MidniteGX

Hello from Edmonton, Alberta!
Black 2002 RX300 with 369,000 kms. She's a nice looking car purchased as my son's first "beater" at a very reasonable price. Previous owner sold it on the condition it was misfiring "as-is".
Knew it could be a little or a lot of work to resolve but took the chance. Why not? It's otherwise in great shape and drives very well other than the misfiring. I've owned Toyotas until over 400, 000km before with little issues. Surely, this little Lexus must have a bit of life left in it? Hope so! Trying to keep costs down so any help would be appreciated!
Got the flashing CEL.

OBD II codes: P0302 P300. Simple troubleshooting going on here. Using the process of elimination. Checked plugs and 2 is definitely fouled. 4 and 6 look great.
Started engine and pulled connectors from the coils on 2, 4 and 6. 2 doesn't make a difference but 4 and 6 certainly do.
Before any money was spent, I cleaned the gunk out of the IAC valve. It helped the idle for sure but didn't resolve anything.
Thought I'd start the spending $ with new plugs: NGK iridium since we're going to start down the maintenance list. Then, went out to the recycle yard and yanked out three fuel injectors from a 2001 Sienna with a 1MFZE. $25.
Was hoping there'd be some ignition coils but didn't have any luck yet. May go back and have another look. They're only $10 and could be OEM.

Installed plugs in 2,4 and 6 - installed "new" injector (hopefully oem) in 2. Still no difference.
Confirmed spark from coils 2 and 4 by pulling coil, inserting new plug and holding the ground strap on the valve cover and starting the engine. Both look the same.
Switched injectors 2 and 4 and pulled coil connectors. 2 still made no difference.
So, have I think I have confirmed we have spark and we have gas on 2.

I'm thinking it still may be a bad coil since I've read that even one bad coil can throw things off. I'm inclined to buy a new Denso or look down at the recycle yard again - especially when it isn't so damn hot.
Amazon may be the way to go here since I can always return it if it doesn't resolve the issue. Still trying to keep costs down.

Beyond that, I'm thinking a compression test is next. But, I'm wondering why cylinder 2 would have had major trauma to the valve seats. Aren't these engines supposed to be tough and reliable? After all, they're top shelf Toyotas. Still leaning towards a bad coil or even something else like those oil solenoid switch thingy's. Sorry, it's late. Can't remember.

If it's time to pull the head and redo valve seats - is that something common? Seems unlikely, but it could happen, I guess.
Anyway, I've read through many threads, watched many Youtube vids and read a few pages of "answers".
Now, I turn to the collective.
Any help would be appreciated.

Audiqv8

An easy test for the coil is to swap the coil from either 4 or 6 to 2 and see if the problem moves with the coil. If a compression test shows low compression, then I would do a leak down test to see if the exhaust/intake valves are bad or if the engine has bad compression rings. Since you indicated that #2 plug was fouled it might be time for new valve seals.

MidniteGX

Did that. Just tried it again this morning after clearing the P0302 code.
Switched coils from 2 and 4.
Still ran rough as the cylinder 2 not firing properly.
Pending and then actual P0302 again.

What's next? I've sourced used Denso coils locally for $40. It's a shot in the dark, but for the price, may be worth trying anyways.
Sourced a decent compression tester for $30. I need one anyway. I'll start there and then do a leak down test.
That should answer any further questions.
If it's the seals, is there anything we can do prior to ripping off the head?

Audiqv8

Since you already moved known good coils to #2 cylinder and #2 still shows misfire then I wouldn't waste time or money with purchasing coils. I would remove the plug from cylinder #2 and inspect it. Is the plug wet, white, black or brownish? You do not have to remove the head to replace the valve seals. Valve seals can be replaced with the head still bolted to the engine but we're getting ahead of the problem(s). A compression test will tell if you have a dead cylinder with very low compression. A leak-down test will tell you if the exhaust or intake valve is the problem or if the piston rings are too worn.

MidniteGX

We're thinking along the same lines. Didn't bother with the coils - went out and got a compression tester.
Cylinder 2 has none. 4 and 6 are good.

So, since we have a timing belt/water pump to do, it looks like we can possibly tie this all together.
Got a price on a used/good head - not unreasonable.
Would have to remove the head to get estimates on repair from a couple of recommended shops.
If replacing the seals is possible without removing the head, it already sounds hard.

salimshah

Please follow the advice being given and not go out of order.
Leakdown test is next.
If the problem is below the head, you may be better off with a rebuilt engine.

Salim

MidniteGX

Valid point and I have been looking for a leak down tester. No luck yet. May have to buy from Amazon or online but have a few other options to try first.
Maybe some of our local rental shops? I’ll call around tomorrow.

Audiqv8

In addition to the leak down tester, you will also need an air compressor to supply compressed air. A 30-gallon compressor set for 90psi or above should be adequate. Watch the following video on how to perform a leak down test.

salimshah

The dilemma is, are you doing the diagnosis or a mechanic will do it. You certainly can save money by eliminating the mechanic in-between, but you have to get the tools/equipment/know-how. I can speak for myself and I would take the vehicle to a mechanic. There is a good change that OP is more qualified than a mechanic ... he himself has to make the call.

I certainly hope that the risk OP took by purchasing a vehicle with CEL pays off at the end. It is the paranoia in me that always says, "if the seller did not fix his Lexus, there is more to it". Any other vehicle brand it may not apply, but Lexus owner typically expects long service after repairs.

Salim

Audiqv8

If I was the OP I would take the car to an independent mechanic and have them perform the test(s). If the intake or exhaust valves are bad then I would remove the head and take it to a machine shop to have them clean, do a mild surface grind to verify that the head is straight and check all the valve guide and replace all the valve seal. The cheap way is to purchase a used head and slap it on.

Misfire caused by low compression.

MidniteGX

Thanks is for the video link. Yes, I run air tools and have a compressor, just no leak down tester - per say.
I might fabricate with parts and build one myself rather than buy a ready made. Renting seems not to be an option locally.

MidniteGX

Thank you, Salim. I feel pretty confident in my mechanical abilities and have tools and experience. Although I’m not a professional, I do defer to pros when I feel things are beyond my scope. Life is an adventure to me and I’m enjoying the time I get to spend with my son (who owns the car) and there are, of course, other benefits as well.

MidniteGX

Ok, did leak down test on cyls 2,4 and 6. Mixed results.
Just for kicks, I pulled off the inspection cover for the timing belt and check the timing. It all lines up good. I have no idea how old tye timing belt is.
Since I’m a newbie at leakdown testing, I’m going to make sure we’re on a compression stroke.

MidniteGX

Ok, made sure cylinders were on a compression stroke by confirming with a balloon and then inserting a rod to visually check. Did 6 first.
Went with 90 psi at first but found it pushed the pistons down. Backed off to 60 psi. Even turned the crank pulley to bring the piston back up to top and felt the compression and saw the needle on the tester go up. I thought that was the most accurate reading and marked it down.
Same one cylinder 4. Both 6 and 4 were about the same. At 60-65 psi, they read a pressure of 45-50 psi. Not sure how accurate my tester is, or me, but I calculated 20-25 % leakage. Exhaust valve seats are probably pretty carboned up? Attached a nitrile glove to the exhaust pipe prior. Both cylinders were puffing up the glove very lightly.
Then, on to the cylinder 2, the one with low compression.
It was tough blowing up the balloon to establish compression stroke but managed to figure it out. At 60psi it leaked down to 12-15 psi. A noticeable air stream was coming from cylinder 6, so I put the spark plug back in to see where else the air was going. The exhaust pipe glove was definitely very inflated!

So, thoughts? Anything obvious jumping out of this?

MidniteGX

Well, moving on. Was hoping for a bit of support from the collective but have a few other resources to draw from. Hopefully, this thread will provide some help to the next RX owner experiencing similar issues.

We have decided to remove the front head and are well into the work. Taking some pics for re-assembly. We have access to the FSM as well,
Once the head is off, we’ll have a good idea on how to proceed.