Procedure for testing fuel pressure with guage kit?

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hal keiner

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I want to check whether the fuel line pressure is within spec on my 2002 ST. I already understand how to connect the gauge to the schrader valve on the fuel rail. What I don't know is what readings to expect on the gauge to determine if everything is OK.



I want to confirm the following:



1) The fuel pump is generating enough pressure.

2) The fuel pump is not leaking and causing a too quick pressure drop after motor is turned off.

3) The pressure regulator is regulating pressure properly.

4) The fuel injectors are not leaking and causing a too quick pressure drop after the motor is turned off.



I expect that I should check the gauge's PSI reading when when the motor is idling and also monitor how fast the PSI reading drops after the motor is turned off. The thing I do not know is what the normal PSI reading at idle should be and how long the PSI level should last before dropping (a few minutes, a few hours, or a few days). Of course, I understand that if the PSI is out of spec or dropping too fast after motor is turned of, then I have to do some further troubleshooting to figure out which component(s) are at fault.



Thanks in advance for any advice.
 
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It looks like you have a permanent installation of a pressure gauge on your ST. I expect that you know what is the normal behaviour of the gauge after the motor is turned off too.



Does the gauge continue to show 60psi for long after the motor is turned off? If it drops, how fast does it drop (in minutes, hours, days, etc.) and how far down does it drop before it stays there until the next time the motor is started?



I appreciate your knowledge on this. If I know what is "normal" after motor shut down, I can then evaluate if the fuel delivery system on my ST is losing pressure and causing my hard starting problems. I already did the IAC cleaning and MAF senser cleaning steps with no success.
 
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It's not permanent, it was a temporary gauge. I actually still have it sitting in the closet. I don't remember the PSI drop rate. One way to test is to crank it up at night and see how fast it cranks when it's warm. Then, in the morning to see how fast it cranks. If it's slow, get it to crank, then let it sit for a few hours, then, instead of turning it over immediately, turn the key to "on" for a few seconds, then crank it up. If it was losing pressure, the few seconds of priming from the pump would be enough to restore pressure and it will crank normally.
 
Your looking for anywhere between 55-65 psi when the engine is running. When you shut it off your pressure may rise depending on hot it is, this is normal. It is also normal for the pressure to drop a little. A little bit is not cause for alarm, now if you lose all the pressure within 10 minutes than you more than likely have a bad fuel pump. If you let the truck sit overnight don't be surprised if there is no fuel pressure this is also normal at times, what's important is how fast the pressure returns when you turn the key on. Pressure should return almost immediately upon turning the key on, if you have to cycle the key a few times to get pressure then you probably have a pump going bad. Injectors nowadays usually don't go bad, if you have an injector that is leaking you can usually tell by the truck acting like it is flooded when you try to start it and is usually accompanied by a rough idle due to too much fuel going in that particular cylinder.
 
LOL..... Mine sits around 70PSI at idle and close to 80 at WOT....



Zabteck sells permanent Gauge kits for the ST...

They will be on sale shortly too !!!



Todd Z
 
Yes.....

Thats why i used the LOL in the front of what I said.....;);)



Todd Z
 
Most fuel pressure gauges just connect to the end of the fuel rail. You start the engine and you will seel the pressure as shown in the photo above.



When you shut the eignie off you should not see a noticable drop in pressure even if the engine sits overnight. If you do see a leak you will need to test just the fuel pump side of the line before it connects to the fuel rail. when you turn on the key it should start to build pressure for about 15 seconds and then the fuel pump will shut off. Again check that it holds pressure. If it does, you fuel pump and fuel line are OK. If it does not, you probably have a leaking fuel pump and it needs to be replaced.



If the pressure holds but the original test showed a leak, then it's probably a sticking injector and you ccan pull the injectors and look for one that's driping, or check the spark plugs looking for any cylinders that are burning too rich with black fluffy carbon build up. I doubt that the injector is you problem because the OBDII system should sense a rich mixture and give you a CEL.



...Rich
 
Guys,



Thanks for the many helpful responses. Knowing what is normal regarding how long the pressure should hold up (after motor shut down) was my biggest unknown. I didn't see this described in the specs section of my OEM Ford shop manual. I hope to do the pressure tests this weekend or next.



If the fuel pressure passes muster, then I'll probably consider replacing the IAC valve. Perhaps it is just defective, as I already cleaned it a few times without success, along with cleaning the MAF senser, replacing the infamous "elbow" on the vacuum line, and replacing the PCV valve.
 
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Richard L



I disagree with your statement that you shouldn't see a drop in pressure if the vehicle has sat overnight. It has been my experience, 20 plus years as an ASE certified master technician with L1 certification, trainer and State of CA advanced emission specialist, that most system will lose quite a bit of pressure if not all if the vehicle has sat overnight. This is usually not a problem as long the fuel hasn't drained out of the line and back into the tank. I just had a car in the shop with a fuel pressure gauge hooked to it overnight to check for extended crank. After I shut the car off fuel pressure held at 50 psi for at least the hour I was there before I went home. Came back the next morning and pressure was down to 15 psi but within 1/2 a second of turning the key on pressure ws back up to 60 psi. The fuel pump turns on for 2 seconds when the key is turned on to prime the system, not the afore mentioned 15 seconds, some systems even run the pump for 2 seconds after the key is shut off to help eliminate vapor lock in the fuel line.
 
Came back the next morning and pressure was down to 15 psi but within 1/2 a second of turning the key on pressure ws back up to 60 psi.
Click to expand...



After I do the pressure test, I'll report back with what I find out. It is problably too early to discuss at length which range of scenarios are definately OK and which range of scenarios definately indicate a problem.



Having said this, I guess that if there is at least some pressure in the system the next morning and it kicks up to the 60psi range quickly after the key is turned on , then things look OK. If the pressure drops to zero and it takes several seconds to kick back up to the 60psi level after the key is turned on, that is not OK and it might explain the delayed start of the motor. The problem is intermittant so it may take some patience on my part to troubleshoot this.



Thanks again to everyone....



 
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Just keep in mind that the fuel pump will only run for 2 seconds when the key is just turned on.
Click to expand...



If the key is instead turned all the way to to start position right away (so that the motor turns over until it starts), I presume the fuel pump would keep pumping until 60 PSI is reached. And that the time from the key first being turned towards start to the time that 60psi is reached is also a good indicator. Is that right?
 
As long as the key is being held in the start position the fuel pump will run, the fuel pressure should almost instantly reach operating pressure.
 
On mine it reaches full PSI in that 2 seconds....



Also some times my Truck will drop all the pressure almost instantly and some times it holds it overnight....



Todd Z
 

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