Hi all. Got the CELs P171, P300, 304 and 306. So, cylinders 4&6 missfire.
Things done:
new air filter
new NGK two electrode plugs
new spark plug wires
new PCV valve
injectors tested good
smoke tested - no leaks post throttle body
timing belt - all three marks are dead on
no overheating even in heavy stop and go traffic
no coolant loss
no white smoke at cold start

Car has annoying vibration at idle. More when in D, slightly less in N, when rpms rise a bit. Can feel that engine doesn't idle smooth. As soon as I start to drive all is ok, good power and engine pulls well. Except slight hesistation beeen 800-1500rpm, when I floor WOT from stop. And that is occasional too.

Have crack on air intake hose right where it ataches the throttle body. I'll change that as soon as it arrives. Might consider that leak post MAF triggers p171 (system too lean) code, but it's not logical to trigger missfire only on two adjacent cylinders.

Every time I clear codes, they come back at same 4&6 cylinders only. Swapped coils N3 and N5 to see if the missfire shift from cylinder 6 to cylinder 2. No luck, still got same CELs. Every time first comes 171 code (system too lean) and after couple of miles come 300, 304 and 306.

Sorry for such a long post, tried to describe every little bit to get clear picture and get some advices. Trying to do things mostly by myself, due to job and lack of free time. So, any input is appreciated.

Have you performed a compression test of the cylinders yet? If not, that's what I suggest you do next.

/\ /\ /\ What Tim said, compression test, and if that's bad, a leakdown test to see where the air is going.

Short of doing a real compression test, it's quick and easy to do a shade tree compression test. Pull the fuel injection system fuse (I think it's #17 under the hood on my '99) so it wont start, and then just crank it over and listen to how much the starter slows down for each compression. If there's an offbeat pattern of slower and faster cranking over each compression then you know there are issues that require further attention. If it cranks very evenly over all 6 cylinders, odds are that you won't find much of a problem with a compression test.

Has it been using coolant, or acting like it's overheating? Another quick test is for combustion gasses in the cooling system. I think Autozone has testers for free-rental. But not all HG failures result in gasses going into the cooling system, just a lot of them.

No, no coolant loss and no overheating, it's rock solid between 176-180 degrees even in heavy stop and go traffic. Monitoring via torque app.
Will buy compression tester shortly and perform the test

Sounds like you need to replace that intake elbow first. Mine came with a crack at the same junction. A compression tester is is an easy way to get a bit of peace of mind. When you say you tested the the injectors, do you mean resistance via an ohm meter?

OEM thermostat puts the engine temp right around 190*F - do you have a colder/aftermarket one installed? (that still won't cause the issues you are seeing)

I'd

Yes. And also swapping them with places

Don't know to be honest. Own the car for only 2 months, it was like that when bought. Seems to me a bit on a low side, but haven't messed with it, since it's not related with my problem

At some point in time, IF you have compression readings within 10% of each other, you plan on keeping this motor, and you're comfortable pulling the plenum and the fuel rails (again?), you might as well replace all the injectors with a MotorWest kit. I am NOT a mechanic and also write run-on sentences, but I had a similar journey recently. Injectors can do weird things.

Did the compression test. Here are the results:

1 - 172
3 - 172
5 - 171
2 - 170
4 - 131
6 - 162

Seems, everything goes to removing driver side head... which I don't wanted at all :///

Here are the pics of spark plugs too, from top to bottom 1-3-5 and 2-4-6. They have less than 500 miles

I'm gonna take a guess and say you have a #4 leaky faulty fuel injector which has caused your plug to soot up from to much fuel. Spraying to much fuel along with the seal being bad causing a lose of compression. May not have to remove head after all. Lots of problems with older fuel injectors on these rigs causing problems.