Thread: Fuel/Octane Issue?

Hello all,
I purchased a 2003 Grand Prix GT 80,000 mi.., no mods, bone stock. Since I owned this car, about a month, I've been putting in 93 octane ( Shell ). Then I read I am waisting my money. I had about a half a tank of 93 and decided to top it off with 87.
Now, I noticed if I accelerate hard up a gradual incline, @ about 5500 RPM the RPM's start to roll off, I hear what appears to be pinging and small backfires and the engine runs like %$@!. If I back off, the engine runs fine. Otherwise it idles very smooth and it accelerates from midrange on as well as expected, for stock.
Could this mean that I'm running too low octane now? I have read here that these 3.8's Series II should run fine with 87. BTW, no engine light at all and has never come on for any reason since I have owned the car.
I'm hoping its not transmission slip.....

New fuel filter
New PCV
Spark plugs ( Autolites ) look fine and the gaps were with spec @ .060, so I did not replace them.


Thanks in advance.....

Good suggestion..... I'm not certain if the original owner had replaced the wires.. The boots on the coil side are stamped "Delphi" would these be the stock wires?

I'll jump on the plug wires tomorrow. Any recommendations on quality wires? I understand and have read here that Auozone lifetime wires are good? Any others better?

It seems odd that I never had this issue running 93 though. @ 5500 to 5800, it would shift smooth into the last gear and continue to accelerate.

I'm thinkin' I got a bad batch of fuel.

1/2 93 Octane - 1/2 87 Octane = bad idea maybe. This car ran strong with 93 Octane, no issues at all, I can feel its down on power and now maybe a full tank of junk fuel. The last tank of fuel, I added seafoam ( about an ounce to a gallon of fuel ), maybe my fuel filter needs another change?
Anyway, has anyone here run 87 or 89 with any performance issues? Could there be something else going on with this motor ( like bad plug wires as mentioned before ) would the addition of the lower octane make the engine run worse if the plug wires where the issue?

Drawing straws here....

Running badly...

Still no Engine light, no codes at all...

Any ideas? Anyone want to chime in?

I'm running seafoam through the tank now, making little difference.

I would clean the MAF, replace the wires with stock AC Delco's (I get mine from the dealer- lifetime warranty) and see what you get.

I run NGK V-power TR55's in my stock 00GT with Delco wires and 87 octane. No ping or knock. Been that way forever. Still runs just fine even under a heavy load.

Try running the Seafoam through a vac line that will get it into the cylinders. May just be carbon buildup. At 80K I doubt it, but you never know. And if your Shell gas is like mine around here my GT runs like sh*t on it. I will drive the extra 5 miles w/ the low fuel light on to get to Phillips 66. This is just my experience, yours may be better.

O/T- BTW, interesting screename. You actually a PBX tech?

Thanks for your reply, very good suggestion.....

I am a PBX/Switch Tech, 25 years ( Nortel, now Cisco ). Used to race boats for a hobby and a second hobby Certified Mercury Marine Mechanic ( had to repair my own MODVP Outboards ). Not too great with car engines, though.

O.k.. So it finally threw a code P0301 ( Fairly useless code, my guess is still better ) narrows it down to, oh lets say, 10 problems, like in a marriage, he, he....
There's seems to be nothing up with CYL. #1, best I could tell from the # 1 plug.....
Funny you should mention Shell, thats where I filled the last half of the tank with ( 87 OCTANE ). Honestly, the engine seems to be running lean with a flat spot at mid range, midrange to 4500 RPM is o.k., 4500 RPM is about it though, then it backfires through the intake and exhaust, timing I presume or very very lean.
This issue has been coming on gradually for the last month, the power curve would roll off at about 5500 RPM now it rolls off at 4500 RPM, the engine now has significant power loss and I am driving it only as needed.
What really seems to have killed it is the last fillup at Shell with 87 Octane. I learned as a mechanic not to "part swap", start with the simple things, it used to be the customer's money, so that last thing I want to do is start screwing parts into the engine in order to resolve this issue.
I'll have my girlfriend drive it to run it out of gas , she's got that down to a science, then go to Mobile or Marathon.

I'm pulling the fuel filter too...

Thanks again for your reply and suggestions.

Have you thought of pulling the ECM fuse to erase the BLM and start over from fresh?

Great suggestion....

I pulled off the positive battery cable, turn the ignition to "start" for over a min. -> turned it back to "off" -> put the battery cable back on.
Would this erase the BLM?
If so, I drove it for a while so it would "relearn" , runs the same...

Taking my car into a very good mechanic in my town... When he repairs the issue, I'll share the results with you all.

On another topic, I'm going to to the LIM gasket. I have a porting tool and thinking I would grind and match the ports to exact gasket size, then polish while I have them exposed.

Any thoughts?

Good ol Nortel. I've been over on the Avaya (used to be Lucent) side of things for 12 years. They own part of Nortel now as well along w/ Cisco.

Changing the fuel filter may help as it may be clogged up. Not sure I'd bother port matching a GT, but don't see why not. Keep us updated w/ the results.

Issue resolved.....

Based on the info I included in the last several posts, would anyone like to submit a good guess as to what the issue was? This is going in my black book of notes...

BTW, the issue was not anything mentioned here in past replies.

No good guess... Too easy 'cause thats what the code threw.

Your good.... That's exactly what it was.
A code P0301 - a plugged cat? Not my first guess.
I was ready to replace Coil Pack, Plug Wires, the list went on and on.
The CAT cost me $500.00 parts and labor, there's no way I was going to do this myself.