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check if the fan is driven from the water pump shaft
if so replace the fan clutch ( viscous fan hub) as it is not locking up to pull the air at idle
If electric ensure that the fans are coming on as required
the problem is a lack of air flow through the radiator at no or low road speeds and that come back to fan operation
first, try to clean your throtle body with a good cleaner an a tooth brush, open buter fly and clean all around, front an rear, when starting it hold your gas pedal down until it starts letgo quickly but keep it running at 2500rpm until engine clears(some smoke may be heavy) this will help with idle problems and clean some carbon from valves and combustion chamber,.second your egr valve may be gettin partially stuck open, disconect hose from top of it, use a small bolt to plug vacum hose and drive it normally if problem doesnot occurs you need a new egr valve,
Had this problem on my 2004tahoe. there is a fan control module that is bad. The module, if i remember correctly, is inside the heat control box on the passenger side. the fan is there as well. the part is about 150? - shop around.
Oil pressure will drop at idle because the pump is driven by the engine. It should not go lower than 20psi. (I prefer to see at least 30) Very low pressure indicates bearing wear and pump replacement will not fix that. Pressure may also be influenced by engine temp and if yours is getting hotter than it should, that accounts for a few psi as well. Your water pump only needs replacement if it is leaking or the shaft bearing is loose. It actually will continue pumping even if both things are wrong with it. Overheating at idle is generally caused by internal blockage of the radiator, bugs, or leaves packed into the front of the A/C condenser, or trapped between the condenser and the radiator. Your cooling fan must be operational as well. electric fans should come on at just above normal operating temp and the mechanical fan must not freewheel when the engine is hot (it has a fluid clutch or spring driven clutch that allows freewheeling cold , and locks up when hot). If your thermostat is new, check that it is not in backwards (spring end goes towards the block) and that it is centered in the hole. When adding coolant, always run the heater while re-filling to eliminate air pockets, and always make sure the thermostat has opened so all the coolant can be added (fill when running) I am assuming that you have no other leaks or problems.
sounds real easy either a bad idle air control solenoid or more likley a blown throttle body gasket. to test fro a blown gasket, with engine running and cool spray carb cleaner or brake cleaner around base of tb were it bolts to intake. when it hits the bad area, should idle down and be normal for a second or 2. iac is possible,try this first since its so common and let me know
30-40 psi of oil pressure is fine for many vehicles including your GMC. I am assuming that the truck has more than 100,000 miles on it and still has the original engine in it. By rule of thumb you only need 10 psi per 1000 rpms in a typical street driven engine to give adequate oil pressure to keep the bearings from oil starvation. The oil pressure is typically higher when initial start up occurs because it is more viscous due to the temperature of the oil as the engines oil warms up the visconsity thins out and the oil flows more smoothly. The drop in oil pressure is a normal occurance. Above 45 is more than required for a daily driven street engine. I believe that the specs on that engine only list 30- 45 psi.