3.4 Misfire at WOT and High Idle

Please help with my 2000 3.4 V6 5MT Tacoma! Myself and at least one other shade-tree mechanic as well as one ASE certified mechanic have attempted to troubleshoot this issue to no avail.

The truck idles fine when cold. Once the truck is warm, it idles high (around 1800 RPM), but will settle down about 70% of the time with a throttle blip. The misfire occurs at WOT regardless of temp. It runs fine up to about 3/4 throttle.

So far, I have:

- Checked for vacuum leaks (spray with soapy water and throttle body cleaner)
- Replaced spark plugs and wires
- Inspected, tested, and cleaned the IACV (possibly already replaced by one of the mechanics, but difficult to tell)
- Inspected and cleaned the MAF sensor (with proper MAF cleaner, mind you)
- Inspected and cleaned the throttle body (complete removal with replacement of gaskets) as well as the throttle cable
- Replaced throttle position sensor
- Replaced fuel filter
- Replaced PCV valve

... and the issue persists.

The biggest breakthrough in troubleshooting came from disconnecting the IACV. This brings the idle down, but seems to cause an extremely rich condition which makes the truck underivable with backfiring. When testing the IACV, I could not find a diagram showing the valve in the fully closed position. In the service manual and in a video I found, it doesn't show it completely. My valve, when "fully closed" has a small slit, about 1/16" wide. Is this normal? Could this be the source of all the issues?

The last thing I can imagine which would cause this issue is the coolant temp sensor. I haven't been able to test this as I live in a very rural area and have not been able to source the copper crush washers on the fuel rail, which are necessary to remove for CTS inspection.

Any ideas? I'm at my wits end...

 

My brain goes to the coolant temp sensor as well.

Do you have an OBDII reader, so you can post fuel trims and open/closed loop status?

Driving with your IAC off is not advised. I'm surprised you even got out of the driveway tbh..

 

Is the CEL on? Any pending codes? Have you checked?

 

Redo all your grounds and add some it doesn't hurt. Possible could be igniter as well, but bad grounds can do that as well.

 

Almost exact same issue but only occurs after truck has sat for a couple weeks. After driving for a while, it corrects itself. My mechanic said he thinks it's a bad catalytic converter. He replaced the in-tank fuel pump and fuel filter and after driving it said not head gasket, IAC or coil packs. Exhaust shop would not agree as he said he does not do diagnostics and in such I'm in a rule out situation. In Hawaii we do not have emissions so he said he could eliminate the Cats but would end up with a check engine light. Just does not add up, why would a bad converter only act up after sitting but be fine when daily driving?

 

OK so not the Cat and the mechanic now thinking it's either a timing issue or the new in tank fuel pump he installed is bad. He can't check it for another week as his workload is so heavy.

 

It was the Crank Position Sensor.