Heat Gauge

I have a 98 Trans Sport and recently as I was driving the heat gauge started to register that the van was overheating. ( I found this strange because 1- I had recently replaced the coolant (no leaks) and 2- this happened within 2 mile and the van was cold prior to driving ). AS i continued to drive the gauge would come back down to the normal range. I took to my mechanic who changed the theromostat as well as the "switch". To no avail. So when I drive the van now I have to let it warm up well before I take off, and as I drive I have to be cautious when the gauge rises not to drive too fast until the gauge come back down. On all occasion there are no physical signs of overheating. Both the mechanic and I are puzzled, so HELP!!!!!

Also, after my last oil change my OIL LIMIT come on everything I start up after an extented stop. To clear this I have to stop and restart the van. Someone suggested that there my be a error message in the computer that needs to be cleared, coul this be so? And how do I clear this?

As a first reaction, I think you have found the solution yourself when you said "I had recently replaced the coolant".
Unless proper procedure is used to vent ALL air from system, emptying and replenishing often leaves air trapped in the system and this has consequences very similar to what you experience. THere is a vent on the thermostat housing (probably) that NEEDS to be opened to let all air escape. ALso, there is a so called "vacuum" fill procedure.
Check for this possibility first before going for other causes. It seems the design of recent cars is more prone to this condition....I wonder why:it is like car makers continuously forget about past flawed design and keep reinventing the wheel.

If you have other signs...foamy white stuff on bottom of oil cap, milky oil, this would point to blown head gasket.....3.4 prone to that..

As a first reaction, I think you have found the solution yourself when you said "I had recently replaced the coolant".
Unless proper procedure is used to vent ALL air from system, emptying and replenishing often leaves air trapped in the system and this has consequences very similar to what you experience. THere is a vent on the thermostat housing (probably) that NEEDS to be opened to let all air escape. ALso, there is a so called "vacuum" fill procedure.
Check for this possibility first before going for other causes. It seems the design of recent cars is more prone to this condition....I wonder why:it is like car makers continuously forget about past flawed design and keep reinventing the wheel.

If you have other signs...foamy white stuff on bottom of oil cap, milky oil, this would point to blown head gasket.....3.4 prone to that..



Luckily, no milky oil. Please describe this 'vacuum fill procedure'. I believe this my have been the method to remove and replace the fluid. Also this problem happen nearly 2 weeks and 300 miles after the fluid change.
Please advise.

While some on this or another forum mentioned the "vacuum fill" technique to expell air or avoid entering air in the system , I did not use it nor needed to. check this link:
http://www.sjdiscounttools.com/uvu570000.html

THe idea is to let new coolant into the system as same time as the old one is sucked out. A variant is to apply succion to one orifice of the drained system and let coolant enter through some other opening.
One precaution GM says if there is a rear heater is to position the system in COOL or VENT mode (and run the engine for a few seconds to activate the vacuum operated valves) before draining the system: this closes the valve that feeds the rear heater and avoids (or reduces - one line still opened) the possibility of trapping air in that section.

About your problem, the GM book says this(cooling sytem diagnosis)
COndition: Hot lamp on or high temperature gage reading; no loss of coolant
Possible cause: faulty engine temperature switch or circuit.

WHich lets me ask this: what did they change? Was it the temperature sending switch or temperature sender? In fact there are some which combine both the high temp alarm contact and the sender for the gauge.

According to my mechanic the van only has one switch which he changed. And when the fluid was changed the procedure was to suck the fluid out the top of the radiator and refill it through the same place.
This may be a dumb question but how can air be trapped in the system? And wouldn't the air be displaced if i ran the van with the radiator cap open?

IT really seems there are some places in the cooling circuit that are higher than the orifices through which the emptying and replenishing goes, so these spots can empty, but keep an air bubble when replenishing. I have experienced this with a DOdge OMni (please do not laugh)...It would take a LONG ...LONG time hearing gargles in the heater core before all the air would eventually be carried out to the top of the radiator where one thinks it could be evacuated to the overflow reservoir, theoretically, but in fact this air bubble accepts to be compressed by the heat expansion of liquid , and may not reach sufficient pressure to pop the overflow trap, so it can stay in the system for a long while. In fact, one has to open the radiator cap often and fill up any void that would show with time, a little bit of air is so expelled every time, but do not think it is all there in one shot, and it does not stay there either waiting: it circulates with the coolant every time the engine runs. So letting the radiator cap open cannot be expected to help.

Oh yeah...one trick I used, which in fact is equivalent to your suggestion to let the radiator cap open, is to block open the little valve in the radiator cap . THis valve is a one way valve that normally opens under a small VACUUM from the inside of the cooling circuit to suck in coolant from the surge tank when the engine cools and the liquid volume retracts. Blocking it open allows air at just above 0 pressure to escape to the surge tank under thermal expansion. Iinstead of waiting 13psi to pop out the cap) AIr surfaces the tank to atmosphere, and when the coolant and remaining air retracts, liquid is sucked from the surge tank to fill up the void. After a few cycles, this will have purged all of the air.
Of course, during this time, the cooling circuit runs at atmospheric pressure so if the thermostat is 195 and outside temperature is high, the coolant could top 212°F and boil out which is no big risk where I live (-4°C or 25°F today). (under 13 psi pressure normal cap release, pure water boils at 245°F). I used a plastic/metallic tie for garbage bags inserted under the little valve to keep it opened.

Thanks, I will pass this info on to my mechanic.

..I'm a little busy now but to-morrow, I'll grab my camera and post a photo of what I mean in case it would convince you to do it yourself.....

Thanks, U DA BEST

Get it at
www.avigex.ca/xport/cap.jpg
To remove the cap from the radiator, it is easy (well, may be sticky at first) to turn it counterclockwise , but then it blocks against notches that prevent "instant removal" and steam blow out if there were pressure still left inside (this if engine is warm); keeping the cap there in depressurised position allows boiling fluid to be diverted downwards thus protecting the inexperienced user. you must press hard in to clear the notches and be able to turn counterclocwise further and remove the cap.

Let me get this straight. Remove rad cap and use twist and do like picture. Then put cap back and close tightly. Then start and run engine.
Question: For how long? (It's pretty cool in NYC)

Yeah you got it right. Well if there is no boiling, which you would see very soon if it is to happen, it is no big deal running it like that for a while...lets say, until the problem you want to correct is cleared. Contrary to popular legend, pressure in the system has no effect on proper cooling and heat exchange. It just allows to run the engine warmer (which improves combustion qualities) without boiling.
I really think 3 or 4 cycles (warming and cooling cycles) should clear any trapped air. If gargles in the heating circuit (well I think you did not mention any of this..but trapped air or exhaust lend to this) or other symptoms persist, then I would investigate for blown head gasket....and this generally leads to true overheating as another cue.

Thanks. How about the "OIL LEVEL" light which comes on and stays on when I start the van. I only goes away after I turn off and restart. The switch in the oil pan was changed. Please dont tell me air in that system (ha). But it did happen afetr an oil change. By the way there is the correct amount of oil in the pan.

MAy be there is an electrical explanation for it all......heat and oil , and all the fuss about trapped air is a no show. For 2 in fact closely related events to occur like this, I'd also think of more than a coincidence...mmm do you have a real oil pressure gauge on that?
BTW, if you do the tie trick, run it around one full turn; the half turn I used for the "demonstrator" would easily slip away...

Well it finally happened, the @#$%$%# van overheated and shut off after bleeding the coolant system twice. It seem as if the head gasket is blown and has to be replaced. I am also changing the water pump. I there anything else coolantwise I have to change? I am also draining the system and refilling it. On the overflow bottle is states the there is a special way of refill the system and it refers me to the manual which only has how to top off the system. How does one flush and refill the system?

Sorry it concluded in that way...., but it seemed pointing to that.
Honestly, I have NEVER had any real problems filling the system with "basic" methods, may be except my Omni....and may be after all, it had a slight head gasket problem that kept gaz in the system but did not real harm other than that. These engines (too) were prone to that.
Usually, there is a vent somewhere ..like the thermostat casing .. to allow air to escape while you fill honestly through the radiator cap. YOu fill slowly, and when coolant flows out of the vent, plug it and finish filling up the rad to the top. THEN, during the next days, check the radiator and top it if needed. It should not be necessary to do that more than one or 2 times. AFter that, it will exchange and finish the job with the surge tank by itself, which coolant level you keep at indicated minimum.
A thorough vacuum fill might do it right the first time, but for myself, I would not care. If you have a rear heater, just be sure to turn it OFF like posted previously.

Ahh..p.s.: disconnect lower radiator hose from radiator to dump the coolant.


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