I drove my '95 to work today for the first time. 55 miles one way. The temp gauge read hot the whole time. I had the heater on and when it was running 'very' hot (according to the gauge) the air from the heater was lukewarm. If the temp dropped, the air from the heater would get hotter. Occasionally the 'low coolant' light would come on but I didn't believe it, as it has randomly come on before and if I jiggled the wires at the surge tank, the light would go out. When I got to work I popped the hood and there was no sign of it remotely running hot. No hissing, popping, nothing. The 'fill' tank (the white plastic one) was full and the coolant was cool. I didn't open the surge tank. The upper radiator hose was hot but not any hotter than you'd expect. I ran my hand over the engine and it felt either cool or 'normal' warm. No sign of it running hot at all. Is it possible I have air trapped in the system? If so, how can I purge it? Or what should I be looking for? My first thought (besides low coolant) was a sticking thermostat but like I said, it shows no signs of running hot other than a very scary temp needle. Thoughts?

 

There are two different temp gauges. The analog that is a needle on the dash and a separate digital one that the computer uses to adjust fuel mixture and fans. Pull up the digital one using your gauge button (between dash and console) and see what it says. The analog one is known to be inaccurate. If the digital is reading a lot lower then replace the analog temp sensor or a least clean its contacts.

 

I went out for a smoke and started it up. I let it run and the digital readout hovered around 189-190-191 while the analog gauge read 240-245. Maybe it's just a crappy connection or the sending unit.

 

That's usually the reverse with mine...

 

The analog gauge is not liner, trust the dig. fan 1 cones on around 225 and fan two joins in at around 235, run the AC and one of the fans runs all the time.

 

LT1 fans come on at 228* and secondary at 230*. It is not unusual nor problematic if the car hits this range in stop and go traffic.

Highway cruising, you should see temps anywhere from 180-200.

As far as air from the heater, this can be the sign of a bad t-stat, heater core. But if you have the auto air temp control, it could just be it doing it's job, unless the car was uncomfortably cold at the time. There are a few things involved with this system that aren't really hard to repair, just tough to get to.

 

I'll leave the digital one going on the way home and see what it reads. Nothing surprises me anymore. My '57 Bel Air pegs to "H" even though I've tried 190, 180 and I think even a 160 thermostat, plus cleaning the contacts on the gauge, plus replacing the sending unit with an Eckler's "guaranteed this is the absolutely correct one" sending unit. It clearly does not overheat but the gauge gave a heart attack.

 

Drove it home this afternoon with no issues whatsoever. I had the digital readout on the whole time and it stayed pegged right on 194 for 80% of the time. I hit a pile of stop lights in one town and it crept up over 200 but went back down to the 190s before I got a mile out of town. The analog gauge didn't match the digital but it certainly wasn't pegged like this morning. Strange....

 

It's not really that strange since the digital readout (and ECM) and the analog gauge are fed by two different sending units. Sending units go bad. I've replaced a number over the years. Usually the end up with full open (reading full cold) or full shorts (reading full hot), but occasionally they gradually go bad or start giving bad values. They're not real expensive and not hard to change out.

 

I also have a 95, and just to clarify; your fans run at the same time Low/High, there is no "Primary/Secondary" fan operation on your car.

Your Analog gauge should read slightly higher the digital gauge.. except during long periods of Idling, where the Analog gauge will climb near the top of it's range before the fans kick in at low speed 228° [digital] The Analog gauge probe is located directly in the R/H cylinder head, there fore it reacts much more quickly then the Digital gauge, which is located at the water pump. As mentioned before, the Analog gauge is Non Linear due to it's temperature divisions, but make no mistake about it, it does read the heat that is being generated inside of your cylinder head.

I suspect you have air in the cooling system, this could be caused by a blocked steam pipe assy, a coolant leak, or a leaking head gasket..

When you start the engine up cold and within a couple minutes or so the coolant line entering the throttle body, and the lower 3/8 hose at the Surge tank should get hot fairly quick, and after the engine warms up you should not be able to keep your hands on these two hose's. I would inspect these two items first....

 

ksdaddy,

There should NOT be that much of a difference between the digital and analog temperature gauges. Generally there should only be about a 10 degree F difference with the analog being LOWER (i.e. 194 on digital, 185 on analog).

Whereas it could be a problem with the analog gauge sending unit, more likely if it is the gauge (and not a thermostat issue -read below) it is with the wire or electrical connection to it.

HOWEVER, do NOT discount it being a BAD (or wrong) thermostat.

The gurgling you heard COULD be that the lower portion of the thermostat has become slightly disconnected from the upper portion and it not allowing the proper coolant flow from the engine block into the water pump mixing chamber (it happened to me on our 94).

LT1 thermostats are special. both because of the reverse flow coolant system, and because they have two plates, one which controls the inflow of water into the water pump (from the radiator) and the other plate controls the flow of coolant from the engine into the mixing chamber of the water pump. The lower plate has a 'cap' which holds it onto the main plunger/plate of the thermostat. This 'cap' can slip off the main plunger and cause the lower plate to either not move or not open/close the bypass area properly. This WILL cause the analog gauge (and actually the engine block) to be hotter -- and this is NOT good.

I tried a few different brands of thermostats, and have found ACDelco to be the most consistent (and it has a jiggler/bleeder which should be mounted at '12 o'clock). The model number is in the title.

 

About an hour ago I started it up and opened the surge tank. It had 'some' coolant in it but barely covering the floor of the tank. It began flowing through the lower 3/8 hose but just sputtering, not a steady flow. I assumed from this that the coolant was indeed low so I trickled some into the surge tank while the engine was running. Not a lot but it seemed to be pulling it out just as quickly as I was putting it in. It seemed to stabilize after a little while so I only filled it to just below the lower hose inlet. The overflow tank was pretty much empty so I put 'some' into it so when it cools down it will be able to pull some if it wants it. Once it got to operating temperature I cracked open the bleed screw at the thermostat neck but only got a little burst of air, then pure coolant. I'm really concerned because it was bubbling at the surge tank... I'm having visions of head gaskets. I mean, if it needs them, fine, it's all in the budget but may max that out. Is "any" bubbling at the surge tank normal or is it completely out of the question? I ask because if the coolant level is low, there will be air in the system, and could that bubbling I'm seeing just be air working it's way out?

 

ksdaddy,

if the surge tank was that low, then there is air in the system and the bubbling is likely normal.

Coolant level in the surge tank should be just below the bottom of the radiator cap lower lip (where the lower rubber gasket contacts) but above the small hose which is the from the coolant bypass/back of the head bleeder hoses.

Remember to not run the engine too long with the cap off, as the pressure build up in the tank in important to help move any air trapped in the system to the surge tank.

Don't forget there is a air pocket between the bottom of the radiator cap lower lip and the top of the tank. This is where the pressure builds as water cannot build pressure, but air can.

 

OK.. good start, get the two tanks filled up, and keep an eye on it. The one thing I would do at this point as well, would be to replace the radiator cap. Don't fret over a possible gasket problem until you get to that point, this may be a simple problem, in the mean time I wouldn't be racing around in it..

 

theadmiral94 I got your PM but your settings won't allow me to reply. Thanks for the info. I topped it to where you suggested and I'm watching it. Just new/old car panic is all.

 

You want to get the surge tank full. It will catch some of the air in your system but not all. As suggested you need to fill it to just below the cap neck. Keep opening the bleeder valve after every run. The problem you saw with the analog gauge going crazy was because of air pockets. Get the air out and things should settle down. After you get the system purged if the analog still acts up just replace the sensor and no more worries.

 

By 'after every run' do I let it cool down first? Sounds dumb but I have to ask.

 

I'm experiencing the same thing on my 90, the overflow tank is full when the engine is cool and the surge tank is low! Is this trapped air in the system or maybe a blocked return line? I never had this problem until I have the system flushed. It's so full, it's leaking out of the cap of the on the overflow tank.

 

There are a good many archive threads on bleeding the colling system. What has always worked the best for me is to raise the front of the car, which puts the bleeder screw at the highest point of the system.

Then I remove the pressure cap and make sure there is coolant in the overflow tank. Run the car to operating temp. If the coolant flow drops in the surge tank I add coolant, and let continue to run. when it no no longer drops, I will open the bleeder screw. At this point, you should get a steady stream of coolant from the bleeder. If so, I close up the system, turn the car off, and let it cool. Then I check the surge tank level the next morning after the engine completely cools.

 

Here's the deal.. when the cooling system heats up it builds pressure, when the pressure exceeds the relieving set point of the pressure cap, fluid is routed to a storage tank. conversely; when the cooling system "cools" the pressure drops back down, and can actually start to draw a slight Vacuum, at this point the same valve in the cap begins to open, and draws coolant back from the recovery tank.. (same pipe)

So having explained this, just imagine what will happen when there is a leak in your cooing system.. you got it!