Loss of Power/MPG on 03 GT

First let me say that this site is great, and I have used it in the past to help diagnose some problems with my truck. This time I need a little help on my 03 Grand Prix GT 3.8L with 98,000 miles. For the past couple of months I have noticed a decrease in power through the RPM range, a loss of gas mileage, and when it reaches 4500 RPM falls flat on its face with a blinking SES light. I recently had a tune up to include new AC Iridium plugs and a new wire set (I had found that the No 6 wire had a nice gash in it) thinking that it just needed a tune up. Got that done and the problem is still there. I checked the fuel pressure and it holds steady at 50 psi, when primed and when running. I had the catalytic converter inspected at an exhaust shop and they said it looked good (they never did a back pressure check or anything). So I hooked up my vacuum gauge to the upper intake and it shows at idle its right around 15-16 hg. When the vehicle is revved up it heads toward the zero mark, but once you hold it at the 2000 RPM range it goes back to the 15-16 mark. So I tend to think that it has a plugged cat, however when I hook up a timing light to the plug wires, I sometimes get some intermittent flashes (I can't rule out a bad timing light yet.) So my question is do you think the cat is the problem, or do I possibly have a bigger problem with the ECU or coil packs provided my timing light is good? Any help would be appreciated.

If you have a blinking SES light get your codes read at a local auto parts (free). I'd guess at this point you need an O2 sensor but with the DTC it'll help narrow it down.

I took it to a parts store and they can't pull the codes unless the SES light is actively flaging a code. The SES light turns off once the RPMs go back down and I'm driving normal. They are unable to pull a code that is stored in the computer. I read in some of the other threads that the blinking SES light means that a multiple misfire condition exists.

Your engine vacuum is on the low side although technically it is in specs. Where did you connect the vacuum tester. At what fitting/connection? Best one to use is the one at the left hand side of the throttle body which has the big rubber tube/reducer that has the two vacuum lines that feeds out to the fuel pressure regulator and evaporative purge solenoid. Disconnect the reducer and take the reading directly off the throttle body fitting with the proper adapter to the gauge. A reading towards zero is normal when blipping the throttle but the reading at the 2K rpm test should be steady at an acceptable vacuum reading. For the reading at a steady 2K rpm in most cases you should see an increase in vacuum to around 18-20 in-hg and it will be higher than the reading seen at idle.


I read in some of the other threads that the blinking SES light means that a multiple misfire condition exists.

The blinking SES/CEL light means a misfire condition has been detected by the PCM module. When on steady an emissions control system problem.

You are correct the code reader that most AP stores have would/can't read a code unless its active.
I know on my 98 if the SES light is blinking it means there is a condition that is occuring that could damage the CC. A misfire could do that.

There is a ton of diagnostics that I could post but it may just have you going round and round. It sounds like you do your own car repairs. Maybe it would be worth it to pick up your own code reader to pin point the problem?