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Sudden Drop in Power and MPG

GT in Kennesaw

GT in Kennesaw

New member
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.
 
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FordMan77 said:
Do you have a way to scan it and read live data? Even if no CEL maybe it would help pinpoint the issue.
Click to expand...

No. I think I'll start by removing the new spark plugs and look for any indications that one or more isn't firing the same as the others. It's just too odd that all this started when I did the "tune up".
 
FOLLOW UP: I replaced the K&N filter with a plain paper element. After one tank, no difference. I just pulled the plugs and they all looked identical—like new. No signs of misfiring or anything. They were all wet (gasoline), but to be fair, I just drove the car into the garage and it's 35° outside. Put the plugs/wires back and put in a new front O2 sensor. I drove the car and, no change. This is just too strange.
 
Do you have the boost bypass hooked up correctly? I believe I had the vacuum line to boost bypass unhooked and plugged or the electical solenoid bypassed. Drove fine but bad gas milage.
 


Shot in the dark. but your getting air and good spark now. maybe not much fuel pressure so its dumping it. when was last time you cchanged the fuel filter
 
also check out your injector orings, i dont think your problem lays here but my car done the same exact thing and thats what it was
 
I replaced those when I replaced the upper and lower intake manifold gaskets. What I've learned about the damage to the 2 plugs I show above is that they indicate a lean condition. I added BG injector cleaner to a tank of gas and there seems to be an improvement in throttle response. I haven't driven out the tank yet to check mpg, but it may have been clogged injectors in # 4 and #6. Interpreting how replacing the plugs may have brought on the loss of throttle response is a little difficult, but it's possible the ECM was compensating for the lean condition by dumping more gas and didn't re-calibrate. I'll know more after I run the second can of injector cleaner through the engine.
 


I finally have resolved this problem. Turns out it wasn't one thing, but three.
First I tried everything that made sense. That didn't work. I tried the few suggestions I got and that didn't help. Strangely enough, there were days that the problem seemed to have cleared up. Then it would go right back. The fuel filter was changed and while it didn't make any difference, it was full of junk. The first item I discovered was a perished O ring in the fuel quick connect at the injector rail.



This is the only one of the 3 that can be linked to changing the plugs. I noticed it when the engine was cold and I had the hood up. I smelled gas and when I looked, there was a puddle on the intake manifold. I turned the key on and it was pumping out. I replaced the O ring and that at least was corrected. It didn't fix the main issue of poor throttle response. That ended up being a combination of 2 other issues. The IAC and the TPS. I cannot believe the TPS wasn't the first thing I checked. Probably because it all happened when I replaced the plugs/wires (which I actually replaced again—I now have a spare set). I also did a compression test and found an average of 184.5 psi, with a high of 190 and a low of 180. Not too bad for an engine with 215,000 miles on it. Anyway, the IAC was replaced when I did the L36 to L26 UIM conversion, but it was a Delphi part. This time I got ACDelco. I've been reading a lot of reviews that state the Delphi parts only last a short while before problems reappear.



I'm not sure if the problem (heavy build up on the valve) is a result of the TPS being bad, or if it is yet another issue, but there definitely was a problem with the idle. That's all back to normal now.

To recap, I replaced the pugs/wires, the coils, the MAF sensor, the fuel pressure regulator, the O2 sensors, the downpipe, the fuel filter and the IAC. I will replace the IAT and ECT sensors when they arrive this weekend. I haven't run a full tank of gas yet but I expect it to improve also. What an ordeal.

I created a spreadsheet with the sensors and the symptoms to narrow down the possibilities. I'll add a link for it as soon as I find a home for it online.
 
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I've enjoyed driving mine since 2005. It just needs some love at the moment. Since i'm not putting 250 miles on it weekly maybe I can start to afford to give it some luvin now. She's getting a new heart soon with something that might make her rumble and lope some ;)
 
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