Cooling Issue

[63glacierblu]

Hi, everyone,

Long-time listener, first-time caller. I just registered for the forum, but I've been relying on your expertise for years, so first of all, thank you! You've helped me keep two Marks out of the shop and on the road since 2014.

I've got a mysterious overheating issue that I need help with. Forgive me if it's already covered elsewhere in the forum--I've been looking for a while but can't seem to find what I'm looking for.

The details and symptoms:

-November, 2021 I lost a VRCM (mechanic mistakenly diagnosed it as a bad PCM, but that's beside the point). I needed to get home and was far away, so I hotwired the fuel pump. Drove fine, but I got caught in a bit of traffic and it briefly overheated.

-I replaced the VRCM and ever since then, the engine runs hot. Not overheating, but hot. Twice now, during long drives, the water pump has suffered a catastrophic leakage at the weep hole.

-Things I've done/replaced:
Water pump (twice), thermostat (twice), radiator flush (nothing too concerning came out), exhaust gas test on coolant (negative). Mechanic also did a pressure test and said it held fine for a couple hours.

My question is mostly concerning what proper temp readings should be at various places. After flushing and filling yesterday, I was getting it up to temp to open up the thermostat and burp it. After about 20 minutes, the temp at the passenger side (top) rad hose was about 190 or so. The driver side hose from the crossover to thermostat housing only read about 140 near the bottom at the housing. The hose from the thermostat to the bottom of the rad was only 108 or so, so obviously it hadn't opened yet.

Are those readings normal, or close to normal? Am I way off in thinking that water is not flowing properly, otherwise the driver's side rad hose should be warmer than that after 20 minutes, thus opening up the thermostat?

I do what I can but am no expert. Any help you can provide would be greatly appreciated. Thanks in advance!

 

[xtriggerman]

A burp area folks over look is the top heater pipe going into the heater core on the fire wall. Seems no matter how well you burp it at the plug, there is usually an air pocket at that pipe joint. Other than that, I'm not sure what else I can tell you since I'm only familiar with the 2nd gen. On that model, that VRCM controls the fan turn on via AC line pressure

 

[driller]

Sounds like you may not have the proper thermostat. The Mark VIII runs a bypass thermostat housing which allows coolant circulation to short circuit the radiator until the thermostat opens which then closes off the bypass between the coolant crossover and the t-stat housing.

The improper t-stat does not close off the bypass which effectively reduces flow through the radiator and results in overheating.

170° PERFORMANCE THERMOSTAT for Mustang Cobra, Mach 1, Mercury Marauder & Taurus SHO by Reische Performance Products

Proper fill/burp procedure(link).

 

[63glacierblu]

Sounds like you may not have the proper thermostat. The Mark VIII runs a bypass thermostat housing which allows coolant circulation to short circuit the radiator until the thermostat opens which then closes off the bypass between the coolant crossover and the t-stat housing.

View attachment 828577283

The improper t-stat does not close off the bypass which effectively reduces flow through the radiator and results in overheating.

View attachment 828577284
170° PERFORMANCE THERMOSTAT for Mustang Cobra, Mach 1, Mercury Marauder & Taurus SHO by Reische Performance Products

Proper fill/burp procedure(link).

Thanks, driller. That coolant flow diagram is especially helpful--I had the flow pattern all wrong in my head.

On that diagram, hot coolant should be flowing from 18 to 15, thus opening up the thermostat, correct? What's confusing to me is that after 20-25 minutes of idling, my temp readings at around points 9 and 15 (ends of both of the hoses coming from the crossover) are about 50-60 degrees different (cooler at thermostat side). Shouldn't those temps be closer? Could I have a blockage somewhere?

Also, two details I forgot to mention: Sometimes, under load, for instance when running up a grade at about 3,000-3,500 RPM, the temp will drop significantly--from near the top of the NORMAL range to the middle or below middle. In other words, the opposite of what one would expect. I've also replaced both temp sensors.

Thanks for your help!

 

[xtriggerman]

I would take a good look at the inside of your radiator. It sounds like it may have corrosion in the rows. It may be just not circulating well at lower RPMs then once its kicked up to over 3K, the increase pump speed forces the coolant threw the corroded rows with higher pump flow pressure. Just a guess. I put a new aluminum rad in my 97 from Ebay and the temp gauge stays a hair lower than middle of the gauge no matter what. It stays welded there at all speeds and outside temps. Oh, never put a fail safe thermo in these cars. The extra flow port will flood the coolant tank before you can get the rad topped off. Dealing with some one elses maintenance habits can lead to problems like this Tap Water brain fart a prior owner had released upon this car's coolant system.

Repairing a rotten lower Thermostat Housing

These old car do not lac for surprise repairs! While in the process of swapping in an all aluminum Radiator, I thought I would check on the thermostat housing since it was dripping coolant. Mind you, this 1997 car only has 69K miles on it but as...

www.lincolnvscadillac.com

 

[63glacierblu]

I would take a good look at the inside of your radiator. It sounds like it may have corrosion in the rows. It may be just not circulating well at lower RPMs then once its kicked up to over 3K, the increase pump speed forces the coolant threw the corroded rows with higher pump flow pressure. Just a guess. I put a new aluminum rad in my 97 from Ebay and the temp gauge stays a hair lower than middle of the gauge no matter what. It stays welded there at all speeds and outside temps. Oh, never put a fail safe thermo in these cars. The extra flow port will flood the coolant tank before you can get the rad topped off. Dealing with some one elses maintenance habits can lead to problems like this Tap Water brain fart a prior owner had released upon this car's coolant system.

Repairing a rotten lower Thermostat Housing

These old car do not lac for surprise repairs! While in the process of swapping in an all aluminum Radiator, I thought I would check on the thermostat housing since it was dripping coolant. Mind you, this 1997 car only has 69K miles on it but as...

www.lincolnvscadillac.com

Inadequate rad flow was my initial guess as well. Both times my water pump started puking, it was after a long highway run. I thought maybe the water pump was having to work extra hard due to increased resistance somewhere. I'll drain the rad again and take a look inside.

 

[driller]

On that diagram, hot coolant should be flowing from 18 to 15, thus opening up the thermostat, correct? What's confusing to me is that after 20-25 minutes of idling, my temp readings at around points 9 and 15 (ends of both of the hoses coming from the crossover) are about 50-60 degrees different (cooler at thermostat side). Shouldn't those temps be closer? Could I have a blockage somewhere?

During warmup, yes coolant should flow from 18 to 15 and the radiator is dead headed at 16. As the thermostat opens, coolant flows through the radiator and the bypass closes off at 15. The wrong t-stat will not close off the bypass and subsequently short circuits the flow through the radiator. In effect it's practically the same as no thermostat.

As far as temperatures, it would be normal for the inlet to the radiator to be hotter than the outlet to the thermostat. It's been a while since I scanned one with an IR thermometer but I would say 20-30 degrees easily, certainly more under load. Higher than that would have me believing there could be a flow restriction in the radiator. That would also fit the symptom of water pump failure (from excess operating pressure?).

 

[63glacierblu]

During warmup, yes coolant should flow from 18 to 15 and the radiator is dead headed at 16. As the thermostat opens, coolant flows through the radiator and the bypass closes off at 15. The wrong t-stat will not close off the bypass and subsequently short circuits the flow through the radiator. In effect it's practically the same as no thermostat.

As far as temperatures, it would be normal for the inlet to the radiator to be hotter than the outlet to the thermostat. It's been a while since I scanned one with an IR thermometer but I would say 20-30 degrees easily, certainly more under load. Higher than that would have me believing there could be a flow restriction in the radiator. That would also fit the symptom of water pump failure (from excess operating pressure?).

Thanks. I'm going to take a closer look inside the rad and will let you know how it turns out.