ORoffroad

My 4Runner runs fine on cold start and continues to run well after it warms up. However, if you shut it down and try to restart it will run really rough.

At one point the check engine light came on and indicated a lean fuel ratio. My repair book suggested a bad O2 Sensor as a possible cause, which made sense because it runs fine when cold. (Doesn't explain running fine after warming up though?) I went ahead and changed the 02 Sensor, the check engine light went out, and it ran great for about three days then started having the same issues until now.

I don't think it's an electrical issue because the cap, rotor, plugs and wires are relatively new. I thought the fuel mixture ratio was preset and shouldn't be modified. I have considered trying to clean the MAF sensor to see if that helps.

Has anyone else experienced this type of problem? Any ideas or advice would be much appreciated.

thook

What brand of sensor did you install?

Just because all those new parts are on there does't mean it isn't electrically related. You have an air temp sensor in the airflow meter, a throttle position sensor, a coolant temp sensor for the EFI (not the temp sending unit), and so on.

Anyway......the first question, eh. ????

ORoffroad

I put in a Bosch OE replacement sensor.

thook

Mmmmm.....first off I wll say more often than not, I talk to and read about people having issues using Bosch O2 sensors on their Yotas. I only know of one person that hasn't/doesn't/isn't. He's the anomoly I can't figure out. All I can say is if you read....run a search on the forum.....about the Bosch sensors you'll find displeasure and then improvement once a switch was made to an OEM Denso unit. This may not be your issue, though. I just wanted to point that out. You can try unplugging the sensor temporarily to see how it will affect running conditions. You will get a CEL, but not to worry. You're just testing. Try this and report back what happens.

Otherwise, it could be....say....the airflow meter, but I doubt it.

ORoffroad

Tried unplugging the 02 Sensor when the engine was running rough. Check Engine light came on and the engine ran even worse than before. I could also smell gas.

I put my timing light on while it was running bad and it seemed to be off, however, the engine was idling slower than recommended (800rpm). I cranked up the idle to the 800RPM and that helped it run slightly better, but the timing still seemed off. I couldn't even see the timing mark on the pulley?

The truck works fine when cold and even after warmed up as long as the engine stays running. The big question is what changes in engine logic take place when restarting with a warm engine. If someone knows that, I think we may be able to narrow down the problem.

Thanks for all your advice.

thook

I think what you're dealing is called starting problems after hot soak. Now, I remember reading some things about that specific here recently, but don't remember the exact possible cause/s. I'll have to find the info again and get back with you.

Later......

thook

Okay.....I found the info I was looking for. There a vacuum switching valve referred to as the fuel pressure up valve connected in line with your fuel pressure regulator. It's suppose allow atomospheric air into the fuel pressure regulator and increase fuel pressure under hot soak conditions upon starting. If that's not working....allowing air in...it can create this problem.

Any questions??? You still around, bubba?

Fishaa

Ive had this exact some issue for years and never figured it out!!!

3 mechanics couldnt figure it out and I was sick of replacing parts on their ideas so I told them if they cant garuntee that it will fix it then I dont want it. They couldnt garuntee it so I just deal with it. One also mentioned a fuel/air mix error came up on one of their "tests" but that lead him to replacing some intake hose that was cracked that didnt make a difference (but cost me 200 bucks)

Over time ive looked at/been told to replace Fuel pressure reg, EGR valve, and the Oxygen sensor.

Does yours also sometimes run a little hesitant/jerky in the winter? Its fine when it warms up but if you only let it idle for a minute or so before driving it seems to struggle/hesitate pretty bad in the cold.

Reedr125

I have had 2 diffrent vehicles do this to me. In both cases it turned out i had a pin hole in my gas line. I think they call it vapor locking, the pressure slowly goes down and fills the line with air. Creating a pocket of air in the line. Try turning it on and letting the pump primes itsself for a minute. I never seen a leak on my truck, but then the line blew out, after fixing it, the truck ran perfect all the time. Gives ya something easy to look for though.

lou94v6

Hello
I just joined to follow this problem. This exactly what my well tuned 185xxx miles 4 run is doing.
Please keep us posted on how to fix the rough hot run/start.
Lou