My grand prix is getting terriable mileage but runs great. Where should I begin checking. There is no codes or lights. It runs good but only gets 18-19 mpg on the hwy. I traded my wifes 97 Bonneville for this car and that big old bonnie got about 26 on the hwy with the same engine. I just assumed the gp would do at the very least the same. What do you guys get with the 98's? I don't know where to start but if it don't get better than that, I'm trading it back for a Bonneville. It has 113k on it and I have no idea what anyone has done to it. I have put a new Air Filter in it and Thermostat. It seems like it does not heat up very much but I guess it is ok. I put the new Thermostat in and it heats up the same. About 1/4 way up the gage. I guess like 160-170?
Anyway, any help would be great!
Rob

Something is DEFINITELY wrong! Have you looked at anything on your fuel system? Fuel filter, fuel pump, pressure, etc? The fact that you say that it's running well is what I find interesting. Is everything on this car stock?

It is bone stock and I have not checked anything at all with the fuel system.
I have not had it long but it needed a rack when I got it and I finally just got ahead enough to have that put on finally.

Do a tune up, plugs, wires ,fuel filter,and seafoam the intake.Check tire pressure.

What was the rating of the thermostat installed? If stock make sure it is the 195 degrees because longer engine warm up times will kill fuel economy and the PCM module will take longer to switch into closed loop mode from open loop mode. You can run a full function odb-ii scanner and check the short and long term fuel trims.

Other issues could be abnormal exhaust system back pressure , i.e. flaky CAT converter or even a degraded pre-CAT 02 sensor which might explain the poor highway mileage.

If you're under financial pressure, the first thing I would do is take off your fuel filter and see how clogged up it is. That can make a big difference and also will give you a good idea as to what the rest of your fuel system looks like. kwk39018 (http://www.automotiveforums.com/vbulletin/member.php?u=562193) mentioned using some Seafoam, which I think is also a great idea. With those two things, you're not out hardly anything and if you see any improvements with those two things, you probably have a good start.

Using Seafoam via the intake system only as a top engine cleaner can be a bad idea too. For one it doesn't properly clean the fuel injectors. Two - All the carbon that blows out the exhaust from the cylinders, pistons and valves may clog the inlet screen at the CAT converter. Three- the chemical content may damage the MAF sensor wire element and thermistors. Four - Since this is a '98 NA SII 3800 engine with the infamous plastic UIM plenum you could exacerbate or cause leaks in UIM to LIM sealing.

Best to take your time and manually clean the throttle body and intake components including the MAF sensor. Use MAF spray cleaner only on the MAF sensor. Use throttle body spray cleaner on the throttle body components so the butterfly plate end seals are not damaged. And don't forget the IAC.

I ran 2 tanks of Seafoam in the gas tank when I got it..in fact on the way home. I also cleaned out the TB but I didn't mess with the maf or anything because I wasn't sure what kind of stuff to use. I can pick up some maf cleaner and a fuel filter today at lunch if you all think that might help. I'll try to pull a couple plugs out of it too and see how they look.
Rob

Oh as a side note they cut off my resonatior?? I think they called it. It had a hole in and they said I didn't need it. Would that have hurt something?

At 113k miles, you're way overdue for a tune-up (as previously suggested) if it hasn't been done. If it has been tuned up, check and see what brand plugs were installed. Avoid gimmicky multi-electrode plugs (read Bosch+2/+4), as they are junk!

I got about 18 city but I have a heavy foot and about 28 to 30 highway with my old engine in my 98 GTP. I now have a new engine with 57000K on it and haven't really checked yet but the mileage seems better. Of course, I have new plugs, filters etc. so it should be better.

They cut off your resonator? The one on your air box?

I'm quite sure he was referring to the one at the exhaust. You know the one after the CAT converter. That should not affect gas mileage that much but I guess you never know for sure since some exhaust backpressure is required and it affects everyone in different ways.

I'm curious about this guy's in city only gas mileage especially if the highway is that bad.

Oh, you mean THAT resonator! :grinyes:

When I had my exhaust done, I took off the resonator as well and it didn't affect anything that I noticed.

:lol:

Mine's deleted too. But my in city average is at least as good as the OP's highway mileage. Plus it is moderately modded too. :eek:

http://i5.photobucket.com/albums/y186/lizzywiz/dicfuel.jpg

Ok Mr. Moderator (and all you other AF old-timers!!!) :worshippy

I posted some VERY helpful threads and no one has responded yet! Go to each of these and put in your two cents....NOW!

http://www.automotiveforums.com/vbulletin/showthread.php?t=907011

http://www.automotiveforums.com/vbulletin/showthread.php?t=907013

http://www.automotiveforums.com/vbulletin/showthread.php?t=907017

:uhoh:

Interesting posts. Not too many of us, if any, ever visit those forums on AF. You'd probably get better replies here at Grand Prix which is probably the most active tech forum on AF last time I checked out the stats with AF Admin.

Let me know if you want them moved here but we can leave a redirect there.

I don't really care where they are, as long as I can get some responses. I've learned more about cars in the last three months on AF than I have in the past 3 years.
I really appreciate everyone sharing their wealth of knowledge and experience.

I don't really care where they are, as long as I can get some responses. I've learned more about cars in the last three months on AF than I have in the past 3 years.
I really appreciate everyone sharing their wealth of knowledge and experience.

Nice to know.

Well, in that case the 3 threads have been moved here. The redirect remains at Cars in General, Automotive Trivia but it will take anyone here. At least you're getting replies now. :wink:

Very good.

I tuned up my gp 3800 with new plugs and wires. No change on my mileage. I finally took it in to a shop today and he just called saying my LIM Gasket is leaking. Would this cause bad mileage? I guess I will have it replaced. This car is becoming a money pit. (I want my Bonneville back) LOL

No change on my mileage. I finally took it in to a shop today and he just called saying my LIM Gasket is leaking. Would this cause bad mileage? I guess I will have it replaced.

Possibly. Just curious how did the mechanic determine the LIM gaskets are leaking? External or internal?

He said it is leaking up by the injectors. Orange dexcool is pooling up up there.

He said it is leaking up by the injectors. Orange dexcool is pooling up up there.

I guess you could have a LIM port gasket do that but in most cases the pooling of coolant on top of the lower intake manifold is caused by a bad UIM gasket or in your case since you have a 98 model possibly the EGR stovepipe issue. Regardless it is best to get the LIM gaskets done and you should consider getting an improved UIM plenum along with gaskets either aftermarket (APN, Dorman) or the improved GM version. I would use the newer aluminum framed LIM gaskets. The aftermarket kits will have a modified smaller diameter EGR stovepipe that mounts on the LIM. Also, GM has one available. Good luck and let us know how it turns out.

Sorry it has been so long since I replied but I got all new gaskets on it. UIM, LIM and a new plentium on it. NO change in MPG. It has been completely tuned up and also had my mechanic put it on the computer. He said he can't see anything wrong but agreed it should get much better MPG. I drive it easy and mostly on double lane to work and home at 70 mph. Some city running around I guess but very blended and mostly hwy. I live 20 mile outside of the "city" haha. Well it is rural Missouri so it's not like St. Louis or something when I say city anyway. Also, I just bought a GM Delco pre O2 and put it on. I have ran 2 tanks of gas out and averaged 19.2. Not much change if any. I don't know what else to do except wait until I can afford to take it to the dealer and have it checked. OR maybe 19 is all I am going to get?? It seems so weird that our Bonnevilles got sooo much better. Very lowest we ever got with one of our Bonnevilles was like 23 I think. That was major running around town and letting it warm up in the morning. I have had 5 Bonnevilles and a couple of them I would get over 30 mpg with. This little GP should do as good I would think. Oh well, any ideas let me know.

My GP GTP even with my mods can get 32mpg highway easily.

It usually averages out to about 27mpg combined.

Double check your tire pressures, 30psi is recommended by GM, but you should be fine increasing a little, maybe 32-34.

Also is your car going into overdrive? What are your RPM's when your driving at 60mph?

Could you post your long term and short term fuel trim readings?

You did try and reset the DIC right? Are you calcing out the MPG the old way at the pump or relying on the DIC?

Do you leave the performance shift on all the time?

Even though it's been a while since you posted your findings, I'll throw my 2 cents in. Since you had a coolant leak issue, I would have the converter checked and possibly check oxygen sensor operation with a scan tool. Coolant on the converter substrate and oxygen sensors can cause a significant drop in mileage.

I don't have a DIC...haha ok...that's sounds funny. At least my car don't ..Hmmm no comment.
I am calculating the old way. I did manage almost 21 the last tank of gas. I just reread everything an realized I forgot to put a fuel filter on it. Do you think I need to do this? I have done everything else. I was going to pull my egr off and see if it is carboned up too. I don't know how to check my cat. Is there some way to check it?
Rob

Oh sorry...I don't have the performance shift either.

Performance shift is only on GTP supercharged models, so that is irrelevant. Your combined city/highway figure of 19.2 mpg is on the low side of normal.

I have purposely varied my driving habits and have come up with the following:

In my 99 GT, I can get as high as 22-23 if I drive like a grandma and don't sit at drive-thru's, but average closer to 20.5 to 21 mpg if I drive normal (and have experienced as low as 19 if I'm a little more aggressive). On trips, if I keep it around 70-73 mph, I can average better than 30 (on a long trip at 65mph though Ohio, I got 33+). At 75 or higher, there is a significant drop in mileage (only 25-26 mpg).

Driving habits have a big impact on economy!

Make sure the perf switch is off. Usally located on the left side of shifter. If on will greatly reduce gas mileage.

Make sure the perf switch is off. Usally located on the left side of shifter. If on will greatly reduce gas mileage.

Are you referring to a GT/SE or GTP? The members that posted in this thread that are having mileage issues don't have performance shift since it is only applicable to a L67 GTP.