I just got my Supra and started to notice this problem. My car starts fine on a cold start. But when i drive it around, let the engine heat up, park....then when I try to start the car again, when the engine is warm, the rpm's drop and the car stalls. This happens 100% of the time. I was thinking it could be the fuel pump or leaky cold start valve, but it doesn't happen when i start it when the engine is completely cold. If anyone has insight into this matter please let me know. I have a 86.5 n/a, 69,400 miles.
Warm Start Problems
- Thread starter excelsupra
- Start date
After the engine is turned off fuel left in the rail can become hot enough to boil. Since engines don't run well on vapor what results on a warm restart are symptoms much like yours. It's most likely to happen the first 15-60 minutes after shutdown. Commonly called "vapor lock".
To avoid it pressure is deliberately trapped in the supply side (between the fuel pump and regulator) of the fuel system to maintain an increased boiling point. It's the same principle used in cooling systems. Toyota calls this "residual fuel pressure" but I prefer the Bosch term.
Anyway, what you need to do is check fuel pressure just after the engine is shut off. It should remain at the minimum stated, for the time stated, in the service manual. Iirc that's at least 21 psi for 20 minutes but most cars will hold higher and longer than that.
Course, it could be something other than vapor. For example a leaky injector (including the CSI) will bleed rest pressure off into the intake manifold. On restart the richness will also cause these symptoms. That said, since such a leak also shows up as a loss of rest pressure, the diagnostic path is the same.
It's a case of one problem being able to cause the same symptom but in two different ways. One is too lean (from vapor upstream of the injectors) while the other is too rich (from vapor or even liquid) downstream of the injectors. The result is the same though: fussy hot starts and a funky idle until the mixture in all cylinders comes back to what it should be.
If you don't have a way to easily check fuel pressure a quick way around it is to jumper B+ and FP in the diagnostic block for a minute before the next warm start. That'll pressurize the system with fresh fuel and displace any vapor back to the tank. If the engine starts OK when doing this you'll know the problem is likely rest pressure related. If it does the same thing look elsewhere.
Lastly, the car has a Fuel Pressure Up System. It's job is to increase fuel pressure slightly during hot starts and up to 90 seconds thereafter. Could be a problem with that but again, if there's no rest pressure to begin with the FPU System isn't going to do any good. That's why you should begin by checking rest pressure.
Or it could be something else. It's not easy fixing cars over the Net you know....
To avoid it pressure is deliberately trapped in the supply side (between the fuel pump and regulator) of the fuel system to maintain an increased boiling point. It's the same principle used in cooling systems. Toyota calls this "residual fuel pressure" but I prefer the Bosch term.
Anyway, what you need to do is check fuel pressure just after the engine is shut off. It should remain at the minimum stated, for the time stated, in the service manual. Iirc that's at least 21 psi for 20 minutes but most cars will hold higher and longer than that.
Course, it could be something other than vapor. For example a leaky injector (including the CSI) will bleed rest pressure off into the intake manifold. On restart the richness will also cause these symptoms. That said, since such a leak also shows up as a loss of rest pressure, the diagnostic path is the same.
It's a case of one problem being able to cause the same symptom but in two different ways. One is too lean (from vapor upstream of the injectors) while the other is too rich (from vapor or even liquid) downstream of the injectors. The result is the same though: fussy hot starts and a funky idle until the mixture in all cylinders comes back to what it should be.
If you don't have a way to easily check fuel pressure a quick way around it is to jumper B+ and FP in the diagnostic block for a minute before the next warm start. That'll pressurize the system with fresh fuel and displace any vapor back to the tank. If the engine starts OK when doing this you'll know the problem is likely rest pressure related. If it does the same thing look elsewhere.
Lastly, the car has a Fuel Pressure Up System. It's job is to increase fuel pressure slightly during hot starts and up to 90 seconds thereafter. Could be a problem with that but again, if there's no rest pressure to begin with the FPU System isn't going to do any good. That's why you should begin by checking rest pressure.
Or it could be something else. It's not easy fixing cars over the Net you know....
Wow, thank you for the very detailed explanation. I will try to perform a pressure test on the fuel system after work today, and go from there.