Howdy y’all,
Got a 99 4Runner, 2WD, 3RZ, 5-speed with 303,000 on the odometer. I’m getting P0300 and P0303 codes about once a week. Spark plugs are NGK BKR5EIX with about 3500 miles on them with NGK wires (same mileage). I pulled the #3 spark plug and compared it to the #2 plug in the pics below. Good #2 on left in the socket, bad #3 on the right. The #2 looks a bit blacker on the ground electrode. Both plug gaps matched @ .032”. Since the plugs/wires are nearly new I’m thinking it could be a fuel injector issue. Any ideas?

Other info: fires right up, 19mpg mixed, 23 hwy, good throttle response, no lack of power (for a 3rz anyway...). It idles rough @ 600 rpm once warmed up, which increases engine and shifter vibrations (I’m guessing due to misfire). I went ahead and swapped #2 and 3 plugs to see if it follows the plug to rule that out, need to test drive it and check.

https://i.imgur.com/pIC7u2U.jpg
https://i.imgur.com/rnRB9ER.jpg
https://i.imgur.com/T5l3ccl.jpg

I agree, that plug is very sooty, especially on the insulator. That would indicate it is running rich.

So the same CEL, P0300, P0303 codes came back today. It did not follow the plug, so I’m going to check the plug wire, and I f that’s good I guess it’s gotta be the fuel injector. If it sooty would that indicate an injector that’s leaking or maybe stuck open?

When talking 3RZ and running funny it is important to know if you valve clearances have been checked. Not to say this is causing your issue, but when have they last been checked?

FYI- I have had the best luck with the Denso copper plugs from the dealer for a couple of dollars each.

Who knows if the valves have ever been checked. All the carfax maintenance stuff was tires, alignments, oil, maybe a clutch. I watched Timmy’s video of the valve adjustment and I think I can tackle that. May as well do a compression test, too.

I feel like I can smell fuel in the exhaust, though.

Very possible. One thing you can check without removing the injectors is to just unclip them and measure the resistance in ohms on the injectors themselves (unclip the wire connectors). See if any of them are out of spec, especially on cylinder 3.

So got me some GB reman injectors from Rock Auto. Good news, they’re IDENTICAL to the ones I pulled out. Man they look rough @ 303,000 miles. Ironically the worst looking ones are in cylinders that are running fine.

New question: new injector on L, old on R.
Is there something left in the head when I pulled the injectors, because the new ones have a grommet on the bottom.
https://i.imgur.com/8OBWfWK.jpg

Edit: NM. Got it sorted out. 4 x new injectors installed, ran @ idle, no leaks. Quick trip out and back, still tight. Updates later on how it runs on the highway.

So a quick trip out in town revealed the PROBLEM IS NOT SOLVED. In less than 5 miles if driving got a flashing CEL, followed be steady CEL and P0303 codes again. Still rough idle and all that. Gonna grab a multi-meter and test the plug wires.

You might get lucky with the plug wires, I hope so.

Very likely you have an exhaust valve that is too tight on that cylinder. It is a VERY common thing on the 3rz. The valves need adjustment periodically. People seem to recommend every 100,000 miles. Google Tacoma sites for a ton of threads. Not mentioned much here because few of us have 3rzs and it doesn't seem to be a v6 issue.

If you can't say for certain when they were last done, stop driving the truck before you damage the valve and head. It is relatively easy to just check, measure and record the clearances. I might be wrong but at least then you will know where you stand. Do not blindly keep driving it, you could cook the head.

FYI- mine had a valve out of spec at 100,000 miles. I adjusted about 8 in total to get everything centred again. And of interest, I have yet to find a 3rz in a junkyard with the head on it because people burn up the originals then need a replacement. Check soon.

Unfortunately I don’t have leads for my multimeter long enough to test the plug wires. That boot is about 6” deep. They’re only 6 months old, so I think I can rule them out. Going to check the resistance on the ignition coils tomorrow.

I might have a shop do the valve adjustment for me.

Do a compression test. If cylinder 3 is low do a leak down test to find out why. You put compressed air into the cylinder with both valve shut to see where it leaks out. Exhaust pipe=exhaust valve, into the intake=intake valve, out the dipstick tube or crankcase breather=worn rings. As the valves wear into the seats they travel further up into the head and take up all the valve clearance at the rocker arm so they don't close all the way. You may just need a valve adjustment, but with that kind of mileage you might need a valve job or an engine rebuild.

Try a screwdriver or long nail/spike. That'll make the inside contact.

Checking valves can be comfortably done in a day on the 3rz. I love this motor for working on. Adjusting (if needed) is considerably more painful with the need to calculate and order shims, especially if you can't leave it apart while you wait for the parts to arrive.

Like MaineRunna mentioned, the compression test will indicate a leak if that is the issue.

Got a quote for a valve adjustment and new valve cover gasket for $400, I think I’m gonna do it this week.

That price seems in line with what I was quoted here in Canada ($5-600). I did mine myself eventually, but it requires quite a bit of downtime to order new shims. Hopefully that resolves it but at least it will buy you the peace of mind for the next 100,000.

See if they will record before and after measurements as well as thicknesses of any shims they touch. This will save a heap of work next time.

Took it in for the valve adjustment. They ran a compression test first just to see what’s going on:
1- 180 psi
2- 180 psi
3- 80 psi
4- 180 psi

Doing a leak down test tomorrow to see if it’s valves or piston rings.