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sprinkles08, ASE Certified Technician
Category: Car
Satisfied Customers: 22659
Experience:  ASE Master and Advanced level certified. Factory trained with 15 years dealership experience
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I have a 2000 4runner 4wd with the 5vz-fe and stadard transmission. I

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I have a 2000 4runner 4wd with the 5vz-fe and stadard transmission. I have an issue with a very rough idle and random misfires, the problem only occurs after a warm start (around 30-45 minutes after it was last driven and shut off), and usually lasts 2-3 minutes after start-up. After 2-3 minutes the car seems to rectify it's self and the misfires go away. The computer threw a PO300 because of the misfires. I have my thoughts as to what it might be, but I would like some professional advice/thoughts.
Hello and welcome to JustAnswer!

It sounds as if you may have a fuel issue. When the engine is hot and you park the vehicle the underhood temperatures stay high for quite a while. The engine heat coming up can cause the fuel in the fuel rail to boil.

In the fuel pump there is a check valve that prevents fuel from going back into the tank once it's in the line. What sometimes happens is the high pressure created by the boiling fuel in the rail will cause a weak check valve to open partially and it will force fuel back to the tank.

When you go to restart the engine it will cause the misfires for a couple reasons. First, there is fuel vapor in the rail but little actual liquid fuel. The engine control software fires the injectors a certain amount of time to inject a calibrated amount of fuel. If it's firing with vapor in the rail then you aren't getting this fuel.

Second, the fuel pump has to run long enough to fill the line and rail again.

If this fuel drainback is your case then the pump would need to be replaced to cure it.
Customer: replied 4 years ago.
Do you know of any methods to test this fuel return regulator without having to drop the fuel tank and pull the whole pump assembly?
It can be a hard thing to test for since drainback will normally cause fuel pressure loss, yet the high pressure in the rail is what is causing the drainback so pressure can appear to remain ok.

One thing that can be done is spray the fuel rail area with cool water like from a garden hose. This can collapse some of the fuel vapor and may help it start easier.

An actual test that can be done is a fuel pressure bleedoff test with a T hose. A T hose can be installed at the fuel rail and then the fuel pressure gauge connects to the T. The T hose allows you to pinch off either side of the fuel system so the gauge can check bleedoff on either side of the system. Isolating the high rail pressure from the supply side of the system may allow you to see some pressure drop on the gauge.

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Customer: replied 4 years ago.
Thanks for your help, and I appreciate your knowledgable and prompt responses.
You're welcome and good luck!