Having major overheating issues here. Car will overheat only when pressurized, and after running in drive- if I let it idle, it will run for an hour or hour and a half without a heat issue. There is no heat in the cabin unless the gas is floored when parked, and once I start driving, the heat will go away in minutes and turn cold, followed by overheating. I have replaced water control valve (new part tested in boiling water and passed), thermostat (also tested good), flushed radiator with hose, flushed heater core, new radiator cap, new water pump, head gasket sealer, pressure tested ok, and tried bleeding many times. NO LUCK. Somtimes, when it starts getting hot, I can rev the engine on an incline and get the temp to go back to nromal, but still get no heat in the cabin. Car never overheats when system isn't pressurized. At a loss, any help is welcome!

What do you mean by the term "pressurized"? What did you use as "head gasket sealer"? How and what was "pressure tested"? "Tried bleeding many times" sounds medieval. What are you "bleeding" and why are you "bleeding" whatever it is?

Time for a new rad.

You said you put in head gasket sealer. Could you elaborate more on why you did that and what you used?

I'll be following this post as I have the EXACT SAME PROBLEM with my '02 QX4.
Last summer the vehicle overheated 100 miles from home out of the blue (I guess you never do expect it, do you). Pulled over and let it cool down. Couldn't get coolant, so topped up with mountain stream water and limped until I could get coolant.

Local mechanic diagnosed stuck thermostat (engine hot; radiator cold) and charged me--get this--twenty-five bucks to pull the thermostat, verify it was stuck, tear the thermostat out of the housing, put the empty housing back on, reattached hoses and fill the system with coolant. (I gave him $60 when he was done. Serious good Samaritan. Loved talking cars, too.)

Anyway, we made the rest of the drive home NO PROBLEM. (It was nighttime by then, so that could have been a factor.) Next week I put in a new thermostat (aftermarket... we'll talk about that later). Everything was great for a couple days until it suddenly overheated AGAIN. Steam/coolant was blasting out the overflow nipple (hose had become disconnected... could it be user error? Yeah, probably. My wife is certainly convinced of it.)

Took it to a specialty shop in our town and they tested the system. Found a leak in one of the hoses. I took it home and replaced the hose myself. Took it back and they flushed/filled the system. All good.

Then I started noticing the temperature being erratic. Would start to overheat, then cool down on its own. Very finicky and unpredictable. The thermostat doesn't seem to understand when to open all the way--the vehicle occasionally gets "above recommended temperature range" although it never "red-lines" (sorry for the tachometer term) like it did last summer.

I bought a genuine Nissan thermostat to replace the aftermarket I had installed. I have procrastinated and it's still in its box. And now it's winter. I'll install it before summer. Swear.

So like I said, it's winter now. I take the rig out and turn the heater on... NO HEAT. Fan blasts low to high, but it's cold, cold, cold.

Is it a coincidence that the heater core wigs out at the same time? Or is it something other than the heater core?

Thanks for any help/insight!

Jack

Treepreacher wrote: ↑

Sat Feb 01, 2020 5:49 pm

Took it to a specialty shop in our town and they tested the system. Found a leak in one of the hoses. I took it home and replaced the hose myself. Took it back and they flushed/filled the system. All good.

So like I said, it's winter now. I take the rig out and turn the heater on... NO HEAT. Fan blasts low to high, but it's cold, cold, cold.

Is it a coincidence that the heater core wigs out at the same time? Or is it something other than the heater core?

Thanks for any help/insight!

Jack

If the indy shop is inexperienced, they probably didn't bleed the cooling system properly, and you still have a lot of air in there.
That's what usually causes the swings in temperature and no heat condition.
Go to the FSM and make sure you follow the proper procedure when refilling and bleeding the cooling system.
You can also try searching this particular forum for tips.
I seem to remember a thread from a few years back about this very subject.

Found this thread from a few years ago that refers to improper bleeding.
Scan through it. It might help.

post6678400.html?hilit=bleeding%20cooli ... m#p6678400

Thank youuuuuuuuuu

Keep us posted. I'm curious now.

The issue there sounds pretty similar to mine. I put Blue Devil head gasket sealer in there because I was out of options- replaced water control valve and thermostat, flushed & back flushed heater core, no leaks present, flow tested radiator, replaced water pump, and I had the coolant overflow tank overflowing with bubbles in the coolant. However, head gasket sealer did nothing, and when I block tested it with that blue fluid, it stayed blue, signaling there was no exhaust gases in the cooling system.

Right now, I've just been driving it without the thermostat and it works fine when you're on the highway. As soon as you come to a stop or the engine goes to lower RPMs (anything below 2000), it will start to overheat. If I rev the engine for a minute or so at higher RPMs (above 2500), the temp gauge will return to normal. Have tried bleeding many times with no luck, and I have no heat unless im running it at higher RPMs for a while. Thinking radiator next?

The old rule of thumb is: if it overheats on the highway, think radiator, if it overheats at idle or in slow traffic with the a/c on, think fan clutch.

Given your symptoms, it is more likely a fan clutch issue I think. By revving the engine, you are making up for the bad fan clutch.

Also, if you haven't yet, might be time for a new rad cap. You need to make sure the system is pressurizing ok.

If it is the fan clutch, would that explain getting no cabin heat unless I'm revving the engine for a while? It's like there's an air pocket that gets stuck in the highest points of the cooling system. The hoses are hot until they get to the heater core, then both inlet and outlet are ice cold unless it's being revved. Same with the hose at the back of the engine against the firewall where the bleeder valve is attached.

Forgot to mention, it only does this when in drive. If I start it in the driveway and let it heat up in park, it will not overheat and I will get cabin heat. The second I put it in drive and go down the block, it loses heat in the cabin and will overheat.

Yes, could still be air in there. The last time I did mine, I hooked a vacuum pump tester to the bleeder tube on the back of the engine and pumped all the air out. You can find some cheap ones on amazon or ebay. I never had any luck getting it to burp the air, so I used the pump idea.

My procedure: Set the system to econ, set the vents to floor only, turn the temp knob to max, and roll down the windows with fan set to 2. I figured this would open the water valve to the heater core up by making it try to really heat up the passenger compartment. Then I removed the radiator cap, and hooked the vacuum pump tester up to the bleeder tube. I think it was around 100 pumps on the tester before I began pulling coolant out of the bleeder tube. But it did get all the air out. Also verified at that point I was blowing warm air out of the floor vents.

You could also do the system self diagnosis procedure in the service manual. It tests your blend doors and other system components to be sure everything is working correctly as far as electronics/ motors / sensors. It's in the heating / ac section.

You did say you flow tested the radiator, so for now, that would not seem to be the issue.

Rockwood wrote: ↑

Wed Apr 08, 2020 9:28 pm

Yes, could still be air in there. The last time I did mine, I hooked a vacuum pump tester to the bleeder tube on the back of the engine and pumped all the air out. You can find some cheap ones on amazon or ebay. I never had any luck getting it to burp the air, so I used the pump idea.

My procedure: Set the system to econ, set the vents to floor only, turn the temp knob to max, and roll down the windows with fan set to 2. I figured this would open the water valve to the heater core up by making it try to really heat up the passenger compartment. Then I removed the radiator cap, and hooked the vacuum pump tester up to the bleeder tube. I think it was around 100 pumps on the tester before I began pulling coolant out of the bleeder tube. But it did get all the air out. Also verified at that point I was blowing warm air out of the floor vents.

You could also do the system self diagnosis procedure in the service manual. It tests your blend doors and other system components to be sure everything is working correctly as far as electronics/ motors / sensors. It's in the heating / ac section.

You did say you flow tested the radiator, so for now, that would not seem to be the issue.

Thanks for the tip on the vacuum pump tester Rockwood- I'll try that next. The only thing that steers me away from believing there's still air in there is that I've bled it many times and then driven it for a few hours on the highway with no issues, which would make me think that driving it for that long would purge any remaining air from the system. It's just within 5 minutes of slowing the RPMs when I get off the highway, it get's hot. It's like coolant is getting blocked by something unless it's being forced through the system when cruising at higher RPMs.

Air is being “forced thru” the radiator when your driving on the freeway.

2500rpm will have just as much coolant flow at 0mph as it goes at 60mph.

Definitely would be looking at how clean are the fins on the AC condense and radiator and wether the fan clutch is actually functioning correctly when hot.

Slumpert wrote: ↑

Thu Apr 09, 2020 1:00 pm

Air is being “forced thru” the radiator when your driving on the freeway.

2500rpm will have just as much coolant flow at 0mph as it goes at 60mph.

Definitely would be looking at how clean are the fins on the AC condense and radiator and wether the fan clutch is actually functioning correctly when hot.

To the best of my knowledge, the fan clutch seems to be working properly. It has a decent amount of resistance, and can spins about a turn or a little less when spinning with the engine off. Fins aren't clogged, I have cleaned a few times now.

Coolant will be flowing faster at higher RPMs- the timing chain is spinning faster and therefore spinning the water pump faster, which pushes coolant through at a higher rate. I've been going down a big hill (coming down the mountains from I-70 into Denver) and have put it in neutral, which sets the RPMs around 1000. The car will overheat after a few minutes if I let it coast down a hill in neutral at lower RPms- so the car remains at highway speeds and has the same airflow, but still overheats at lower RPMs.

x2002Qx4x wrote: ↑

Thu Apr 09, 2020 1:43 pm

To the best of my knowledge, the fan clutch seems to be working properly. It has a decent amount of resistance, and can spins about a turn or a little less when spinning with the engine off. Fins aren't clogged, I have cleaned a few times now.

That's not a good test. On BMWs, which I have much more experience with, but probably these, a fan clutch test is to get it hot, not overheating, but hot, then try to stop the fan clutch with a rolled up newspaper. A bit amateurish, but a very effective test.

Coolant will be flowing faster at higher RPMs- the timing chain is spinning faster and therefore spinning the water pump faster, which pushes coolant through at a higher rate. I've been going down a big hill (coming down the mountains from I-70 into Denver) and have put it in neutral, which sets the RPMs around 1000. The car will overheat after a few minutes if I let it coast down a hill in neutral at lower RPms- so the car remains at highway speeds and has the same airflow, but still overheats at lower RPMs.

Now that's odd. I'd replace the water pump, it should pump more than enough going downhill, coasting to keep it cool. That's very odd.

Thanks Mike, I'll have to try that newspaper test today and see how the clutch responds.

Additionally, I replaced the water pump with an OEM one from Nissan about a month ago. I thought for sure this would solve the issue, but to no avail.

Can't think of the last time I heard of an engine overheating while going downhill in neutral. So that is a weird symptom given all the work you have already done.

The only thing I can think of is maybe an air pocket - it would move to the back of the engine with the vehicle nose pointed downhill. So maybe there is air trapped there. It is the one thing about the R50's that causes overheating.

I will try the bleeding procedure again, this time with a vacuum pump. Hopefully that works

Rockwood wrote: ↑

Fri Apr 10, 2020 2:52 pm

Can't think of the last time I heard of an engine overheating while going downhill in neutral. So that is a weird symptom given all the work you have already done.

The only thing I can think of is maybe an air pocket - it would move to the back of the engine with the vehicle nose pointed downhill. So maybe there is air trapped there. It is the one thing about the R50's that causes overheating.

Agreed. I've never heard of a car overheating going downhill in neutral.

Little bit of an update here:

Let the car run in my driveway (a decently steep incline) for 3 hours this morning, no issues, and was blowing hot air the whole time. Tried stopping the fan with a rolled up newspaper when it was at operating temp, and I couldn't stop it. Took it out for a drive and within 30 seconds of driving down my street, I lost heat to the cabin. Drove on the highway and my heat returned, but as soon as I slowed to get off the highway, I immediately lost heat to the cabin again. Pulled over and found the overflow tank was filling with coolant, so got back on the highway and things returned to normal until I got back off the highway and pulled into my driveway.

Rereading the thread, this raises a red flag

and I had the coolant overflow tank overflowing with bubbles in the coolant.

You shouldn't have bubbles. I know you said you tried a tester, but bubbles in the coolant tends to be from a cracked head or blown HG. Now one can also have turbulence, which is similar, and it can be hard to tell the difference, but you shouldn't have bubbles. Maybe try a different tester or take it to a shop with a smog machine to sniff the coolant.

Going back to the beginning, how did it start? Was it just fine, then all of the sudden a problem or did it start gradually or after an event? I know pulling the plugs on these things is a PITA, but they might give you a clue. If one is steam cleaned, it points to a problem.

So, I got in a front-end collision going about 25mph back in November. No real damage, car drove normally for 2 1/2 weeks after the collision and then one day randomly on the highway, I lost cabin heat and minutes later it overheated. Tried replacing thermostat that day, bled the system with front end lifted, overheated after about 10 minutes of driving. Then tried the water control valve, then the water pump then flushed heater core, then flow tested the radiator, and ultimately drove it without the main thermostat for a while, but left water control valve in. Drove ok like that! haha

I thought for sure the "bubbles" meant a cracked head or HG. It's hard to tell if it's bubbles or just ripples from the coolant being forced into the overflow tank. I've tested with two different testers now actually, both in the overflow tank and the radiator itself. When I tested in the overflow tank, I waited until it got completely full of coolant and was noticing what looked like a decent amount of bubbles. I unscrewed the cap and tested it 4 or 5 times while it was full and bubbling, and the test still came back negative. I had also tested in the radiator about 5 or 6 times when it was blowing out steam, and all tests came back negative (fluid came back blue instead of yellow).

I also pressure tested the cooling system, letting it sit for 5 hours- never lost any pressure during that time, so that came back ok. Don't have any other signs of a head gasket failure as well. I also don't lose any coolant, besides what excess is pushed into the overflow tank.

I really think you have a blown HG, even though the tests suggest otherwise. Regardless, I think I'd pull the stat, drill a medium size hole in it, maybe 3/16", reinstall it and see what happens. That would help purge air when the stat is closed without completely bypassing it.

Mike W. wrote: ↑

Wed Apr 15, 2020 11:04 pm

I really think you have a blown HG, even though the tests suggest otherwise. Regardless, I think I'd pull the stat, drill a medium size hole in it, maybe 3/16", reinstall it and see what happens. That would help purge air when the stat is closed without completely bypassing it.

Haha actually took the thermostat out and drove it for a while, but left the water control valve in. It drives fine without the thermostat, only will overheat if I let it idle for a couple of minutes after a long highway drive, so that might be my move. Assuming you're probably right about the head gasket, although none of my tests seem to want to agree!

Another update here: tested again for the head gasket problem with another block tester. Waited until the engine was up to temp, and coolant coming into the radiator looking all foamy/bubbly. Test came back negative again, even with the coolant looking all foamy and bubbly- I did it about 6 times, all negative. Did a compression test later, and all cylinders tested right at around 130PSI, so good to know she's a healthy happy engine despite the cooling system issues.

What seems to be happening, after going over the system with a Nissan master tech, is that air is somehow getting into the cooling system and going to the back of the engine to the highest points in the cooling system, forcing the coolant to the front and away from cooling the engine. The question is how is the air getting in without any visible leaks...where could it be coming from? Going to put a vacuum on it later this week to be 100% certain all air is removed and then test it out again. The tech was just as thrown for a loop as I am.

Well, if you pressure tested the cooling system, and it held pressure for five hours, then you simply do not have a leak.

You may have some residual air at the back of the engine from the heater core flush you did. So yes, I would do the vacuum procedure to be sure there is no air back there.

The three things you did not test when you did the pressure test are the radiator cap, hose to the overlow tank, and the tank. My guess is the system is sucking in air thru one of those items as it cools down. So, I would examine those very carefully, and a new name brand cap might be a good idea.

All very cheap to do, which usually makes it a good place to start. I always do the wallet busters last.

I was leaning against it being the cap since it's only a year old and it's a Nissan cap, but I'll try that next. Upon visible inspection, the only thing that appears different in the overflow tank and with the hose is the fact that the top of the tank doesn't align with the grill anymore. I also have an 04 Pathfinder, same engine, and the hole at the top of the overflow tank rests right on a plastic brace from the grill, practically covering the hole. On the Qx4, that hole is exposed since I got in a collision and it knocked things a little off, but I wouldn't think this hole (which appears to be an overflow hole if coolant gets all the way to the top) would cause the big issues I'm having.

Another thing I noticed is one hose/grommet that's on the coolant return pipe (right where pipes go up to the heater core) going back to the radiator is rock hard. I had to replace that specific piece about 5 years ago when it blew a hole after a few hours of highway driving in over 100-degree heat, and now that replacement hose is bulging and is rock hard. I'm going to try replacing that and see if that makes a difference, and then do the vacuum procedure.

Quick update: out of nowhere, the car seemed to be fixed after I reinstalled the thermostat and replaced my front bumper (driver's side was missing due to the accident). I drove it for about two weeks and would just rev the gas if it started to overheat, which would eventually bring the temp gauge back down (not the best but whatever). It started overheating less and less and now it drives perfectly normal, no issues.

My theory is either air was trapped deep inside and took a while to get out, or else that head gasket sealer took a while to work. It's also strange how as soon as I replaced the bumper, it hasn't overheated since...just happy I have a working car again!