[DrakeL]

This car has stumped me, so I’m turning to the collective intelligence of the interwebs.

2004 Cadillac Deville DHS
238k miles
Bought at 100k miles, owned for seven years.

It has a history of overheating if the outside temperature is 90+ degrees fahrenheit. Downshifting to run the engine (and water pump) faster will cool it down temporarily. This problem is worse in town than on the highway. The fans work and the radiator is not obstructed. The car is fine below 90 degrees.

I replaced the thermostat after discovering that the radiator was not heating up. The old thermostat was definitely bad - I tested it - and I replaced it with a tested new thermostat. Approximately one gallon of coolant drained from the engine during replacement; this was clean and clear of debris. There was a little gunk on the radiator side of the thermostat housing and hose due to lack of flow, but nothing serious.

The radiator now heats up as anticipated, but the car still overheats - just not as quickly as before.

• There is plenty of coolant in the overflow tank. The car has slowly lost coolant since I bought it, but I always keep it topped up with all-purpose or dexcool.
• The purge line is not blocked and has good flow.
• Block combustion test was negative.
• No signs of water in the oil.
• Oil consumption very low - less than one quart per 5,000 miles or so.
• Cabin heat works quite well, occasionally gurgles quietly when starting/revving engine. It’s quite adorable.

Potentially relevant codes:
IPC U1016 - history - Lost Communications With Powertrain/Engine Control Module
PCM P0171 - current - Fuel Trim System Lean Bank 1
PCM P0174 - current - Fuel Trim System Lean Bank 2
PCM P0300 - history - Cylinder misfire detected random cylinders

I did have a vacuum leak a couple of years ago (PCV line), which also caused P0171, P0174, and P0300 codes. It does have a rough idle again and I suspect another vacuum leak, but I haven’t been able to find this one.

Here’s where things get interesting… This afternoon, I went for a test cruise in town to induce toasty mode - mission accomplished. Having got the temp gauge up almost to the red line, I parked, grabbed an infrared temp gun, and started taking measurements.

It appears that coolant/heat flow is BACKWARDS from what it should be! Picture with temperatures attached. Each area was measured 2-3 times, with and without the engine running, and the results averaged.

Could the previous owner’s mechanic have installed a water pump with the wrong flow direction at some point?
Is there anything else that can cause this consistent overheating pattern?

Thanks for reading this saga of a post, and thanks for any suggestions y’all may have!

 

[Dave The Rave]

Gurgling could be air in the cooling system, very important to get all air out. I also believe the Radiator needs a replacement.

 

[Ranger]

Have you checked the condenser for an accumulation of dead bugs blocking the air flow?

A gurgling heater core definitely indicates air in the system. I have found that sometimes it takes several drive cycles to purge it all with a top off each time.

The car has slowly lost coolant since I bought it

That could be a crossover manifold gasket. Somewhat common in the 2000+ engines.

 

[basscatt]

This car has stumped me, so I’m turning to the collective intelligence of the interwebs.

2004 Cadillac Deville DHS
238k miles
Bought at 100k miles, owned for seven years.

It has a history of overheating if the outside temperature is 90+ degrees fahrenheit. Downshifting to run the engine (and water pump) faster will cool it down temporarily. This problem is worse in town than on the highway. The fans work and the radiator is not obstructed. The car is fine below 90 degrees.

I replaced the thermostat after discovering that the radiator was not heating up. The old thermostat was definitely bad - I tested it - and I replaced it with a tested new thermostat. Approximately one gallon of coolant drained from the engine during replacement; this was clean and clear of debris. There was a little gunk on the radiator side of the thermostat housing and hose due to lack of flow, but nothing serious.

The radiator now heats up as anticipated, but the car still overheats - just not as quickly as before.

• There is plenty of coolant in the overflow tank. The car has slowly lost coolant since I bought it, but I always keep it topped up with all-purpose or dexcool.
• The purge line is not blocked and has good flow.
• Block combustion test was negative.
• No signs of water in the oil.
• Oil consumption very low - less than one quart per 5,000 miles or so.
• Cabin heat works quite well, occasionally gurgles quietly when starting/revving engine. It’s quite adorable.

Potentially relevant codes:
IPC U1016 - history - Lost Communications With Powertrain/Engine Control Module
PCM P0171 - current - Fuel Trim System Lean Bank 1
PCM P0174 - current - Fuel Trim System Lean Bank 2
PCM P0300 - history - Cylinder misfire detected random cylinders

I did have a vacuum leak a couple of years ago (PCV line), which also caused P0171, P0174, and P0300 codes. It does have a rough idle again and I suspect another vacuum leak, but I haven’t been able to find this one.

Here’s where things get interesting… This afternoon, I went for a test cruise in town to induce toasty mode - mission accomplished. Having got the temp gauge up almost to the red line, I parked, grabbed an infrared temp gun, and started taking measurements.

It appears that coolant/heat flow is BACKWARDS from what it should be! Picture with temperatures attached. Each area was measured 2-3 times, with and without the engine running, and the results averaged.

View attachment 583267

Could the previous owner’s mechanic have installed a water pump with the wrong flow direction at some point?
Is there anything else that can cause this consistent overheating pattern?

Thanks for reading this saga of a post, and thanks for any suggestions y’all may have!

==========================
Could the previous owner’s mechanic have installed a water pump with the wrong flow direction at some point?
NO - the pump only goes in one way - and the belt only moves one way -

a tiny coolant leak that can't be found is probably the crossover gaskets -
this is the one and only time I - we - recommend using the stop-leak tablets -

--------------

what else causes "overheating"?
the MOST common is a clogged PURGE LINE -
which you said IS flowing coolant -

next common cause is the lack of pressure in the system -
a weak pressure cap can allow pressure to bleed off -
caps are cheap - around $10 - should be an 18lb cap -

a coolant leak will also bleed pressure off -

you should do a cooling system pressure test -
you can "rent" the pressure testing kit for free from autozone -

pump the system up to 20psi COLD -
the system should maintain that 20psi forever - test it for at least an hour -

---------------------

reduced airflow through the radiator is another cause for running hot -
dead bugs and just road debris can block BOTH the AC condenser AND the radiator -
especially if these are the original parts at 240K miles -

the BEST way to clean BOTH would be to remove the radiator and use a pressure washer to blast the crud out -
HOWEVER -
DO NOT use full power at close range -
you can actually bend the aluminum fins - thus further reducing airflow - rather than increasing it -

----------------

lastly - the radiator itself -
some of the cooling tubes could be clogged up - so water doesn't flow through them -
back flushing - or chemical flushes RARELY work -
the best solution is to replace the radiator -

if you do replace the radiator -
I recommend the SPECTRA PREMIUM CU2491 - $103 - with free shipping -

Amazon.com

-------------------------
It does have a rough idle again and I suspect another vacuum leak,
but I haven’t been able to find this one.
rough idle along with those two codes points to a split in the PLENUM DUCT -
the little bellows shaped thing between the throttle body and intake log -

they usually split in the groove - at the bottom -
use a spray can of carb cleaner in that area with the engine running -
if the idle changes - you found the vacuum leak -

the DORMAN 911-010 kit includes 8 new silicone intake seals -
$23 - including shipping - from rockauto.com

 

[DrakeL]

Have you checked the condenser for an accumulation of dead bugs blocking the air flow?

Yes sir - there's a few dings and dents from the occasional road debris, but nothing serious.

A gurgling heater core definitely indicates air in the system. I have found that sometimes it takes several drive cycles to purge it all with a top off each time.

I should mention the sound is very quiet, and it's been there since I bought the car. You can only hear it occasionally in a quiet area with the interior fans off. It also didn't get more noticeable when I drained 1+ gallons of coolant when replacing the thermostat.

That could be a crossover manifold gasket. Somewhat common in the 2000+ engines.

Thanks! Would that be enough to cause a heat issue without failing a combustion test? Just to make sure I have the right part in mind, are we talking about these guys? 2004 CADILLAC DEVILLE 4.6L V8 Exhaust Manifold Gasket | RockAuto

 

[DrakeL]

a tiny coolant leak that can't be found is probably the crossover gaskets -
this is the one and only time I - we - recommend using the stop-leak tablets -

Ah, the dreaded tablets... I've been leery of using them, but it might be worth trying...

next common cause is the lack of pressure in the system -
a weak pressure cap can allow pressure to bleed off -
caps are cheap - around $10 - should be an 18lb cap -

Thanks! I have an spare pressure cap on hand; I'll try swapping them out.

a coolant leak will also bleed pressure off -

you should do a cooling system pressure test -
you can "rent" the pressure testing kit for free from autozone -

pump the system up to 20psi COLD -
the system should maintain that 20psi forever - test it for at least an hour -

Adding this to the list of things to test...

lastly - the radiator itself -
some of the cooling tubes could be clogged up - so water doesn't flow through them -
back flushing - or chemical flushes RARELY work -
the best solution is to replace the radiator -

I'm curious about this, because in theory the radiator should be working significantly better now that there is a working thermostat in the vehicle. I'm also seeing a 23 degree temperature differential across the radiator. Should I be seeing a greater temperature drop than this? I couldn't find any data on this specific to this vehicle.

Thanks for your suggestions!

 

[basscatt]

Yes sir - there's a few dings and dents from the occasional road debris, but nothing serious.



I should mention the sound is very quiet, and it's been there since I bought the car. You can only hear it occasionally in a quiet area with the interior fans off. It also didn't get more noticeable when I drained 1+ gallons of coolant when replacing the thermostat.



Thanks! Would that be enough to cause a heat issue without failing a combustion test? Just to make sure I have the right part in mind, are we talking about these guys? 2004 CADILLAC DEVILLE 4.6L V8 Exhaust Manifold Gasket | RockAuto

=======================
Just to make sure I have the right part in mind, are we talking about these guys? 2004 CADILLAC DEVILLE 4.6L V8 Exhaust Manifold Gasket | RockAuto
NO -
this is the COOLANT crossover -
directs coolant to the two sides of the engine -
these are MISERABLE to replace - even with the engine removed -
which is WHY we don't mind trying the stop-leak tablets -

 

[basscatt]

Ah, the dreaded tablets... I've been leery of using them, but it might be worth trying...



Thanks! I have an spare pressure cap on hand; I'll try swapping them out.



Adding this to the list of things to test...



I'm curious about this, because in theory the radiator should be working significantly better now that there is a working thermostat in the vehicle. I'm also seeing a 23 degree temperature differential across the radiator. Should I be seeing a greater temperature drop than this? I couldn't find any data on this specific to this vehicle.

Thanks for your suggestions!

=======================
I'm curious about this, because in theory the radiator should be working significantly better now that there is a working thermostat in the vehicle. I'm also seeing a 23 degree temperature differential across the radiator. Should I be seeing a greater temperature drop than this? I couldn't find any data on this specific to this vehicle.
too many variables to have any specific data - ambient temp - air flow - engine demand - etc -
and the only way to inspect the individual coolant tubes would be to remove the plastic tanks -
not worth the effort -

 

[DrakeL]

this is the COOLANT crossover -
directs coolant to the two sides of the engine -
these are MISERABLE to replace - even with the engine removed -
which is WHY we don't mind trying the stop-leak tablets -

Ah, thanks for clarifying. Yes, that looks like a royal pain!

I will do a pressure test in the morning and report back.

 

[Submariner409]

Your coolant flow description is backwards. Coolant leaves the engine via the upper hose and enters the water pump via the lower hose. The thermostat is acted upon by the block/heater coolant temps.

 

[DrakeL]

Your coolant flow description is backwards. Coolant leaves the engine via the upper hose and enters the water pump via the lower hose. The thermostat is acted upon by the block/heater coolant temps.

Well... I'm an idiot! Didn't realize the Northstar was a reverse flow system. Thanks for pointing that out.

Still trying to find a pressure test kit that will fit the tank. None of the universal adapters I tried would hold enough pressure to do the test. Will try a couple more places tonight. Thanks again for all the suggestions!