2002 Grand Prix MPG improvement and Check Engine Light gone!!! fixed by chance

[mtsav8or]

I've owned this 2002 Grand Prix 3.8 NA for about 4 years now and it now has about 160K miles on it. I've put about 70K of those miles on it and it's been rock solid dependable. However, I've consistently only got about 20-22 mpg and have had a check engine light for that entire time until recently.

I've replaced the Cat. Converter, all O2 sensors, spark plugs, cleaned the MAF (twice!) and had finally given up that I was ever going to resolve this problem and would have poor fuel economy. It's always ran great so I figured I could live with it.

A few weeks ago, I was getting it ready for a 400+ mile cross-country trip. You know, checking fluids, changing the oil, cleaning the air filter, checking tire air pressure.

While I had the air cleaner assmbly apart I thought I'd use what was left of my MAF cleaner to clean the MAF sensor one more time for S&Gs but ended up using most of it on what I think is an air intake temp sensor on the fresh air plenum.

So I'm about 6 hours into my trip and I'm watching the gauges and notice that my tripometer is at around 350 miles and my fuel gauge is still at around 1/2 full. OK, the most I've ever got out of a tank of gas is 370 miles so I'm panicking and looking for a fuel station. I figured the sending unit in my fuel pump is flaking (it does have 160K miles on it) and I'm about ready to run out of fuel.

While fueling up, the pump kicks off early (if the car was empty) and I'm starting to wonder if something else is wrong. I checked my mpgs after I finish fueling and I was shocked! 29 MPG!!!! The previous owner used to get this mileage all the time but I had never got better than 24 mpg.

So thinking this was a fluke, I've been keeping track and my mpgs are consistently higher with the last tank of combo driving at around 26 mpgs. As an added bonus, my check engine light has reset itself and has been out for a couple of days now.

So, if you've got a light that won't go away and getting poor mileage, buy a $5 can of MAF sensor cleaner, clean your MAF sensor and the air temp sensor on the the plenum. If you've got a K&N or similar filter, clean it and oil it only lightly.

Good luck, it worked for me and maybe it will for you too!

 

[tcman49]

good to know, getting my cat replaced, might do the rest as well, if the codes come up

 

[mtsav8or]

watch your fuel mileage! My codes didn't have anything to do with incoming air temp and the reported temperature (I've got a ODBII/USB interface for my laptop), while high wasn't outside of what I thought it might be under the hood and didn't fluctuate too wildly, 10 degrees or so.

Oh, at the end, the engine was running a little rough after start up which is why I was cleaning the filter (which was pretty clean already) and decided to clean the sensors anyway.

 

[benner]

You realize had you scanned the car for free at an auto parts store and seen the AIT code you could have fixed that 70,000 miles ago.

 

[mtsav8or]

You realize had you scanned the car for free at an auto parts store and seen the AIT code you could have fixed that 70,000 miles ago.

Thanks Buddy!

I forgot to post on this forum that I scanned the codes early on and they were always the same and were pointing to the O2 sensors and CAT which already had been replaced. I don't remember the exact codes since I stopped reading the codes 2 years ago. I also read the the realtime data from the sensors which were also not helpful in this situation. Sometimes, the computer doesn't always tell you what to do and sometimes you just get lucky. Hence, the point of the post.

Just trying to help out somebody else that may be looking for help.

Oh, and there's no point being a jerk. I may be new to this website but don't assume I don't know what I'm doing working on a car. I've been repairing cars for 20+ years.