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V8 Fuel Pressure


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Hi,

Can anyone confirm how long the pressure should remain in the fuel lines after the engine has been switched off,I am loosing most of the pressure after 24hours and I'm not sure if this is correct or not.

I have checked both the primary and secondary injectors for leakage all is okay here,the pressure regulator has been replaced,so if the pressure is dropping then it has to be via the pumps which I assume must contain a flow back device.

I obviously want to perform a pressure test but don't have a gauge,I am in the process of making one when I can find a source for the schreader port check connector.

The symptoms with the car have been difficulty in starting ie excessive cranking,I am suspecting fuel problems as the car has recently had new plug leads,plugs and coil packs.

Any help would be appreciated.

Dave

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do both pumps run on starting ? ..as far as I know there is a note in the workshop-book that both pumps are working together on 'start' , and on 'fuel dump' situations with engine getting hotter

if you turn off your ignition, and manually override the relay soccets of both pumps with paperclips (relays are RH side rear trunkarea .. ) wile you turn your key into start again and let the engine start ..does it start faster ?

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Hi Gunter,

Thanks for your input

Yes you are right both pumps run on start up ,the secondary one for exercising purposes and thereafter only with speeds above 130 mph

The secondary pump does draw current for a few seconds but without a pressure gauge to monitor what is going on in the fuel rails during starting I am only guessing at the moment

I managed to find some shrader fittings and a 0-100psi gauge which are waiting to be collected from the local post office,I am going to pick them up shortly and construct the gauge tonight then hopefully I can take some readings of the fuel pressure plus the sustained duration

Thanks

Dave

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I don't think they should hold for ~24 hrs...

The manual states that a pressure drop of 6-12psi, after the pumps stop, is normal.

On my 89SE I often turn the key twice to prime the pump before starting the car, to make sure the rail is fully pressurized.

Travis

Vulcan Grey 89SE

 

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Okay, I have tested the fuel pressure and it is reading 55-60psi after the 3 second prime,there is no noticeable drop after the pumps stop,the pressure is more or less the same whilst driving the car

I tried you're method Travis,but as the pressure was up to spec after the 3sec prime it didn't make any difference

Conclusion,my diagnostic skills need a bit polish up and the problem appears not to be related to poor fuel pressure

Back to square one!!

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so at least you know that your first thought about *drop in pressure* is not right -what in opposition is with a 'drop in current/voltage' in you system while starting the car ??

as we all know our cars are mainly plastic, and as so it means there are a lot of additonal wires fitted to aid the current going its way. Inspect how much your battery goes down in voltage if you turn the key and the starter turns over, maybe it just needs to much electricity and the ECM shuts off ? ..you can also undo all the cables and contacts on the frame to engine (big cable on lower block end & rear engine side the small ones for the ECM/injector ground) and inspect the oxidisation there on the surfaces. Same is for inspecting the starter itself -undo the plenum and undo the starter -you can dismantle the starter of the V8 easylie by seperating the two main section [ gear drive/ rotor ] ..have done this years ago and turned out to have a fully oxidised and wet starter, covered with leaves and having worn brush contacts ..long story, easy repair!

*********************************************************************

to name the things if I see them, that's what I call integrity..

*********************************************************************

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Hi Gunter,

Some valid points and I will check them out

When starting the engine does crank over fast and steady,it used to turn over for a few seconds then burst into life,now it's as if the cylinders are joining in one by one,this is why I suspected fuel pressure

I am now thinking the injectors may not be pulsed long enough during starting possibly due to one of the critical sensors MAP,BARO,TPS,COOLANT and IAT giving a slight incorrect reading but not enough to trigger the MIL

On a side note I changed primary injectors last summer for the FiveOmotor sport EV2 24lbs 240cc types

If the starting problem persists I am going to put back in some OEM Rochesters ,my originals have long gone but good old Mike S is coming to the rescue yet again

Regards

Dave

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  • 2 weeks later...

Hi Dave,

many thx info, now I know the maker and the number. (Just found out I can get it locally). The reason I asked because I didn't want to buy a complete midlock set of more than 300 euro's which is suited for almost every car.

Best regards

Freek

Esprit Freak

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  • 2 months later...
  • 1 month later...

I was having to double prime (with the ign key) then crank for 3 or so seconds until It started. I too noticed that the fuel pressure was leaking down quickly. I just replaced the stock Rochestors with the RC Eng SL2-240 (240cc) Lucas injectors and now my cranking time without allowing the full 3 second key-on prime is around 1 second - huge improvement.

I wonder if the regulator is allowing fuel to "return" and not holding the presure - pressure bleeding down via the "return" circuit (The regulator is a very simple, not very high quality looking part)... If it's not the regulator & not the injectors then it would have to be the pump - assuming there are no fuel line leaks, right?

I think it's the regulator - I'm going to fiddle with mine - possibly replace with high quality unit.

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Eddie, just bock your pressure regulator with something like a ball from an old bearing --to see if it helps starting

..but be carefull, don't let the pump 'prime up' fuel to long until you start the engine ;)

*other more easy way -use a clamp and lock up the return pipe ..

Edited by Günter

*********************************************************************

to name the things if I see them, that's what I call integrity..

*********************************************************************

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