1999 Pontiac Grand Prix very poor gas milage

FORD719

MEMBER
1999 PONTIAC GRAND PRIX
6 CYL
FWD
AUTOMATIC
150,000 MILES

I have a 1999 grand prix 3.8 that is not supercharged. Im getting 13 mpg with it and I know it should be much higher than that. The car runs great, no missing no hesitation.I changed the O2 sensor before the converter also I changed the air filter and fuel filter. I did a fuel psi test nd with the key on not running I had 48 psi with the engine running it drops to 42 psi. It ahs no codes in the computer, I took to local shop and they said it was running lean then go rich and then lean but stayed to the lean side the most. I can not find any vacum leaks but when I put my ear close to the upper plastic intake manifold I can what sounds like air rushing in its quite loud but I have never listened to one that close before so im not sure if that is normal or not. The other thing I found odd is that if I unplug the MAF sensor I get no codes, so then I unplugged the MAF, TPS, IAT sensors all with the car running and still no codes. The only way I got a trouble code was to pull off a spark plug wire then I got a misfire code. What am I missing or not seeing with my problem?

5 Replies

JAMES W.

MECHANIC
2,394 POSTS

I'm not one to jump at blaming the computer, but if you didn't set a code after pulling the sensor plugs you mentioned, it has to be the ECM. Unless you drive the car awfully hard, you should get at least 20mpg on the highway. It sounds as if the ECM is in limp mode and just stays there. When was your last tune up? Worn plugs and wires can run the engine smoothly yet still push a lot of gas out the tail pipe. Please advise.

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Monday, March 9th, 2009 AT 8:43 PM

FORD719

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3 POSTS

The plugs and wires were replaced with in the last year. Is there a way to check the ECM to be sure?

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Tuesday, March 10th, 2009 AT 3:34 AM

JAMES W.

MECHANIC
2,394 POSTS

Not really. If the ECM is not setting any troulbe codes, this means it thinks everything is OK. The only thing you might do, is connect a mobile diagnostic unit to monitor the sensor perametors as you're driving to see what ranges they are working in. Even this would be difficult as you would have to be watching the monitor and writing instead of watching the road. Question; Do you do a lot of city driving verses highway? This would be a major factor in fuel consumption. I'm not saying changing ECMs will instantly increase your mileage. There are too many "if's" here. If, it were possible to "borrow" another ECM, just to try it, would be interesting. There are, so called, performance Eproms "chips" available in the aftermarket that claim improvement in economy and performance. But, I've personally had little experience with them. You may want to check these out. Hope I have helped you in some small way.

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Tuesday, March 10th, 2009 AT 10:36 AM

FORD719

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3 POSTS

Its driven mostly in the city. But I know it should get better than that as Ive had other 3.8s that did much better. You said it sounds like its limp mode? What is that like open loop?

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Tuesday, March 10th, 2009 AT 4:10 PM

JAMES W.

MECHANIC
2,394 POSTS

Yes, it is basically the same as open loop. The ECM processes information as if all the input sensors were in mid-range. This mode is usually triggered by the failure of any one of the input sensors and restores when the input is corrected. Your ECM may have into "limp" computation and never came out. Keep in mind, this is just a theory and I haven't seen a case of it since the early 80's when the ECMs first came out. You might be able to test this theory by connecting a mobile unit to the diagnostic port and watching if the sensor inputs vary with driving conditions. Let me know.