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Old 12-28-2004, 12:34 PM   #1
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Default Need help with 1990 Thermo replacement - Please!

I would greatly appreciate advice on how to install the thermo / housing back on my 3800 1990 Bonneville. I cannot get the left hand side (side w/o bolt) of the thermo housing to sit flush against the manifold; even tapping it with a wood block and rubber hammer... no luck. The side with the bolt goes flush with the manifold and looks correct. I cannot for the life of me figure out why the other side is 1/16 (approx) gapped. The thermo is new and I'm using the old gaskets. Is there some trick to getting the housing back on correctly? The housing fits very snug to the manifold when installed by hand. I'm use to working on 350 small blocks so, this is a different beast for me. I was surprised that only 1 bolt was used to hold thermo housing on. Unfortunately, I dont have a service manual or chiltons available. And - if I remove the thermo, the housing will lay flush on both sides with no problem. Old thermo still will not go flush for whatever reason. The two thermos look identical so I dont think that'* the problem. Should I just ignore the gap and see if it leaks? I was hoping to get it right the first time but this one has me confused to say the least. Thanks for the help. I will try and get some pics later on if needed.
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Old 12-28-2004, 12:49 PM   #2
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opensourceguy just went through the same thing....
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Old 12-28-2004, 01:22 PM   #3
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Default Thanks - I have emailed OpenSourceGuy

OpenSourceGuy - you have mail or please leave me a message here if you can help me out. Thanks!
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Old 12-28-2004, 02:53 PM   #4
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Yep, same exact problem I had. Do you have two gaskets or one? Some 87-91 Bonnevilles have two, some have one. Mine has two, one that goes between the Thermostat and housing, and another that goes in between the housing and the intake [an o-ring]. If you have two, keep both, but inspect the one that goes between the tstat and housing for cracks or whatever [mine was in perfect condition]. If all checks out on that one, then there is the o-ring. Does yours fit tight around housing, or is it loose? If it doesn't fit tight, then it needs to be replaced [the source of your problem]. Go to a place that sells o-rings [Lowes, Autozone {or any auto parts store I can think of}, etc..]. Take the housing with you, and make sure the oring fits TIGHT around it, with no play.. this is VERY important. After you have that, test fit the housing without the bolt, or tstat. Make sure it fits nicely around there, and is not loose, or does not fit on. Some tapping may be necessary, but don't bend or break the housing ear, so take it easy. If all that checks out, clean everything perfectly [trust me on this]. If you have not drained the coolant [like you should], then do that now. Unless you replaced the coolant within the last 6months, now is a good time to do a flush and fill. Then, once you don't have any coolant sitting on the intake [just a little bit of coolant needs to be removed for that.. you can skip this if you REALLY want to, but I suggest it]. Make sure all of that area is clean from any debre [you don't want garbage going through your cooling system, do you?] Then, place the tstat IN the intake [NOT on the housing]. Place the housing over, and check all your spacing. If everything is nice and tight, you can bolt it down, then. Start up the car, fill up [or top off] your cooling system, and check for leaks. It will only leak above idle with the tstat closed. So, check it right away.

My, that was a long post.


-justin
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Old 12-28-2004, 03:51 PM   #5
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Default Thanks Justin!

Justin - thanks for your excellent description. I will follow these steps to the letter and hopefully, the housing will go back on flush to the manifold. I bought a new "O" ring but did not use it. I will check the fit and give it a try. I actually broke the original thermo housing at the edge getting it off. I did manage to get an original GM replacement part from my dealer. Mine does have two gaskets: both looked in perfect condition although I didn't check the tightness of the O ring. The surface of the intake where the thermo goes in might be suspect too.... I gave it a light cleaning but nothing to heavy. I am use to Chevy small blocks that have plenty of tolerance and forgiveness when it comes to the thermo housing. The cooling system has already been flushed; I had to replace the radiator last month and probably should have done the thermo then. Live and learn.
Thanks again for your help. Jim Miles
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Old 12-30-2004, 06:47 PM   #6
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Default Installed new thermo. Still have a gap. Not leaking so far.

OpenSourceGuy: I followed your instructions to the letter, even replaced the "O" ring. Cleaned the manifold area real well. Drained the coolant down. When I tighten the housing bold down, the other side still gaps about 1/16 of an inch. So far it is not leaking after several test drives. I'm not sure what else to do at this point and I needed the car back on the road (out of time). The parts I used are all GM... just doesn't make sense to me! Thanks again for your help. If you have any other suggestions, please let me know. For now, I'll see how it holds up. The new thermo definitely is working correctly.
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Old 12-30-2004, 06:57 PM   #7
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I replaced my thermostat last year, and mine also didn't seat quite right. I just tightened the housing bolt as much as I dared (that'* aluminum you're going into, remember). You might try to tap it down gently with a dead blow hammer, or regular hammer and a block of wood, and then see if you can tighten it more.

But really, as long as your o-ring is seated okay, it should be fine. Mine hasn't leaked a drop yet.
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Old 12-30-2004, 07:26 PM   #8
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JMFC is right, you may want to try some of his suggestions.. the oring sits pretty low in the housing, you should be alright. I don't know why it still has a 1/16" gap.. but as long as it seals, I wouldn't worry about it. Mine has a litle bit of a gap, but it'* less than 1/16".


-justin
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Old 01-10-2007, 01:40 AM   #9
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I just did mine too, and it sits cockeyed too, some of the guys at my shop said this happens with these motors, tighten it, but not too tight, and if it doesnt leak, then your good.
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Old 01-10-2007, 11:49 AM   #10
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You need to get the right sized oring. once you get the right sized oring, it will either sit flush on it'* own, or it will easily sit flush by tightening bolt, mild hammer on other side, tighten, hammer, etc... slowly until it is good. Also make sure you put the gasket between the tstat and housing in there, i hear that takes the heat a lot longer to warm up.
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