Temp Gauge Spiking then Falling

[tturner0626]

Hi all.  I've used this forum alot, always very helpful reads, but first post I've made.  I have an '04 Subaru Outback (3L LL Bean Edition).  It has 148,000 miles on it.  I replaced the transmission on it about 18 months ago, and last year replaced the thermostat.  Recently, I have noticed that the temperature gauge will spike into the red very rapidly (within 30 sec), hold there for a few seconds, and then drop back down instantly to the halfway mark.  This does not happen on highway, only when doing lots of stop and go driving.  I live in a flat area, not hills, and there does not seem to be any rhyme or reason to it.  The transmission tends to jump slightly at lower speed, and I feel some clunks when backing up on colder days, and the catylytic converters are going, unfortunately (not sure if these issues are related).  Otherwise the car runs fine.  No fluid leaks, all fluids at good levels.  From what I have read it seems like it could be one of three things.  Either there is air in the coolant, the thermostat is bad, or it's that dreaded headgasket problem.  I was wondering if anyone has had this specific issue, and what the first steps I should take are?  Should I try flushing the coolant first, or get another new thermostat?  Or is there something else I missed.  Thanks for any advice!  

 

*Edit: When I replaced the thermostat the mechanics showed me where the previous owner had bored out the center.  The reason I replaced mine originally was because the car was running cool.

Edited May 17, 2017 by tturner0626

[1 Lucky Texan]

not hopeful that the prev. owner did that - perhaps covering-up a problem.

make sure to check coolant level in the radiator.

does the car push coolant into the overflow bottle?

do the fans both work (kinda seems like one or both may not if overheating only presents whe the car is stopped or perhaps in very slow traffic)

thermostats for our car must be OEM style;

 

 

 

[DaveT]

What 1LT wrote. Get an oem thermostat in there now.

Go through the burping procedure, which should be detailed in numerous threads.

Closely monitor coolant level aline the overflow and air in the upper hose. This can take a week of checking before every drive.

 

The spiking is not good. It can slowly cause the headgaskets to fail.

[tturner0626]

Thanks for the quick responses!  Just ran out to my car to check some things.  To answer 1LT:

 

Coolant level in the radiator is low.

 

Overflow coolant is pretty full.

 

I will have to check the fans after work.  I know one works when I first looked at this, don't remember if I saw the second one come on.

 

I will also check the receipt for the thermostat to see if the part is OEM or not and let you all know.

[DaveT]

Radiator must be full or you can have air get trapped in the water pump. Never drive if the level in the radiator is low. A little air in the upper hose is sometimes ok, but anything less than full is not good.

[tturner0626]

Alright, so fans are working.  As I said the coolant in the radiator is low (even though the overflow is full).  Im not sure if the thermostat is original or an aftermarket part, the receipt from the last replacement doesn't say.  I think I am going to try flushing the coolant and installing an OEM thermostat, hopefully that will work! Looks like it won't cost too much to give it a try.

[golucky66]

After you do that, if you still have issues, get your hands on a scanner and check the coolant temperature that the ecm sees. On my 04 Baja, the coolant temp sensor has two different wires, one for the ecm, and one for the gauge.

If the gauge goes high and the ecm's temperature remains normal, you have a wiring issue.

If they both go high, then the above posters comments are true.

[golucky66]

The fact that it spikes high for a second or two and then immediately drops to normal doesn't sound like an over heating to me (just my opinion) it sounds more like an electrical issue of some sort.

[1 Lucky Texan]

except the rad is low and the o'flow is high.

I'd install a new radiator cap too I think.

make sure the overflow tube is clear, cut the bottom of it at a 45* angle, use 2 small zipties to clamp it to its nipple at the rad's neck..

when you refill, consider getting the car's nose uphill or on ramps, rad full and overflow on the line or a above - make noted of its level - and idle it till the fans come on, let it cool, then check the level again in the overflow - top off if necessary.

Edited May 18, 2017 by 1 Lucky Texan

[Bushwick]

I ^ 2nd a new radiator cap. Make sure it's an oem pressure rating. Although uncommon, they can fail or stick, and in your case, it might be unable to pull fluid back into the system from the over-flow.

 

As far as the modded thermostat was concerned, that's a rather old trick to getting a carb'd (or at least a non modern OBDII that doesn't track engine warm up times) engine running cooler and it might lower some working resistance of the water pump, though unsure.

 

Can try wiggling the connector on the temp sensor with it running and have someone watch if the gauge fluctuates.

 

Can also use a multimeter on the actual sensor and see if it's reading steady or erratically.

 

Although unlikely, if you have say a long gravel driveway, drive constantly down rusty roads, let salt build-up in winter, the AC condenser and radiator and get packed up and loose their efficiency over time. I like to take a nozzle OFF the hose and (carefully) hold it close to the fins, and work left/right, etc. The water will be dark if there's gunk and clears up almost right away. More of an issue with thicker 3" radiators, but worth the 5 minutes while messing around under the hood.

[potter2010]

I had the almost exact same thing happen to a 2001 Legacy of mine. After much investigating and lots of help from this forum I ended up finding out that the hose going to the rad from the over flow tank was plugged/frozen up. I took the line off, blew it out and cleaned it well. The overflow tank was pretty grody so I dumped that out and gave it a good clean then filled it back up to the right level. I got a new rad cap just in case and then put a screw clamp on the end of the hose attaching to the rad in case a bit of air was sneaking in. After that no issues. I think it was sucking in air because it couldn't get coolant from the overflow since the hose was plugged. I recommend taking the hose off and giving it a look, super easy and only takes a couple minutes. Never know. Good luck!

[tturner0626]

Thanks for the help everyone!  Just an update:
 

I got a new radiator cap, cleaned out the hose going from the radiator to the overflow, and zip tied it onto the radiator.  So far no problems.  I am driving the car about 50 miles a day, at all speeds, and havn't seen the temp gauge move over halfway at all.  I think it was the same issue potter2010 described here.  And the trick of putting the nose uphill and idling before topping off the radiator was very helpful 1LT.

 

I am extremely happy this wasn't a bad headgasket or any other serious issue!

Edited June 6, 2017 by tturner0626

[1 Lucky Texan]

fingers crossed

 

 

(over the next few weeks - make a point to check coolant level in the overflow AND the radiator, overflow tank alone can't be trusted in any incidence where coolant loss or overheating has occurred, too risky)

[tturner0626]

Yeah, learned my lesson here about trusting the overflow tank.  I will definitely be checking the radiator as well now every time.