Fluctuating temp. gauge - part4?

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.

Snow,

You saying that the gauge pegs at 250-260 when you start it up 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

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

I replaced the pump on Monday afternoon.

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

On initial warmup, before the drop, what is the highest reading you get?

After its' done the startup fubar routine and come down to normal, does it spike again? If so, under what conditions? How high does the temp get?

Have you tried backflushing the entire system after yer' initial flush? It is possible a few passages in the cyl. head are now partially clogged due to crud you may have loosened up. When the T-stat opens and there is increased flow, it's enough to get by the blockage and get the gauge reading down. When the t-stat is closed, lack of pressure causes the fluid in that one spot (where the temp sender is) to overheat.

Just a wild stab....................

I've e-mailed you an R vs. Temp chart for the CTS.

Doc

got the chart. thanks Doc. other replies inline...

Yes, up to 240 or so upon acceleration (stop sign to stop sign).

haven't done this and don't know how. Can you give me some tips on this one because it sure makes sense that it could be the problem origin?

It

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

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.

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

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

FYI

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.