Fluctuating temp. gauge - part4?

'94 K1500 Suburban 350/4L60E
I'm still getting a fluctuating temp. gauge. The Tech II at GM says 197 degrees on the ECM sensor, but the gauge still reads high. My local GM
dealer had the truck nearly a week and couldn't find a problem with the electronics. They, first, diagnosed a bad gauge cluster, but they replaced that and it wasn't the fix. Then they changed the temp sender for the gauge (in the head) and that wasn't it. They hooked up my engine to another truck's gauges and that gauge did the same thing mine was doing.
Upon a cold startup, the temp gauge will progress over to the 250-260 mark before coming down, then go up and down. Mostly goes up during acceleration. When cruising at 55 mph constant, it will remain about 180-190 and sit there indefinately.
I had an exhaust gas test on the cooling system done this weekend and it showed no problems. We, then, tried burping it with a burb funnel and it didn't do anything. The coolant kept coming up, then down, then up, then down. We revved the engine to about 1500 RPM, then held it there. When we let it go back to idle, the coolant gushed out the top of the radiator.
I put a new water pump on yesterday afternoon, just for the heck of it and it still acts the same. I let it sit overnight without the cap on and had to add quite a bit of a-frz. this morning to bring it back up to the top.
What does this sound like? I'm stumped!
I've ran both heaters to try to open anything that may have air trapped, but it doesn't seem to help much. Any other suggestions? This started shortly after I ran a coolant flush, so that is why I was guessing it was an air pocket.
Snowman
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"Snowman" <somethingorotherdotcom> wrote in message

replaced
gauge
we
but
shortly
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I recently heard of a problem very similar to this that was coming from bad water pumps. At low RPM's everything is normal, and at high RPM's the temp jumps up, and when you slow down the RPM's everything cools down again. Does this sound familiar? There are 2 temperature sensors in your engine one that feeds to the dash, and one that feeds to the computer that you can see with an electrical diag tool. What was happening is that at low RPM's both temps looked normal, and at high RPM's the dash showed hot, and the computer showed normal still. The whole problem was linked to bad water pumps that at high RPM's does not deliver water to half the engine, but at low RPM's it is able to seep through still. Give that a thought and see if it makes sense in your case.
"Snowman" <somethingorotherdotcom> wrote in message

replaced
gauge
we
but
shortly
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I replaced the pump on Monday afternoon.

mark
it
then
and
had
top.
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"Snowman" <somethingorotherdotcom> wrote in message

replaced
gauge
we
but
shortly
Snow,
You saying that the gauge pegs at 250-260 when you start it up <COLD> and it takes a few seconds to come back down to below 100*F before it heats back up again?
Elaborate on this for me:
"Upon a cold startup, the temp gauge will progress over to the 250-260 mark before coming down"
How long "before coming down" are we talking, and just how low does it go before it "comes back up" again?
Doc

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Doc,
I leave my driveway, and drive 55 down the road and the temp goes up slowly until the 210 mark, then it seems to go up faster. All this takes about 2 miles or so. When it comes down, it comes down at what I would expect to be normal speed when the t-stat opens. It drops down to about 180 or so at this time. It's a new OEM t-stat. I didn't boil it to see where it opened, so I am only assuming that it's working.
Snowman

mark
it
then
and
had
top.
it
up
mark
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"Snowman" <somethingorotherdotcom> wrote in message

197
GM
the
it
When
radiator.
trapped,
air
and
back
go
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got the chart. thanks Doc. other replies inline...

the
another
250-260
about
and
and
it
and
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On Thu, 04 Sep 2003 07:32:21 -0400, Snowman wrote:

Sounds familiar. I think mine was fluc before, then I bored a small hole into the tstat. I also plan on replacing my radiator with a higher cap one. I think I will also go with a oil cooler and alt tranny cooler.
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On Sat, 06 Sep 2003 13:38:46 GMT, "Robert Kenneth Loyd"

That does sound familiar. I replaced the thermostat in my tractor and then all hell broke loose. It overheated when first run and upon shutting down for a few minutes it cooled a bit and then would be OK after that with a normal temp. The next time I ran it after a complete cool down it overheated again. When left to cool awhile the thermostat would open and then if re-started all would be well and the temp would stay at 180. I asked around and found out that the thermostat (in this case anyway) needs a small hole in it so I drilled it with a 1/8" hole and the problem went away. Different application but the problem may be similar.
-- Regards Gordie
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When I changed the water pump and t-stat in my truck I had that same kind of problem. All I did was "burp" the system several times over the next couple of hours. I imagine a hole in the t-stat would allow the air to pass back to the rad without having to heat the engine up to 195F and let the t-stat open.
Ed
wrote:

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Snow I'm late into this thread...forgive me long winded reply follows Have you replaced your thermostat? Have you ohmed the cyl head sensor and watched resistance values change as the truck warms up? R values can be found in almost all manuals.
I suspect a tstat going south on you ,also just maybe the heater or bypass are plugged with dislodged junk after the flush and therefore your difficulty burping the system.
My '93 acts much the same though the temp peaks around 220 until the tstat opens then the temp will cycle up and down (light loading).Under heavier loading the temp rises slightly but will remain stable.
My theory is that tstats of a given range coupled with variations in rad capacity have different flow characteristics and will sometimes cause temp indications to cycle Rolf
"Snowman" <somethingorotherdotcom> wrote in message

replaced
gauge
we
but
shortly
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My 92 Z-71 has an inaccurate temp gauge too. Although it doesn't fluctuate to the degree that yours does it is off by about 40 degrees. Sometimes its right on.
Seems to me that it has something to do with the connection behind the cluster. I have had 2 clusters and both did the same thing. After I cleaned the connector behind the cluster and reseated it and it was great for about a month.
I also have a tack that jumps between 2000 and 2500 rpm intermittently on the highway. disconnecting the mileage stepper motor fixed that but I had to reconnect it of course, but at least I know where the problem is. One of these days maybe I will shield that connection and motor. Good Luck JRE
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On Tue, 2 Sep 2003 08:00:40 -0400, "Snowman" <somethingorotherdotcom> wrote:

FYI 1993 Chevrolet Suburban 2500 4x4 454
Same problem as you.
Moved "new" temp sensor from the head and installed in the water pump. My pump had an extra plugged port. Installed "cheap" mechanical sensor in head. No more problems, and both guages read within a few degrees of each other.
Possibly the proximity to the exhaust manifold was causing inaccurate readings. I don't know. The mechanical sensor is nearly fluch with the head so less likely that the exhaust manifold influenced it. My two cents worth. and after 2 days of hard driving and testing, mine seems fixed!
Just so you know I bought this truck super cheap becuase the previous owner was tired of spending money at the dealer fixing this problem.
My initial clue was becuase I placed an old "mech" unit directly into the radiator while the display was readin 250. it read 195. and it was obviously not boiling over.
Good Luck. Steve E.
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