okay.

I was just browsing a textbook and it states that at idle, max back pressure should be less than 1.5 PSI, and it should be less than 2.5 PSI at 2500 RPM. I'm pretty confident that pressures slightly higher than that fall within specs still so I wouldn't condemn a cat or call an exhaust restricted if the numbers were a couple PSI higher.
 
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