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#4701511 - 03/21/18 07:26 AM random overheat
joaks Offline


Registered: 12/28/10
Posts: 853
Loc: San Diego, CA
Hey guys, I need some feedback- the shop where I took my car is stumped.

Short version:
My 97 subaru 2.2 will overheat randomly when I come to a stop, but will return to normal once I get up to speed. The heater does not work at this time but will work otherwise. Everything else works.

Long version:
About a month ago I got a bubble in my radiator (overflow tank got too low). I refilled and burped. Worked fine but notice I had small leak in radiator. Got the radiator fixed.

Now the car will overheat but seems to happen only when I get off of the highway on my way to work in the morning (after about 20 min of driving). I brought the car to the shop. The car does not overheat in the hands of the guys at the shop, although I brought it in hot yesterday. They have checked:

Compression
Thermostat
fluid levels
Fans turning on
Heater core (based on something I found on another forum).
Water pump


Everything is fine. They do find if they let it run with the radiator cap open that occasionally bubbles will show up. Sometimes fairly big, sometimes tiny.

Current guess is hairline crack in head or head gasket but the issue there is that the bubbles are not consistent (I guess they are supposed to be consistent with head/head gasket issues) and the car passes all other tests.

I've had the car at the shop three times to try to figure this out and everyone is really stumped.
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#4701513 - 03/21/18 07:30 AM Re: random overheat [Re: joaks]
Eddie Offline


Registered: 12/07/03
Posts: 9382
Loc: Florida, Cape Coral
Do a pressure test on the cooling system.
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#4701518 - 03/21/18 07:32 AM Re: random overheat [Re: joaks]
spasm3 Offline


Registered: 05/30/10
Posts: 8417
Loc: North Carolina
Water pump vanes eroded?
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#4701521 - 03/21/18 07:33 AM Re: random overheat [Re: joaks]
mk378 Offline


Registered: 09/27/15
Posts: 1325
Loc: USA
Heater does not work at idle means that there is a bubble in the engine. When you rev up the water pump is turning fast enough to slosh the coolant around. The bottom line is lack of coolant where it needs to be. Either there is a leak to the outside or a leaking head gasket.

A block tester can be used to see if the bubbles contain CO2. That means they came from the combustion chamber and you have a leak in the engine. The only symptom of small combustion chamber leaks is displacement of coolant and eventual overheating. There will not be noticeable loss of compression or steam from the exhaust. Subaru heads crack easily when run overheated.


Edited by mk378 (03/21/18 07:43 AM)

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#4701529 - 03/21/18 07:39 AM Re: random overheat [Re: joaks]
Virtus_Probi Offline


Registered: 06/25/15
Posts: 3789
Loc: New England
Maybe the fans are intermittent or not spinning fast enough?
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#4701531 - 03/21/18 07:41 AM Re: random overheat [Re: joaks]
Snagglefoot Offline


Registered: 12/31/17
Posts: 1278
Loc: Alberta
You can pick up a kit to test for the presence of combustion gas in the radiator to help diagnose a cracked head.

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#4701535 - 03/21/18 07:43 AM Re: random overheat [Re: joaks]
Chris142 Offline


Registered: 06/05/03
Posts: 16514
Loc: Deplorable in apple valley, ca
Do they have a smog machine to test for hc in the coolant or at least the chemicle test?
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#4701536 - 03/21/18 07:43 AM Re: random overheat [Re: joaks]
joaks Offline


Registered: 12/28/10
Posts: 853
Loc: San Diego, CA
Thanks for the replies so far!

The fans work fine (tested different ways by different people)
Water pump is supposedly good.

As I look more online it looks like a really small head gasket leak. It passes the coolant system pressure check and compression test but somehow air is getting into the coolant system.
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#4701537 - 03/21/18 07:43 AM Re: random overheat [Re: joaks]
michaelluscher Offline


Registered: 10/07/12
Posts: 2379
Loc: Staten Island, NY
Burp the system with a spill free funnel, then take it from there
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#4701538 - 03/21/18 07:46 AM Re: random overheat [Re: joaks]
Trav Offline


Registered: 11/20/06
Posts: 18234
Loc: MA, Mittelfranken.de
Its either air bound or probably has a bad head gasket. A exhaust sniffer will detect combustion gasses in the coolant or have them do a leak down test with the radiator cap off.
You say they fixed the radiator, did they nip off any tubes?
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#4701542 - 03/21/18 07:49 AM Re: random overheat [Re: joaks]
dlayman Offline


Registered: 12/21/14
Posts: 849
Loc: pennsylvania, usa
My 03 Civic displayed exactly the same symptoms (other than the no heat at idle), and passed all the tests, including the test on the coolant. Head gasket. You could drive it around town all day with no problem, but take it on the highway and when you exited, the coolant temp would take off.

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#4701558 - 03/21/18 08:04 AM Re: random overheat [Re: mk378]
Ducked Offline


Registered: 10/25/12
Posts: 4055
Loc: Taiwan
Originally Posted By: mk378
Heater does not work at idle means that there is a bubble in the engine. When you rev up the water pump is turning fast enough to slosh the coolant around. The bottom line is lack of coolant where it needs to be. Either there is a leak to the outside or a leaking head gasket.

A block tester can be used to see if the bubbles contain CO2. That means they came from the combustion chamber and you have a leak in the engine. The only symptom of small combustion chamber leaks is displacement of coolant and eventual overheating. There will not be noticeable loss of compression or steam from the exhaust. Subaru heads crack easily when run overheated.


As above, except you MIGHT not actually have an integrity problem. I got these symptoms after replacing the coolant and as of now it SEEMS to have been down to trapped air, despite running the engine with a large funnel in the radiator filler port several times.

Coolant was blowing out of the overflow tank in operation, which I thought was either combustion gases or steam from overheating.

Since testing for combustiontion gases may not be available here, my punk technology diagnostic trick attempt was to put a coolant-filled inverted plastic bag in the overflow tank, theory being steam wont show bubbles.

No bubbles, but also no more coolant loss, so it could just be that most of the trapped air is gone.

You might promote burping by applying negative pressure to the system.

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#4701590 - 03/21/18 08:33 AM Re: random overheat [Re: joaks]
Drew99GT Offline


Registered: 10/11/02
Posts: 22086
Loc: Colorado Springs
If the shop has a vacuum filler for cooling systems, try that, or, if you don't want to pay that labor, like someone above said, try the spill free funnel. Fill it, run the car and try to run in until the thermostat opens with the funnel attached. Then, let the car sit for several hours and as it cools the vacuum will pull coolant into the air bubble prone areas of the cooling system and the bubbles will escape. Do this with the heater on.

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#4701602 - 03/21/18 08:49 AM Re: random overheat [Re: joaks]
pcoxe Offline


Registered: 05/14/07
Posts: 445
Loc: South Carolina
I once had an intermittent fan motor that caused the problem you describe. Probably was a worn commutator brush sticking. It tested good too until I caught it in the act while parked.

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#4701688 - 03/21/18 09:53 AM Re: random overheat [Re: mk378]
dlundblad Offline


Registered: 09/30/13
Posts: 9646
Loc: Indiana
Originally Posted By: mk378
Heater does not work at idle means that there is a bubble in the engine. When you rev up the water pump is turning fast enough to slosh the coolant around. The bottom line is lack of coolant where it needs to be. Either there is a leak to the outside or a leaking head gasket.

A block tester can be used to see if the bubbles contain CO2. That means they came from the combustion chamber and you have a leak in the engine. The only symptom of small combustion chamber leaks is displacement of coolant and eventual overheating. There will not be noticeable loss of compression or steam from the exhaust. Subaru heads crack easily when run overheated.


Would heat not work with an eroded WP impeller too?
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