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Toyota Service, Toyota Expert
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I have a 1998 sienna, it had a bad oil leak coming from rear

Customer Question

hi, I have a 1998 sienna, it had a bad oil leak coming from rear v/c gskt. r&r int plennum while apart I repl plugs, wires, pcv valve after reassembly eng. runs rough. keep getting codes po300, po301, po303, po305
JA: Have you ever replaced the gaskets? Maybe the hoses?
Customer: repl plenum and throttle body gskts also ckd for int leaks with carb cleaner
JA: Are you fixing your Sienna yourself? What have you tried so far?
Customer: yep just me, so far repl all 3 coils and boots tried diff set of wires no diff could bad bank 1 02 sensor cause this?
JA: Anything else you want the mechanic to know before I connect you?
Customer: no
Submitted: 4 months ago.
Category: Toyota
Expert:  Andrew Martin replied 4 months ago.

An O2 sensor would not cause that issue. It sounds to me like the spark plugs or the wires are the issue. Double check that the wires are connected to the correct cylinders and that the spark plugs are tight and the gap is correct on them. Cylinders 1 3 and 5 are all on the back bank of that engine, so its not a coincidence that the codes are misfire codes for those cylinders.

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Customer: replied 4 months ago.
Disappointed stated that I had already tried 2nd set of wires ckd plugs several times also was told on other websites that o2 sensor could be problem just wanted to confirm before wasting money like on this website 2 stars
Expert:  Toyota Service replied 4 months ago.

Hello. Different expert here. Reading your statement of the issue, and, to answer your question, no. An oxygen sensor failure will not give you P0300 etc codes. The P0300 codes are cylinder misfires. This means that the cylinders are not firing at optimum efficiency (as read by the Air-Fuel sensor on the exhaust pipe). If you have the plenum off, I would be concerned that the plenum gasket is not properly centered on the 1-3-5 side of the engine, there is a tiny bit of gasket left over from the removal of the old gasket (you did fit a new plenum gasket? if not, thsi is your problem). I would recommend pulling the plenum off and rechecking for any type of debris or particles that is allowing air to enter that side of the intake. Sure sounds like you have a problem with your plenum gasket to me. This is what I would be looking at if you brought this vehicle to me.

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