98 Grand Prix GT 3800 Bad mileage

fritcr · 06-09-2008, 09:51 AM

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

00GTP4ME · 06-09-2008, 11:44 AM

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?

fritcr · 06-09-2008, 07:46 PM

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.

kwk39018 · 06-09-2008, 07:56 PM

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

BNaylor · 06-09-2008, 08:14 PM

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.

00GTP4ME · 06-10-2008, 09:37 AM

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 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.

BNaylor · 06-10-2008, 10:02 AM

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.

fritcr · 06-11-2008, 10:36 AM

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

fritcr · 06-11-2008, 10:39 AM

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?

richtazz · 06-11-2008, 02:06 PM

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!

Rocket1964 · 06-11-2008, 09:51 PM

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.

00GTP4ME · 06-12-2008, 10:51 AM

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

BNaylor · 06-12-2008, 10:58 AM

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.

00GTP4ME · 06-13-2008, 11:11 AM

Oh, you mean THAT resonator!

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

BNaylor · 06-13-2008, 11:25 AM

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.

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