Quote Originally Posted by Duke George V View Post
I've been getting less than stellar fuel economy out of this car recently. Before this all went down, I ran two tanks of fuel through it, one on the trip back from Ontario, and one around town. The highway tank netted 24 mpg, which, while low, isn't so bad when you consider the car probably sat for a while before I got a hold of it, and had really gross oil. The city tank ran 19 mpg, which is bang on for the EPA city rating. Now though, even when factoring in AC usage, it's terrible. Anywhere from a low of 14.99 to a high of 17.3 mpg. A 3.1 V6 car should not get worse mpg than a 3.8 van (I always got 18 city out of the van no matter how I drove it).

So I had a couple of theories. Perhaps I am not actually getting bad economy, because the miles may not be reported correctly by the odometer due to incorrect tire size. I got a GPS speedometer application on my wife's phone and drove around for a bit. It suggested that the speedo in the car is fast by 1 mph, which I believe is typical for GM cars. That reading was consistent across the whole range, from 25 mph residential, to 45 mph city, to 65 (and higher, shhh) on the highway. So yes, the miles are being correctly reported.

The other theory is that the spark plugs are no good. This ended up being partially correct. The spark plugs I put in are ones I had laying around from when I had big plans for my old Celebrity. It had a 2.8 MFI, which is related to the 3100, but obviously not the same. They fit, and it ran, and that was all I cared about. Two possibilities: they are not working well enough because they're not specifically for the 3100, or they're not gapped correctly.

They weren't gapped correctly... for the 3100. They were all gapped to about 0.040" or 0.045", which is fine for the old MFI engines I believe, but the 3100 calls for 0.060". Gap reset. Let's hope it helps.
Man what a job! I know this is an old thread but I have conquered the same problems also recently with my 96 century wagon. I was lucky and my heads were not ruined.

bought the car from a dealer actually, 97k miles very very clean, and actually had the lim job done, and I thought it was ok. first trip over 15 minutes and pow that little elbow hose broke and it started spraying out coolant, im surprised I didn't smell it, all I knew is the ac was kicking on and off all of a sudden, when it kicked on it would cool down to the normal 3/4 it would start to rise when kicked off, since I was in the middle of nowhere I just kept kicking the ac off and back on when driving well it got to the point where it was pegging hot and I pulled over and landed in some guys yard off the highway, found out that the compressor was getting hosed down with water causing it to slip and I was bone ass dry, we filled it up and let it come done from being totally pegged out then shut it off, that's when I found the leak, he was nice enough to lend me some old heater hose and I looped it around so it wouldn't kink and limped it home. replaced all hoses and nicked the damn radiator in the process so I had to change that shit too. seemed to be running and not getting hot but there was one problem...

the temp gauge would raise very quickly and peg out, hmm ok so bad gauge since it ran hot. well the damn thing was cross threaded and the LIM cracked a chunk out when removing it! ARGGGGGG, so I went on a hunt for a LIM, found one buried in the dirt from a junkyard way out in the land where nobody owns a mower. this one had to be just right because after this year the 3100 was different. went through all the same things you did with the exception that I didn't go down into the heads but when I popped the LIM I noticed the new metal gasket had been torqued so badly it was tearing away at one water port, then of course I found a little separated oil in the water but didn't notice much of anything in the oil, put the SOB back together only to find out they sold me the wrong gasket, I didn't know about the plastic metal gasket issue just figured I got the better one, well this gasket had plastic rod guides and I also had metal ones, the plastic ones where causing the valves to not respond just enough to make it run like total shit but not hurt the motor.

So back all the way in and all the way out, I feel your pain, I went to sleep with cuts and panting like a dog, when I went in the second time I replaced every damn thing in sight I could get to sensors and I even had to rewire the side crank sensor and the speed sensor since the wires were melting. looks like the original wire loom cracked away. I double loomed it wrapped in ET and zip tied it up good. all back together and running well no oil water contamination, now it doesn't run like a singer, its a little shaky sometimes barely but always fires right up never dies. started smelling gas up front. hmmm I thought shit I forgot a fuel injector o ring and left an old one on! but nope it was that stupid purge canister valve hose, easy fix (oh forgot to mention the first thing I noticed gas smell at back before any of this stuff, the evap elbow was ripped on the tank too.

ok so good running motor. runs 1/3 the way normally moving or with ac on keeping the fan running, but without it creeps to half way then starts the fan and cools down and cycles like that when sitting, seems fine to me.

Now I did shocks and struts, struts were a pain and I kept them sideways when claming the spring as to preserve my balls ive it were to snap loose, found a busted cv axle on the driver side, swapped it, replaced breaks and drums/rotors and had new tires put on, alignment guy says we cant align it your front end needs work, so then I started checking and everything was loose, replaced all 4 tie rods, rebooted the R&P, replaced lower ball joints as one was bad (this was a bitch too, picture how floor in the garage with fireworks of metal shavings flying to get the damn rivets out. some more stuff I can even remember and I had a new suspension system on the front end. alighment guy comes back and says it needs a special alignment since parts were changed 50 bucks more so I was like fine, then he came back and said we got it pretty good or as good as we could but needed some more special alignment since the entire front end was new (I told him this at first) I said well just let me pay and get out of here if you think its close. ok . seems to drive fine, not the smoothest rider but not bouncing anymore.

now while under there I notices fluid all over the underside, hmm oil, no there was a small oil leak on the pan, and even after the dam distro dummy gasket replace it still gets gummy up there a touch, arg. but it never drops on the stick and ive only seen at most a nickel sized drop. transmission, it was somewhat low, still visible on stick but under the range. ok so hmm pan bolts were loose, well some of them, I tightened it all up, but that didn't explain all of it because that was minor, turns out the guy who worked on the transmission didn't even tighten down the TC cover up at all hardly, it has a cork gasket and seemed not too bad, so I tightened it up all the way, filled the tranny fluid up. the fluid is burgundy with a little tiny blackish residue. not the best but not burned up smelling. and nobody wants the responsibility of changing the fluid so this makes me weary of even doing it myself.

all in all I have a nice champagne colored wagon, no wood, that has almost perfect paint, very nice interior and a so so headliner, not falling but not soft anymore, fingers kind of stick around in it. I can play my tapes and everything works but the drivers door lock button, unlock works ok, I disassembled switch and tested, its ok, tried manually triggering it, nothing, figure I got a bad wire in the bunch under the door to chassis rubber. fuck it, ill just lean over.

my only last nagging issue with this is the way it drives, it seems to clunk in and out when I let off gas or give it gas again, but only sometimes, sometimes its fine. every shift is pretty solid, not bang but not slip just a smooth transition. when this problem happens it is not shifiting. feels like it drives rough sometimes but no engine misfire codes and nothing suggesting the engine is having issues, oh btw all wires plugs coil packs and module are new, so are cam and crank sensors I could get too, for now the 2nd crank sensor on the crank pully is still original, no check engine light, engine doesn't move around but an inch or a little more when changing from p to r so mounts I think are ok. for now I am driving it as my daily driver since my Honda is in the body shop. hasn't let me down yet, wondering if im looking at transmission issues down the road though.