I'm actually posting for my parents, who are completely and hopelessly computer illiterate. Hoping to figure something out, I appreciate any help.

They have a 98 Deville they purchased from my grandfather, and the thing keeps overheating. It has 90K on the clock.

It overheats if it travels over about 30 miles distance, for shorter trips it is ok. Basically it gets hot and cooks the coolant out if you keep driving too long.

Background:
No foam in oil pan - I presume headgasket it ok, but correct me if I'm wrong.

Had the dealer replace the little coolant pipe on the back of the block that connects the coolant channels (what kind of stupid design is that!), replaced the thermostat, replaced the water pump, replaced the hoses, flushed entire system and refilled with the specified coolant/water mixture.

Both the cadillac dealer and the GM dealer have no idea, they are stumped. Is there some strange quirk that needs to be addressed? I know my bmw has issues with the valve timing, is there something similar with this model Cadillac and it's cooling system?

I appreciate any help. I think the only thing that has not been replaced is the radiator, but the dealer said it was ok, I'm not sure what type of test they used to verify this.

 

How hot does it run?Does engine hot message come on?

 

Slowroll, given this car is likely to have been driven gingerly seems possible that carbon build up in the EGR passages could cause the engine to run hot. What do you think about that?

 

I don't have the temp, but it gets hot enough to blow the coolant out of the expansion tank. The engine overheat message does come up.

Last time it popped it took over 10 liters to refill it. the total capacity is only 12 if I'm remembering correctly.

From searching I saw that there are some problems with the heater core, is this only when running the heat since they are not using the heater yet?

It also does not always overheat, I took it our and ran it quite hard for 20 minutes and it stayed in the normal zone.

 

I want to jump in and suggest a exhaust gas test in the cooling system, but the info is to vague.
We be taking shots in the dark. please read this thread:

https://www.cadillacforum.com/m_9792/tm.htm

 

Restricted egr should turn ses and egr system code should be set. As to running hot,things to look at : Missing baffle that attaches from lower edge of bumper to the core support - it helps force air thru radiator, Leaves or other debris built up on condenser or between condenser and radiator - Can block air flow thru radiator, Restricted exhaust - exhaust holds the heat in, pipes will normal turn orangish to red - should turn ses light on but not always, Cooling fans when coolant was blowing out, both fans should have on and you should have felt hot air from both.they also should have been loud like a window fan on high speed, Partially restricted radiator - flushing sometimes doesn't always get all the sediment out. A infrared temp meter/gun may help find restrictions or hot spot in cooling system. This car was built when dexcool was not so good, if cooling system got low it would breakdown and turn to mud. They did fix the problem with dexcool since then, that's why GM still uses it. If these suggestions don't show anything, have a show run CO test on cooling system. If it passes, head gaskets should be ok. Radiator would be my choice of cause

 

I will tell you now i have a 99 deville and it was overheating i was hoping it was the headgasket and so i change the thermostat and flushed the radiator and still it was happening...........well this was my promblem and you might want to look i have 4.6l northstar and under the hood on the right side of the motor check your tensionr pulley the drives the water pump mine bearing seased up and the pulley broke and threw the belt of $60 later new pully and 30min my car is as good as new

hopes this helps

 

OK, here are the updates on the overheating caddy.

The radiator seems to be ok, verified by analysis with a thermal camera when the car was at running temperature. No hot or cold spots to indicate blockage inside the radiator.

On the heater core, the tubes going in and out are all very close to the same temperature when the car is running and at temperature. Also the heat works well, so I believe that means the heater core is good.

Everything has been cleaned of leaves, debris, and verified that the fans work properly, no ducts are unhooked or anything.

I have not tried anything with the egr valve, but there are no codes other than overheat so I think this rules out the egr valve.

What next? Again, thanks for your help.

 

did thermal camera give you temp reading? what does temp read on dic while driving, both highway and in town?

 

the temp gauge,once the car is warmed up varies from 195 to about 118, occasionally spiking up to 220.

I can drive it just fine for quite a while, then all the sudden it spikes to 265,the engine overheat and service engine indicators come on,normally it blows the radiator cap off also.

Once it cools down,you can fill it back with coolant and it is just fine,no lights or anything.

I find it strange that it only overheats after 30 min of driving. on shorter trips it is fine even though it still hits operating temp on the shorter trips.