my coolant temp gauge only rises to about half when you start the truck do(where as the other gauges rise all the way and fall back down on startup) and then when the gauge settles, it doesn't go any higher than one bar above cold. what could be causing this?

 

Failing gauge. Should sweep thru its full range when you turn the key to start.

 

ok, is this something that can be rebuilt or replace a gear or something or should I look for a new gauge online?

 

I don't think parts are available to repair it. Likely you would need to replace the entire cluster. You could try pulling the cluster, seeing if you can get access to the temp gauge, and a couple drop of 3 in 1 oil might just solve the issue.

 

ok I gotcha, could a failing sensor cause this or no? and if so, is there a way to that the sensor and/or gauge?

 

Plug in with a scanner, and see what temp the PCM thinks the engine is..... and if it bears any similarity to what the gauge is telling you.

 

ok, how would I go about doing that? as far as I know, there is no obd2 scanner port on it?

 

What year is your truck?

 

it's a 1989 d100

 

That one is OBDI. There are scanners out there that will do OBDI, but, they are getting harder and harder to find. If you plan on keeping the truck, investing in something like a Snap-On MT2500 wouldn't be a bad idea. (just make sure you get all the cartridges, and personality keys....) You can find 'em on Ebay for a couple hundred bucks, and it will do OBDII up to 2006, I think.....

Being able to talk to the computer makes these things a LOT easier to work on, and figure out what's up.