Late this summer the Temp gauge in my Malibu jumped from the normal 190/200 to around 210 the first time I used the heater. It also started fluctuating between 210 and 230 while driving. When driving if I back off the gas it will climb to 240/250 in a second, then when you get back on the gas will drop back down.

This is a mechanical temp gauge and I have tried another known good gauge with the same results.

With the Rad cap off there appears to be a good flow to the coolant and no noticable air bubbles show in the top tank.

I haven't touched the thermostat, as it appears to be opening at around 180, where it is suppose to and I don't see how it could cause the rapid changes that the gauge is registering.

The engine is a re-built 406 running World Torquer heads with the 400 type steam holes drilled. The waterpump is a new GM, the same as used on the ZZ3. The rad has a new 4 core, with fan shroud and 6 blade fan. The temp gauge bulb has always been installed in the rear plug in the head, on the passenger side. Running a 50/50 anti-freeze mix. This combo has about 5000 trouble free miles on it until this started happening about a month ago.

HELP !!! Anyone have any ideas on what could cause this sudden change?

JimC

?? the thermostat is only opening a little bit and the revs force the fluid through...but that seems a bit weird...

I had it happen once that a wire was shorting against the mech tube to my guage and caused some weird things to happen to the guage....

Where is the sensor, head or intake? Could there be air trapped in the block??

[This message has been edited by Bow_Tied (edited 11-04-2002).]

[This message has been edited by Bow_Tied (edited 11-04-2002).]

Maybe your water pump is tightening up on you?

First, have you recently opened the radiator warm/hot and then closed it without letting it cool all the way down? This is a no-no and produces an actual vacuum at lower temps which can translate to hotspotting which produces temperature fluctuations. Also on a related issue, do you have a wire coil in the lower hose. It sure sounds like the lower hose is collapsing. Are you loosing coolant? I would also change the thermostat as cheap insurance.

Is your heating problems happening at idle/slow speeds or cruising? Low speed and idle overheating is usually lack of airflow across the raditor. Overheating at speed, esp quick variations are due to air in the system and/or hose collapsing possibilities.

FWIW, the GM stock pumps are not a good high performance selection. An aftermarket pump such as Tuffstuff or Stewart's Stage I or II are much better performers! I would look at that.

On your fans. If you are having overheating problems at idle and slow speeds, airflow is a likely culprit! First, I do NOT know of any 16" or 14" fans rated for over 2700+ cfms. If you REALLY have a fan that moves around 2700cfms, it will be sucking down over 22 amps to accomplish such. This means twice that for the two....sorry I just don't think so.

I would look for the best fans for the money which are Spals. The twin Spal fan set up is a product that works and moves alot of air! Over 2800CFMs! Another alternative is a Mark VIII fan. That is a big sucker if you can make it fit.

Otherwise....n

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Steve Jack
-Concept One Pulleys and Brackets
-Marketing Technologies Limited
-Northern/Southern Rodder Magazine's "Jack'Stands" technical column author/originator

"Overheating at speed, esp quick variations are due to air in the system and/or hose collapsing possibilities".

Will check the hose out, it doesn't have a coil in it. How would the system get air in it? That could explain the very quick temperature changes. How do you get rid of it?

This car has been driven all summer in heavy traffic and on the highway with no problem. The problem showed up when the cool weather came and the heater was turned on. Speed in the range of 30+ MPH. problem still appearing with temperatures below freezing.

I know a GM pump is not a major HP item, but this is a nearly stock engine, bit of a cam, 10/1 pistons and WP heads. the fan is a 6 blade mechanical with a custom full molded shroud.

Thanks

If your running a clutch type fan the clutch maybe getting ready to take a dump.

I think that your first sentence may have the necessary information to fix this one. The return hose from the heater core (with the associated hoses and heater control valve) flows into the intake manifold right next to the thermostat. You probably dislodged something which then got stuck in the thermostat while it was open and is now holding it in that position. It flows enough to warm up but not enough to keep it cool under load or slow water pump speeds.
Maybe take out the thermostat and visually check it? That would be the first logical step in my opinion.

Good luck,

Louie

JImC..... did you drain the system before the season and not open the heater core circuit to amply supply the coolant when refilling. IF not, this can hold air until you do open it.

If the lower hose is in "iffy" condition, in other words softish, the lack of a support spring may indeed be the culprit at speed. If you think about air being trapped in the heater core, then all of a sudden opened and forced/injected with coolant from a constant volume in, .....what has the ability to reduce the coolant volume reservoir....the hoses will slightly reduce their overall volume by slightly collapsing to make up the diff. At idle, this may not be a problem, but add suction at the lower hose which already has a negative pressure on it, and it tips the hose to collapse to a certain extent with the addition of RPMS.

Just a guess.....but an educated one. I would definitely put a sping in when you change the thermostat.

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Steve Jack
-Concept One Pulleys and Brackets
-Marketing Technologies Limited
-Northern/Southern Rodder Magazine's "Jack'Stands" technical column author/originator

Steve
Your ideas sound good. The cooling system was filled when the new 406 was put in and the heater had not been used since and was probably not purged either (who can remember, just wanted to get his puppy on the road). Know the lower hose did not come with a internal spring, but can't remember if I put one in.

Unfortunately before I could try anything that white stuff we get for what seems like half the year showed up and temperatures dropped to nearly the coldest in Canada. The car is in a unheated storage shed where it spends the winter. So solution may end up waiting till spring. 30 Coupe is taking up all the workshop space.

Will update in the spring.

Thanks
JimC

Along with the otehr suggestions, don't overlook the water pump belt. It may simply be loose. Also, make sure the vacuum check valve is holding. It may be alternately opening and closing the water valve as you apply and let off on the throttle.

Thanks for the ideas Randy, Belts are good. No vacumn operated valves on this car, Early 64 Canadian car

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http://www.chevelles.com/feature/jan99.html

Here's another thought. Have the radiator checked for the presence of combustion gases. A lot of the independant shops have them. The one I'm talking about uses a blue liquid and a squeeze bulb that pulls in vapor from the inside of the radiator through the liquid. If combustion gases are present, the liquid turns yellow, almost like beer. If there combustion gases present, you probably have a head gasket leak that's allowing combustion gases into a water jacket.

Also, is there a chance the water pump is for a reverse flow system and got installed by accident?

Hey guys... remember the simple stuff. Someone stated "how's air getting into the system?". How 'bout spending $6 and getting a new radiator cap. This little gizmo is the pressure control device for the whole system.
Check the condition of the sealing surface in the radiator neck as well.

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"Atomic Batteries to Power...Turbines to Speed"

(The Minnesota John D)

'64 El Camino
383 - TH-350 - 12bolt - 4wh pwr discs
Bedded down for the Winter.

[This message has been edited by John D (edited 11-18-2002).]