: idle? and mpg?

05-22-05, 09:11 PM
how many rpms should the idle be set at on a 94 northstar, it idles at 800 right now, but what Should it be...and btw...I'm sooo excited to have my car finally running because i bought the car with the engine apart......Its a 94 seville sls ....it is sweet. Oh....and one more thing...It says im only getting 6 mpg....whats up with that?....is there a leak?...or something?...hmm.

05-22-05, 10:41 PM
The idle speed is controlled by the PCM via signals sent to the idle speed control motor...that small can shaped device on the throttle body with the plunger acting against the throttle lever. The idle speed varies depending on conditions but 800 seems a bit high for a warmed up engine idling in drive. Open the hood and watch the idle speed control motor plunger when you turn the key on and then off. Does the idle speed control motor move the plunger in and out or is it inactive. It should move the plunger when the key is cycled so that is a quick check to see if it is working.

If it is moving then idle the engine. Hold the RPM at about 2500 by holding the throttle lever open with your hand. While the RPM is elevated to 2500, depress the plunger on the idle speed control motor. when it is depressed it should start to retract. Retract it fully and then freeze it in position by disconnecting the wire connector to the idle speed control motor. Return the throttle to the closed bore position. If the idle speed control motor plunger is fully retracted there should be about .030 clearance between the plunger and the throttle lever when the throttle is closed and the engine should barely idle at about 450-500. If there is no clearance then the idle speed control motor is misadjusted. Turn the plunger to screw the plunger in to make it shorter to get the correct clearance. If there is no clearance between the plunger and the throttle lever when the ISC motor is fully retracted then the idle speed control system cannot work correctly and the idle will be too high.

Are you looking at the Instantaneous MPG or the Average MPG readout....??

The Average MPG is the average MPG since it was last reset...so if you have been idling or doing a lot of work on the engine idling and not driving far then the average MPG will reflect this and be very low. Reset it by pushing the reset button the next time you drive it and it will then show the average MPG from that point on. The instantaneous MPG readout is highly variable depending on the driving conditions. 6 MPG is normal for an acceleration and city driving. At idle it is zero. The instantaneous MPG will be much higher when cruising on the expressway at a steady speed. You just have to allow for conditions and the operating mode with the instantaneous MPG.

05-23-05, 02:54 AM
Awesome. Thanks for the idle info. Yeah I found out about the mpg thing. .....

But yeah...I ran my car today and i drove for about 10 min and the engine overheated beacuse the coolant was hot,(thats what the dislpay said)...I know its time for an oil change..but it seems I am loosing coolant fast. Is there an overflow container that recycles the fluid back into the engine?...because right now, the overflow tube isnt connected to anything, it jus leaks to the ground. is that right?.....:hide: ....OR could it be something else??

05-23-05, 08:22 AM
there is a hose on the surge tank, just below the coolant cap and that hose runs to the side of the tank and stops, just dumps to the ground in front of the motor. there are 2 hoses that run to the surge tank as well beside the overflow hose. one on the bottom thats kinda big, about 3/4" and one that comes from the top of the motor that is like 1/4" hose.

05-23-05, 08:52 AM
nice, yeah i found out why my engine is overheating. ...a blown head gasket. errr....so this will take 4ever to repair.