1989 GMC Sierra clogged cat?

TKENDE

MEMBER
1989 GMC SIERRA
V8
2WD
AUTOMATIC
208,000 MILES

My truck starts right up when the engine is cold, and idles fine. After driving long enough for the temperature gauge to reach normal operating temp, it starts to skip, and wants to die. If I floor it quickly, it responds, but giving it gas regularly makes it stall. It won't re start until it is completely cooled down. If I start it and let it idle until warm, the rpms increase noticeably as soon as it is warm, and it does the same thing. It can sit and idle all day, but the motor revs high, and you can't kick down the idle without stalling it, then it won't re start.

1 Reply

RASMATAZ

MECHANIC
75,992 POSTS

Test and monitor the fuel pressure and scope the ignition system

Too fast an idle speed. If an engine without computerized idle speed control is idling too fast and refuses to come down to a normal idle speed despite your best efforts to back off the carburetor idle speed screw or air bypass adjustment screw (fuel injection), air is getting past the throttle somewhere. Common leak paths include the carburetor and throttle body gaskets, carburetor insulator spacers, intake manifold gaskets, and of course, any of the engine's vacuum fittings, hoses and accessories. It is even possible that leaky O-rings around the fuel injectors are allowing air to leak past the seals. Another overlooked item can be a worn throttle shaft and a defective idle speed speed control motor/valve stuck in the extended (high idle speed) position/throttle position sensor. Also the throttle plate could be binding in its bore and kinked accelerator cable, coolant temperature sensor might not be operating properly misleading the computer that the engine is still cold and computer throwing fuel at it raising the idle speed.