here’s an interesting one for you….

[Mike]

Hi there,

have an interesting case here for those up to the challenge….

Car is an 2009 Nissan 370Z with 157,000 miles on it, all maintenance up to date. now on to the issue, i have been having a little bit of an odd overheating problem rearing it’s head ever since i had a nissan dealership flush the cooling system. car was completely fine with no overheating issues even in 100+ degree whether with AC on full blast before coolant system was flushed, fast forward about 3 months after the system was flushed and out of no where on a 90 degree day with the AC running just sitting waiting for a friend for about a half hour i look up to find that the temperature gauge is sitting at just below the danger zone on the temperature gauge. needless to say i immediately shut the car off and let it sit for about 20 minutes before i turned it back on. car started back up no issue but was still a little on the warm side when it came to the temperature gauge so i turned the ac off. the moment i did this the temperature gauge shot back down to normal range within about 5 minutes so the first thing that came to mind was the radiator cooling fans. opened up the hood to check them out, nothing out of the ordinary and both fans functioning normally at their low setting where they normally would be without the ac on. just to be sure fans were functioning on their high setting as well i turned the AC back on and sure enough both fans charged up to their high setting blowing full force. shut the AC off, drove home and put the car away till i could investigate further. fast forward a couple weeks and after researching some, decided to try the simplest thing which was to change out the thermostat, everything went smooth, filled the system back up with coolant and bled the system of all the air in it, things seemed to be ok after that…until the next hot day came along when i had to use the AC again. same conditions as first overheat, just sitting idling with AC on both cooling fans going full blast and the temperature gauge proceeds to climb into the overheating range, fortunately i had my bluetooth scan tool with me at this point so i hook it up and go to check the coolant temps…..234 degrees and climbing so i shut the AC off just like the previous time and just like the previous time the coolant temps return to normal. research some more and decide to try the next simplest thing that could cause an overheat, the radiator cap. Source a brand new radiator cap directly from nissan, test it before i install it, everythings good with the cap. install it and seems to help, until the next hot day when again, same conditions, same overheating issue with the AC on until i shut it off and everything goes back to normal. at this point im thinking maybe there’s something blocking the flow of air, take off my front bumper and look all around the radiator only to find nothing, no debris, radiator fins are all in relatively good shape considering the age of the car, so i move on to it maybe being a flow issue such as the radiator being clogged. grab some prestone radiator flush from the nearest auto zone and follow the directions exactly, now the overheating issue seems to be exacerbated as oddly enough the cooling system seems to be able to cope with the AC being on at temperatures up to mid eighties before i added the flushing solution and now after the flushing fluid it can be mid seventies outside and it will start to overheat. thinking the radiator truly is clogged i decide to source a brand new OEM radiator on the recommendation of my mechanic and have the old one changed out and at the same time have the AC system serviced since the condenser and radiator are one unit on this car. radiator gets changed out, coolant system bled and AC system serviced, low and behold it seems to have worked! for the next 2 weeks i enjoy nice cold AC and coolant temps seem to finally be in check….until the same thing happens again idling in the parking lot at my friends place with the AC on during another 90+ degree day. at this point im thinking maybe im getting a false reading from the engine coolant temperature sensor so seeing as it’s a pretty inexpensive part i decide to pick one up, get it installed, bleed the cooling system of air again…..and again, same overheating issue with the AC on during hot weather. so i go back to it maybe being a coolant flow issue, decide to take the heater hose off to see if im getting flow through the system and sure enough i get a nice steady stream of coolant out of the heater inlet hose while the engine is running and can feel coolant flowing through the upper radiator hose. my thoughts immediately jump to it possibly being the worst thing imaginable on an engine….THE HEAD GASKET! ok, so now what do i do….decide to pressure test the system and rent a radiator pressure tester from autozone to see if there’s either a leak in one of the hoses that’s letting air in or worse yet, a leak in the head gasket. pump the tester up to 16 psi and wait…and wait….and wait…..this goes on for about 10 minutes and low and behold…..no pressure loss, no coolant squirting anywhere, nothing. just to double check and make sure i get a block testing kit from autozone and run that test on it also and again, also nothing…..no fluid color change, no sweet smelling exhaust fumes, nothing to indicate a failing head gasket. so now here i am at the mercy of all you wonderful people to ask…….WHAT THE HECK IS GOING ON WITH MY COOLING SYSTEM?! lol

Topic

[Richard Kirshy]

After reading your detailed post.
There appears to be a single common thread through out..
You state that the vehicle over heats with the AC on.
I’m assuming that the overheating issue does not occur under the same conditions with the AC off ( please correct me if I’m wrong on that )

My thought is that, possibly the AC is putting some excessive load on the engine causing it to overheat.
Perhaps a failing compressor or maybe AC belt tensioner.

[Mike]

That made sense to me too at one point, until i decided to test that theory out and hopped on the highway while the ac was running. Car cooled itself off more than fine and i get nice cool air. That and as long as the outside temperature is below a certain temperature the car cools itself off fine with the ac on. It only seems to be when the outside temperature is over 80 degrees that i start getting an overheating issue. On the chance that it is the ac compressor though, how would i go about checking that as i dont visually see anything wrong with it and it’s not making any noises out of the ordinary.

[Richard Kirshy]

If the compressor was loading the engine you would see / feel a major drop in the RPM
This still for all intent and purpose sounds like a flow issue.
Might be a damaged or slipping water pump impeller

Or perhaps a partial blockage with in the blocks water jacket.

I understand you did a flush on the system.
But a blockage may still be there.

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[Mike]

Pertaining to the ac causing a major drop in rpm, that is one thing it does not do, it does cause a slight drop in rpm, but then the rpm climb back up to around 800 rpm. Also, is it still possible that it’s a flow issue even though it cools itself off fine without the ac on? I have performed a second flush on the system, but did a back flush the second time around, but didnt seem to help although in the back flush i have to admit i did not use any flushing chemicals.

[Richard Kirshy]

I did have a instance when the outside temperature would be below 80 degrees, the vehicle operated with no issues.
Stop and go traffic, highway or city driving it ran fine.
But once the air temperature was above 80 degrees, that is when it would begin to overheat.
That turned out to be a flow issue.
Given one particular set of conditions the vehicle had enough cooling capacity.
Increase the surrounding air temperature and you would breach the limit of the cooling capacity.

I performed a head gasket job on my brothers vehicle and noticed that some of the water jacket passages were partially clogged.
After reassembling the engine, I went through a 7 step process

Ran the system with clean water for 30 minutes.
Drained and refilled with water and DAWN liquid soap and ran for 30 minutes.
Drained and flushed with clean water for 30 minutes.
Drained and refilled with water and radiator system flush and ran for 2 hours.
Drained and flushed with clean water for 30 minutes.
Drained the entire block ( pulled the water jacket plugs )
Refilled with 50% distilled water and anti-freeze.
The car can barely maintain its rated thermostat temperature now, even on the hottest days with the AC on full.

[Mike]

I will try your procedure this weekend report back any findings i have

[Todd Smith]

Some things you may want to check the next time this happens:
Make sure both the upper and lower rad hoses are too hot to touch. They should both be too hot to hold if the thermostat is working properly.
Also, check for a collapsed rad hose, older hoses can suck shut under certain conditions and will return to normal once you’ve turned the engine off.
Also make sure the heater core hoses are both getting hot. If not, this will indicate air in the system.
Finally, drive the car on the highway for 20 minutes or so. Then let it cool down overnight and open the rad cap and see if the system is still pressurized. This would indicate a head gasket failure.

[Mike]

thanks for the tips tod, I appreciate them very much. that being said pretty much everything you mentioned I have done to the cooling system all the way down to checking all the hoses with an infrared thermometer. all hoses including the heater hoses get up to around 200 degrees, there is close to a 10 degree drop from the inlet on the radiator to the outlet, but other than that there’s not much more I can say other than what I’ve tested so far. Also I have taken the radiator cap off after letting the car sit overnight after running it for over 20 minutes…no hissing sound or release of pressure from the radiator cap when I open her up so as far as I can tell with the pressure test, and block test that I’ve done on the system the head gasket is doing just fine.

[Mike]

Question: the dawn you are talking about, is it the liquid hand soap that you use to wash dishes by hand or is it the liquid detergent that you put in the dishwasher?

[Todd Smith]

Then I’d say you’ve covered all your bases. Weird one for sure. I’d almost want to say failing compressor but without seeing it in person that’s just a guess on my part. Hope you’re able to figure it out.

[Richard Kirshy]

[quote=”Z_ealot” post=189461]Question: the dawn you are talking about, is it the liquid hand soap that you use to wash dishes by hand or is it the liquid detergent that you put in the dishwasher?[/quote]
The liquid dish washing soap.

Does a good job at breaking down the built up deposits.

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[Rich]

It’s not clear from your post whether the cooling system has ever been properly burped since the overheating issue began.

Air bubbles in the cooling system can wreak havoc and cause overheating. Bubble near a temperature sensor are the worst because they will cause the ECU to react to the wrong engine temperature.

So… have you yourself – or a different mechanic than the one you initially took the car to – ever properly burped the coolant system? Lisle funnels are extremely helpful for this procedure. More here:

[Mike]

To answer your question, yes i have bled the system with the lisle funnel on all occasions and jacked the front of the car up as high as i could safely do to get all air out of the system.

[Mike]

Well, went through the entire procedure you described to a tee. Did the final fill and burped all the air out of the system. With hood up and ac on coolant temps stayed at a cozy 183. Decided to close the hood and see if it stayed that way, mind you this was at a very mild 76 degrees outside temperature with fans going full blast. At first coolant temps stayed at the previously mentioned 183, but within about 15 minutes slowly but surely the temps started to climb and eventually seemed to settle at 194 after running the ac for about another 40 minutes. Seeing as this was a fairly cool day i think i might be safe to assume that if outside temps were higher in the same situation my coolant temps would be much worse. The strange thing is i neither hear nor see any of the telltale signs of a water pump failure.

[Richard Kirshy]

A corroded impeller would not make noise or show signs of leaking.
A slipping impeller may make noise.

Being that all other avenues have been addressed it would appear the water pump is the only item left under suspicion.

[Author]

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