: 95 STS Cooling System Quirks



caniceman
01-11-08, 06:36 AM
Hello All..........

I'm new to this forum, and have some rather strange and interesting things going on with the cooling system on a 1995 STS.

The CHECK COOLANT LEVEL message started being displayed on the driver INFO screen about 3 weeks ago. I checked the level in the surge tank, and it was slightly low, so I topped it up. I test ran the engine to check for leaks, and none were readily visible. While letting the engine idle, I noticed that both cooling fans started up in low speed mode at an indicated coolant temperature of 175 degs F, and then switched to high speed mode at 185 degs F. This occurred with the HVAC switch in either AUTO, ECON, or OFF modes. (Prior to this, the fans were coming on between 222-235 degs F). The engine shows no signs of overheating or coolant loss. At cruise, the indicated coolant temp will read between 199-212 degs F, and in stop and go traffic, it will remain fairly stable between 203-207 degs F. (Before the fans started running full time, the coolant temp in traffic would creep up to 222 or greater before they came on, which I understand is normal).

Once the engine is up to operating temperature, the upper rad hose is warm, the thermostat housing is hot, and the lower rad hose is cool. Also, the back draft from the fans feels cool. This would lead me to believe that either the thermostat is sticking, or I have an air lock in the system. If so, the engine should be running extremely hot or overheating, which doesn't seem to be the case.

I'm wondering if the ECT sensor is defective, and reading lower than it should. Is the ECT sensor that provides the coolant temp display on the instrument panel the same one that the ECM and/or BCM reads to turn the fans on/off, or are there 2 separate units involved ?? Also, I was wondering if the ECM will turn the engine fans on at a lower temperature than programmed when it repeatedly detects a low coolant level ??

Any insight into what may be happening here would be much appreciated.

Thanks in advance............

Ranger
01-11-08, 06:12 PM
I'm wondering if the ECT sensor is defective
That was my first thought.


Is the ECT sensor that provides the coolant temp display on the instrument panel the same one that the ECM and/or BCM reads to turn the fans on/off
I believe so.


Also, I was wondering if the ECM will turn the engine fans on at a lower temperature than programmed when it repeatedly detects a low coolant level ??
My guess is no.

stskenny
01-12-08, 03:55 AM
Hi, I had a similar problem w/ my 93 STS. I was initially perplexed because it seemed to me that I wasn't loosing coolant. But actually, I was. I was misled by not seeing any leaking in my driveway, or when I stopped, and seeing that my hoses were ok. My problem turned out to be my water pump. By the time I found the problem... I had to replace my radiator too. My coolant wasn't expelled until the car heated up for about ten minutes, and most of it was left on the highway. When I stopped, It didn't leak either (initially). I'm not sure what may have changed in 95... but my advice would be to remove the radiator cap and top the coolant off with the car cold. Then drive the car until you get the 'check coolant level' message. Let the engine get cold again. Open the radiator cap, and see how much coolant it takes to top it off again. With most cars, coolant loss is very obvious to you (and everybody else) by clouds of white steam. I never saw steam at all. N*s are designed to resist destruction by coolant loss. But when it says it has lost coolant... I suggest you believe it! The water pump is constructed in a modular way. It has a housing bolted onto the block (which I did not have to remove), and a replaceable pump core. Mine was leaking through the seal between the core and the housing, and was further concealed by the water pump pulley. I had to remove lots of covers and the air filter box (and use a mirror) to finally see the leak. If this is your problem... it's a fairly easy fix. If you do it yourself, be advised that you will need a special tool and a half inch breaker bar to remove the pump core. I got the tool from Advance Auto Parts for under $30 bucks. I think I paid about $80 for the core. Be sure it comes with the O ring seal, and be sure to replace the $5 water pump belt too, because I feel that's the weakest link. You don't want it to break on you.

Anyway, that's my story. I hope it helps.

Ken

caniceman
01-12-08, 06:47 AM
Hello Ranger and Ken....

Thanks very much for taking the time to reply, - I appreciate it !!!!

The engine has not lost any coolant since I topped up the surge or header tank, - at least, the coolant level in the surge tank has not changed. However, the CHECK COOLANT LEVEL message is still being displayed daily, - not on initial engine start-up, but as soon as the car begins moving down the road. Also, the fans come on as described previously, ie, in low speed mode at 175 degs F, and in high speed mode at 185 degs F. Once the fans come on, they do not cycle off until I shut the engine down.

Ken, - I was wondering if your problem with the water pump on your 93 STS was mostly one of coolant loss, which required you to keep topping-up the coolant level, or did you experience overheating, or any of the peculiar symptoms I have with my car ??

If I have diminished coolant circulation related to either the water pump, thermostat, or an air lock, I would still expect to experience overheating at some point. So, either the engine is overheating, and the ECT sensor is displaying erroneous data, or coolant temp is within limits, and the fan control module or BCM/ECM is/are defective. I don't know which. I'll let you know if I figure anything out here.

Regards....... Colin.

Ranger
01-12-08, 01:20 PM
Have you checked the purge line to be sure it is not clogged and has coolant flow? How have you determined "diminished coolant circulation"? Have you checked the water pump belt tensioner?

stskenny
01-12-08, 07:54 PM
Yes Colin, coolant loss was my primary concern. I was/am really leary of overheating engines. My only indication of trouble was the Check Coolant Level messages, and the mysterious coolant loss. So I proactively kept coolant in the car and stopped to top the system off soon after seeing the message. I later read that N* are designed to resist overheating due to coolant loss, by switching to one of it's many 'limp' modes. It supposedly will shut down some cylinders and use them to air-cool the head enough to get you home. But I'm planning never to test that. Ranger's sugestion to check the belt tensioner is key. I personally believe that little belt and pulley is the coolant systems weakest link. If it slips, coolant ain't circulatin'. Sensors can't properly sense if the coolant around them hasn't been in the engine for a while. I'd remove and inspect the belt, and while it's off... hand rotate the pulley that's part of the water pump impeller to see if it feels right. I checked my fluid flow by running hose water through the upper and lower radiator hoses, the thermostat housing, as well as the water pump orfices, once I had removed the pump core. I believe the pressure from the trapped air in my system eventually cracked one of my radiator side caps. But I think my problem originated from my coolant being forced through my failed water pump seal. There are some ferrous parts of the pump core that are in contact with the aluminum housing. I detected some slight signs of corrosion inside my pump from a previous owner not using the right type/mix of coolant. The coolant needs to be suitable for aluminum engines, and GM specifies some sort of additive tablets for some models.

creeker
01-12-08, 08:06 PM
Have you checked the purge line to be sure it is not clogged and has coolant flow? How have you determined "diminished coolant circulation"? Have you checked the water pump belt tensioner?

Does the purge line run all the time,if not,how do you test it? tnx.

Ranger
01-12-08, 08:57 PM
I later read that N* are designed to resist overheating due to coolant loss, by switching to one of it's many 'limp' modes.
Only one limp home mode. It kicks in around 265 deg. and as you mentioned, alternately disables one bank of cylinders to air cool them.


I personally believe that little belt and pulley is the coolant systems weakest link.
That is actually a pretty reliable part of the system, but a crucial one.


I believe the pressure from the trapped air in my system eventually cracked one of my radiator side caps.
I doubt it was pressure. The side tanks tend to crack over time and many heat cycles. They are actually a weaker link it seems than the belt and tensioner.


I detected some slight signs of corrosion inside my pump from a previous owner not using the right type/mix of coolant.
I would suspect that was more from neglect (not changing coolant).


GM specifies some sort of additive tablets for some models.
The tabs or cooling system supplement designed to seal possible porous aluminum castings. Same exact product as Barleaks tabs (HDC) or the powdered version (G12BP).


Does the purge line run all the time? If not,how do you test it? tnx.
Yes. Just pull the 3/8" purge line off the surge tank while idling (cold so as not to burn yourself). you should see coolant flow. It should increase with RPM. If you see coolant flow, quickly plug it back on and reattach the clamp. If not, locate and clear the blockage.

stskenny
01-13-08, 12:22 AM
Thanks for the corrections and clarifications.
The more I can learn about N*s, the better!

Ken

caniceman
01-13-08, 05:06 AM
Hello All....

Thanks for the continued feedback, info,and suggestions. All very much appreciated !! No, I haven't checked the purge line for flow, nor have I inspected the water pump drive belt and tensioner for condition and proper operation.

I suspect I have diminished coolant flow because as described in my original post, the upper rad hose only gets warm (to the touch), the lower rad hose remains fairly cool, the thermostat housing gets hot, and the fan blow back air from the radiator feels cool. My experience with other vehicles having these symptoms tells me to suspect a thermostat that isn't opening completely, an air lock, other physical restriction or blockage, or poor water pump output. I used an infra-red thermometer today to get some sample readings, and although I know these things aren't entirely accurate, here are some of the temperature readings I'm getting following a highway run:

Thermostat housing: 190-195 degs F, Upper rad hose: 158-170 degs F, Lower rad hose: 115 degs F. The CHECK COOLANT LEVEL message persists despite no apparent change in the coolant level, and the cooling fans continue to come on at approx 175 degs F (info display indicated), and stay on until I shut the engine down.

If in fact, I do have diminished coolant flow, then surely the engine should be running much hotter than indicated, but this doesn't seem to be the case.

This all remains a mystery for now. When I get things sorted out, I'll let you know what I find.

Regards........ Colin.

clarkz71
01-13-08, 11:18 AM
I don't know off hand about the fans coming on early but the
"low coolant level" when it's not low is usually a bad level sensor.

A bad PCM could cause both problems.

I would try a new thermostat for the "circulation" problem.

stskenny
01-13-08, 11:40 AM
BTW...

When I referred to 'many' limp home modes, I was admiring how well my Cadillac was designed. I've personally never owned a better car, and I'm impressed with the effort that Cadillac applied to engineering redundancy into my car. In addition to a water cooled engine, that can operate as an air cooled engine, there's the 'limp home mode' of the transaxle, that incorporates enough redundancy to enable it to use 2nd gear to get the car off the road. That philosophy of versatility is also evident in the advanced anti-detonation design that prevents damage to the high performance N* from low octane fuel, and performance vs fuel economy versatility of an engine that can switch between 4, 6 and 8 cylinders!

Ranger
01-13-08, 01:16 PM
Definitely a head scratcher Ken. I would check the obvious first. As Clark mentioned, thermostat, then purge line, tensioner etc. Keep us posted. Inquiring minds want to know.