can a bad cat cause a low compression numbers?

[Gary Allan]

Lean burns things because it burns off the insulating layer of unburned fuel on the margins. It's cooler. I always thought it ran hotter before being kindly corrected. That's why they use in inert material to cool the combustion with EGR. All it basically does is take up space that you would normally have to fill with Ox/Nitr air.

 

[The_Jeff]

You could run a compression test with the upstream o2 out. They say you should run compression tests at WOT; having had a bad cat act as an effective jake brake, it would throttle things down on the other end.

"They say" compression tests are most useful for difference between cylinders b/c of all the variables in technique. But I bet compression will be up with the o2 out.

 

[gathermewool]

leaner = hotter. Anything richer than stoich runs cooler because energy is required to vaporize the excess fuel that does not burn.

Some of us turbo subie guys run aftermarket tuning, and one of the major effects of this is running 11:1 AFR in open loop while boosting. The stock map runs much higher than this - closer to stoich. The result is a much cooler burn, and the ability to up the target boost with out worry of detonation.

To the OP - Find out why he had the cats replaced. Unless there was a cat CEL code there's no reason to. Compression is with valves shut to determine how much leaks past the rings and/or valves. IMHO cats shouldn't affect it.

 

[ms21043]

All cylinders between 75-85 and only 38k miles? Did someone mess with the cam timing? could be late, belt jump a tooth?

 

[bdcardinal]

Originally Posted By: Onmo'Eegusee
120ish is still very low. Did they do it right by removing ALL the spark plugs before testing? A leak down test will be more specific as to where the compression is going. Rebuild time!


on fords you dont have to pull plugs or open the hood to do a compression test. just plug in the factory scan tool and crank the motor with the throttle open.

 

[gathermewool]

Originally Posted By: bdcardinal
Originally Posted By: Onmo'Eegusee
120ish is still very low. Did they do it right by removing ALL the spark plugs before testing? A leak down test will be more specific as to where the compression is going. Rebuild time!


on fords you dont have to pull plugs or open the hood to do a compression test. just plug in the factory scan tool and crank the motor with the throttle open.


That's a neat trick. What senses the pressure?

 

[bdcardinal]

its a relative compression check. it factors compression based off of cranking rpm variation for each cylinder.

on warranty jobs thats all ford will pay for unless for some reason they want a physical compression check.

 

[mechtech2]

If all the cylinders are low, suspect a timing belt problem.

BTW, running lean won't raise combustion temperatures, it lowers them. Same with rich. The highest temp is at stoichiometric.

Normally , a bad cat is a result of something, and would not cause low compression.