98 Grand Prix GT 3800 Bad mileage - Page 2

00GTP4ME · 06-13-2008, 12:40 PM

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

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/vbul...d.php?t=907011

http://www.automotiveforums.com/vbul...d.php?t=907013

http://www.automotiveforums.com/vbul...d.php?t=907017

BNaylor · 06-13-2008, 01:22 PM

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.

00GTP4ME · 06-13-2008, 01:36 PM

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.

BNaylor · 06-13-2008, 01:51 PM

Quote:

Originally Posted by 00GTP4ME

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.

00GTP4ME · 06-13-2008, 02:20 PM

Very good.

fritcr · 07-30-2008, 10:57 AM

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

BNaylor · 07-30-2008, 04:03 PM

Quote:

Originally Posted by fritcr

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?

fritcr · 07-30-2008, 05:00 PM

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

BNaylor · 07-30-2008, 06:44 PM

Quote:

Originally Posted by fritcr

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.

fritcr · 09-26-2008, 10:56 AM

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.

tblake · 09-26-2008, 02:55 PM

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?

00GTP4ME · 10-07-2008, 09:27 PM

Do you leave the performance shift on all the time?

richtazz · 10-08-2008, 09:54 AM

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.

fritcr · 10-09-2008, 10:40 AM

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

fritcr · 10-09-2008, 10:41 AM

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

POST REPLY TO THIS THREAD