99 I30 DTC Codes and Coolant Leak

[loystock]

The Check Engine Light (CEL) came on my son’s 99 I30 with 64K miles. DTC codes were P0105 (Absolute Pressure Sensor) and P0158 (Rear Right Bank O2 Sensor, red cable). Per FSM guide, cleaned engine ground connectors and re-secured them. Inspected, cleaned and applied dielectric grease on all connectors related to the Absolute Pressure Sensor and O2 Sensor as well as the MAF sensor. Checked air intake line and then reset codes. Also I found out that my son has been using medium grade (Octane 89) gas off and on to save money. So I added a can of BG44k and filled up tank with Shell Premium (Octane 91) gas. Drove around the neighborhood and re-started the car several times – no more CEL.

Question 1: I know crappy fuel can affect the O2 sensors but why was the APS triggered at the same time?

During inspection of the rear O2 sensor, I noticed a small coolant leak under the engine, near the exhaust pipe just close to where it is mated with the transmission. It seems to be coming from an area with a small metal plate. I cleaned it up and didn’t see anymore leak after the test drive. My son also drove it this morning and didn’t observe any sign of leak.

Question 2: Is it possible that the lower grade fuel caused early detonation on the VQ (high-compression) engine for the temp to go higher than normal and thus the leak? Note, there was no overtemp observed when the CEL was on? If this leak comes back again, does it mean that the cylinder head gasket needs to be replaced (major job)?

Thanks,Rod

[NutriaforBreakfast]

#1 Dont know

#2 If you have a bad head gasket you could hardly keep thevehicle going most likely. Pull your plugs and look for "wet"ones-that MIGHT indicate a blown head gasket. Cheap fuelby itself wouldnt necessarily cause a blown head gasket butit could be a contributing factor. Usually overheating causesblown head gaskets from my experiences.