: 2001 Deville overheating... block test OK

07-17-13, 11:13 PM
Hi all, my name is Cole and I just purchased an 01 Seville that is overheating if driven at highway speeds for 15 minutes or more. The car doesn't overheat while idling or running at slower speeds. I know that this "typically" a tell-tale sign of pulled headbolts... But I have done a block check test 3 different times and the fluid always remains blue which indicates there are no compression.gasses in the coolant. I am loosing coolant somewhere... If I let the car sit overnight the coolant tank will loose several inches of coolant, but there is no leak visible and the oil level is unchanged (no signs of milkiness either).

The fans kick on and run just fine, and the car overheats when the coolant is full. I will change the thermostat and do a coolant system pressure test tomorrow morning, but what do you guys/gals think I should check next? I am a competent home mechanic with a fully equipped shop, but this is my first northstar so I am asking for assistance due to my extremely limited knowledge with these engines.

1. If the block test comes back negative for combustion gasses is that a fairly safe bet that my head gaskets are fine at this time?

2. Any other tests I should be running in order to pinpoint the problem?

thanks in advance for the help... Any advice is appreciated!

07-17-13, 11:43 PM
If your test is negative, that typically tends to be a good sign. Run it one more time after an overheat episode to be sure.

Northstars typically do not put coolant in the oil.

What kind of coolant are you running? Dried dex leaves a nice chalky white residue to indicate leaks. Does the car loose coolant during an overheat episode? What does the exhaust smell like?

If you're positive the block test is good, your next step would be a cooling system pressure test. If the block test is coming back good, you're overheating, and you're losing coolant, that suggests a leak.

Lastly, define losing coolant. Full coolant level is halfway up the tank with the engine COLD. If you're filling it all the way to the top, naturally the extra coolant will run out the excess drain pipe.

Good luck. I'm rootin' for ya.

07-18-13, 09:07 AM
I ran the block test again...still no change in color (blue) of the rest fluid. I would think if I had bad head gasket/s it would turn green pretty quick???

I will get the coolant system pressure tester today and give that a go.

I don't see any external leaks anywhere in the engine, I cam tell where there is some old coolant residue on the passenger side of the block underneath the ps pulley, but it looks like it has been there for quite some time.

Is there any way the waterpump could have reduced flow but still function fine at idle and low speeds? I know that normally if they are turning and not weeping they are good, but I'm just trying to cover my bases.


Sorry, I missed a question... Loosing coolant meaning that if I let the car sit for a few hours after running I can add 1/2 QUART of fluid or so to the overflow to bring the cookant level up to around 3/4 in the tank. I will make sure it's at half full cold and go from there.

07-18-13, 10:28 AM
Define "overheating" - and have you read the 3 sticky posts on cooling/temperature/gauges just above this - as well as study the cooling system articles way up ^^^ in the top black bar - "Cadillac Technical Archive" ?

Copy this - and many coolng system parts in my 2 albums - the Community tab, Albums, the red Seville.

07-18-13, 10:45 AM
The fans kick on and run just fine, and the car overheats when the coolant is full.
Have you checked the purge line and verified that it is clear and flows coolant?

07-18-13, 01:39 PM
The purge line is clear, how do I check that it flows coolant?


I have read all of those posts/articles... thanks for the info.
When I say overheating I mean gauge pegged and car say overheating on the dash.

07-18-13, 01:53 PM
Purge line - car cold - open the surge tank and carefully remove the rubber purge line from the surge tank nipple at the upper side of the surge tank - NOT the filler neck overflow - the purge line. Holding the end of the line in the open filler neck, have an assistant start the engine at normal cold start idle. A couple of spits, then steady coolant flow from the line ??? If not, shut down and remove the beauty cover and the entire line - blow it out or rod it out if it is clogged. Otherwise, the metal bolt/nipple at the water pump cover area is clogged. Rod it out with a straightened piece of clothes hanger......... rags under to prevent coolant from pooling on the transmission top.

The purge line is a constant high point bleed back to the surge tank (which rides on the cooling system through a 3/4" hose at the bottom) and is the method by which air and gases are removed from the coolant in order to prevent coolant aeration which leads to water pump cavitation which causes overheating.

07-18-13, 05:40 PM
Yeah - what Ranger and Sub wrote about the purge line. Check or change the thermostat - also, check the water pump drive belt and tensioner pulley...make sure there is a good, reliable drive connection. Remove the coolant pressure cap and smell it - does it smell like exhaust? Also, you never did tell us how you define overheating...???

07-18-13, 08:23 PM
When I say overheating I mean that the temp gauge is pegged at the top and the car says overheat. I am not just saying that the bars get 3/4 of the way to the top, I mean an actual "overheat" situation. I can let off of the gas and let the car coast/idle and bring the temp down in short order, but it's not something that I want to do on purpose.

07-18-13, 09:13 PM
Are you actually overheating and sure it is not just an indication problem? Is the coolant level rising? Are you overflowing coolant?

Another though is if you are in fact loosing coolant, but can't see a leak, consider a cracked radiator side tank. Coolant seeps out and evaporates before it drips. Remember that the condenser is sandwiched up against the front of the radiator so you cannot see front the radiator side tanks.

07-19-13, 11:53 PM
Ranger - I am not sure that I am actually overheating, but I am fairly certain that is the case. For the gauge to act the way that it does would be very consistent with the car actually overheating (amount of time it takes, on the highway under load etc...). I have every inclination to believe that I have blown headgaskets and the car is loosing water internally, but the block test has come back good several times now...? I have not had the opportunity to do a coolant system pressure test or change the thermostat yet, but I am getting good coolant flow through the purge line and the WP belt and tensioner seems to functioning correctly.

I will try and test the cooling system pressure tomorrow and throw a new thermostat into it...

07-20-13, 09:35 AM
A pressure test can help find an external leak, but is useless in determining a HG failure. I hate to ask and don't mean to underestimate you, but are you sure you did the block test correctly and at the proper time (right after an overheat episode)?

07-20-13, 03:41 PM
Ranger - I am positive I performed the test correctly... The kit i used had the squeeze bulb rather than the vacuum setup, so I made sure and pumper for over three minutes rather than the suggested two minutes. I did it after an overheat episode, but it was probably 45 minutes later so that I could safely open the overflow cap. I understand that it is always best to do it immediately after, but I am unsure as to why I wouldn't get any signs of combustion gasses even if I waited an extended period of time...? On one test I revved the car at around 3K rpm for 30 seconds or so and then immediately tested... same result, fluid remained blue. I know that these tests are 100% confirmation either way, but I am sure hoping not to have to pull the engine :)

07-20-13, 09:35 PM
fluid remained blue
That's a good thing. Can't bitch about that.

Thermostats are pretty reliable, but in your case it may not be a bad idea to throw one at it since they are not expensive. Just be sure you get the proper one.

07-22-13, 05:07 PM
Check the water pump drive belt and tensioner. Check the thermostat.

If the coolant is low after it's been sitting, it's getting "sucked" back into the block. If it's been "overheating" that means the coolant is boiling off. Therefore it's going to need replenishment from somewhere. That means you have to add.

The block test if performed correctly comes back negative, and the thermostat is good, purge line is clear, the water pump is functioning....... your restriction is the radiator. It maybe partially flowing but not up to capacity, therefore inducing the overheat condition.

Don't flush it..... replace it.

Good luck

07-24-13, 05:09 PM
I had the same problem with my 01.. I know no caddy guy wants to hear it but my car had bad head gaskets and stripped head bolts as well as some crank shaft problems.. Same exact problem! But i paid $5000 for the block to be re-machined from soup to nuts. Thermostat, gaskets, heads, etc. but my car runs great and i love it.. I don't want to say its head gaskets cuz it sucks but it may be.

07-24-13, 08:36 PM
My 2000 Deville passed the block test but the head gaskets were bad..I did that test like 4 times and it never changed color...I dont think that test is all that reliable imho...I think that test would be best done at a higher Rpm. When my head gaskets went I could let the car idle for hours and the temp never went up..but as soon as I got on that gas pedal the car would overheat..

07-24-13, 11:09 PM
Make sure you have strong blue fluid.