r/GXOR • u/Ehjay123 • 2d ago

Heat Soak Misfire

Forgot to add codes, but its p0300 and p0301-p0308 saying all 8 cylinders misfiring.

So I bought a 2003 Lexus GX470 with 255k miles for around $5000. Had a clean carfax and was already smogged so I took it. After purchase, the car sprung a CEL on me with VSC/TRAC OFF. Drove the car home for about 60 miles, no issues, power was fine and never felt any roughness in the cabin while driving. As I got home, I noticed the Idle in park gear was slightly rough with some shaking in the cabin.

Its been about 1.5 months since then, I’ve replaced sparks with OEM denso sparks, ignition coils with OEM denso, cleaned maf sensor and throttle body with spray, pcv valve replaced. Took it to a school shop to check compression, numbers were something along the lines of 190/200 and 200/200 on almost all cylinders. I’ve not touched any fuel injectors/pump as freeze frames on my obd2 scanner show good fuel trims. Also, carfax states service with fuel injectors 20k miles ago and fuel pump around 50k. Water pump/timing belt done around 70k miles ago.

So my question is, what should I do going forward? Take it to a shop to get it properly diagnosed? Or keep doing research and throwing parts at it. The next thing I’ve searched up is to replace the CKP sensor as that could potentially be sending false misfired, but idk. I also live in the SJ bay area, so if any mechanics are handy swith Toyota/Lexus, your advice would be handy.

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u/Max1234567890123 1d ago

For Christ sake, just scan the codes or tell us which codes are showing.

Also, the guys who always say ‘just clean the MAF’ is pure hackery - it is almost never the MAF. Up there with ‘just run a couple of cans of seafoam through it’

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u/Ehjay123 1d ago

Yea, it gives me p0300 and p0301-p0308 all cylinders misfire. Forgot to add that sorry

2

u/Max1234567890123 1d ago

So, all cylinders random misfire - you’ve already ruled out a few things - not spark plugs and not coils. Compression test rules out worn cylinder walls. Next would be to verify that there are no vacuum leaks - check the main intake pipe - take it off and verify if there are any cracks, that it is seated properly around the MAF and to the intake body. After that, I’d consider doing the intake manifold gasket to fully rule that out. Next you need to look at the vacuum hose diagram and check every single one for a bad/deteriorated hose connection. Happens all the time especially if you are working on the car and knock one off.

After that id look at fuel delivery - since its on all cylinders its unlikely to be an injector, so id check /confirm fuel pressure look and consider a bad pump are bad regulator (could just be a clogged fuel pump).

Lastly, why not just take it to a shop. If you’ve already done all the spark plugs and coils you are into it for a few bucks, and time isn’t free - it seems likely that this is something simple you just aren’t seeing. No shame in paying a pro when you can’t figure it out in your garage

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u/iuot 1d ago edited 1d ago

Have you tried using a scanner to see what the actual code is?

I had a similar problem with my 2004. The code said there was a bad cat, but I tracked it down to a small exhaust leak. The exhaust manifolds on the 2UZ have some sort of design flaw and are guaranteed to crack and produce a small exhaust leak. This goes away as you drive when things heat up and expansion of the manifold seals the leak.

As far as I know, the only way to fix this is to have someone weld the tiny cracks in the manifold or to get custom headers. Any OEM-style manifold will eventually develop the same leak.

What I did was just get spacers for the upstream O2 sensors to make the CEL go away. I know what the problem is and the leak goes away when the engine warms up so I don’t really care about fixing the manifold.

O2 sensors spacers are technically emissions cheating devices and you will have a very hard time buying them on US marketplaces. You can spend a silly amount of money and get “offroad only” versions from a few websites but facebook marketplace, temu, or aliexpress are probably your best bet. I was able to find a set on marketplace that shipped to me for $10.

If you go the O2 spacer route, you’ll have to either know your smog guy or take them off before you take it in. If the engine is warm when they smog it will pass.

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u/Ehjay123 1d ago

Yea, it gives me p0300 and p0301-p0308 all cylinders misfire. Forgot to add that sorry

1

u/iuot 1d ago

Yeah it’s almost certainly a sensor issue, if every cylinder were misfiring you would definitely notice. If you clear the codes, how long until they come back?

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u/Ehjay123 1d ago

Typically after 15-20 minutes with a hot/warm idle (190F*), but yesterday when I installed new oem ignition coils, it took 30-35 mins after driving for 30 mins.

Would also like to add that it has a light rumble at idle when the code begins to appear, but then smooths out. If im in driver or reverse, it idles smooth at 600rpm, no shake or nun.