highwaydude50

Here is the problem I have. My temp gauge has been and still is running just above 185 while driving. Today when I parked it I heard the boiling sound coming from the radiator. I let it cool down some and opened the cap and noticed the water was about 1/2 way down from the top. I checked the overflow container and it was right at the top. Here's hoping someone can tell me the problem. I had the radiator cleaned out and new antifreeze installed about five months ago and I believe a new thermostat as well. I have a 94 c4. Thanks

MissileDoc

It sounds like you have a weak radiator cap. Have it pressure tested or replace it with the proper new one.

highwaydude50

Thank you sir. Will be my first stop tomorrow morning. Looks like it has been on there awhile.

lt4obsesses

Get a good one from a Chevy dealer, not a generic autoparts store unit. I tried to cheap out when I started having occaisional issues with the overflow getting full. Then I had the heater control valve split on me, then the lower hose came off. Could all be coincidence, but it just seemed strange. Put the Chevy unit on and so far so good.

But for good measure, I would rent a chemical block tester to make sure you don't have combustion gasses in the coolant system.

don hall

May be two problems......

1. Low level in cooling system, and high level in overflow bottle
indicates a breach in the 'closed system'. Caused by a split hose end, loose clamp, rad cap gasket distorted/torn, rad fill neck
cracked, anything that will allow air into the system and prevent a siphon return during cool-down.

2. Temp sensor may be faulty.

A 'closed system' means that coolant expands into the overflow bottle at temp, and returns to the cooling system during cool-down.

Check all hose ends, clamps, especially the rad cap gasket.

You will learn that most rad caps are made by STANT, and packaged in various brand boxes including ACDelco.

highwaydude50

Should I empty the reservoir and fill the radiator up to the top then run the car or should I pull out enough out of the reservoir to where the level should be then fill up the radiator?

don hall

Always fill the reservoir to the brim, and the overflow bottle to the
'cold' level, at least. It should end up that way after driving and cool-down. If the levels change..... air is getting into the system.

don hall

Good point..... exhaust gases forced through the head gasket into the cooling system would cause a rapid rise in temp, and the expulsion of coolant into the overflow bottle. A ruptured gasket would prevent a siphon from occurring, and the overflow bottle would remain full.

A stuck closed t-stat would cause a similar condition.

highwaydude50

Thanks all for the info. Put an oem cap on and it seems to be holding. At least the reservoir is not rising. Next thing would be to replace the thermostat

highwaydude50

Going on a corvette club run this Saturday. Will be an excellent test. Will bring a few water bottles just in case.

don hall

What reservoir are you writing about? The tank with the rad cap is
an extension of the radiator (expansion tank). That tank should be filled to the brim. When the engine heats to op temp, the pressure will exceed 16#, open the cap valve, and allow expanded coolant to flow to the overflow bottle.

The overflow bottle level should increase when coolant is hot, and decrease during cool-down.

highwaydude50

The reservoir I was referring to is the overflow bottle located next to the radiator. I haven't driven the car enough to see if the new OEM cap is working but on a few short trips to town, 30 minutes of driving the temp gauge is setting just above the 185 mark and the overflow bottle still stays at the cold mark. ??

don hall

In reviewing your thread.... post #1: temp gage at 185 - radiator boiling. That makes the temp sensor suspect. The digital temp sensor
is located in the water pump..... that should be replaced.

If the level in the overflow bottle doesn't increase as the engine temp increases, either there is insufficient coolant in the system at start-up, or the engine has not reached operating temp.

Coolant is going to expand, and pressure is going to build with a hot engine. Your cap is probably rated at 16#. When the pressure reaches 16#, expanded coolant is going to flow to the overflow bottle.

If you are reading the analog gage (dash needle), that gage is just an approximation. The analog sensor is located in the passenger side head.

highwaydude50

Again thanks for the info. Very helpful!! If the digital gauge is in the water pump, is that something that can be changed or do I need a new water pump?? I will check the level of antifreeze at startup and let the engine run for 30 minutes or so and check it again.

don hall

No, you don't need a new pump.... the sensor is a simple swap (easy access). Just be aware that you will lose a little coolant, but if you are prepared for the swap ( have the new sensor ready to insert) the fluid loss will be minor.

Some stores refer to the sensor as a 'sender', especially the analog sensor.
You want a 1994 LT1 digital temp sensor that fits into the water pump.

I sincerely encourage you to check-out your cooling system before taking your planned weekend cruise. No need to be stranded.

FASTAZU

Have you changed your thermostat to a lower temp? Had your fan on temps changed? I ask because 185 on the low side for a stock LT1.

Your temps should be in the 190-200 range and can get as high as 235 in stop and go traffic, as seabright mentioned I would have the sensor checked.

don hall

Low level of coolant in the expansion reservoir, and a high level of coolant in the overflow bottle after cool down indicates there is air entering the cooling system, which is negating the siphon that should occur during cool down.

With a properly functioning system, a 15# cap will raise the boiling point of coolant to about 257F ( 3 deg per 1 lb. of pressure -http://www.heat-transfer-fluid.com/pdf/techpapers/pressure-boiling-point.pdf). You should not hear boiling coolant.

Check for air leaks..... hose ends, loose clamps, check the neck of the expansion reservoir for a crack, any area that would allow air in.
Run the engine to op temp, and determine if moisture is present any where in the cooling system. I had to run my finger nail around the surface of the expansion tank neck to find a crack.


To create a siphon, the system must be closed (no outside air).

Since you have replaced the cap, you have eliminated a common place for air to enter..... through a distorted or torn cap gasket.

If not corrected, coolant will be displaced into the overflow bottle, and NOT returned via a siphon. Each start-up will have less and less coolant in the system, and overheating will occur.

highwaydude50

Thanks a lot. I ran the car for 35 minutes with the ac on and the temp got to 190. I have noticed in traffic it does raise to 235ish. When I shut the car down the overflow bottle did raise almost to the hot water level. I added a small amount of antifreeze to the line. I checked the radiator with the cap off and it was just below the rim on the pic here. I will drive it today for the real test. Again very valuable info.

don hall

Sounds like a little progress..... cap is working (allowing expanded coolant into the overflow bottle)

Do you hear the fan activate at about 226F?....the second fan at about 235? If not, check the fan fuses.

highwaydude50

I ran the car at an idle for 40 minutes. I did not hear the fans come on. The temp got up to 250ish. I turned on the ac and the fan came on and cooled down to 190. turned off the ac and turned on the fan switch inside and the fans came on also cooling down to 190 or so.