Intermittent High Idle

[Celsiorous]

Hi, I have a 90 Celsior/LS400. It is the second Celsior I have owned and both have had this same issue.
Basically the issue is that it idles high, almost like the auto choke is engaged even if the car is hot. For example I can take it on a 2 hour drive so the temperature is up, I will pull into my driveway and untill I have been stationary for about 5 seconds the engine is idling at 1100-1300rpm. After the 5 seconds or so it will idle low again (700rpm) and remain like that.
When driving around town at 50-60kmh I will take my foot off the gas and the car will not slow down as the idle is high enough to not get any engine braking so to speak, then after a few seconds the idle will drop and it will behave normally.
I was told that this is most likely caused from a dirty, sticky throttlebody. I have checked the throttle body and its clean and does not seem to get stuck up on anything.
Also the temperature gauge does not read accurately, when the engine is cold the gauge reads about where it should when it is warm, about 1/4 of the way up. It also does not cold start correctly, I assume that this is because the temperature sensor is not engaging the cold start injector as it thinks the engine is warm. This is a complete guess but seems to make sense to a non-mechanic.
Any suggestions would be great.
Cheers

 

[V8superlux]

Check that you don't have an air lock in the cooling system at the idle speed control valve (17mm bolt on top of engine near thermostat if air bleed/filling point) if this is still a problem can't hurt to take idle speed control valve out carefully disassemble and clean.

 

[Celsiorous]

thanks for the reply, sorry im a noob, how do I check if there is an airlock?

 

[V8superlux]

With the engine cold undo the bleed plug (as described above) and check that it is full of water/coolant. If not fill up system from this point refit bleed plug and take vehicle for a drive with the heater on for a good ten minutes, then let the vehicle completely cool down (o'night) and recheck. If you have a big air lock it may take a few times to get the system competely full and remember to renew the aluminium sealing washer when finished.

 

[Celsiorous]

awesome, thanks for your help superlux ill check it out tonight.

 

[Celsiorous]

cheers superlux, there was an airlock. I have filled it at the bleed plug point twice but i suspect I will need to do it a few more times. It has allready made a difference, the idle is not rising half as much as before.
Thanks again.