Coolant Temp Sensor

[jmburton1993]

Well I sold my lifted 85 Wednesday and bought a stock 89, runs a lot better although it has some engine noise but I'll get that looked at eventually. Anyway its idling pretty high (roughly 1500 rpm) so I'm thinking it may be the IAC, however I read somewhere a bad coolant temp sensor could be the issue and my temp gauge doesnt give a reading at all, does anyone know how tough it is to change and if it could be causing my high idle?

 

[jmburton1993]

heres some pics of the new truck

 

[Slammin Sam]

Coolant temp sensor is an easy fix. Its on the front of the engine by the thermostat. If your gauge isn't working, pull the wire from the sensor and ground it. If the needle jumps, your gauge is good. The sensor is less than $15.00 When you replace it don't put sealant on the first few threads. They need to be clean for a good ground.

 

[RonD]

Yes, that is the gauge temp sender, a 1 wire device.

There is also a 2 wire sensor, the ECT sensor, it should be near the temp sender.
The ECT sensor is only used by the computer, it tells the computer when the engine has warmed up so yes it does effect the idle.
But if the engine has not warmed up after 15 minutes the computer will turn on the check engine light with code 21 or 51, ECT temp out of range.
ECT sensor rarely fail, not never, and they can be tested in place with an ohm meter.

But if outside temp is cold and the thermostat on the engine has been removed or is stuck open the engine may not be getting warm so idle is staying high.
Start cold engine and feel the upper rad hose, it should stay cold for about 4 or 5 minutes then heat up pretty quick when t-stat opens, if upper hose warms up slowly with the engine then t-stat needs to be replaced.

To check the IAC just unplug it's connector while engine is idling, if idle drops to 500rpm or stalls then computer was holding the idle high, if idle doesn't change then IAC is stuck, clean it.
If idle only drops to 900-1,000rpm then you have a vacuum leak

 

[jmburton1993]

ok thanks I'll check all this out asap, hopefully it will be one of these, I'll keep yall posted

 

[RonD]

Oh, two other things because of the age and probability the engine has had a few DIYers working on it

On the throttle linkage, cable side, there maybe a screw adjustment, this is to set minimum idle, not "the idle".
To set it correctly you warm up the engine, then unhook the IAC valve, idle should drop to 500-600rpms, if engine stalls use screw to open throttle a bit more, so it will barely idle at 500-600rpms.
Plug IAC back in and idle should go up to 700-750rpms, might be 800 with an automatic.

Other thing to check are the throttle cables.
Warmed up engine and engine off, open throttle manually and remove both cables, second cable is if it has cruise control.
Now start engine and see if idle is at the normal 750rpm, if so then someone may have done "the throttle cable mod".
Over the years the throttle cable can stretch this makes Full throttle unavailable, so under the dash where the throttle cable attaches to the gas pedal people install zip ties on the cable part to, in effect, "shorten it".
This is fine to do and recommended for better performance, but if done incorrectly you force a high idle but shortening cable too much.

If it has cruise control, leave that cable off and see if idle is normal, if it is then cruise control module maybe pulling the cable a bit when engine is running, it gets it's pulling power from engine vacuum which is highest at idle.