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Thread: "What could possibly go wrong?" Let's Play: GM 3100 Head Gasket Job [blind]

  1. #136
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    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.



  2. #137
    Senior Member Duke George V's Avatar
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    Thanks for sharing your experience doing this job. I know how aggravating it can be to get the wrong parts. That's why I made sure I did my homework and got the revised metal gaskets for the intake manifold and good head gaskets.

    I don't know if you've been checking out other parts of the forum, but I'm currently in the midst of preparing for an engine swap in this car. I've been documenting most of it in this thread but that was more about my other two Centurys, so I'm going to be posting about the swap itself in this thread. I am pretty certain it will be the first swap of its kind into these cars, and I hope to show that it is not that difficult of a job. Sure, there are some custom parts that have to be made, and a modified computer to be installed, but it wouldn't be worth doing if it were easy.

  3. #138
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    well here we are 3 years later, ive had the car in the shop a few times being sick of some small issues, the guy aligned it, put new tires on it, apparently i had one bad you could never tell though, he replaced the motor and transmission mounts, probably my clunk, i repaired the dogbone bracket rubber by making a full silicone seal around the weak parts in it, basically its full of silicone (this part is no longer made anywhere at all and nobody has NOS parts anywhere) its different from the one that has two dogbones, this one has one and its bigger. he was trying to find out why my car is a little idle shakey and said fuel injectors were bad, he replaced all 6 and it made no difference so he basically didn't charge me for that, he did a transmission fluid change, but didn't tighten the pan bolds ARG! 3 days later it was leaking so i tightened them up, why does nobody use silicone, i changed a 3400's paired transmission 4 times and always used silicone on the gasket and snugged it up tight and mine never leaked, well i stopped the leak by tightening them up real good in criss cross patterns several times until tightening one did not make another feel loose, i put some torque on those things but with a baby ratchet so i don't think anything was over tightened, nothing broke or stripped. I had him change my power steering pump and replace all hoses, this was something i was going to do myself but it wasn't bad so i just kept putting it off until then, of course its fluid was flushed, i had him flush the coolant and brake fluid, the brake fluid was crapped out but the radiator had been flushed by me like 4 times back when i posted about this.

    so now i have a perfect car except two f-n problems nobody can figure out. its still shakey at idle and the temp gauge kind of fluctuates from 1/4 to 1/2 at 1/2 big fan goes into high speed, only does this sometimes its weird, sometimes ill granny it to work and its 1/4 all the way, sometimes im on the highway going 75 and its just back and forth from a half to a quarter (the thermostat is the right recommended temperature and new as of my first post) laser temp gun confirms it is rising above 195 a bit when its at 1/2 its like 210. not sure why my fans wait before trying to cool it back to 195, once i was stuck in traffic for like an hour running the AC in hot as hell weather and it stayed right at 1/4 and never climbed up.

    the mechanic said i have no signs of head gasket failure. i am not sure i totally believe him, afraid of the possibility of a very small pinhole leak that's barely adding air to the coolant causing a bubble up top, but i just don't know, i loose 0 water ever its dead on i marked it with a sharpie and it now has 119k on it like 20k miles since, the overflow doesn't bubble.

    tranny shifts fine no more kick (think it was the trans mount) steers and rides like a champ, AC is still cold and has no trouble cooling the car down on the hottest day after its had time to cool the ducts and work at dropping the air in the car down. all problems are gone besides those two arg!

    been driving it for those 20k miles just watching the temp gauge roll up and down from time to time and just dealing with the slight misfire feeling at stop, i mean it doesn't die or even try to. ive replaced sooo many sensors trying to solve this since my decent scan tool shows nothing and the guys at the shop wont point him in any direction.

    i do believe its internal engine issues with the head gasket that are just so small they cant be detected the usual way, i think one day it will blow, at that point im having a good engine specialist rebuild mine, id rather have someone rebuild mine and use all the top quality gaskets than to buy one with cheap shit on it.

    anyways still rolling in it, its outside my office right now, i don't drive it everyday i have a 2 year old honda that does that but i drive it enough to keep gas flowing through it regularly and needing fill ups to keep it from issues from sitting up in my possession like it did with the original owners.

    we shall see, but im just going to ride it out knowing eventually it will need major motor work, and when the time comes ill pay the 4 grand for it, ive always had a fetish for stationwagons and this one is simply just cherry, i mean it looks like maybe a year old, paint shines like brand new, interior is perfect with exception of a headliner still dry but not falling at all and a spot where someone spilled something hot and it left a half dollar size spot where the fabric kind of left a melted scratchy spot but its not discolored at all. oh i also changed all the gas struts, it nice for the glass and hood to stay up and the hatch to actually lift itself with just a tug on it to get it started. god everything is seriously like been changed id be stupid to not pay for a superior engine job when that time comes.

  4. #139
    Senior Member Duke George V's Avatar
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    From the factory, most cars are designed to run hotter than you would expect. This is for the sake of better emissions. So your fan won't kick on until close to 220 degrees. Only ways to alter the behavior are to change the computer's programming, or to hard wire the fan to a switch you can control.

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