1990 4Runner
22re, 4wd, manual trans, sr5.

Was running great, has newer engine with new Toyota fuel filter, new denso injectors, new denso o2 sensor, new denso cap, rotor, wires, and plugs, all components are denso except MAF is an autozone unit, has been in about 10 years now. Compression test is 175 psi across all cylinders. No CEL codes stored in ECU when checked.

I have CA smog coming up and I usually do a little tune up before hand, new plugs, air filter, and oil change. When I swapped air filter it was super dirty and it seems a little dust got past the element so I decided to clean the MAF sensor with a little MAF cleaner. Pulled the TB to clean it too. Put everything back together and when it fired up it cold idled normally. Once it was warmed up I could hear an intermittent misfire while idling. It’ll randomly miss up to about 2000 rpm while parked. Not a consistent misfire. Drives normally and as far as I can tell it doesn’t misfire at all while under load. Gas mileage is normal at 18-20 mpg. Still has great power for a 22re, cruises great on the highway at 65-75. I have a wide band o2 sensor and while on the freeway it’s normal AFR of 14.5-15.5 sweep like it always is. Idle is a different story. Idle varies between normal 14.5-15.5, but sometimes the idle increases or decreases and the mixture leans out pretty good when I come to a stop light. Sometimes as high as 18:1. The lean out usually accompanies an idle that’s lower than normal. It’s always had an inconsistent idle as long as I’ve owned it, over 10 years now, but it’s been a lot erratic now and the AFR change is new. Almost every time I come to a stop light now the idle drops, but it doesn’t always lean out when the idle drops.

since this started I decided to check stuff that can be effecting it. Can’t find any vacuum leaks, MAF tests okay per FSM. Fuel pressure was 48psi idle and 55psi with the vacuum line removed so I swapped that for a new denso part. Psi is now back in spec per FSM. Checked EGR and that passed all tests in the FSM. ECU coolant temp sensor checks out good. Coil checks out good, igniter doesn’t have a test in the FSM but from what I’ve read if the igniter is going out it’ll be prevalent more when under load or it just won’t run at all. PCV felt a little sticky so I swapped that with a new Toyota unit with a new grommet. TPS checks out good. Dipstick is tight in the tube. Distributor pickup gap and resistance in spec per FSM. Nothing I’ve done has helped at all. I cannot find a new reman MAF around so I ordered a “good tested” denso unit on eBay that matches the part number than comes up for my vin/build code.

today I tried a couple things to see if I can find the problem and noticed:

it will not misfire while the AC is on, my AC idle up is set at 900 rpm, slightly higher than my normal idle.

misfire still there if I unplug TPS. TPS is in working order, idle drops when diagnostic port is shorted per FSM, timing correct, and timing advanced as soon as the throttle is opened, so TPS is fine.

engine dies immediately if MAF is unplugged while idling.

idle smooths out immediately and misfire disappears if o2 sensor is unplugged while running. CEL will illuminate if engine gets revved over 2000 rpm. Idle drop does not happen when coming to a stop and idle overall becomes very consistent with the o2 sensor unplugged but the truck runs very rich, 12.5:1 at almost all times. Plug the o2 back in and within 5 seconds the idle misfire comes back, can see motor shimmy slightly and feel it in the shift lever, CEL goes away as well. When checked at the diagnostic port it sweeps correctly per FSM.

up until this unplugging testing no ECU codes were stored.

im kind of at a loss but my guess is that the MAF is the problem. Cleaning the TB has only ever helped the idle smooth out in my experience. I’d only cleaned the MAF once as far as I can remember, but I’m guessing something dislodged or something when I sprayed it down? Didn’t use much cleaner though. Waited about 2 hours between spray and startup so dried enough, but it still tests in spec per FSM.

anyone have any ideas? I’m not sure what else to check or try at this point.

thanks