2002 Grand Prix GT Coupe getting poor MPGs low 20s in mixed Driving

[mtsav8or]

First post in the Grand Prix board. I've got a 2002 that we've owned since new. It's always got about 30 mpg on a trip and 25-26 mixed driving. It has about 125,000 miles now but has been maintained really well. The best mpgs I've had lately (last 25K) are 20 mixed and 24 highway.

New Cat at about 90K miles.
Clean K&N drop in.
New Plugs and wires.
Replaced both O2s, still getting a low code every now and then.
Fuel filter done recently.
Tires at 40 lbs.

I'm not sure why I'm still getting the codes on the O2 but things look normal when checking the output numbers on the PCM/computer connection.

Does anyone out there have any suggestions. This car still runs great and has not missed a beat since day one except when the cat was clogged. Even with the poor MPGs it still starts, drives great. Throttle response is also excellent. I'm really stumped since I'm not seeing any other codes or abnormalities.

Thanks, MTS

[grandprixgtx00]

are you figuring your MPG's manually...or going by the DIC?

[mtsav8or]

Manually Fill-up to Fill-up. Miles/Gallons

[Scrapper]

i got a friend that cleaned k&n filter and her son put to much grease on it it pluged up maf sensor and showed up o2 so replaced and all is well.no more k&n filters for her.

[jessep28]

Could this be an OEM vs. Aftermarket part issue?

[BNaylor]

Originally Posted by mtsav8or

I'm not sure why I'm still getting the codes on the O2 but things look normal when checking the output numbers on the PCM/computer connection.

Welcome to the forum.

What specific 02 sensor DTC(s) are you getting? And how was the mileage before the K&N drop in air filter? Was the CAT converter OE or aftermarket?

Get it scanned with a full function scanner to see what the short and long term fuel trims are.

BTW - You are better off with a paper WIX or AC Delco air filter. Anecdotal testing has proven that K&N filters do not help give better gas mileage over the stock paper types.

[troy1]

With the K&N installed I would try cleaning the MAF with CRC MASS AIR FLOW SENSOR CLEANER. and you need a 20 or 25 security torx to remove the MAF. also put vacuum line off the FPR a few times and see if its leaking.

[mtsav8or]

scrapper1 - That sounds like a good thing to look at. The K&N has been in there a while and this issue crept up before I started driving this car.

BNaylor - O2s are DENSO. I heard mixed reviews of the replacement AC Delcos and Boschs. Many good reviews of the DENSO though. I don't know about the CAT. It was taken to an exhaust shop by my in-laws and I don't have the records. I don't think there was anything wrong with my O2s though as the car is behaving exactly the same as before replacement. I will try replacing the K&N as that's a cheap and easy test.

I have a PC interface to my car's computer control and am reading the sensor outputs realtime on a laptop. Everything "looks" within normal parameters when I've viewing it but I don't have any "bench" or "default" values so I may be missing something.

I'm going to try using a paper filter and cleaning the MAF. At the very least it can't hurt.

I shouldn't have bragged on the car so much as now I'm hearing a little bearing noise from somewhere in the drivetrain. I think it's a wheel bearing as it doesn't change with engine speed.

Thanks for all the responses!

[BNaylor]

Originally Posted by mtsav8or

BNaylor - O2s are DENSO. I heard mixed reviews of the replacement AC Delcos and Boschs. Many good reviews of the DENSO though. I don't know about the CAT. It was taken to an exhaust shop by my in-laws and I don't have the records. I don't think there was anything wrong with my O2s though as the car is behaving exactly the same as before replacement. I will try replacing the K&N as that's a cheap and easy test.

I have a PC interface to my car's computer control and am reading the sensor outputs realtime on a laptop. Everything "looks" within normal parameters when I've viewing it but I don't have any "bench" or "default" values so I may be missing something.

The Denso 02 sensor is fine. That is what I use as the pre-cat 02 sensor. You stated you get 02 sensor codes but do you recall what they were?

Since you have a PC/Laptop based odb-ii program you should be able to check the short and long term fuel trims which may tell you something better than guessing. After sufficient engine warmup the 02 sensor will switch rapidly ranging from .1 mV to around .8 mV.

Also, keep in mind it is not uncommon to lose a few miles per gallon in the colder months since the engine takes longer to warmup.