Author Topic: Temp guage drops when driving... Why? (Read 1098 times)

My96A4Quattro · « **on:** February 05, 2007, 08:02:31 PM »

Ever since it started to get really cold outside my temp gauge is acting funny. It takes the car along time to warm up but I know that is normal in the below 0 weather. My problem is that once I start driving the car the temp gauge goes down. I was thinking of just covering up some of the front of the radiator so that it all doesn't cool off.

I know someone said that the thermostat might be stuck open. But I really dont feel like tearing apart the car just to check that.

Any ideas or clues on why this would happen?

Thanks
Dave

Castle · « **Reply #1 on:** February 05, 2007, 09:18:07 PM »

Well on the CQ the gauge temp sensor is on the radiator whereas the engine temp sensor is on the block. The radiator cools when air gets thru it so my gauge goes down. The engine still runs at the same temp but the gauge goes colder till I stop.

My96A4Quattro · « **Reply #2 on:** February 05, 2007, 09:38:25 PM »

But it feels like the heat gets colder when i am driving apposed to when i am sitting still for a while. Should I block off some of the radiator?

Castle · « **Reply #3 on:** February 05, 2007, 09:41:21 PM »

no, you may want to look at your thermostat then. If it is staying open you'll loose heat with increased cold air flow.

Timmy! · « **Reply #4 on:** February 05, 2007, 09:43:18 PM »

Mine drops quick too, but I never seem to experience any heat problems.

My96A4Quattro · « **Reply #5 on:** February 05, 2007, 09:53:31 PM »

Well checking the thermostat requires taking off the timing belt correct?

Rocket3004 · « **Reply #6 on:** February 05, 2007, 10:05:15 PM »

That may be model specific, but most of the time, the thermostat sits in the upper or lower radiator hose.

I had the same problem (but w/ a 1.8T). I came to the conclusion that it was just due to going fast on the freeway and super cold moving air hitting the front cooling it even further. I didn't have any lack of heat that I can remember though.

edit: I have to say that it hasn't happened w/ any other car I've driven though, so it might be something goofy w/ the way the sensors/gauges are placed in the Audi (which would make sense what Castle wrote)....

Jarett Hoffman · « **Reply #7 on:** February 05, 2007, 10:29:38 PM »

A few things your radiator could be frozen. I am guessing you do not have a 50/50 mixture. Also your thermostat could have went bad. Blocking off your radiator does nothing on our cars. The basics of a cooling system is radiator>waterpump>block>thermostat then if it if up to a predetermined temp the thermostat will open and send the coolant back into the radiator. If it is cold the coolant will be sent back to the waterpump and back to the block until it is hot. Hope that helps.

My96A4Quattro · « **Reply #8 on:** February 05, 2007, 10:49:56 PM »

Well I dont mean to sound like an a$$ but I work for Mobil 1 so I do all my fluids for free. So I do have a 50/50 mixture of coolant. Please dont take the the wrong way either.

If I remember correctly the thermostat on the 1996 2.8L A4 Quattro is located on the passenger side just below the Head on the block under a cover.

I spoke to Castle and I am going to try what he said to do.

Theta Tau 87 · « **Reply #9 on:** February 06, 2007, 09:54:47 AM »

When it's this cold it is hard for the t-stat to keep the temp perfectly balanced especially when driving with a lot of super cold airflow over the radiator. Once the engine gets up to temp the t-stat will open and allow flow throught the radiator. Imagine how cold that coolant is in your rad when driving in weather this cold! That super cold coolant quickly brings the engine temp down below normal at the sensor, but the temp at the t-stat is still hot enough to keep it open a bit. The coolant may also be still cool when it goes through the heater core causing the reduction in heat output.

Blocking part of your rad may help some. All this does is reduce the cooling capacity of the rad. In weather this cold the portion that is uncovered should be more than enough to cool the engine. The t-stat will just open a little more to allow more flow through the rad. The blocked part of the rad will help balance the huge temp differential between the hot coolant coming out of the engine with the super cold coolant returning from the rad.

Jarett Hoffman · « **Reply #10 on:** February 06, 2007, 10:45:57 PM »

Quote from: Theta Tau 87 on February 06, 2007, 09:54:47 AM

When it's this cold it is hard for the t-stat to keep the temp perfectly balanced especially when driving with a lot of super cold airflow over the radiator. Once the engine gets up to temp the t-stat will open and allow flow throught the radiator. Imagine how cold that coolant is in your rad when driving in weather this cold! That super cold coolant quickly brings the engine temp down below normal at the sensor, but the temp at the t-stat is still hot enough to keep it open a bit. The coolant may also be still cool when it goes through the heater core causing the reduction in heat output.

Blocking part of your rad may help some. All this does is reduce the cooling capacity of the rad. In weather this cold the portion that is uncovered should be more than enough to cool the engine. The t-stat will just open a little more to allow more flow through the rad. The blocked part of the rad will help balance the huge temp differential between the hot coolant coming out of the engine with the super cold coolant returning from the rad.

I talked to my teacher in school today about over cooling and he said the outside temps would have to be much colder(it's been between 0and-10 here. The 2 biggest problems we have been seeing at the school has been 1. Bad t-stat 2. Bad mixture 3.Bad waterpump

Legato · « **Reply #11 on:** February 07, 2007, 10:27:01 AM »

Mine does this too. It's just the cold air cooling off the coolant faster than you can heat it. Not too big a deal.

Castle · « **Reply #12 on:** February 07, 2007, 11:30:29 AM »

Quote from: Legato on February 07, 2007, 10:27:01 AM

Mine does this too. It's just the cold air cooling off the coolant faster than you can heat it. Not too big a deal.

yes but you don't loose heat output into your car. I think that is the issue that Dave is concerned about.

ChiMaster · « **Reply #13 on:** February 07, 2007, 11:39:18 AM »

It's more than likely your Thermostat that's stuck open or broken. (I've had one that cracked).
On the 96 its a 12v V6, the Thermo is on the passenger side behind the timing cover.

Now, I have changed the thermostat without moving the timing belt but it was very tricky. The Thermostat is part of a larger plate that has more than two bolts on it. This was also on a 92 100CS which had more room to work but it is doable.

jm · « **Reply #14 on:** February 07, 2007, 11:47:05 AM »

It was happening to me at idle when the temp's were below 5 deg. Heat was still coming out though. Regardless, mid-point on our coolant gagues in 190 i think. So basically, if your coolant temp sensor is above one single notch off cold, you should still be able to get air warmer than ~70 degrees.

Coolant temp sensors changed position on the B5 like crazy. The 96 was different than late 97's and it changed again for 99's and again at least once for the b6 platform IIRC.
On mine its behind the block and extremely easy to change. I had to replace mine at about 110000 miles.

A bad thermostat is a possibility, but i would imagine that you'd only see temps spike cold (thermo stuck open), or an overheating issue (thermo stuck closed). Not some sort of interesting medium.
Same for a broken waterpump.

Adjusting your mix could be an interesting solution. I don't know the heat characteristics of coolant, but... If having a water rich mixture or a coolant rich mixtures causes the system to be less efficient at heat transfer that could help (in cold temps).

Dark Lightning · « **Reply #15 on:** February 07, 2007, 03:16:20 PM »

My 98 1.8t (AEB) is having very similar issues, but I can add a few my symptoms. At a steady cruise:

Under 3000rpm, temps behave normally and the engine gets up to operating temp.
3000-3500rpm, temps will hold steady but will not rise.
Above 3500rpm, temps drop consistently until there is virtually no heat coming from the climate control.

I have done the following with no improvement:

Replaced the thermostat (but, admittedly, with a Stant I believe; not an OEM part).
Replaced the engine temp sensor.

Maybe still a bad t-stat?

ChiMaster · « **Reply #16 on:** February 07, 2007, 05:58:40 PM »

Sounds like your fan is not disengaging.

There are two fans, one is electric, one driven by the belt, the one driven by the belt is thermostatically controlled with its own spring, its only supposed to be active when the car reaches a certain temp.

I've never heard of one sticking in the on position but I suppose anything is possible.
Do not test the fan to see if its freewheeling with the engine on.

Dark Lightning · « **Reply #17 on:** February 08, 2007, 08:54:49 AM »

Oops, one small additional note:

I yanked the mechanical fan about 15k miles ago and threw in a high-flow electric.

But following along the idea of too much cooling, I unplugged the aftermarket fan a few thousand miles ago without any resulting change in my engine temperature behavior. Also, the A/C fan is behaving normally (on with A/C) and doesn't seem to be a factor.

My96A4Quattro · « **Reply #18 on:** February 08, 2007, 07:51:54 PM »

yeah... I am more concerned about the heat output that I am getting inside the car. I dont like freezing my butt off on my way to work. I know it is not the water pump because it was just replaced in September. I am hoping that it is just the weather outside cooling off the coolant faster then it can heat up.

I will wait and see how it goes over the next week and see if I want to go and replace the t-stat. I might do it anyways since I dont know when it was last done.

Castle · « **Reply #19 on:** February 08, 2007, 08:21:07 PM »

I won't be the outside air, Audi is made for this weather. I've got two radiators in my car and I get TONS of hot air now. I'd put money on the t-stat.