I meant cylinder compression and cylinder leak down test. There is nothing wrong with doing the fuel pressure leak down test but I Don’t think that’s your problem

George

 

you can read my fuel trim article found in my signature. Negative isn’t good or bad. Large numbers are bad. Changing numbers under different conditions help us find the problem

George

 

So, fuel pressure leak down was good, thew the O2s on scope again, and wasn't happy at what I got

 

exhaust restriction from a bad cat can cause your symptoms and running rich can cause a bad cat. What is the decode on those graph colors?

George

 

Do you know how to test for a failed cat?:

Exhaust pressure gauge
Vacuum gauge
Remove O2 sensor
IR gun on cat
...


George

 

red and green are the two upstream and the blue is downstream

 

It sure looks like you might have a bad cat. If that is confirmed the question then is why? The biggest causes of premature failure is:

Physical damage
Burning oil
Coolant leak
Unburned fuel (running rich)
Poor quality replacement

You will need to get to the bottom of why your truck is killing cats.


George

 

Thought I would throw this into the mix.....remove the EGR valve, clean it, and the two small intake holes....look for any small pieces of carbon that could be keeping the EGR valve open.

Also, a question to George...would removing the upstream O2 sensor, then try rev'ing engine, see if it goes past 1400rpm....would this be a valid quick test for a clogged cat? CEL will be set as the O2 sensor reading will be wrong. Can only do this for a few seconds.

 

There are a number of ways to test for a cat induced exhaust restriction. The most conclusive test is to remove an O2 sensor and measure the back pressure at the O2 port. There is also temp measurements, vacuum measurements, tapping on the cat with a mallet, the O2 sensor readings and yes, temporarily creating an exhaust leak by removing an O2 sensor or loosening an exhaust pipe fitting to see if things improve.

George

 

I forget about the EGR valve because my 4.3L does not have one. It’s worth a look.

George