Intermittent Temp Increase - Diagnosis

[kzite29]

Hey All,

Hate to make another one of these diagnostic threads since we have so many, but I can't seem to find any past posts that really capture my interesting issue. (If there is, a link would be appreciated.)

Anyways, the Cobra runs great, idles fine, sounds amazing. Heat is hot, air is cold, plenty of power and consistent gas mileage. Mostly stock, couple of bolt on's and suspension work, nothing that's given me trouble in the past.

The issue is that when I'm on the highway in 5th gear with revs at or below 2000 the temp gauge starts to climb super slowly. I haven't let it get more than 2/3 of max but based on what I've seen so far it would probably take 30 minutes or more to reach the bad zone. As soon as I notice it, I'll just downshift and it cools itself off.

Now here are my thoughts:
- Seems like some sort of flow issue. When the water pump is spinning faster the temps drop, regardless of speed. Maybe the radiator is restricted? It does have 20 years and 85,000 miles on it.
- OR, maybe it's just getting upset at low speed high load and generating way more waste heat than it should. I don't know what would cause that. Sticky valves maybe?

Here's what I've dismissed so far:
- Maybe there's an air pocket that only impacts at lower coolant flow rates? Nope. Burped the system over 3 days of heat cycling.
- Maybe air is getting in through the reservoir? I don't think so. I replaced the cap with a new motorcraft one and issue is still present. No visual warping or leaking in the reservoir.
- Faulty sensor? Probably not, given that this is a repeatable issue under specific engine conditions.
- Bad thermostat? Well if the stat was stuck it would overheat at all speed/loads, unless maybe it's stuck partly open?

Any thoughts greatly appreciated.

 

[Mojo88]

Well, first I would probably check with IR temp gun. Drive around a bit to get to normal temps, then stop and measure temps in a few spots (write them down), then do the highway drive thing and when gauge climbs, quickly stop and measure same spot(s) and compare with previous temps to confirm they are actually hotter. If they are still the same temp, then you have been blessed by the Gods and it's just a sensor (hopefully).

My other thought is possibly there is dirt, or crud, or old leaves, or plastic bag, or whatever; restricting air flow over the radiator cooling fins. Hold a light against the radiator from the engine side and look through to see if all nice and clean.

Of course, I'm sure you checked the fan.

A bad or faulty catalytic converter can disrupt the normal combustion properties of an engine, and force the engine to work harder in order to emit the burned exhaust gases, causing overheating.

Hope this helps. It's a weird issue. I'll be interested to see what it ends up being. Good luck in the fix!

 

[01yellercobra]

Given that you can cool it down by increasing the RPM I'd say you have a coolant flow issue. The question is, is it a restriction in the radiator or something with the pump? I'd take a hard look at the radiator. Years ago my wife had a Honda Accord that suddenly had an issue where if we were at a light it would start to heat up. As soon as I raised the RPM it would cool right down. When we got home I went to check the fluid in the radiator and half of the top tank came off in my hand. Had the radiator cleaned and repaired and never another issue.

 

[DSG2003Mach1]

Given that you can cool it down by increasing the RPM I'd say you have a coolant flow issue. The question is, is it a restriction in the radiator or something with the pump? I'd take a hard look at the radiator. Years ago my wife had a Honda Accord that suddenly had an issue where if we were at a light it would start to heat up. As soon as I raised the RPM it would cool right down. When we got home I went to check the fluid in the radiator and half of the top tank came off in my hand. Had the radiator cleaned and repaired and never another issue.

I would tend to agree. Personally at this age with the car Id probably replace the pump and depending on what local shops charge inspect/dip the radiator or replace it. If it's all original a fail safe thermostat wouldnt be the worst idea.

 

[Coosawjack]

Probably air in the system...."burp" it and see if that works??

 

[FIREBALL]

How long have you had the car? have you flushed out and replaced coolant? My guess is that the radiator is partially plugged up, if someone has added a lot of stop leak in the past that could have plugged it up. Also could be a bad thermostat that is only partially opening. The first thing I would do is flush cooling system and install new thermostat.

 

[kzite29]

Wow, thanks for all the responses guys!

I've had the car since 2017 and been on half a dozen drives of 15+ hours, so it likely isn't an old stop-leak buildup.
I remanned the oil cooler this spring and did a full coolant flush (the long/correct way) when I put that back on. Coolant is still spotless after 6 months so I'm not thinking there's a "clog" per se from debris but maybe some bend or other restriction in coolant flow.

Fan is good. Hoses are good. Radiator has some dings in the fins, but I feel like an air flow issue probably have the opposite of "highway only" appearances. Might just go over to a Mishimoto anyway as insurance for auto-x.

Thinking about the Cats, wouldn't that throw me an emissions code? I know these cars aren't too "emissions sensitive" but I thought usually some DTC comes out of unwanted backpressure especially from the converters. Maybe this is a good excuse to grab me a new H-pipe anyway!

I did burp it extensively when I reassembled everything, but maybe the old rad cap was letting a bit of air in the system, been burping the past 2 monrings since discovering this issue and not getting much of a difference, maybe 1oz of coolant add.

Not sure what the best way to diagnose the water pump would be given that it's cooling fine at idle. Definitely open to recommendations here! It has a slightly larger pulley on it from the Steeda kit, hasn't given me issues in the past but maybe this is contributing?

I'll burp one more day and update you guys. If that fails, start with a new thermostat (probably grab a Motorcraft from dealer unless you guys have other recommendations) and see if I can pour some water through the radiator to "flow test" it while I have the coolant out.

Cheers.

 

[FIREBALL]

Stant--Super Stat, not the regular Stant, the Super Stat is a high performance upgrade, I use a 180, engine stays at normal operating temp in 100 degree heat, even with my underdrive pulley, although I also installed a Stewart Warner pump.

Stant superstat #46138 its a 180 that fits, 180s came on the 03-04 cobras
More Information for STANT 46138

 

[RX1Cobra]

Are you burping it at the cross over or by running with the cap off?

 

[hotcobra03]

I had issues for awhile with running hot to overheating after long drives

Mine was the radiator no flowing enough

Test I did was a water hose on radiator

Sitting still with ac on car would overheat
Water was the only thing that cooled it down air did nothing

Was a cheap repair

 

[kzite29]

Are you burping it at the cross over or by running with the cap off?

Burping at the crossover.

 

[Coosawjack]

With the car pointed slightly UPHILL??

 

[DSG2003Mach1]

With the car pointed slightly UPHILL??

never had to do this... I also dont think air in the system explains it going away with an increase in rpm.

 

[Mojo88]

......Thinking about the Cats, wouldn't that throw me an emissions code? I know these cars aren't too "emissions sensitive" but I thought usually some DTC comes out of unwanted backpressure especially from the converters. Maybe this is a good excuse to grab me a new H-pipe anyway!........

I suggested the cats because the car is old enough, plus it has a few miles, plus it likely gets used and then put up for a while (helping the cats deteriorate), plus it likely gets used HARD once in a while, haha, and this definitely puts a strain on the cats.

I bought my heavily modded Roush from a great guy on this forum. The aftermarket "hi flow" cats on the car had just a few thousand miles on them, and when I took exhaust apart to swap some pipes, I could see right away that the cats were getting blown apart from the 28psi of boost.

Your cooling issue should be somewhat 'easy' fix. I'm curious to see what the problem is. Please be sure and post the eventual solution. Some good tips from the guys here, no doubt!

 

[DSG2003Mach1]

I did burp it extensively when I reassembled everything, but maybe the old rad cap was letting a bit of air in the system, been burping the past 2 monrings since discovering this issue and not getting much of a difference, maybe 1oz of coolant add.

Not sure what the best way to diagnose the water pump would be given that it's cooling fine at idle. Definitely open to recommendations here! It has a slightly larger pulley on it from the Steeda kit, hasn't given me issues in the past but maybe this is contributing?

1) the cap on the overflow not sealing wouldn't cause this, it might cause a leak and for the coolant to boil over quicker because it's not under the correct pressure to raise the boiling point. Coolant doesn't actively move through that tank, there's not really a way for it introduce air into the system.

2) water pump - if nothing else get another WP gasket and when you replace the thermostat pull it and look it over. The obvious failure point being water through the weep hole.

Is it hotter out now than when you normally drive? It's brutal hot down here right now. Personally I'd get rid of the underdrive water pump pulley - I mean it's worth what, one hp maybe?

 

[kzite29]

Thanks again for the continued support guys.

To answer some questions:
- It's not too terribly hot up here in Illinois at the moment, but I'm driving the car down to Texas in a month! To answer your question I don't think the ambient is appreciably different than times when the car has not overheated.
- The U/D pulley for the water pump is more to balance the total serpentine length so you can use the stock belt and not have any issues with the tensioner. It's almost the same size as stock, so yeah, not much gain, but plenty of convenience. Will probably leave the same if I can get away with it.
- Yes I jacked the front up a bit to burp the system.


Here's today's update:

I opened the crossover tube to top off after sitting for 24 hours and coolant pushed out the hole as I pulled the plug. Is this normal? There weren't any air bubbles, just coolant. That hasn't happened before.

Ordered that Stant 180 deg thermostat and a water pump gasket off RockAuto today.
Also noticed that Motorcraft water pumps are way cheaper than expected and all Radiators are way more expensive than expected. Fingers crossed on that one...