|
Why do I have high fuel pressure...
First let me say that this by the dash gauge, not verified by checking the pressure with a pressure gauge. But, assuming it's not the gauge...Kirkham with Southern 427. Single Holley 4160/vac secondaries (Holley #0-3310C). Both mechanical pump and electric Holley.
I've owned the car for nearly 15 years and it's always run about 6 PSI at idle with either the mechanical or the mechanical + electric. On the road, same until about 3K RPM when, under load, the mechanical pump falters, dropping the pressure to 2-3 making the car stumble. Switching on the electric pump solves that problem and the pressure returns quickly to 6 and remains there, as long as the electric pump is on. Been that way forever, but now.... The car idles and cruises at 9 PSI, e-pump off. But still stumbled until I flipped on the e-pump and then it ran great. Weirdly, the gauge then showed about 8.5...a sl. tick lower, it seemed to me. Also, my idle seemed to have dropped from about 1050 or so to 850-900...causing the car to die without increasing the idle speed back to 1050. Any thoughts, I understand high blood pressure way better than I understand high fuel pressure. thanx steve meltzer |
There seems to be some conflicting data there - in one place you said it runs about 6 psi at idle with mechanical or electric pump. And later it idles and cruises at 9 psi. Or did it used to run 6 psi in past years and now it's running 9 psi?
I assume this is a mechanical fuel pressure gage? If your pressure is falling off to 2 - 3 psi at 3000 rpm you either have a mechanical pump going bad or some restriction somewhere. Also, possibly your tank or cap is not venting and it's taking the electric pump to pull it from the tank and push it to the engine. When opening your fuel filter do you notice any pressure being released? |
Quote:
I would first test the mechanical pump for both pressure and volume. The way you do this is right out of the service manual. Put a new pressure gauge, petcock, and tube in line right before the carb. Run the engine and, of course, you should have 4.5 to 6.5 psi. If it passes that test, then check the volume. Spec's call for it fill a pint jar in 20 seconds. |
"There seems to be some conflicting data there - in one place you said it runs about 6 psi at idle with mechanical or electric pump. And later it idles and cruises at 9 psi. Or did it used to run 6 psi in past years and now it's running 9 psi?
I assume this is a mechanical fuel pressure gage? If your pressure is falling off to 2 - 3 psi at 3000 rpm you either have a mechanical pump going bad or some restriction somewhere. Also, possibly your tank or cap is not venting and it's taking the electric pump to pull it from the tank and push it to the engine. When opening your fuel filter do you notice any pressure being released?" Sorry about the confusion, but yes it used to fine (except for the drop in fuel pressure, occurring only under load, and solved by kicking on the e-pump....never understood why that has always been that way) meaning running about 6 PSI. The increase to 9 PSI is a new problem and my concern. No, opening the fuel cap doesn't result in an audible sound. I'll probably have to check the pressure using a fuel gauge but looking for other causes, than a bad gauge. thanx. s |
And how are your two pumps plumbed? In series, in parallel? What size are your lines?
|
The rear mounted e-pump feeds into the mechanical pump. I would call this series and I dunno the ID of the lines, but they're all original. They are braided; I can measure the OD when I'm next out there. They feed a fuel log/rail with dual feeds to the Holley.
|
Quote:
|
I second the mechanic fuel pump not putting out 9 psi. Gauge has got to be reading a little high. Holley high volume mechanicals shut off at 7.5-9psi. Me thinks you should bag the mechanical pump and go with the electric or vice versa. I can't imagine the need for a redundant fuel system like that in which you have to physically flip the switch to make the car run right. I'm sure it's possible and may be your case but I don't recall where I've ever read that a mechanical fuel pump failed on a Cobra, simply because they are not daily drivers and don't see that much action.
Maybe I'm missing something, anyone else out there have two fuel pumps? I can see where one might run a second electric pump with a dedicated line from the tank to a fuel nitrous solenoid, but two pumps to a vacuum secondary carburetor for normal or spirited driving seems like a lot of ambition. Simplify |
Probably won't get to this for a week or so, but will post the data. thanx so much. All of this makes sense to me. thanx again. s
|
Quote:
I've owned the car for nearly 15 years and it's always run about 6 PSI at idle with either the mechanical or the mechanical + electric. Perfectly normal; as it should be. On the road, same until about 3K RPM when, under load, the mechanical pump falters, dropping the pressure to 2-3 making the car stumble. Switching on the electric pump solves that problem and the pressure returns quickly to 6 and remains there, as long as the electric pump is on. Been that way forever, but now.... Hmmmmm, maybe the mechanical pump is collapsing the rubber fuel line coming in to it, thus causing the output pressure to drop to 2 to 3. Then when you switch the electric pump on it opens the collapsed rubber line back up and the pressure is restored. That would do it... and it would be a damn tricky gremlin to find. ... and now The car idles and cruises at 9 PSI, e-pump off. But still stumbled until I flipped on the e-pump and then it ran great. Either the gauge is wrong or the mechanical pump is now broken. If you have a little Go-Pro camera, it would be interesting to see if the rubber supply line, either right by the fuel pump or maybe back at the tank itself, is collapsing on load when the electric pump is off. But, regardless, we'll figure it out. There's not that many things it could be. |
Yep Patrick, you got it perfectly. I'll get to check the pressure ASAP, but I don't have the GoPro, tho' i do know what it is! If it's the mechanical pump, maybe I just put a plate over it and run the line to the fuel rail. Do they make sucha plate so I can just ditch the mechanical pump....tho' it's nice insurance should the e-pump fail. thanx again. more later. s
|
Sure - it's just a block off plate. Like this: https://www.summitracing.com/parts/p...view/make/ford but the next time you stroll through the garage, bend over and see if you pinch the black fuel hose going to the mechanical pump close with your fingers. If it feels "squishy," then it may be collapsing on you.
|
Quote:
I'm just saying ...gremlins do exist with a 15 year old pump... |
Quote:
|
OK, the pump is only about 5 years old (the original Holley died), maybe 3-4, and the failure of the mechanical pump (which only occurs under load...can't replicate it revving it up in the garage) antedates that.
I installed the engine and all, with the help of my friend who's an excellent mechanic, so I don't think it was junk hose to start. Thinkin' about bypassin' the mechanical pump, but let me get out there (not sure when!) and at least see if it's the gauge. thanx for all of the good help. steve |
Patrick brings up a good point about any rubber line feeding into the mechanical pump could be damaged from Ethanol and collapsing under suction while the electrical pump pressurizes it and forces it open. Something still wrong with the 9 PSI however.
Yes, feeding a mech pump with an electric one is not unusual. I've had that arrangement on a Corvette for 25 years now. You can use the electric to prime the carb before starting if the car has been sitting a while. |
I've got to square that (a collapsed hose) with the fact that it's been this way from day 1 (and I'm the original owner). When I turn the car to the "on" position, before the 9 PSI fiasco, the initial fuel pressure reading was 3 PSI (regardless of how long the car had been sitting). I too, always kick on the e-pump and watch the gauge until I have 6 PSI, then start the car. Thanx for hours of entertainment here...i'm learning a lot and appreciate the schoolin' s
|
Quote:
|
Quote:
|
Quote:
|
| All times are GMT -7. The time now is 09:50 PM. |
Powered by vBulletin® Version 3.8.0
Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
Search Engine Friendly URLs by vBSEO 3.6.0
The representations expressed are the representations and opinions of the clubcobra.com forum members and do not necessarily reflect the opinions and viewpoints of the site owners, moderators, Shelby American, any other replica manufacturer, Ford Motor Company. This website has been planned and developed by clubcobra.com and its forum members and should not be construed as being endorsed by Ford Motor Company, or Shelby American or any other manufacturer unless expressly noted by that entity. "Cobra" and the Cobra logo are registered trademarks for Ford Motor Co., Inc. clubcobra.com forum members agree not to post any copyrighted material unless the copyrighted material is owned by you. Although we do not and cannot review the messages posted and are not responsible for the content of any of these messages, we reserve the right to delete any message for any reason whatsoever. You remain solely responsible for the content of your messages, and you agree to indemnify and hold us harmless with respect to any claim based upon transmission of your message(s). Thank you for visiting clubcobra.com. For full policy documentation refer to the following link: