02 sensor and gutted precats

thundering02 · #1 09-16-2005, 03:59 PM

Ok the question is that if I gut the pre cats will the O2 sensors still work properly or will the system need to be modified.....Long story short I bottomed out and mangled the main cat a little and am now looking to redo the exhaust either header back or at least precat back.

immortal_suby · #2 09-16-2005, 04:14 PM

I had read on here before some speculation that without the precats the 02 sensors do not get hot enough to read correctly.

I have been running with no precats for a few years now and have been getting 25-26 highway mpg and I don't have any CEL problems. So I don't think it will be a problem based on my experience.

thundering02 · #3 09-16-2005, 06:48 PM

As is I'm only getting 23ish at best So thats good to hear, also right about when its up to temp if I stay low in the revs I can get a check engine light for the driver side O2 sensor, do I need to go with the Subaru $100 Oem one or can I just use a $40 universal and be just fine?

immortal_suby · #4 09-16-2005, 07:42 PM

Alot of people will tell you to only use OEM sensors and they are probably right.

But - last time I changed mine I went with the splice in sensors and I haven't had any mileage decrease or CEL's since. Only been about 6 months with them so time will tell if they do not last as long. They seem to be working fine so far.

Budfreak · #5 09-17-2005, 06:07 AM

Quote:

Originally Posted by thundering02

As is I'm only getting 23ish at best So thats good to hear, also right about when its up to temp if I stay low in the revs I can get a check engine light for the driver side O2 sensor, do I need to go with the Subaru $100 Oem one or can I just use a $40 universal and be just fine?

i have the exact same problem with my pass. side sensor. just had them replaced last oct. at the dealership too.

thundering02 · #6 09-17-2005, 09:48 AM

After you replaced them or did that fix it?

Budfreak · #7 09-17-2005, 03:32 PM

Quote:

Originally Posted by thundering02

After you replaced them or did that fix it?

after i replaced them. it's only been 10 months and they are bad again.

Beav · #8 09-17-2005, 03:59 PM

No matter how many times I tell people they still don't get it. Codes don't necessarily mean that the identified sensor is the actual problem. Changing (or just changing) a sensor because of a code is the Autozone approach. They'll read the code for free and even sell you a sensor but that usually isn't the root cause of the problem. It ends up costing you money, to their profit as most people just buy more parts in attempt to correct the problem - called the shotgun approach. Consumers get mad as hell when their dumbass mechanic ("but he's cheap, he has to be good") takes this approach but they seem to be willing to accept it when they are the uh.. underinformed party.

In particular O2 sensors are pretty reliable devices, especially considering the environment they operate in. By far and large they will survive the vehicle they reside in. What causes those that fail are bad gas, excessive fueling caused by other engine or engine management malfunctions, chemical pollution - such as silicone sealers or antifreeze contamination and physical damage. Then, of course, some just fail due to age and/or manufacturing inconsistencies but these probably comprise the smallest percentage of failures. That being said, when an O2 sensor fails there is almost always something else to blame and that root cause needs to be addressed in order to prevent subsequent failures.

In answer to the original question, gutting the pre-cats will not affect the O2 sensors ability to reliably perform their intended function. Reduction of back-pressure in the exhaust can promote sluggish response or extended warm-up periods but just gutting the pre-cats, and leaving the main cat in place, should have little to no effect on that.

thundering02 · #9 09-18-2005, 01:19 AM

Ok then what about the accompanying Driver side knock sensor CEL? Also did the O2 sensor and I think fuel milage has already gone up.... 280 with 1/4 tank left and I usually get about 275.

Budfreak · #10 09-18-2005, 02:13 AM

i've been throwing a code 32 for awhile now. i had new o2 sensors put in in oct and i immediately got another code 32. took it back and got another sensor put in and it was fine till the day i adjusted my brake band and then it started lighting up the code 32 again. i figured my cam timing might be off so i checked it and sure enough it was. i fixed that and now i'm still throwing a code 32 and threw a 37 once too. i was thinking my pcv valve being clogged and filling my intake with oil or the cam timing may be what screwed up the sensors and the cats which are now semi clogged and need fixed. what do you think beav?

thundering02 · #11 09-18-2005, 02:17 AM

I do know that my PCV needs replacing and have had one for some time now guess I'm gonna hafta go to sears. (for a tool or two.) BTW that makes sense to me.

Budfreak · #12 09-18-2005, 02:20 AM

Quote:

Originally Posted by thundering02

I do know that my PCV needs replacing and have had one for some time now guess I'm gonna hafta go to sears. (for a tool or two.) BTW that makes sense to me.

buy yourself a set of metric crows foot wrenches. they come in handy every now and again and they make the pcv a snap.

yeah it made sense to me too.

Beav · #13 09-18-2005, 09:13 AM

I've not seen an O2 sensor go bad from oil contamination that wasn't also putting out an easily noticeable cloud from the exhaust. It also takes a LOT of oil burning to clog a cat, even partially. Until someone does a backpressure test and/or actually removes the cat and looks inside there is little reason to suspect clogging. Cats generally only go bad from design (usually the 'guts' work loose and rattle) or from gross negligence/abuse. Generally a car would be running fairly poorly for quite a while to have a cat to go down as a result. I would be inclined to think you may have a wiring issue since the problem recently returned after you were in the vicinity, adjusting the band.

Budfreak · #14 09-18-2005, 02:17 PM

Quote:

Originally Posted by Beav

I've not seen an O2 sensor go bad from oil contamination that wasn't also putting out an easily noticeable cloud from the exhaust. It also takes a LOT of oil burning to clog a cat, even partially. Until someone does a backpressure test and/or actually removes the cat and looks inside there is little reason to suspect clogging. Cats generally only go bad from design (usually the 'guts' work loose and rattle) or from gross negligence/abuse. Generally a car would be running fairly poorly for quite a while to have a cat to go down as a result. I would be inclined to think you may have a wiring issue since the problem recently returned after you were in the vicinity, adjusting the band.

i thought about the wiring thing and checked that all out and it looks good. cleaned the connections also.the cel only happens when just about warmed up and under low rpm steady load and as soon as i change the throttle position it will usually go out. my reason for suspecting the cats is cause my car will have gobs of power and then it won't and i hear the little tell tale hissing sound when i get on it while it's feeling weak. don't notice the hiss as much when it's running good. plus i get the rotten egg smell every now and again that's just awful, so i'm pretty sure there clogging. i've had 2 cars that clogged there cats up and they acted the same way too. plus the fact that i have no idea how long the cam timing was off before i bought it. i alone put 12k miles on it with them out of time(pass.side was advanced a tooth and drivers side was retarded a tooth. they didn't even use the white lines when they put the belt on, just eyeballed it.)

so i figure it probally cooked them. it's also been bottomed out on the pipes by the looks of it so they need replaced anyway. is there anyway to test the o2 sensor at all? i hate to spend the money for them if there not actually bad. would the timing being off help screw up the sensors? i've only gotten code 37 once since the sensors were replaced and that was shortly after fixing the timing. hasn't happened again though. get the code 32 maybe once or twice a week under the previously described conditions. mileage ain't that great either but it goes up and down. i can get 320-350 miles to one tank and get 220 to another. i just got 130 miles to a half tank this time.

14 mpg.yay.

Beav · #15 09-18-2005, 02:42 PM

The only way I know to check an O2 sensor is with a graphing multi meter or o'scope. Warmed-up sensor, engine @ 2000 rpm, voltage should rise and fall between ~.2vdc & ~.9vdc at least once every five seconds, the more the merrier.

Hissing could simply be leaking gaskets. Rotten egg smell comes from excessive fueling/poor combustion (which are the same thing as far as the cat is concerned.)