My B5 idles kinda rough, but everything seems to be okay except for the engine temperature. It is always cold but the heating works great.
If the car is idling the temperature gauage will be in the middle. In city driving it will be between the middle and the cold indicator. However when I take the car on the highway it always shows cold! Even if the the car is idling and the temperature is perfect, it only takes about 5 mins on the highway for the gauge to drop all the way to Cold.
....................any ideas?

 

Thermostat is stuck open, when was the last time the timing belt, water pump, and thermostat changed?

 

Faulty temp sensor. Common problem. The sensor is on the back of the block on the passenger side. If you have small hands, you can get at it by removing the snorkel of the air intake on the clean side of the filter. Check it to see what color you have so you can be sure to get the right replacement, as there are three to choose from.

As for the rough idle, replace your vacuum lines asap. A rough idle is a sign of a vac leak, which will make you run lean, which does more harm than good over time. Vac tubing is cheap, and replacing the lines is a piece of cake. Well worth the 5-10 minutes you'll spend swapping them out.

Best of luck and welcome to the forums

 

Actually, auditech79's suggestion sounds more feasible. While the sensor is still a common problem and could be contributing to the problem, the tstat being stuck open is more likely if it's cooling off on the freeway that much.

And I would trust a tech's opinion over my own any day, I'm still a nub with these cars, even with as much as I've learned in the last year and a half, I'm not even on the chart with what a tech would know.

 

^^^Well i wouldn't say you're wrong but how would you know if its actually running cold?.........Checking the temp of the hoses by hand is the best way to find out if its just a gauge malfunction or a thermostat/water pump issue. So what he should do right now is actually verify its actually running cold.

 

Mines the same, replaced sensor and no dice. Going to do Tstat, think on the 2.8 it's a bit rougher to do than a 1.8t.

 

I would lean towards the tstat as well; the coolant temp sensor shouldn't affect the temp as regularly as is described. Tstat being stuck open makes good sense since you are running higher & consistent RPMs on the highway....I am almost positive I have a faulty temp sensor but my engine temp is normal, it shouldn't go down from half way once it has reached full temp.

 

Here's what's involved, from doing mine a little over a month ago:

The thermostat goes in the small hole in the center of the picture:


And a more general overview of the level of disassembly:

 

This is interesting. I've only had my 98 for a couple months and assumed that the temp gauge typically sat on the left side. The oil temp gauge reads fine, but when I'm driving (city, highway, etc) the temp gauge is always on the left. If I park and shut the car off, when I come back out the gauge will be 1/4 or 1/2 way and then go back to full left once I start driving again. Is this not normal?

 

Nope, with everything working right the temp gauge should climb to vertical and stay there. If your heat works great while the gauge is pointing left, it's likely the sensor. If your heat works great at a standstill but not so well while driving, with the gauge toward the left, the thermostat is stuck open and needs replacement.