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88 Olds 98 3800 Runs Cold


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Is this common? For years my Olds would run cold on the highway, ok in town, fans operating correctly. In cool weather, it would slowly drop temp on the highway until it would unlock the converter. I put in a new thermostat, 195?, and it will still do that.

Obviously there is coolant leaking by and circulating through the radiator to overcool, but where or why? Anything peculiar about the 3800 cooling system? I hate to keep buying thermostats.

Thanks for any feedback. TJ

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There were some GM serv bulletins about "temperature overshoot", where when waming up from cold, the temp would go high and then settle down to the normal operating range. Seems like "the fix" was to drill a 1/4" (or slightly smaller) hole in the thermostat's flange plate. Thermostats usually already have that hole and/or a small metal "wire" rattling around in that hole to restrict the flow through the hole a bit. One thing I might suggest is to change the sending unit. Reason is that on my Y2K Impala (with the Chevy 3.4L), I recently started replacing things (like getting the cooling system flushed and replacing the upper and lower hoses . . . followed a few weeks later by a water pump, tensioner, and belt, plus a power steering pump and upper hoses, as it was leaking and in the same vicinity as the leaking water pump) . . . along with a GM fan unit (as one was making noise). When the fan unit was replaced, I toyed with the idea of going ahead and getting a new radiator at that time, but I gambled and lost as the radiator had obviously been seeping on the lh side for a while before it finally "leaked" per se.

After that initial clean and such, I was pleased as after leaving Gateway Buick (where we'd had our meeting that month), in LBJ "wreck ahead" traffic, the temp gauge actually went down toward 170 degrees. As things progressed, the temp swings got wilder over the next few weeks, until I was sitting thei driveway and it "went wild" between about 140 and 230 degrees. So I had the thermostat replaced and it was noticed that the sending unit was broken. New sending unit and now things are back to normal, pretty much. On the Impala, it has the basic gauge cluster, with the gauges (temp and fuel) as big as the speedometer, so even a 5 degree variation looks very significant.

Prior to this last bunch of "swings", I became a little concerned about the ECM seeing a "cold engine" and would go into "open loop" mode rather than "closed loop" mode, which could affect fuel economy and such. Oh, this last time, the radiator tank on the lh side started leaking, so that was the final reason for the trip to the serv dept.

Just some thoughts,


Edited by NTX5467 (see edit history)
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NTX5467: Thanks for those observations.

Yes, I have seen that "overshoot" problem before, usually a thermostat design issue. Most notably on Cummins 6B and ISB models, we had a fit with those for awhile.

My guage has always been rock steady and doubt that my issue is a sending unit.

I will shop for another thermostat, maybe I can find one without the air vent hole or jiggle pin. If the jiggle pin seals the hole under pressure, it should not bleed coolant to the radiator. I think it may be a quality control issue with the "jiggle pin" design.

I will update this thread if I find a solution.

Thanks again for the reply Willis, TJ55

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