A few weeks ago, I changed the plugs and wires and serviced the air filter on my 2001 GP GT (209,000 miles) and got a noticeable drop in power and lost 3 mpg for my troubles. I could use some help.
I noticed the change immediately. The SES light is not on and there are no codes. There have been several tanks of gas and I'm consistently getting 17-18 mpg now, not the 20-21 I've been getting since the dawn of time. I'm driving the same and haven't changed when and where I'm driving. The car will get up to speed, but acceleration is poor (low, mid-range and high). There is the slightest bit of roughness at idle, which is amplified by the poly bushing in the torque arms. The car starts up immediately and does not stall.
What I did:
I changed the plugs and wires, using stock AC Delco part #s (iridium plugs). This is the second time I've replaced the plugs/wires, using the exact same parts. I cleaned and recharged the K&N air filter. That filter has been in the car for a very long time and I've cleaned and recharged it many times. I've checked that the boots are seated correctly on the plugs and that the plugs are properly seated. I use a large U-bolt in place of the torque arms to hold the engine in a "rocked forward" position when I perform this service.
What I noticed when changing the plugs/wires:
When I pulled the plugs, the #4 and #6 cylinder plugs were badly worn (see following photo). The other 4 looked as good as the new plugs I replaced them with—gaps still correct. And yes, I know the plugs are stamped NGK. They're the correct AC Delco plugs.
#4 and & 6 plugs
I also noticed what looked like mud or rust on the coil end lead of the #4 wire, as well as in the socket of the coil (#4 only). I cleaned the socket out before installing the new wires. Yes, I have them in the right order.
Addition Actions Taken:
1) Since then, I've replaced the coils. No change.
2) Removed and cleaned the MAF sensor. Precaution taken in case the recharge oil got to the sensor wire. The wire appeared clean. I used MAF spray cleaner, the same cleaner I used when I rebuilt the upper and lower intake manifold.
No change.
3) Cussed a lot.
Still no change.
What I Suspect:
1) O2 Sensors
2) Cat (It's been replaced twice under warranty, but the symptoms were different than what is happening now)
3) Knock Sensor(s) There is no pinging. If anything, it acts like there's not enough advance.
4) MAF Sensor
I suppose I could just replace all of those items. Given the mileage it's probably only a question of time before they fail anyway. But, I'd rather pinpoint the problem and replace/fix whatever the real problem is.
Has anyone experienced this problem before and/or can anyone suggest something I haven't thought of yet (please don't suggest I check my tire pressure or make sure I've released the parking brake)?
Thanks.
