: Could a bad catalytic converter also decrease my gas mileage?



ejguillot
07-16-07, 02:43 AM
Hello,

I have a 2000 Seville STS, with 105,000 miles on it, that just set code P0420. From reading the various posts, it appears that a new catalytic converter is in my future.

I've also noticed that my average gas mileage has declined about 2 mpg (from about 19-21 to 18-19), and this happened after running a tank of Shell V-Power premium gas (supposedly with more additives, to help remove deposits). Normal gas diet is regular from the local Mobil station, which the car likes just fine.

Since the shell gas, I've also noticed decreased power some of the time (a partially clogged cat?), especially at/near WOT.

So... Is bad gas mileage a symptom consistent with a bad cat (or is there another problem I should look for first?)

Also, any suggestions for a higher flow aftermarket cat, that the onboard diagnostics won't throw a hissy fit over?

AJxtcman
07-16-07, 06:40 AM
If you remember my story about the my customer that had his heads reworked and had little to no performance gain. It was an 02 Eldo. I had always thought that the converter was restricted. It had been an oil burner before i replace the pistons and installed the heads. I suggested replacing it with a good converter, but he sold the car.

ejguillot
07-16-07, 08:05 AM
I'll be looking for a shop in this town (Tampa Bay area) that would drop in an aftermarket high flow cat. I was thinking of getting a cat with 3" in/out (and have them adapt it as needed), to prepare for an exhaust upgrade down the road. Any suggestions as to brand?

jadcock
07-16-07, 12:02 PM
No high performance catalytic converter discussion is compete without a mention of Random Technologies. They are typically considered "the best" aftermarket catalytic converters.

http://www.randomtechnology.com/

ejguillot
07-16-07, 04:43 PM
I decided to clear the code to see if it would come back, and put 1/2 a tank of the regular Mobil gas. Drove the car about 1 an hour and 30 miles, the code stayed away! :bouncy: And most of the symptoms seemed to have cleared themselves too. Actually, the engine seems a little smoother, and the missing power seems to be back. Jury's still out on the gas mileage.

Is it possible that the V-power gas dislodged so much junk that the cat was temporarily clogged?

I'll keep Random Technology in mind when I get the $$$ together for an upgraded exhaust.

jadcock
07-17-07, 09:33 AM
Is it possible that the V-power gas dislodged so much junk that the cat was temporarily clogged?

No; all major brands of gasoline are going to have a similar amount of fuel system cleaners. Shell advertises their "V power". Chevron advertises their "Techron". It's all detergent. Like Tide vs. All, or whatever brand you like.

Your 2000 STS was optimized for regular fuel. Using a higher octane than required CAN cause decreased performance (just like using a lower octane than required can cause the same thing).

I always use regular 87 fuel in mine and it runs like a beast. It sounds like yours runs best on 87 also, and that's certainly what I'd keep using.

ejguillot
09-25-07, 07:35 PM
The code is back! :mad: But with no surplus budget to replace it, i'll have to live with it for a while. Also, does anyone have the part # for the OEM part? Looked at gmpartsdirect, and the part #'s they have are for a complete exhaust system, from the converter to the muffler tips.

Submariner409
09-26-07, 12:08 AM
:sneaky: Changing to Shell whatever octane has absolutely nothing to do with your perceived gas mileage change. WNL (within normal limits) Shell, BP, Chevron, any top tier gasoline will give you practically the same performance results. Either 87 or 115/130 avgas. If Mom's Gas could instantly give you +2mpg, over anything else, Mom would be rich, and Big Oil would buy her out.....Just how much "crud" would a gasoline detergent have to "dislodge" in order to clog a cat ??? Scrape all the crap out of the downstream combustion areas of a modern V8 engine and see how full you can pack a teacup. You're running a computer-controlled powertrain, and the tuning options, either by computer or yourself, are NOT infinite. Don't start vast projects with half vast ideas........

FrankT
09-26-07, 10:34 AM
does anyone have the part # for the OEM part? Looked at gmpartsdirect, and the part #'s they have are for a complete exhaust system, from the converter to the muffler tips.

Cat by it's self, OEM Part #19152008=$905.00
The cat needs to be cut off the pipe, then clamped back on.
yea I know :rant2:

ejguillot
09-27-07, 12:03 AM
:sneaky: Changing to Shell whatever octane has absolutely nothing to do with your perceived gas mileage change. WNL (within normal limits) Shell, BP, Chevron, any top tier gasoline will give you practically the same performance results. Either 87 or 115/130 avgas. If Mom's Gas could instantly give you +2mpg, over anything else, Mom would be rich, and Big Oil would buy her out.....Just how much "crud" would a gasoline detergent have to "dislodge" in order to clog a cat ??? Scrape all the crap out of the downstream combustion areas of a modern V8 engine and see how full you can pack a teacup. You're running a computer-controlled powertrain, and the tuning options, either by computer or yourself, are NOT infinite. Don't start vast projects with half vast ideas........

I know... I'm suspecting an intermittently sticky EGR valve, and/or an intermittent MAP sensor connection. The EGR valve I removed and cleaned out per the tech tip, but even after cleaning it would sometimes stick a bit when pushing it in. Had a code for the MAP sensor (have another thread going on that) so I'll carefully reinspect the wiring again.

The cat problem I'll have to deal with when my money situation stabilizes. Since the OEM cat is so expensive, has anybody actually put in a Random Technologies cat in their Caddy?

derone
10-27-08, 12:56 PM
No, but I used a Magnaflow 46006 on mine after wasting bucks on an Eastern 'direct-fit' replacement converter. The eastern barely got outta of the garage before the light was back on. The Magnaflow has been good for over 1k miles now...no light(P0420code). Jeg's had it drop shipped for $210, but they've gone up a little since then. The 46006 is ODB2 certified and California emissions ready.
Here's a link: http://www.car-sound.com/02product/universalCARB/460.asp
The dealer price is ridiculous for the cat and yeah I noticed about a 1-2mpg increase that I'm thinking once it runs with all of the readiness indicators good in the PCM, it becomes more efficient.

Submariner409
10-27-08, 05:16 PM
You're correct. Once the fuel/air/exhaust system is again performing within the specs set by GM (or whoever) the fuel trims recalculate to run at the highest efficiency.

A "city car" will almost always seem to perform better toward the end of a long highway trip due to adjustments in the long-term fuel trims and, of course, the very fact that an internal combustion engine is most happy at highway speeds. At 60-80 mph long hauls everything is warmed up, there is no expansion/contraction between parts, cooling operates as designed, lubrication is near perfect, and mechanical wear almost ceases. The exhaust system also gets murderously hot and burns off all the city nasties.

thu
10-28-08, 04:07 AM
When my cat plugged last year, the dealer ordered a replacement. When it showed up, it had an entire exhaust system attached to it. The only thing the dealer had to do was cut the tips off the old exhaust and weld them onto the new one.

Cost to me was $0 - it was performed under warranty. You might want to check with a dealer if this part is covered under the emissions warranty on your particular car. Emissions warranties, I believe, are good to 150,000 miles.

ewill3rd
10-28-08, 07:41 AM
A P0420 means the catalyst is not working, it doesn't mean it is plugged.
I have seen partially plugged cats not set codes also. The two things indicate a faulty catalyst but for different reasons. One does not necessarily mean the other.
Traffic and weather can play a role in mileage, a variance of 2 mpg over a short, or even long period of time can be due to so many factors it is ridiculous. Blaming it on using shell gas instead of mobil gas is just silly really.
By all means use what you think your car runs best on.

For best results and easy installation use an OE cat.

derone
10-29-08, 12:04 PM
Yea, I did go to the dealer first for the emissions warranty at 76k miles thinking I was within the emissions warranty period of 8yrs/80k miles. Found out I was 2 months over the inservice date, so I was reseting that code about every 50-150 miles until dropping some bucks on the cat replacement. I've got a little over 90k on it now. The OEM replacement at gmpartsdirect is ~$850. Way high for a replacement part that isnt that unique. Even after buying 2 aftermarket ones...I'm still better off.