Joeyofblades

iTrader: (2)

(I'm sorry if this is the wrong section, but it kind of falls under multiple categories and I'm not sure what the best place to start this topic would be)

Background: '98 LS1, 10.7:1 compression with PRC 2.5 heads and a BTR Stage 4 cam, racetronix 255 pump, 38# procharger injectors. Car was tuned in March and recently started throwing knock sensor codes. Changed knock sensors and harness, still have codes coming back. I'm diagnosing the issue but one thing that came up was in October, around the time these codes started coming up, is about the same time I started putting Costco 93 in my car instead of Shell 93. Obviously it's cheaper, and it's rated top tier so I figured why would it be bad? My 5th gen LS3 Camaro (2010) has had 0 issues with Costco gas but it's stock.

So to the point: some of my friends are saying Costco gas could be setting off my knock sensors. Because it's cheap and not agreeing with the car. Maybe not combusting right or something? So has anyone had any experience with Costco gas being bad for a heads/cam LS car? I thought it would be just as good as Shell or BP because it's top tier. Am I wrong? Am I buying crappy gas for my car and hurting it? I'm going to switch back to Shell and see if it fixes my knock sensor issue, but I wanted to see if anyone else has had problems like this or similar using Costco gas because if it's bad I'll stop using it but if it's fine, I'll keep getting it for cheaper than Shell/BP.

The reason I ask for personal experiences is because I don't want people to say "Oh it's obviously bad just because it's Costco! Stop being cheap!" I want actual assurance that it's a bad idea to use Costco instead of Shell/BP/Mobil/Exxon other than people's superstitions. I am not a tuner and can't look at fuel trims or wideband information but I'm sure someone here is. My friend insists his wideband acts up when he uses any gas except Shell, but I'd like more opinions on the matter.

y2keglide

iTrader: (2)

Well I'd go to a Chevron and fill it with premium a couple times and see if it doesn't clear things up. If it doesn't you'll know it wasn't the Costco gas, if it does clear up don't go back to Costco.

Joeyofblades

iTrader: (2)

This is the current plan, I'm going to see if that does the trick and I'll post back with my results. Just wondering if anyone else had a similar issue and can say for sure. I know using below 93 would cause problems and I'm sure the car wouldn't take well to gas from say, 7-11, but would it actually throw knock sensor codes?

98redorangeta

iTrader: (10)

From my understanding the octane rating is independent of the "top tier" rating so the main difference should just be detergent packages and things that help prevent build up and clogging of the fuel system.

As far as being rated the same I think there is still plenty of variance even in the top tier ratings.

Along with changing back to shell I would pull the intake and spark plugs and take a look and see if you have any issues.

Joeyofblades

iTrader: (2)

I was thinking of pulling the spark plugs, but wouldn't a bad plug cause a misfire? the intake manifold looked fine when we did knock sensors yesterday but codes came back after 15 minutes of driving.

Would running some injector cleaner help once I switch back to Shell? Or should I just put Shell in it and leave it alone and drive for a while?

RixTrix

The CostCo in my area adds Techroline / Techron to their bulk fuel. Ask them?

Joeyofblades

iTrader: (2)

I doubt the Costco employees know what's in the gas... and if I asked the engineers they'd give me a technical answer I wouldn't understand. I need a tuner or someone who has had the problem before to just be like "Yeah stop doing that, use Shell" or "No, Cotsco gas is perfectly fine for any car even a highly modded one that accepts 93"

y2keglide

iTrader: (2)

Costco gets their gas from a variety of suppliers even within a certain area so you never know what you're getting and what you get there is probably different than what I get here. I'll put their gas in my S10 go to work beater but for my Camaro or Harley I buy name brand premium, yeah it's costs a bit more but a dollar or two a tank full is worth the piece of mind IMO.

Joeyofblades

iTrader: (2)

Ah, crap, that basically invalidates this thread then. Well, I think with that I'm just going to stop getting Costco. You're right about peace of mind and it just seems like you never know what you're getting with them. That doesn't sit well with me.

y2keglide

iTrader: (2)

Probably the best move IMO,do the math and you aren't saving a whole lot of money anyway especially if it's not a daily driver like mine but either way really. For the cost of a bottle of Coke it's worth it IMHO.

BrntWS6

iTrader: (7)

Years ago I started getting gas at a 7-11 since it was close. After the second fill up i got the rotten egg smell, at the time my car was basically stock. Switched back to shell and the smell went away, not costco but just an example of inferior gas.

98redorangeta

iTrader: (10)

Mostly just look for a lot of deposit or build up. I would think injector cleaner would help if you do have build up.

You can also run some 110 unleaded and see if the knock stuff goes away to rule out any actual knock.

Marc 85Z28

iTrader: (8)

Fuel cannot cause knock sensor DTCs.

Any DTC set for the knock sensor is a circuit problem.

You did use GM sensors, right?

SikkyMFG

As said previously in this thread with costco they buy gas from a pretty wide variety of suppliers. You really can't guarantee the quality of the fuel like you can with a shell, exxon or BP.

Joeyofblades

iTrader: (2)

Just spoke with my tuner and he uses Costco in everything that doesn't take E85. I appreciate all of the feedback in this thread but given that my tuner has been doing it for 20 years and is very well-known and reputable in Maryland with LS engines, I suspect that the Costco gas has caused 0% of my knock sensor issue (especially since I switched back to using Shell for the past 3 tanks and the issue hasn't changed at all).

I think an important takeaway here is that the quality control of Costco as a wholesale buyer is probably going to be less reliable than that of Shell/Exxon/BP. I definitely believe that there are "bad batches" of gas out there and think it's not unreasonable to think the frequency of bad batches might be a bit higher with Costco than other top tier fuel stations. However, I don't think it's common enough that anyone should avoid Costco.

I think that's all I have left to say on this topic, so hopefully it helps someone besides me and gives some insight.