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Deville
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Been having erratic overheating. Seemed like it would do fine then just overheat. Probably coincidental but the last 2 times I filled up with gas it overheated. The temp would be all over the place and would especially get bad on stop and go trips. Oddly enough most of the time I could drive to work and it wouldnt overheat. When it did though I would get the overheat message and the coolant would be boiling out of the purge tank.
So I tried the exhaust gas test twice and both times it was negative. Yesterday I was messing with it and pulled the return line off. It wasnt flowing. Traced it back to the casting that the upper radiator hose connects to. pulled it loose there and the clog was in the fitting that screws into the casting that the return line hooks to. I removed the fitting and cleaned it out and put it all back together. This morning the temp was rock solid at 199 degrees the whole way. What a relief!!! It only has 59k on it.
I still think its odd that most of the time I could drive and it wouldnt exceed 220. Thanks to everyone for prior posts that I read while researching problem. I would think as small as that passthru is and that GM used stop leak that this isnt more common than the dreaded head gasket. Just thankful I can drive my car with confidence again.
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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You have just discovered why just about every "overheating" thread advice reply begins with "Is the purge line flowing ?"

The Northstar cooling system is somewhat different from a conventional OHV engine in that not only is the thermostat mounted in the coolant return line, but the system flow depends on the heater circuit and a water crossover on the rear end of the block.

Any air bubble or "air binding" in that system and coolant pump flow drops radically, creating hot spots in the block which may boil the coolant locally, raising system pressure above design relief valve limits with the result that coolant vents out of the system, leaving more airspace, compounding the problem.

The purge line exits the system at close to the highest point in the system and will feed air and gases to the airspace over the coolant in the reservoir, assuring full flow to the pump and thereby preventing pump cavitation = full coolant flow.
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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This is from a 2000+ engine, but the basic flow is the same for Northstars......
 

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Cadillac 2000 DTS & 1999 Lexus LS400
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I am having a similar problem with my wife's 2000 DTS. It threw code DIM Pl 1536, engine coolant HOT - AC disabled. She just returned from a 20 mile drive, sat idling with AC on for ~5 minutes, started back down the street and got the DTC code, above. She pulled into a parking lot and shut it off before reaching the red line on the temp gauge.

The overflow is about 3/4 full (i.e. over full), but no pressure on it. I checked the return line - it flows from the aluminum housing to the coolant overflow tank. The top radiator hose is blasted hot and the bottom hose is warm.

I drove it home after it had cooled down. I drove it 4 blocks and it had already heated up to the middle of the temp gauge. Doesn't appear to have any water running through the engine. Thermostat or water pump would be my guess.
 

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2010 DTS
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Forget the water pump. If it is turning (belt not broken) and not leaking, it's doing it's job. Thermostat is possible, but not probable. If the purge line is clear and flows coolant, you might want to borrow a block test kit from Autozone and run the test to see if there is any exhaust gases in the surge tank. Open the cap (after it cools down) and smell the tank. Does it smell like exhaust?
 

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Cadillac 2000 DTS & 1999 Lexus LS400
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The coolant doesn't smell like exhaust.

I ran the car today after pulling the connector nipple and verifying it was clear, too. While the nipple was out, about a quart of coolant leaked out. The coolant is clear as new. I added coolant back up to the full mark on the overflow tank.

There was a small flow through the purge line to the overflow tank when I first started the car. The stream of coolant didn't change as the car warmed up. It warmed up to the 12 o'clock position pretty quickly (less than 5 minutes at an idle in the garage - ambient temp of about 65F).

I took it out and drove it a few miles, did 3 or 4 WOT runs in 1st gear, did 2 or 3 WOT runs in 2nd gear and finally a WOT all the way to the top end. During the WOT runs, the temp stayed around the 12 o'clock position. But, when I coasted back down to 60 mph, the temp climbed to the 3/4 mark. I continued to drive it a couple more miles real easy and the temp fluctuated around the 3/4 mark (a little above and a little below).

When I got back to the garage, the temp stayed around the 3/4 mark while it idled with the A/C on. The overflow tank level was about 1 1/2 inches above normal (must have added a little too much when it was still cool). I ran the heater, it blew warm, but not hot as I would have expected with the engine temp at the 3/4 mark - that could have something to do with the climate control, though.

I will see if I can pick up a head gasket leak kit and check it, hopefully tomorrow.
 

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Do not check the coolant level when it's hot. You will get false reading. (Cooland expands when it's hot)

Do a block test kit like Ranger suggested. Sounds like air(exhaust gas) is being injected into the cooling system. You should do it after a "running hot" or overheating situation. Block test will be always negative when there's never-been-used coolant in the system.
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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There was a small flow through the purge line to the overflow tank when I first started the car. The stream of coolant didn't change as the car warmed up. It warmed up to the 12 o'clock position pretty quickly (less than 5 minutes at an idle in the garage - ambient temp of about 65F).

This sequence is normal.

But, when I coasted back down to 60 mph, the temp climbed to the 3/4 mark. I continued to drive it a couple more miles real easy and the temp fluctuated around the 3/4 mark (a little above and a little below).

Temperature should have stayed within 1/8" of the 12:00 mark.


When I got back to the garage, the temp stayed around the 3/4 mark while it idled with the A/C on. The overflow tank level was about 1 1/2 inches above normal (must have added a little too much when it was still cool).

Coolant expands and the reservoir will give you a false (high) reading when hot - normal

I ran the heater, it blew warm, but not hot as I would have expected with the engine temp at the 3/4 mark - that could have something to do with the climate control, though.

If the A/C was ON, then the climate control system will keep discharge air temps at whatever you set the system to.......you want COLD air, you set it to 60. You want HOT air, you set it to 90.


I will see if I can pick up a head gasket leak kit and check it, hopefully tomorrow.
As previously posted, do not do the block exhaust gas leak test with new coolant - the engine must have been run for several hours if you have added any appreciable amount of new coolant.
 

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Cadillac 2000 DTS & 1999 Lexus LS400
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Well, I drove my wife's car to work today, a 60F day. Drove 25 miles each way to work and the needle was straight up at 12 o'clock position the whole way.

I did put about a quart of coolant in it this morning before leaving. The WOT runs must have pushed through the air pocket. If the coolant level is good tomorrow, I will just drive it for a while until I experience another heat up. If that happens again, I will do the head gasket leak test.

I love the sound of the WOT run. How often is the car suppose to be run like that? Not often enough, is what I tell my wife!!!

Thanks!
 
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