can a bad cat cause a low compression numbers?

[Stanley Rockafeller]

just wondering as the shop who was working on an svt focus I'm very interested in, said the car failed due to bad cat(s) which are currently being replaced under warranty.

I know how a 4 cyl motor works...just not sure if that's the actual cause of the low compression numbers

 

[Colt45ws]

No. What are the numbers?

 

[c3po]

I would be interested in finding out why the converters went bad and what was causing your low compression numbers.

 

[Colt45ws]

Agree. Though sometimes its just a bad cat for whatever reason. I had a cat replaced on my Vic at 31k since it was rattling.

 

[brianl703]

If the cat breaks up pieces of it can get sucked back into the cylinder and score it. Or so I've heard.

 

[chevrofreak]

My bet would be that it's the other way around; the cats failed because of the low compression.

Was the car using much oil?

 

[Colt45ws]

Hmm, Ive never heard of that. This would be a permanent failure though, you can't fix it by putting on a new cat.

 

[Familyguy]

Originally Posted By: brianl703
If the cat breaks up pieces of it can get sucked back into the cylinder and score it. Or so I've heard.


LOL! Um....no.

Poor compression can easily kill a cat, though. You wind up with unburned fuel in the exhaust which will cause the cat to glow cherry red and destroy its innards in a fairly short time frame.

 

[Colt45ws]

Actually, you know what is funny is the exact opposite problem can do the same thing. Say the engine goes lean. Not really lean, but lean enough to damage the cylinders. This can also kill the cat from the extra heat.

 

[Stanley Rockafeller]

the numbers were b/t 75 and 85 psi across the board

but when the sellers dealer did a test, they yielded numbers around 120ish supposidly

also, the seller didn't mention the car usign any oil either. it shouldn't esp at 38k miles

I guess we'll soon see, as i await the results after the new cats are installed/

 

[Colt45ws]

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!

 

[Gary Allan]

Quote:
LOL! Um....no.


It won't happen on something where the cat is part of the exhaust system, but some engines (they may have been Nissan) have the pre-cats right off the exhaust manifolds. Under vacuum breaking (it's not really compression breaking, folks) the debris from the shattered lattice got sucked back into the head and damaged valves.

 

[JHZR2]

yeah, may turn into a better deal if a rebuild is done...

 

[SrDriver]

I would be looking for another vehicle.

 

[Familyguy]

Originally Posted By: Stanley Rockafeller
the numbers were b/t 75 and 85 psi across the board

but when the sellers dealer did a test, they yielded numbers around 120ish supposidly

also, the seller didn't mention the car usign any oil either. it shouldn't esp at 38k miles

I guess we'll soon see, as i await the results after the new cats are installed/


85psi implies about 5.7 to 1 compression ratio, no? Seems a bit low.

 

[tomcat27]

find another car. there's certainly more to this story than meets the eye. low compression AND bad cats? I smell abuse. There will be another car out there for you...

 

[punisher]

Originally Posted By: Onmo'Eegusee
Actually, you know what is funny is the exact opposite problem can do the same thing. Say the engine goes lean. Not really lean, but lean enough to damage the cylinders. This can also kill the cat from the extra heat.


No, just no.

The exhaust actually runs cooler on an engine running lean.

 

[flacoman]

Relatively common problem way back on the Zetec motor . Cat crumbles and gets inhaled through the PCV system. Visit contour.org for more p00p on it . Gotta love Ford though, had the same problem for 10 years and not fixed it yet!

HTH Jorge

 

[Colt45ws]

No. There is a reason they go so rich at WOT. 12.5:1 makes more power but 12:1 is usually safer for the engine. Ive seen dyno sheets showing 10:1 on stock vehicles. Dumping fuel in to cool the engine and the catalyst.
How does CAT get inhaled through the PCV? That system is connected to the intake.

 

[HM12460]

my 1st thought echoed what chevrofreak and familyguy said. i think i'd pass on that one. good used focus' are easy enough to find.