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91 Thermostat vs other years.


Guest wally888
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Guest wally888

All 4 years use the same Thermostat however the 91 uses an entirely different method of mounting , a different gasket and seal.<BR> The seal, not an O ring, but similar, is GM part #25535126 or Advance Auto 35486.<BR> The 91 Manual instructions are, " place 2 "O" rings above the Thermostat flange. I suppose this would work but originally there was a 25535126 there. The picture of the method of mounting is also in error.

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I just stumbled on service bullitin 89-6-3 that reads......<BR>At the start of 1989 production, all 3800 engines will be assembled with a new rubber thermostat gasket..........<P>The previous paper gasket #25534927 will be replaced by #25535939.<BR>it is placed on top of the thermostat.<P>There could be several later service bullitins that cover this subject.....<BR>One of the last rubber seals I purchased (not from GM) was "U" shaped and fit over the thermostat flange, sealing both the top and bottom of the thermostat.

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Guest wally888

GM part #25535126 or Advance Auto 35486 are the numbers that apply to the Seal Barney describes below.<BR> "One of the last rubber seals I purchased (not from GM) was "U" shaped and fit over the thermostat flange, sealing both the top and bottom of the thermostat."<BR> The Manual says to put two "O" rings on top of the thermostat although the one removed is identical, one piece, to the one described by Barney.<BR> I tried using only one "O" ring but will have to remove and add another or the U shaped seal, car won't get to operating temperature if the OAT is not above 60*.

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Note that at least the '88 FSM shows the thermostat installed upside down. Make sure that the side of the thermostat marked "To Radiator" is toward the hose and not the manifold.<P>Not able to reach operating temp if below 60F sounds like a thermostat that is stuck open (have had several).<P>I like a 180F in the Reatta, pleanty of heat and "warm enough" (runs in the 177-183 range on a cool day and you much reach 146F for the transition from closed loop (13-15 mpg) to closed loop (19-22 pmg) to take place.<P>Before real warm weather I hope to have a PROM that will turn the fans on much earlier (low at 185, high at 189).<p>[ 02-24-2002: Message edited by: padgett ]

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Don't tell me let me guess: The design engineer responsible for the intake mold made the recess for the larger thermostat as used in the SBC (one size fits all, right). Meanwhile the outlet does not come even close to clamping the thermostat in place. Hence the need for two gaskets else there is too much flow around the thermostat even when closed.<P>Hmmm. Takes about two miles and a stoplight from home form mine to reach 146F (closed loop) at 60F for me. Is this slow ? <span style="font-style: italic">Think</span> I put two gaskets in (L-shaped on thermostat and O-ring on outlet).

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Guest wally888

Padgett. We lost you! The saga of the 91 thermostat, proper gasket and seal (or maybe double "O" ring) has spread through several post threads. My fault. To recap, the 91 uses a U Seal, as Barney calls it, and a gasket on a flat flange plus is a two bolt hold down. The Manual says to install two "O" rings, both on top of the thermostat flange ( evidently to seal and to keep water from passing around the thermostat. A paper gasket goes between the flanges of Thermostat cover and engine.<BR> I had only one "O" ring so tried that configuration. Does not work! No external leak but cool water is evidently passing by the thermostat internally, thus low water temp.

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