I have a 1996 4Runner and I'm hoping somebody here can help me, I'm a pretty experienced tech but for some reason I can't figure out this intermittent misfireA short history on the problem
I recently did a bottom end rebuild and put the motor back in, for a full day the truck ran perfectly but the last time I drove it during the day it developed a bad misfire, but a couple days later I installed a new mass air flow and it went away, but now it comes back at seemingly random times. Every time the misfire comes back it's on a different cylinder, so far I've had it on three, four, two, six, and one


A list of tests I've already performed
Brake cleaner to try and find an intake leak
Smoke test to try to find intake leakDisconnected and capped all vacuum lines
I tried running the truck with pressure regulator vacuum line disconnected and cappedUnplugged each cylinder to try to narrow the misfire down to one placeNew mass air flow
New plug wires
Fuel pump pressure and flow test, about 43 psi and 48 gphOil pressure at 15 psi at 700 RPM idleCoil packs have less than 10,000 mi on them, Toyota OEM spark plugs replaced less than 5K ago

​​​​​​Currently I have very high fuel trims, but only at idle, short-term is usually between 15 and 19 at idle, long-term can be between 19 and 25, sometimes if the truck hasn't had a misfire for a little while the short term will drop down to around 3 at idle, anything over 2000 RPMs and both short-term and long-term drop very close to zero
I drained and dropped the fuel tank earlier today, I didn't see any water, I'm about to put it back in, I'm at a complete loss for what to do next, does anybody know what it could be?

 

Are you getting the PO300 code for random misfire ??

Any other codes pulled?

Does the code change as the misfire moves .

Are these still the original injectors ? Did you have them serviced when the engine was out?

Mouse nest in the intake they can get past the throttle body but not back out .That one just about drove me crazy till it was figured out..

Any pinched or rubbed wires during the pulling and installing??

 

The igniter wires got pulled on slightly but I've tested all those to make sure nothing broke, The codes I keep getting are p0301 through p0306 usually a day or two apart, I got the multiple cylinder misfire once
These are the original injectors as far as I know, I've tried to rule everything else out before messing with the injectors because I figure if it was an injector issue it would be on the same cylinder every time

The only other code that I get is system lean fuel trim
All parts for the engine were store inside, and I inspected and cleaned everything as it went on, The intake tube is brand new and I made sure to blow out the air filter and make sure that the air box was clean, and just for the sake of testing, I made a temporary intake tube that used a k&n style pod filter so it's definitely not related to a rat's nest

 

Agreed, if its an injector it should be consistent, if its random and changes I would inspect the reluctor/Tone ring on the crank, maybe a few teeth got broke off during the rebuild?, that would cause what you describe..

 

Injector failure during heat soak does not manifest consistently. There was a rash of injector failures about 3-4 years ago. Multiples dying at once or testing fine and then dying after reinstallation.

my vote is injectors. The high fuel trim is a hallmark

 

I'll have to test that, after cleaning the gas tank the misfire went away, if it stays gone then I'll go ahead and order new injectors, The heat soak would make sense, because most of the time the the misfire starts back up it's after turning the truck off for a bit, thanks for the tip

 

If you read my build thread I go thru a bunch of my steps in tracking it down.

 

Well I tested my injectors today, they all performed perfectly, I even heated them up in the oven to see if it was a heat soak related issue, on the upside I found out that the o2 sensors are the same in the front and back of the cat, and I still have the after cat O2 sensor from the parts truck that I'm going to try tomorrow