Check engine light goes off after 3rd tank of 91

Ok so I was pulling a Cat code or high idle code about 4 months ago. I used a Cat cleaner and high octane Chevron gas on a low tank as suggested. I never cleared the code manually.

Check engine light stayed on all that time. I went back to running just low octane gas for a while and then recently I noticed some pinging and just not great performance so I went back to 91 Chevron gas. After my 3rd tank the check engine light went off on its own and I just passed CA smog! Soooo for those who say premium gas is a waste of money I don't think so!

Remember my truck has 386,000 with original Cat. For the first 10 years of its life My dad gave it only premium gas in belief it would keep engine and ignition coils cleaner. Hey dad now I'm a believer!

 

@ 18mpg and .25 per gal delta, you would have spent $5,361 over the 386k miles.

Hardly worth it to save a $150 cat.

Detonation can come from many things. On a vehicle designed for regular fuel, detonation means something else is wrong.

Unfortunately, premium fuel is just a bandaid.

Good your light is out and you passed emissions!

 

Good point sir. But glad to hear you passed and no light. My factory cat clogged at 210k and it wouldn't let the motor over 2200rpm. Crazy but true.

 

91 is still a waste, IMO. Anyone who claims more power/efficiency from high octane gas is purely anecdotal/confirmation bias. The added cost of high octane over shaddow any theoretical (and marginal) increase in efficiency.

I'd rather see someone who has their wife/sig-o fill up their tank for a month or two, not telling which they put in, then comparing the results to see if you actually feel a difference vs. the "I paid $5 more for this tank, I *should* feel a difference."

Unless you have a supercharger, you shouldn't need to run high octane. Your knocking is likely due to other issues (still running orig injectors at 386k miles?).

Just run a bottle of injector cleaner through it every couple of months. That's far cheaper than running high octane all the time.

 

Correlation does not imply causation, you're masking a real detonation issue. These trucks are pretty low compression and designed for regular fuel.

 

All your answers as expected...

BTW California cats are over $1000 and not available for sale anywhere. Plus these have 2.

Also if everyone reccomends high octane fuel when trying to pass smog why would it not give you better performance every day?

Plus Chevron gas already has Techron so I think injector cleaner would also be a waste correct?

 

I do agree on detonation issues. I was planning to clear the code after I changed the spark plugs and cleaned the throttle body / iac. My surprise was that the code cleared itself before I even got a chance to do that

 

So if my cats go out, I'm stuck buying used ones? Or are they literally "not available for sale anywhere"?

Sorry but I haven't seen any recommendation for premium fuel to help pass an emissions inspection. Maybe true, I don't know... seems to have worked for you! Very good to see success. Smog checks can be a real pain. I do know that an engine runs best with the fuel it is designed burn, assuming everything is in good working condition. Our 2.7s are relatively low compression and need regular gas only. I suspect you have a carbon buildup issue, or a heat/cooling issue, ignition system issue, or something else that would cause regular fuel to pre-ignite.

Finally, I would not add Techron additive to Chevron fuel. I would (and sometimes do) add it to Safeway, Circle K, or any other "no-brand" brand fuel LOL. For some reason, I believe the level of treatment adding a bottle of Techron to a tank of generic brand fuel is more that what you get at the Chevron pumps. Maybe true, maybe not... I just can't imagine the fuel tanker trucks dumping that much (ratio-wise) into their mix when they fill up at the refinery. I'm probably wrong.

 

I'm fairly positive you can't buy used cats in CA (at least not to legally pass smog), but I have seen some aftermarket CARB ones that are much cheaper than OEM. But they have to be CARB approved or else the government's gonna getcha come smog check time.

 

A full tank of high octane fuel and a good run on the highway before a smog is what people have always told me anyway.

As far as getting a new Cat the suppliers usually will not ship them to CA to a non mechanic, you are usually stuck going through a mechanic or dealer and at their mercy. Selling used Cats is illegal also in CA like most things other than pot

 

Higher octane does not mean more explosive, it's actually more resistive to combustion for engines that run high compression and hotter cyl temps. It is not cleaner or more efficient than regular, just for a different engine. Some engines have fuel mappings for both, ours do not and are designed to run on regular. Hence why you are likely running less performance (probably not very measurably) but less chance of spontaneous combustion causing issues.

 

Italian Tuneup. it's legit if it's a strong motor, just old with carbon buildup

 

That was more relevant in the days of carburetors and heavy carbon deposits. But can confirm it does wonders when my old Honda bike acts up haha. Actually does okay in the truck too but mostly because it gets the cat up to efficient temp.

 

I remember muffler guy (in Sacramento CA) said it is illegal for them to install used cat. The new aftermarket CA legal was less than 200 dollars installed. SMOG guys have a database now with list of CA approved cats that allowed to be installed on particular year, model, engine combination. It the cat model is not there the SMOG will not pass (legally). That is of course on additionally to sniff test, fuel cap test, ODBII and visual inspection.