Cold start issue

[Pentium]

I have 2004 4Runner 4.0L that has a cold start problem while warm start is OK. Every morning it took 4 to 6 cranking to get it starts, sometimes I am afraid it will damage the starter or flywheel doing that. After many times scanning the engine with portable ODB scanner it gave false reading as P0351, P0352 to P0355 (not P0356). I replaced, swapped the ignition coils, clean the MAF sensor, installed all new spark plugs but still have the same cold start problem. Last week my mechanic scanned the engine with his $10K scanner and found no DTC code other than P0351 (false reading again?), it detected no sensor error.

Yesterday, when the outside temp is around 45 degree, I got in the car and waited for my wife, I turned the key to to ON but not starting the engine. After 4 minutes my wife got out of the house, I then cranked the engine, holly molly the engine started immediately with one crank, I did not have this in the past 4 weeks. This tells me the code P0351 (primary/secondary circuit) is false again as this is not electrical, ignition problem, right?

I think there is a problem with fuel or air or vacuum mixture but I have no clue. there is no misfire, no hesitation, still drive strong, that is how it gets me hard.

For now at least I can bypass the cold start problem but I'd like to find and fix the issue since winter is coming hard soon. Is it Throttle issue, Coolant Temperature Sensor issue, or else? The bottom line is the car runs well, only cold start issue, warm engine starting is fine.

Please help!

 

[Brian3676]

If turning the key of for a few seconds will make it start right away, and it will always start when warm, that can mean the fuel system is not holding pressure. Usually this is a failing pump check valve, slowly bleeding the pressure back into the tank.

 

[Pentium]

Thanks for quick reply. My friend said I might have a bad fuel pump, and it will be a 4,5 hundred dollars to replace. May I ask how do I check to make sure it is the faulty fuel pump, and what is the difference between the pump and the check valve? Where is the check valve, in the tank, or somewhere in the engine? Is it replaceable as a single unit?

 

[Brian3676]

The check valve is built into the pump, so you replace the entire pump, that's likley why your friend said the pump is failing. You can check with a fuel pressure gauge. Hook it up inline with the fuel feed to the engine and start the car for a few minutes. Shut the engine off and watch the pressure, see if it drops over a few hour period

 

[Pentium]

As I read some online threads, it seems the check valve is not part of the fuel pump but it is a single part that locates near the pump, is it right? How do I know whether the check valve or the pump itself is bad? Both cost a little bit and the labor is not easy, if I made a mistake by replacing the wrong part it will be costly and time consuming. Therefore knowing which one needs replacement is helpful.

For the past few days I employ the tactic of turning the key on and wait for 4,5 minutes then crank the engine, it works. However, I notice if I crank the engine within 3 minutes it won't start. While it tried to start I pressed the gas pedal a few times (pumping fuel?) but it stills does not start, a couple more cranking then it will do. Does it indicate something?

Pent