Misfire on cylinders 1, 3, 5

Hi, took apart interference on top of the engine and decided to clean up oil in the vicinity of the spark plug wells. Cleaned all that up and the car is running a bit smoother, and are now getting the check engine light for misfire on three only. Random multiple misfire disappeared from the list and cylinder one and five misfire disappeared from the list. While cleaning up coil pack on two was switched with coil pack on four, coil pack on four powers cylinder three. From this, I am concluding the problem is not coil packs wires, or spark plugs; all of which were replaced about 30,000 miles ago. Onto the injectors and injector wiring as a possible cause.

One odd observation: Reset all the codes before starting the car, with the AC running, the check engine light came on immediately. Turned AC off and reset again, and the light did not come on until turning the AC back on. Not sure why that happens, but it does. The car ran rough at idle either way, the misfire on three appeared to be there all along.

Hi, decided to do a quick test. With car running, unplugged electrical connector on cylinder one fuel injector (it is easy to access). Car runs much rougher when doing so. Plugged it back in and unplugged the connector on cylinder three, there is no difference in the engine roughness. I will replace the injector on three, and probably all others while I am at it.

A side question, the valve covers leak oil, I will replace those gaskets while I have all the interference off. I have seen gasket kits that include tube seals. It appears this 3.0 L straight six has no tube seals, the spark plug wells are machined into the head. Am I correct? Thanks for all the help, this site has been a great resource.

And, there is voltage at that injector. Thanks for the recommendations JIS001!

Hello, just finished the work today, was traveling on business all last week. Replaced injectors, and replace valve cover gaskets. Before taking it apart misfire 1 and 5 returned, there was a lot of oil in the center well again. Cleaned that all up after replacing injectors and replacing gaskets on valve covers.

But, the car is still running rough and Shows P301 Cylinder 1 Misfire Detected. And P1120 Accelerator Pedal Position Sensor Circuit Malfunction is stored. So problem not fixed.

Any recommendations?

I looked at Live Data, the ST fuel trims changed quite a bit. The were hovering at 0, now in the mid teens.

Yes, MAF has been replaced and the throttle bore has been cleaned. I have not recently checked compression. Have had the oil leak problem on the valve covers for a while, I am thinking that the oil has damaged the coil pack driving cylinder one. Had misfire on 1, 3, 5 and after cleaning up all the oil, misfire went to just cylinder 3. After a couple of weeks of light driving, misfire came back on 1, 3 and 5. Took apart the top end interference to clean up oil and change valve cover gaskets. Put it all back together and now I have misfire on 1. I did not keep track of which coil packs went where, so I am thinking the bad pack went from 3 to 1. I will swap coil packs and see if the problem moves. The code on the accelerator position is new, might have damaged the connector or something while assembling or disassembling.

Hi guys, thanks for all the useful information.

In addition to the locked in misfire 1 code, I started getting intermittent misfire on 2 and 4. Replaced all coil packs and plug wires and engine is running smooth no codes. That accelerator position sensor circuit problem went away, I did not do anything but clean and all the wire harness connectors on the throttle body.

So, the lesson learned on this engine is that an oil leak on the valve covers causes misfires. It was interesting, prior to replacing all the coil packs and wires, replacing valve cover gaskets and cleaning all the boots and wires with electronic cleaner, the boots would have an oil sheen after sitting on the work bench a couple of hours. After cleaning again, and installing and running, two weeks later when pulling them all for replacement, the boots were all wet with oil again even though the valve cover gasket change prevented new oil from getting in the the center well. Those plastics are like sponges, absorbed then kept weeping out oil.

I replaced the injectors previously to the coil pack and wire replacement, probably did not need it, but I am sure the engine appreciates the new injectors.

Thanks again for the support.

Had the same kind of issue after replacing my oil control valve and the catalytic converters on my 01 GS300. Misfire codes popped up (random misfires on cyl 1, 3, 5), disconnected the battery then 6 miles later O2 sensor code popped up. Did extensive searches on the internet (Lexus forum) which was very informative and helpful, and ended up finding the culprit after a thorough inspection of the harness which happened to trigger the computer trips those codes. A little snapped off harness in the harness pack out of the bank1 O2 sensor connector was the cause (suspected a vacuum leak as well). The harness has been reconnected and no more CEL after about a 1k miles now.
Hope this helps.
PS the engine never idled rough when all the codes where on, hint that was telling me that the issues had nothing to do with some real misfiring issues.
Hope this helps.

A repair facility indicated that I need to take the car to a dealer. The two codes they registered were PO 120 and P 1346.

My car was misfiring changed all plugs and wires changed all ignition coils ran great for a month now its misfiring in1, 3, &6 and check engine light flashing

P0300, p0301, p0303, p0306, p0135, p0135, p0141