I made a thread awhile ago about my car not reach operating temps, or taking a long to do so. Well I had replaced the thermostat and bled the cooling system. The car starting doing it again. Bled the system, and that seemed to do the trick. The car does have a very minor leak. I can't find the leak. I can only smell it once in awhile.

Anytime I bleed the system, the problem seems to disappear for a few weeks.

Today the problem came back. If I drive normally, the car reaches operating temp and stays there. If I go part boost, 10 psi or so to make a pass on the highway or just accelerate briskly, the temp gauge drops, maybe just above the "C". As soon as I lift off the gas and go back to cruising speed, the temps raises back to normal. Same happens, when I'm going uphills. This is an intermittent problem.

I was thinking that when accelerating or going uphills, that maybe the coolant was sloshing away from the temp sensor, but I do know one of the temp sensors is located toward the rear of the engine bay in the back of the thermostat housing, so the coolant would be sloshing into it. So that doesn't make much sense.

I'm stumped. It's not a thermostat, not a fan issue, it's not low on coolant, despite the coolant leak. My only idea is it keeps getting small air pockets, or the temp gauge is going bad.

 

assuming theres no other issue, try water wetter. maybe you still have some bubbles stuck in the system, water wetter will reduce water surface tension and break up those bubbles.

 

hm thats a tricky one, i had this same issue but it was my thermostat. only thing i can say is see if your radiator gets cooler when the gauge drops if it doesnt than it might indeed be the gauge. but i dont think thats the issue.

 

If all the air is out of the cooling system and then later there is air, it is most likely coming from the HG.

 

+1

 

But wouldn't that cause the car to overheat when on the throttle instead of the temp gauge to drop? Also, when bleeding with a funnel, the coolant doesn't raise or bubble. It only bubbles when air is being released from bleeding the system. After the system is free of air and I raise the RPM the coolant does not raise or surface any more bubbles. And I'm not getting any oil and coolant mixing.

You may be right, I'm not necessarily disagreeing. I understand how a bad headgasket would allow air into the cooling system, but I would think traditionally the car would overheat. It's not overheating. I'm not experiencing any tell tale signs of a bad headgasket.

I hope it's not the headgasket. I'm only 24 years old and I already have a bad back from working on cars

 

places to check for coolant leak under neath throttle body towards the back of the valve cover is a coolant line for the throttle body, if not there check the two heater core hoses, if your car is dropping temp when you get on it, either the thermostat is stuck open, or put on wrong...best bet to test where coolant leak is from is to get your hands on a coolant pressure tester, pressurize system with coolant full in system, watch for a leak...
good luck

 

Change the gauge sender on the front of the thermostat housing. If you want to double check before spending, find someone local with a tactrix cable and get them to log coolant temps when this happens. The reading comes from the ECU temp sender on the back of the housing, not the gauge sender.

 

How slow does the gauge moves? Its possible to be connection to a sensor. Check and clean wiring.

 

Thanks for the tips guys.

The gauges moves fairly quick. It doesn't jump all over the place, but it can drop from the operating temp down to "c" in 2 seconds maybe? Hard to describe the speed.

And again, this is intermittent. I was out driving and it was doing this then it kind of stopped. It stayed at operating temps just fine. Then sometimes it won't drop all the way, only about a 1/8" under the operating temp.

When this problem first started, it was exactly the opposite, leading me to believe it was a bad thermostat. When I'd go WOT up to illegal speeds, the car would warm up very quickly. The second I slow down back to cruising speeds or to a stop, the temp would drop. Now it's opposite. Keeps warm when cruising and the goes cold at part boost.

I'm leaning towards a bad sending unit since it's not a consistent problem and not consistent symptoms.

I don't know anybody with a tactrix cable or I would have done that already. I could make a quick trip to CBRD and see if they can take a look. I'm lucky enough to live under an hour away.

 

Looks like coolant temp is actually fine from what you describe. Temp cant change fast like the gauge changes its position. Therefore it must be an issue with gause, sensor or connection.

 

replace the gauge sensor. It s actually quite cheap. Make sure there is no corrosion on the tab causing intermittent contact.

 

You know... that actually makes sense

 

Your case is odd and it does point to a sensor problem. However, I did hear of one other situation like yours. In that case so much air was in the cooling system that the sensor was dry and not indicating coolant temperature.

 

check to make sure all of your grounds are connected

if you leave the ground off of the intake manifold, it can cause erroneous coolant gauge readings on some cars

cb