aboodster

Hi all, I have a 2004 Lexus GS300 with 295k miles that needed maintenance, so I did it all this summer and now there's a cylinder 4/random misfire once warmed up that's baffling me. List of relevant done (other stuff was brakes, suspension, tires, fluid, filters, etc):



-Timing belt and water pump kit

-Cam/crank seals replaced (rear main not done)

-VVT-i gear rebuild kit

-VVT-i solenoid screen filter replaced

-Coolant bypass pipe o-rings replaced

-Thermostat replaced

-Valve cover gaskets replaced

-PCV valve replaced

-Air filter replaced

-Air intake hose between air box and throttle body replaced

-New pigtails and connectors for ignition coil harness due to corroded wiring and insulation

-Front knock sensor connector replaced

-Radiator hoses replaced

-A/C compressor and condenser replaced + recharge



Now it runs and drives perfect, but only when it's cold. Once it warms up and I drive for probably 15-20 minutes, I start getting hesitation when trying to accelerate moderately, and if I try to drive uphill for example, I can feel it sputtering, and eventually the CEL will come on and give me the codes, P0300/P0304. Not sure what the cause could be. All of the work was done mostly by me at my mechanic's shop under his supervision and with his help.

I'm thinking of removing the throttle body to re-seat the coil connectors and re-check, but not sure if I should also swap the coils around. Usually with a coil or spark plug problem, the code is contained to one cylinder and swapping it with another moves it to the other cylinder, but the addition of the P0300 code has me stumped. Is it still worth trying to swap the coil with another? Cylinder 4 shares a coil with cylinder 3 via wasted spark, so shouldn't the code be for cylinder 3 and 4 if it was the coil? My mechanic is really good and I don't have any reason to doubt his work on replacing the pigtails and connectors, I just want to see if I can figure this out at home since he helped me out so much on this project without charging me reasonably.



More background info below.

The car would run before the work, but I didn't drive it since the timing belt was swollen from oil, cracked, and stretched. Once the work was done, it would crank but not start, we suspected the coil connectors since we could see green on the copper through the insulation and the wiring behind the pins was exposed, so I bought some pigtails off of Amazon and my mechanic twisted them together and heat shrinked them. The car started and ran perfect afterward, but it mostly idled in park and it was a short ride to my house, temp gauge got to the middle but I'm sure it wasn't fully fully warmed up like from more consistent driving.

I then started getting knocking codes a few days/weeks later, found the front knock sensor connector was cracked with poor contact, so I replaced it with one from a junkyard, (the wire and pin was good, clicked in nicely). That solved the knocking code.

Then I started getting that cylinder 4/random misfire after driving a bit and had noticed earlier that the air intake hose was corroded and probably causing a vacuum leak post-MAF, so I replaced that with a new OEM hose and replaced the PCV to intake hose as well. Same symptoms again, so now I'm lost.

I do see similar problems with the throttle control motor as we had seen with the coil connectors, like the copper at the pins is visible and slightly discolored, but if that was causing a problem currently, I'd be having throttle body/drive-by-wire issues and codes, but that works fine for now. I do have a cleaner harness from the junkyard for that as well, but haven't gotten to replacing it yet.



What could I do? Thanks.

Aegean

Swapping coils would not only move the code but often would illuminate the main code P0300. I would start with swapping coils. If code remains identical check other things like injectors but if code changes to another cylinder or even P0300 it is definitely the coil.