: 97 SLS Overheating - I've checked everything and need ideas

06-19-13, 08:56 AM
I need some help from one of you Northstar experts on overheating issue! Thank you in advance for your time and input!

I bought this 97 SLS from a guy who got it on trade but suspected it had a bad head gasket. I bought it as the price was right, even if the headgasket is shot. Below is the history and symptoms (sorry for the length but I wanted to be 100% accurate with the order and procedure I have taken thus far):

Prior owner had his mechanic put in a new water pump and thermostat. This didn't fix it. Also the car has check engine light on, he said his mechanic said the code was pointing to an O2 sensor issue. His mechanic wanted to put in a new radiator but he didn't want to spend any more $$ on the car as he had already lost money on the trade he made, which is when I bought it for $500.

1 - There is no white smoke from exhaust
2 - The oil doesn't look milky
3 - Cannot smell exhaust from resevoir
4 - Resevoir has quite a lot of floaty crap in it (I'm thinking someone put that stop leak junk in it)
5 - I verified the fans kick on correctly when temp reached with AC off and kick on immediately with AC, so I assume my Coolant Temp Sensor is good.
6 - I verified that the new thermostat is working (used pan of hot water to ensure it opens at 180 degrees)
7 - I rented block test kit from Autozone and did two tests below:
a) followed instructions on kit, with card cold started it and did test, which was negative
b) I let the car heat up really good so it begin spitting coolant from resevoir. Shut car off to let it cool a bit, turned it back on and tested again and test was negative
8 - I rented pressure test kit. The cap pops at 16# as it should. I connected to resevoir and it held 15# fine for 3-5 minutes. I assume that that was sufficient time that either it would hold pressure or it won't. Hope this is correct assumption?
9 - I verfied that the purge line has good steady flow of coolant back to resevoir.
10- After the engine warms up, checked the following:
a) I felt around the radiator to see if there were any spots cooler than others and all the spots i could reach were hot.
b) I felt heater hoses and both were hot
c) Verified heater working inside car. It did but did not feel very hot; however, this may be because the temerature outside that day was 90+ degrees and quite humid, so I think my perception of the air from the heater was skewed because of the ambient temp.
11 - I used a garden hose to flush the cooling system as follows:
a) Removed resevoir cap and disconnected hoses from water pump and thermostat, removed thermostat as well as disconnecting the purge line from resevoir
b) sprayed water into resevoir
c) sprayed through both upper and lower hose to flush radiator
d) sprayed through water pump and thermostat housing (verified that purge line was flowing well too)
e) reconnected all hoses, filled with water (thermostat still out) ran car until it came to temp (left heater on hot with fan on high)
f) let engine cool down until i could touch without burning hand
g) flushed system again (same as above) with garden hose
h) reconnected everything, put thermostat back in and filled system with coolant. Started car and let it idle for close to ten minutes as I added coolant as needed until it remained at the top of the neck.
i) Time for test drive (ambient tempature was 92 degrees) I started engine and after idling with AC on for 5 minutes it came up to midway on analog gauge. I took it for a spin but temp was immediately rising past midway on gauge, after only 3 minutes or so of driving i returned home as guage was beginning to head to the red mark on gauge.
j) After car cooled completely down I checked resevoir and it was about 3-4 inches below the neck.

I am at a loss here and the only thing I can figure is that either my pressure test or block test was a false negative or that I'm facing an issue of a partially clogged radiator and/or heater core due to all that floaty junk I originally saw in the resevoir.

Is it time for me to take my car to a mechanic to get an expert's diagnosis? Or how can you guide me next? Thanks again! cal

06-19-13, 09:43 AM
Read up on Northstar cooling system and temp gauge operation and maintenance in the sticky posts just above. Study the cooling system articles way up in the Cadillac Technical Archive - top black bar.

Proper coolant level, cold, is about halfway up in the surge tank. Northstars do not normally mix oil/coolant or vice versa due to failed head gasket(s).

Water pump drive system - tensioner - operating correctly ?

Two block tests, both negative, should be good news.

06-19-13, 11:53 AM
WOW! Sounds like you've covered all the bases. When you put water in the upper radiator hose, did it flow out the bottom?

06-19-13, 02:55 PM
Yes, water flowed from upper to lower radiator just fine. One thing that I did notice is that the flow of water through the radiator was not as fast as it was through the block, but I'm guessing the radiator has more restriction due to the nature of it.

Tonight when i get home from work i'm going to start it and ensure that the resevoir is at half full when at temperature because the articles I've been reading all say that it should not be overfilled as it can cause issues. So, I'll give that a shot and post an update in the morning.

06-19-13, 10:15 PM
Overfilling it will cause only one problem. It will overflow as the coolant heats and expands. Once it pukes out the excess and finds it own level, it should stay there.

06-20-13, 08:22 AM
Thanks for th input on overfilling. So does anyone have any ideas/suggestions of where I go next in isolating this overheating issue? Thanks in advance!

06-20-13, 11:15 AM
I would take the car out and get it to overheat. THEN come back and do a block test

07-02-13, 07:59 AM
Ok,, you saved me some work, I havnt done block test yet which is next step. Mine is in a 2000 Deville.