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Kenny Z., Ford Senior Master Tech
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1990 ford thunderbird sc: hard start condition...electrical

Customer Question

i have a 1990 ford thunderbird sc. hard start condition. its the electrical side of fuel systen. turn the key on with fuel pressure guage hooked up and pressure jumps two 33 psi. after a second and a half it dumps. crank on it and theres no fuel pressure for about 5 seconds then it jumps up and car runs. what is causing this problem.
Submitted: 3 years ago.
Category: Ford
Expert:  Kenny Z. replied 3 years ago.
Hello and thanks for your question.

If you cycle the key from off to run the fuel pump will run for approximately 1 second. If the PCM does not see a crank signal it will shut the pump off. When it sees the crank signal when starting it cycles the pump back on. If the pressure drops after turning the key off then there is a bleed in the system. It should hold fuel pressure with the pump off for at least 5 minutes before it starts to bleed down even then it is a very slow drop.
There are 2 things that can cause the bleed. The fuel pump has a check valve that can go bad and cause rapid bleed off after the pump shuts down. A fuel injector can also cause bleed if it can't close completely. This is hard to diagnose. Usually if its an injector bleeding that fast it will cause the vehicle to run rough after started due to the excess fuel in the cylinder(s). If the pump is bleeding down then usually once it starts it runs good.
Kenny Z., Ford Senior Master Tech
Category: Ford
Satisfied Customers: 4853
Experience: Auto Trans, Drivability and Electrical Specialist
Kenny Z. and 4 other Ford Specialists are ready to help you
Customer: replied 3 years ago.

ok. im not sure its running right anyways. it doesnt smoke upon startup but occasionally misses a coworker suggest fuel pressure regulator just now. its just mind boggling how you turn the key on and wam. 33 psi but it takes so long for it to get back up while cranking like somethings neglecting to tell the pump to engage while cranking. and it holds 33 strong while running. cant test it under a load tho. i dont know

 

Expert:  Kenny Z. replied 3 years ago.
If you turn the key on and it pressurizes and then drops fast after the pump shuts off then that is definitely a problem. It could have other issues that would take it longer to build pressure when cranking. Its going to build slower when cranking since the injectors are cycling while its cranking. It could need a regulator but that usually won't cause a rapid bleed off they usually rupture internally and the pressure is erratic and lower when running. Usually when they rupture if you pull the vacuum line off the regulator it will have raw fuel present in the vacuum port.

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Kenny Z.
Kenny Z.
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Auto Trans, Drivability and Electrical Specialist