1994 GMC C1500 engine idle speed

CARHOP

MEMBER
1994 GMC C1500
112,000 MILES

I noticed that when slowing down to a stop the engine rpms would take a second or two longer than usual going down to the normal idle speed of 650 rpm. This happened intermittently. During the last few days this changed to inconsistent idle speed, also intermittent. It may idle at 750 at one stoplight, and 800 at the next. It may idle normally the next five stops. Idle speed is always stable but at abnormal rpms. I cleaned the throttle body with throttle body cleaner and a toothbrush. Ran a bottle of fuel system cleaner in the tank. I also cleaned the idle air control valve with throttle body cleaner(it was replaced two years ago). Truck has new plugs, wires, cap and rotor. TPS is original factory part. What should I try next? Thank you.

5 Replies

JACOBANDNICKOLAS

MECHANIC
109,700 POSTS

Check the idle air control valve (IAC) which is on the throttle body. It is responsible for maintaining the engine idle speed.

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Saturday, October 26th, 2013 AT 5:01 PM

CARHOP

MEMBER
21 POSTS

I replaced my idle air control valve with an ac delco valve. Now I get high high idle at start up and idle speed always settles down to normal 650 rpms. Thank you for your advice. I still have two drivability issues. Intermittently when I slow down to 10 to 15 miles an hour while preparing to stop I feel like the engine is getting too much fuel and air(my foot is on brake at this time). The engine is not surging but pulling more than it should. When this happens and truck is stopped completely the engine rpms settle down too slowly to normal idle speed. I changed the throttle body positioning sensor to a new ac delco sensor and got no improvement. Please help! Carhop.