Overheating (only) under load

[TripleZeroFilms]

Hey guys,

I've been dealing with a tricky issue as of late and figured it might be worth throwing it out to the think-tank.

THE PROBLEM:

I've just recently repaired my A/C system after fighting it for the last year. Yay! But I now have a new problem: overheating. I live in GA and it is pretty warm here during the summer. If I start driving my car, completely cold, MAX A/C, my engine temp will begin hitting 220 within ~20min (and will rise forever unless action is taken). If I bump it down to NORMAL A/C the temp will drop to ~195 and slowly climb back up to 220+. If I again drop it to A/C OFF (i.e. hot box in the summer), the dial will yet again fall only to rise back up. Turing on the heat at full blast will result to the temp dropping to normal levels and remaining there so long as it is engaged. As soon the heater is turned off, up goes the temp. All this occurs only while driving.

ADDITIONAL INFO:
New water pump and hoses throughout.
New thermostat.
Both of my (original) fans are working properly.
Radiator is NOT plastic.
There is NO coolant leaking/being expelled from the car at any point, no matter the temp.
Coolant level is full.
The system is fully bled.
While at idle, A/C OFF, NORMAL A/C, or MAX A/C, the temperature is perfect.
There are NO bubbles in my coolant tank.

My current thoughts:
Faulty (new) thermostat?
Engine timing/vacuum advance issue?
Radiator plugged?

I am open to all thoughts and suggestions on the topic!

[DMCMW Dave]

My current thoughts:
Faulty (new) thermostat?
Engine timing/vacuum advance issue?
Radiator plugged?

I am open to all thoughts and suggestions on the topic!

Usually really weird overheat issues are head gaskets on the way out. But don't go there first. That problem is typically accompanied by expelling coolant from the cap.

If it seems to get hot way to fast (per the gauge), I've often found that means that there is air in the engine and the sender is fooled by steam. If this is the problem the engine won't seem to be all that hot. But if you have had it on the road at speed that should be long gone. Burp the thermostat bleeder again anyway.

When it is overheating like this, how hot is the line coming from the radiator at the right front lower?

I'd bleed the radiator (again, but trust me on this). Open the small hose at the upper right end of the radiator and let the air out. Coolant header bottle cap should be off (open). Coolant can be cold.

The only difference between regular A/C and MAX A/C is that hot water is circulating in the heater core, so when you switch from MAX to normal you've introduced a second radiator of sorts Aka the heater core, and it was full of relatively cool water.

If this doesn't do it, I'd check for a plugged radiator but that's actually pretty hard to test for. What radiator is it? You said "not plastic", is it one of the aftermarket brass ones? How old?

Air in the radiator will act exactly like a plugged radiator. I've honestly never seen a plugged radiator in person, although I've heard of a few.

One more check - do this cold if you have the metal header tank. With the cap off the tank run the engine. Stick your finger in the tank and see if you feel water coming into the tank from the small hose that goes to the back of the engine. If you don't, the tee at the back of the engine is blocked and you are getting air trapped in the engine. That hose bleeds the engine block.

[Bitsyncmaster]

I would check the current draw on your fans. If you have fused jumpers, bypass the fan circuit breaker. I have seen one set of OEM fans draw 10 amps and others (my old fans) draw more than 15 amps each. Maybe your fan circuit breaker is popping open since running the AC keeps the fans running.

If the fans are not the problem, I would suggest you try one of the cooling system flush kits.

[TripleZeroFilms]

Usually really weird overheat issues are head gaskets on the way out. But don't go there first. That problem is typically accompanied by expelling coolant from the cap.

If it seems to get hot way to fast (per the gauge), I've often found that means that there is air in the engine and the sender is fooled by steam. If this is the problem the engine won't seem to be all that hot. But if you have had it on the road at speed that should be long gone. Burp the thermostat bleeder again anyway.

When it is overheating like this, how hot is the line coming from the radiator at the right front lower?

I'd bleed the radiator (again, but trust me on this). Open the small hose at the upper right end of the radiator and let the air out. Coolant header bottle cap should be off (open). Coolant can be cold.

The only difference between regular A/C and MAX A/C is that hot water is circulating in the heater core, so when you switch from MAX to normal you've introduced a second radiator of sorts Aka the heater core, and it was full of relatively cool water.

If this doesn't do it, I'd check for a plugged radiator but that's actually pretty hard to test for. What radiator is it? You said "not plastic", is it one of the aftermarket brass ones? How old?

Air in the radiator will act exactly like a plugged radiator. I've honestly never seen a plugged radiator in person, although I've heard of a few.

One more check - do this cold if you have the metal header tank. With the cap off the tank run the engine. Stick your finger in the tank and see if you feel water coming into the tank from the small hose that goes to the back of the engine. If you don't, the tee at the back of the engine is blocked and you are getting air trapped in the engine. That hose bleeds the engine block.

I will attempt to bleed again from both ends. That being said, I have done so many times in the past few days. I should also mention that I have an auto-bleeder on the rear of the car (which can be closed manually). Also, as I haven't been burping with the coolant cap off, I will try that. What is the advantage of removing it before bleeding?

As for the temperature, it doesn't get hot particularly fast, but it creeps up over time (20 minutes or so at speed). This makes me wonder if it could be steam related, as doesn't that typically result in the temp increasing quickly?

Regarding the radiator, it was installed by PJ Grady in 1996 (via the previous owner). The invoice only lists it as "Brass Radiator" with a part number of 100486G. Next time I get the engine up to temp I will check the hose leaving the radiator, but I will have to do so by hand as I don't have a temp gun on hand for specific readings.

[David T]

I will attempt to bleed again from both ends. That being said, I have done so many times in the past few days. I should also mention that I have an auto-bleeder on the rear of the car (which can be closed manually). Also, as I haven't been burping with the coolant cap off, I will try that. What is the advantage of removing it before bleeding?

As for the temperature, it doesn't get hot particularly fast, but it creeps up over time (20 minutes or so at speed). This makes me wonder if it could be steam related, as doesn't that typically result in the temp increasing quickly?

Regarding the radiator, it was installed by PJ Grady in 1996 (via the previous owner). The invoice only lists it as "Brass Radiator" with a part number of 100486G. Next time I get the engine up to temp I will check the hose leaving the radiator, but I will have to do so by hand as I don't have a temp gun on hand for specific readings.

From your description of the symptoms it would seem using the heater core to help cool the motor off is just enough "extra" to cool the motor. This tells me the radiator is not transferring enough heat. That can be because of a couple of things. If there is any air in it, it will reduce it's transfer capacity. Loosen the small hose on the top, right corner and let out any air. Next, if there is ANYTHING interfering or blocking the inlet duct you will not get enough air. Is there a license plate or anything mounted near the inlet to the rad duct? Next, you did say both fans are working but check if the fans are firmly attached to the motor shafts. If they are loose they may be connected but not spinning at full speed and drawing enough air through. Also check that both fans are blowing in the correct direction, from front to back. Finally, it is possible the air side is plugged up with debris either the condenser core, the rad or between them so enough air isn't getting through. I would also check the timing and mechanical and vacuum advance to be sure they are all set correctly and working.

[Flash66]

DaveDMCMW is spot on. If running the heater at max cools the temp at speed, it's air in the system. No doubt. First and foremost, change your thermostat! Second check for flow into the bottle from the rear of the engine for blocked T. Now to bleed a pesky air pocket out of your car, this method works every time and uses a little science. Get a long 10" thin funnel (Walmart auto) and put it in the tank with a rag or two around the neck to keep it in place and somewhat sealed in. Then (in a DeLorean) jack up the "rear" of the car as high as you can and start it. This will help the air in the system to rise to the highest point. Keep adding coolant through the funnel with your bleeder open until it's burped as much as possible. The funnel helps push the coolant through with gravity. Next, jack up the front of the car and repeat the process with the upper RH hose at the top of the radiator. I promise this is your fix.

[David T]

DaveDMCMW is spot on. If running the heater at max cools the temp at speed, it's air in the system. No doubt. First and foremost, change your thermostat! Second check for flow into the bottle from the rear of the engine for blocked T. Now to bleed a pesky air pocket out of your car, this method works every time and uses a little science. Get a long 10" thin funnel (Walmart auto) and put it in the tank with a rag or two around the neck to keep it in place and somewhat sealed in. Then (in a DeLorean) jack up the "rear" of the car as high as you can and start it. This will help the air in the system to rise to the highest point. Keep adding coolant through the funnel with your bleeder open until it's burped as much as possible. The funnel helps push the coolant through with gravity. Next, jack up the front of the car and repeat the process with the upper RH hose at the top of the radiator. I promise this is your fix.

Actually the easier way is to use a coolant system pressure tester and loosen the small hose on the top right side of the rad to get the air out of it and the bleeder on the thermostat housing to get the air out of the motor. Any left after that will work it's way out. If you keep getting air you have leaks and/or a blown head gasket.

[Flash66]

Any left after that will work it's way out.

By that time he will have blown his head gaskets. The point here is that air is trapped and it's already overheating under load. The engine isn't going to keep taking the abuse before his heads warp or worse. At this point it's best to get all of the air out NOW and not wait for it to work itself out. Using a pressure tester just introduces more air and creates pockets between the engine rear and the heater core ( hence running the heat at full to cool the engine)

Using the science of elevation, combined with gravity, heat and natural pressure (and adding water, not AIR) is the most effective solution to expelling the air from an engines cooling system. Research, Infiniti and Mercedes, I guess I'm the first to introduce this method to the DeLorean community and you'll thank me for it.

How I learned of this: My Infiniti QX4 was suffering from the same exact issue. The dealer told me I'd need to spend 4-5k to repair my engine because they've tried everything and the symptom would keep returning it did keep returning under load especially in hot weather. Running the heat at full blast would start bringing the gauge down from H almost instantly and they swore my head gaskets were the problem. They've bled and bled to no avail. Using this method not only cured the problem, but also confirmed for me to always be your own advocate when it comes to your engine. Thanks to Ferrari and Mercedes forums, I was able to save myself a boat load of money and tell the dealer to kiss my ass.

Be persistent. When you feel all of the air is out, do it again anyway.

[David T]

By that time he will have blown his head gaskets. The point here is that air is trapped and it's already overheating under load. The engine isn't going to keep taking the abuse before his heads warp or worse. At this point it's best to get all of the air out NOW and not wait for it to work itself out. Using a pressure tester just introduces more air and creates pockets between the engine rear and the heater core ( hence running the heat at full to cool the engine)

Using the science of elevation, combined with gravity, heat and natural pressure (and adding water, not AIR) is the most effective solution to expelling the air from an engines cooling system. Research, Infiniti and Mercedes, I guess I'm the first to introduce this method to the DeLorean community and you'll thank me for it.

How I learned of this: My Infiniti QX4 was suffering from the same exact issue. The dealer told me I'd need to spend 4-5k to repair my engine because they've tried everything and the symptom would keep returning it did keep returning under load especially in hot weather. Running the heat at full blast would start bringing the gauge down from H almost instantly and they swore my head gaskets were the problem. They've bled and bled to no avail. Using this method not only cured the problem, but also confirmed for me to always be your own advocate when it comes to your engine. Thanks to Ferrari and Mercedes forums, I was able to save myself a boat load of money and tell the dealer to kiss my ass.

Be persistent. When you feel all of the air is out, do it again anyway.

Even using your method you can't get ALL of the air out. It takes several heating/cooling cycles and the system can handle it. What it CAN"T handle is the large initial amount of air from filling the system. My method (and it is used by all of the venders too) is quicker and more through. The problem is if you keep finding yourself having to get air out. That means more is getting in and no amount of bleeding is going to get it all out and keep it out. I consider the auto-bleeder a Band-Aid and if you need it to keep your motor from overheating the problem should be found and fixed.

[Flash66]

Actually using this method will get all of the "trapped" air out and doesn't take several cycles. Your old method will. You are right about finding a problem if there's a leak, but that would be obvious. The air is trapped and all I'm suggesting is regardless of how it's been done in the past, there's always a better way and this is it. The dealer I spoke of used a pressure tester also and guess what? His air bubble location is unique to this situation and requires tackling it from another perspective. He already bled it several times and it's not working. I'm not saying you're wrong David but a little science can make all of the difference.