How long should an EFI system maintain fuel pressure?

[toddz69]

I've had two instances lately where my truck didn't start on the first cranking cycle and subsequently required 5-8 cranking cycles to start. Unnerving and frustrating to say the least. Both cases were when the engine wasn't totally cold - it had been sitting for several hours.

I haven't had a chance to check for codes yet but I did hook up my fuel pressure gauge last night to verify I had proper fuel pressure (35 psi in my case) and I did. For grins, I left the gauge hooked up last night and checked it this morning. The pressure this morning was about 8-10 psi after sitting for about 9 hours.

So...my quick internet research didn't pick up much on my question on how long should a system maintain pressure? In my case, a quick turn of the key will quickly return the system to full pressure but I'm uncertain whether this much pressure loss over 9 hrs. time is indicative of other problems I should be looking for. Most info I found online stated that quick loss (within minutes) usually means there's something wrong.

Any thoughts appreciated.

Thanks,
Todd Z.

 

[Viperwolf1]

I haven't seen a spec on the time but I believe it should maintain some pressure for at least a couple hours. 10 psi after 9 hours sounds real good. Time to chase the spark.

 

[welndmn]

I think it was it only needs to hold pressure for 10mins.
It pretty much leaks or it doesn't. A leaky injector would never hold pressure after a key cycle.

 

[siderbox]

IIRC the test according to Ford in the mid 90's when the SBDS came out was no more than 5 psi drop in 90 seconds.

Man that brought back some memories.

 

[toddz69]

Thanks for the inputs, guys. Other things to investigate now.

Todd Z.

 

[Rustytruck]

I would Ohm check the distributer pick-up when cold and check it again when hot and see if it falls out of spec. Hot restarts is usually when the start showing sighns of failure.

 

[toddz69]

I would Ohm check the distributer pick-up when cold and check it again when hot and see if it falls out of spec. Hot restarts is usually when the start showing sighns of failure.

Got a link handy on how to accomplish that?

Thanks,
Todd Z.

 

[Ourobos]

i haven't seen a spec on the time but i believe it should maintain some pressure for at least a couple hours. 10 psi after 9 hours sounds real good. Time to chase the spark.

x2........

 

[Rustytruck]

http://351winsor.com/joel/Ignition_Systems/tfi2.html

 

[pbwcr]

Hi Todd,
On my 96 which I now consider normal my pressure decays much faster than yours. When I turn on the key the pressure immediately goes back to the low 30s. On my 72 even after sitting for 6 months it starts with little cranking. I still wish it had a FP Gauge. I like seeing how it works.
For sure with all the issues I have had with my 96 - My 72 will have a fuel pressure gauge on the dash.
You are the correct track to look for other issues.
What is you battery resting voltage after is sits for a few days?
PaulW

 

[toddz69]

Hi Todd,
On my 96 which I now consider normal my pressure decays much faster than yours. When I turn on the key the pressure immediately goes back to the low 30s. On my 72 even after sitting for 6 months it starts with little cranking. I still wish it had a FP Gauge. I like seeing how it works.
For sure with all the issues I have had with my 96 - My 72 will have a fuel pressure gauge on the dash.
You are the correct track to look for other issues.
What is you battery resting voltage after is sits for a few days?
PaulW

Hi Paul:

Yes, a fuel pressure gauge is on my 'to-do' list as well. Lars has a nice Autometer unit that I plan to duplicate on my truck at some point.

I'm not sure what my resting voltage is but I haven't had any cold start cranking/starting issues yet. I have a 1 year old Odyssey battery in the Bronco right now.

Todd Z.

 

[toddz69]

http://351winsor.com/joel/Ignition_Systems/tfi2.html

Thanks! I reviewed this webpage and realized my Rotunda EEC-IV ignition testing apparatus might be helpful in ruling a few things out tonight. So I hooked it up and checked things out. It said my PIP and TFI module are functioning correctly (at cold). I still have other things to check and codes to run.

Todd Z.

 

[toddz69]

What is you battery resting voltage after is sits for a few days?
PaulW

Checked this last night, Paul. 12.9V across the terminals.

Embarrassingly, I also found a loose connection where the ground cable from my battery connects to the engine block. That's what I get for being too smug about my vehicle prep/condition . Cleaned up a few other grounds 'just because'.

Todd Z.

 

[ransil]

One thing my Bronco does is not start on the first try, if I cycle the key once it fires right up, I would fix it but it runs so damn good this little hiccup does not bother me.

try to crank a rev or 2 then turn the key to off, then try starting again. Mine will fire up very fast when done this way.

 

[briancave]

Mine doesn't hold pressure for long at all, maybe 10 sec. I still need to figure this out by checking injectors, regulator or fuel pump. BUT it runs and starts great. If I didn't have a fuel pressure gauge I would never know there is a problem. Might look to other causes for your problem.

 

[Rox Crusher]

Mine doesn't hold pressure for long at all, maybe 10 sec. I still need to figure this out by checking injectors, regulator or fuel pump. BUT it runs and starts great. If I didn't have a fuel pressure gauge I would never know there is a problem. Might look to other causes for your problem.

If you have an in-tank pump, you might check the hose connections.

I had the same condition and it left me stranded one day.

Turned out that the hose connectors were very loose so it wouldn't hold pressure. The hose finally came off.

 

[casadejohnson]

Todd, I found myself looking for an answer to the same question a while back and was never able to find any "Official" time that fuel pressure should be maintained. I was however given a rule of thumb from two different mechanics ( One a Ford tech and the other a VW/Audi Master tech ). Both told me that you should maintain 20 psi for 20 minutes.

 

[73azbronco]

Working on BMWs, perfect world, you could shutdown, come back next day and still have 40psi. Real world, if it held more than ten after an hour it was OK. But, you had a leak, injectors, fuel pump, regulator.

 

[pbwcr]

One thing my Bronco does is not start on the first try, if I cycle the key once it fires right up,

=====
Fuel pumpo going south

 

[ransil]

=====
Fuel pumpo going south

FP is OK it's electrical.
My first conversion probably a rookie mistake made somewhere.