eallanboggs

iTrader: (1)

Suspect a clogged cat. Ordered the tester. I figure I'll do the check at the pre-cat bung first and then at the post-cat bung. The way I understand it I shouldn't see more than 1.25 psi at the pre-cat bung. If I do the cat has blockage. If I see more than this amount at the post-cat bung the muffler is blocked. If the value reaches 2 psi or more the vehicle is having a asthma attack. Do I understand the procedeure?

Formulated

iTrader: (16)

I think you have the procedure down. I would be concerned about your acceptance criteria, though. What speed and load do you expect to see 1.25 psi at? At WOT / peak power, 5 psi pre-cat would be a really good stock system. Typically, cats like GM use have about a 2 psi pressure differential at WOT, peak power. Add to that some pressure loss for the tubing and muffler. The muffler itself is usually good for at least 2 psi.

I guarantee, you have way more than 1.25 psi at WOT !

eallanboggs

iTrader: (1)

This is my first time doing this test as you might have already concluded. The 1.25 value is at 2500 RPM. Do I conclude the cat is clogged if I get a lower reading at the post-cat than I do at the pre-cat? This is a 3.4 Chevy Impala V6. I got that 1.25 value from Jim Wilson who is a GM ASE Master Tech. He has a driveability website called flatrater that I subscribe to. Am I on track saying the cat is clogged if I see a high pre-cat value(say over 2 psi at 2500 RPM or over 5 psi at WOT) and then see a reduced value at the post-cat bung? To me that says the cat is blocked and the the muffler is not. It would be a lot cheaper if the reading was just as high at the post-cat bung as it is at the pre-cat since mufflers cost a lot less than cats. That would mean the muffler or resonator is blocked instead of the cat. Am I at least in the ball park on my procedure?

Formulated

iTrader: (16)

It would probably be worthwhile contacting Jim Wilson at his website to clarify his 1.25 psi conditions. I'm sure that spec is based on something, you just need to clarify what that condition is.

I would do the test anyway, and post up your results to get feedback.

eallanboggs

iTrader: (1)

I need to know how to isolate the problem if I get a reading that is too high. Do I start unbolting exhaust parts until the reading goes down(meaning the pressure has been relieved)? For example if I get a high reading at the pre-cat bung do I automatically assume the cat is bad or do I have to unbolt the exhaust behind the cat to see if the pressure stays up. In other words I want to be sure it's the cat and not the muffler. The cat is $800 while a muffler is a lot cheaper. Chances are it is the cat, but I want to be SURE before I buy a cat that isn't clogged

Dan Stewart

Differential pressure is not greatly affected by changes in flow downstream of the measured point. Only absolute pressure changes.

So, if you measure a 3psi delta across the cat, the muffler is not going to affect the delta in a noticeable amount.

Example:
5psi pre-cat - 2psi post-cat = 3psi differential. (Clogged cat, free-flowing muff).....high diff, low absolute
8psi pre-cat - 7.5psi post-cat = 0.5psi differential (free-flow cat, clogged muff).....see how the diff stays low but absolute is high?
2psi pre-cat - 1.5psi post-cat = 0.5psi differential (free-flow cat, free-flow muff).....once again, low diff and low absolute

If you really want to understand instrumentation, I recommend a copy of "Process Measurement and Analysis: Instrument Engineer's Handbook" by Liptak. Everything you need to know about instruments.

eallanboggs

iTrader: (1)

Thanks Dan for the detailed explanation. With that information I think I'm actually getting a grasp on the exhaust back pressure stuff. I'll post the test results and use this data to find my blockage.

Dan Stewart

Not a problem. As soon as you have all the data post it up.

I've got a couple of "extra" dp transmitters sitting here at work and doubt they would be missed. Thinking of making a homebrew dp test setup to check radiators, intercoolers, exhaust, oil flow, etc. Now all I need is a 1:2 transformer for a 24v supply....

eallanboggs

iTrader: (1)

Here are the back pressure test results:

Engine Speed (RPM) Pre-Cat Pressure (psi) Post-Cat Pressure (psi)

Idle 0.5 ZERO
1.000 2.5 ZERO
1.500 3.0 ZERO
2.000 3.5 ZERO
2.500 5.5 ZERO

As you can see the cat is completely blocked. Funny thing is there was no SES lite, but you could feel decent pressure and heat with your hand at the tail pipe with the cat blocked. Both of those things have me confused. If anyone can explain how you can have a blocked cat (ZERO psi), but not have a DTC set and still have pressure at the tailpipe I'd like to hear it. The car is running like new now. I managed to figure a way to releve the pressure after a trip to Home Depot. A new cat was $800.00 plus labor whereas the tool from the hardware store was $12 and there is still no SES. Go figure.