Engine compression test help

[Doubeleive]

you can just pull the fuel pump fuse then turn it over a few times and then it will just turn over and not have any thing in it to make it run, I have seen compression test's done with or without plugs. my cat's were not showing any outward excessive temp's but I was getting about 8mpg for a while and then it finally threw a cat code so I replaced them both and it shot right up to 10+, a lot of that has to do with the way I drive (heavy foot) and not many longer trips most of my driving is a few miles of highway and then about the same in city driving, if you have a good enough scanner you can monitor your cat temps with a live data feed.

 

[Tim10]

Now the complete story. By the way, where did you come up with lacquer thinner in the fuel? I have never heard of doing that. Is that an old wives tale? Not saying there may not be a good reason for it but never heard that before. The fuel filter suggestion by me was me rambling on. No fuel filter on the outside, like my 2002. Yours is in the tank with the pump. Change to mid range fuel at least and even the premium to see if you get a big enough jump in MPG to notice. You can always go down to mid grade. You did not say what fuel you use though. Tell me it isn't Shell. I did not know it till on this forum but there are some horror stories on here about that fuel. I see the temp you posted for the cat(s) and that looks good. If was way different that would indicate a clog. Miles on your engine? Miles on the oxygen sensors? Need to monitor the alternator output while driving. You can charge the battery to 14.6 but while driving and under load conditions you need to see what it charges. Load by lights on, fans on, radio on.....basically anything you can turn on. Do you have a Tech 2?

Once I got p0420 code I started to search online and the first thing came up was YouTube channel Scotty Kilmer where he suggests in the first place to use lacquer thinner or remove cats and soak them overnight in soapy water and lots of comments that for lots of people this method worked. I fill up at Costco gas station, they have 2 options regular and premium.
105,000miles on the engine, I don't know the mileage on o2 sensors, I got this Tahoe 5000 miles ago, but I think o2 sensors still from factory
live data shows sensors working properly. I don't have tech 2, just some good OBD2 scanner with lots of features and live data monitors

 

[Tim10]

you can just pull the fuel pump fuse then turn it over a few times and then it will just turn over and not have any thing in it to make it run, I have seen compression test's done with or without plugs. my cat's were not showing any outward excessive temp's but I was getting about 8mpg for a while and then it finally threw a cat code so I replaced them both and it shot right up to 10+, a lot of that has to do with the way I drive (heavy foot) and not many longer trips most of my driving is a few miles of highway and then about the same in city driving, if you have a good enough scanner you can monitor your cat temps with a live data feed.

Yes, I do have a live data monitor and it shows a stable temperature 1200F bank one and around 1100F bank 2. What I found so far online that inlet temperature should be lower by at least 100F than the outlet and if it is the same then cats need to be replaced is it true?

 

[Doubeleive]

Yes, I do have a live data monitor and it shows a stable temperature 1200F bank one and around 1100F bank 2. What I found so far online that inlet temperature should be lower by at least 100F than the outlet and if it is the same then cats need to be replaced is it true?

I don't know mine were reading the same temp in & out also and I was also getting 1200 deg I was very suspect of the cat's and then when the code came up that was it I just bit the bullet and had new ones put in magnaflow makes some aftermarket ones.

 

[OR VietVet]

I would have expected just the opposite. Where online did you find this?

 

[Tim10]

I would have expected just the opposite. Where online did you find this?

One of the sources
https://axleaddict.com/auto-repair/How-to-Test-a-Bad-Catalytic-Converter


“If the catalyst has stopped working, the front temperature will be about the same as or higher than the rear temperature.

In a good catalytic converter, the rear of the converter will be 20+ degrees hotter than the front.”

 

[Tim10]

I don't know mine were reading the same temp in & out also and I was also getting 1200 deg I was very suspect of the cat's and then when the code came up that was it I just bit the bullet and had new ones put in magnaflow makes some aftermarket ones.

Strange that MagnaFlow don’t make cats for 2016 and up Tahoe’s, only back exhaust.
Found that AP makes good cats for very reasonable price

 

[OR VietVet]

That's what I get for thinking but as old as I am I would have checked that info too. You may have cat(s) in your future.

 

[iamdub]

On the '00-'14 models, you can hold the accelerator to the floor when cranking and it'll kill the spark and fuel when cranking. I don't see why this wouldn't apply to your generation. A proper compression test is engine warm and with all spark plugs removed.

 

[Tim10]

Did compression test today, checked 3 cylinders on driver side. Was approximately 170 in each cylinder, so I’m happy with results.
Then decided to change transfer case fluid.
Almost 1.25 dark fluid leaked out and I filled with almost 2 quarts. I see a bit improvement on MPG.
Will transfer case fluid change can improve MPG ?