My GMC denali is not idleing well and lossing oil pressure. Why?

GATORDOC

MEMBER
1999 GMC YUKON
170,000 MILES

Hello, I have a GMC 1999 Denali with 170K. Unsure of the previous maintenance although was told its was done pretty religiously. The car has been running extremely rough at idle and has experienced a loss of oil pressure when hot at idle. The SES light has been on and blinking at times with noted codes of P0300, P0307, P1131, P1137. I have cleaned the TB, Mass Air Unit, replaced O2 sensors, changed oil and replaced with thicker viscosity (20W50). Also have tried to test for a intake manifold leak by spraying cleaner near gasket but noted no change. I also notice a "knocking" in the motor that seems to go away at RPM's above 2000 or so. This knocking is there even when the engine is colder before the oil pressure begins to fall. Any ideas would be appreciated

1 Reply

HMAC300

MECHANIC
48,601 POSTS

Check your fuel pressureit should be 60-66psi. If a 5.7l. Do not run 20w50 in that engine, 10w30 is ok but the other will make your rings stick in pistons. As we cannot hear noise it may be piston slap and check oil pressure with a manual gauge first as yours may be off. It may be an oil filter that is causing the low pressure as well.