temperature swinging wildly?

[neo]

I just changed pumps to the Kennedy hi volume v-belt pump and a new aluminum radiator and now my guage will go to a little over the 200 marka nd then swing back down significatly below the 200 (near halfway between 100 and 200) then when driving in traffic will go back up to a touch over the 200 mark. I have searched without any precise results. I know from other threads the GM guage is far from accurate, but it never did this before. Now, the aluminum radiator was not an exact fit, but after it was the second one (two pass as opposed to the brass three pass) the parts store got me (first one leaked) I had to get it in and move the truck. It was about 1" short, but had all the proper hook-ups. I had to make little polymer spacers but the pressure from the "boss" was on to get it out of the front yard. Does it sound like the radiator is not able to keep up with the heat load? ANy ideas are appreciated.

[JohnC]

My guess is either there's an air bubble in the thermostat housing or the thermostat is sticking. Did you bleed the system?

[JeepSJ]

I agree with John - sounds like an air bubble.

[HammerWerf]

I vote stuck thermostat.

If there is a Robertshaw Hi Perfromance thermostat, they are self purging via a small leak port in the seat. Also sound like it is sticking during the closing phase of temperature control.

What I have noticed is that the guage, while not great, is reasonably accurate in the middle third of its movement. If you have an old one go very high, it probably won't read correctly. I've got one in my 83CC. Normal operating temp now says 150F. Industrial grade aftermarket says 180F, like the thermostat installed.

Another spot to check is the connection at the sender unit, left front of motor, next to #1 injector. I've seen this connection degrade down to a couple of strands of copper holding the whole thing together.

HammerWerf

[neo]

Yikes! I did not "bleed" the system. Should I just go ahead and repalce the t-stat and then bleed the system? How do I bleed it? Boy am I a dumb a$$! Thanks!

[a5150nut]

Yikes! I did not "bleed" the system. Should I just go ahead and repalce the t-stat and then bleed the system? How do I bleed it? Boy am I a dumb a$$! Thanks!

Chalk it up to "learning curve". Look for a little brass fitting on the thermostat housing. Let the air out while running.

[jerry598]

Yep, My 95 k3500 scared me in the dead of winter with wild swinging temp changes (according to dash guage). After starting cold, or after idling for awhile in cold temps, the stat appeared to stick for a full minute ( probably just seconds) and the temp would scream to the top of the guage, then suddenly back off to normal. At the same time, the cab heater would begin to blow cold air, but once the temp returned to normal, the heater would also return to normal. The check coolant light was also coming on occasionally during the same timeframe, but not at the same instant as the wild swings on the coolant temp guage (and I didn't think that could possibly be a problem in the middle of winter!). Could have been a combination of everything - low coolant combined with more air in the system, and then maybe air bubbles around the stat, who knows? I didn't wait to find out, I looked at the price of a new 180 thermostat and convinced myself to install the dual 180 system with an HO water pump. Hope the temps still run stable after its done (tomorrow). And hope it runs cooler of course. I'd like to be able to pull heavy loads up steep mountain grades without any worry (I've never tried it). Maybe this will do it. Who knows. Wish I could trash this truck, but I love the pile of junk and all my retirement money is invested in it.

[88 K30]

I thought the '89 burbs (I assume this is the truck we are talking about?) had the old design setup and v-belts like my '88 crew. There is no bleed for them that I am aware of?

I have seen vehicles with a cooling system that actually works *too* well behave in this way for the first 15-30 minutes of driving until it could balance itself. The engine coolant heats up and the thermostat opens, then the coolant from the radiator rushes in and close the t-stat again. The radiator does such a good job of cooling the hot coolant, that by the time the engine heats the coolant it now has, the radiator's coolant is really cool and will cause the gauge to drop significantly when the t-stat opens and it re-enters the engine. It's most noticeable on cooler mornings, but since diesels don't run as hot as gassers anyway and this one has a nice, efficient aluminum radiator, I can imagine the symptom could be magnified?

How long did you drive it while this happened? Did it seem to drop a little less each time?

I would also say, NEVER believe the factory gauge in a truck of this age, especially if the sending unit is original. I'm putting a mechanical aftermarket unit in when I put my new Banks kit on my truck.