gauge dies after working initially

[SteveO_71]

On my 71 mach 1, 351c 4v engine, I have a fuel pressure gauge between the mechanical fuel pump and the carb. Initially when new, it reads in the 6 - 8 psi range or thereabouts.
BUT, at some later time it stops working for whatever reason. I have fuel flowing, car runs fine but gauge reads 0.

I realize that there is a lot of vibration in the engine area as well as any mechanical fuel pump fluctuations. With those issues in mind, I usually buy liquid / oil filled gauges.

Shouldnt a fuel pressure gauge work when used in that location?? Is there a particular gauge that can be recommended for this application? I have tried different manufactures with the same result.

Thanks!

 

[Cleveland Crush]

Liquid filled under hood gauges are notorious for giving inaccurate readings when they get hot. If you very lightly press on the gauge lens while the engine is running sometimes it will get the needle back in action. My under hood fuel pressure gauge (liquid filled) agrees with my in cabin pressure gauge (used with a fuel isolator) when cold. When the under hood gauge gets hot it acts up. I've been through a couple different oil filled fuel pressure gauges with this issue.

 

[mrgmhale]

Liquid filled under hood gauges are notorious for giving inaccurate readings when they get hot. If you very lightly press on the gauge lens while the engine is running sometimes it will get the needle back in action. My under hood fuel pressure gauge (liquid filled) agrees with my in cabin pressure gauge (used with a fuel isolator) when cold. When the under hood gauge gets hot it acts up. I've been through a couple different oil filled fuel pressure gauges with this issue.

We have a fuel pressure gauge, bourdon tube, not electrical) on the metal fuel line that feeds out Holley 750 CFM Vacuum Secondary carb. It is always rock solid in what it shows. I have never tried to run an electrical fuel pressure gauge in these older pony cars, but I can see the appeal of not having liquid fuel being run into the car's cabin becoming a concern. What Cleveland Crush reports re: the electrical fuel pressure gauges not behaving well when they get too warm at first caused me to cock my head for a moment. But, in his prior writings I have found him to be beyond casually knowledgable, and not one to point to a situation and say such and such is the problem without a plausible reason and/or calling on his personal observations as in this case.. So, I take him at his word, in this case also.

That said, If you are perplexed by this and want to chase it down a bit further my first suggestion would be to check all connections related to the fuel pressure gauge. I do not know if using a tee pipe feeding the electrical sending unit to connect to a bourdon tube mechanical pressure gauge under the hood would be worth the while, but if I was trying to verify what was happening that is what I would be doing. I would also get a 2nd fuel pressure gauge to wire to the sending unit under the hood on a temporary basis, enough to check the pressure reading under the hood when watching the mechanical underhood fuel pressure gauge. Then when the pressure gauge in the car begins to act up connect the 2nd electrical gauge and see what it does, as well as look at the mechanical gauge for what it is doing. With all that going on there ought to be enough "magic" going on to help you identify the cause of the problem.

For me, frankly, if I was not racing the car on a track I would not bother with a fuel pressure gauge on the dash, although I am sure it looks pretty cool having it there. I do like having the mechanical gauge on our Holley fuel inlet tubing, first because it looks cool, and second were I to run into a fuel pressure related issue it is one less thing for me to have to hook up diagnostically...

Gawd these are fun machines to tinker with!

 

[SteveO_71]

Well the gauge i'm currently using is not electrical so I thought the simplicity of the set up would make it even easier to troubleshoot. The gauge is screwed into an NPT connection tee that is installed in the rubber fuel line. No sending units or electrical connections so it sounds almost like the Holley 750 set up you are using.
What manufacturer is the gauge you are using underhood?

 

[Cleveland Crush]

Hi Steve,
I have two mechanical fuel pressure gauges, under hood (Mallory) and in cabin (Stewart-Warner). Just my experience is sometimes, under hood liquid filled fuel pressure gauges can give false or no reading when the liquid gauge gets hot. On your non functioning gauge push gently on the gauge face, this action may free the internal diaphram via increased pressure and get the gauge needle reading again.
These photos show my two gauges agreeing with each other when the engine (and under hood gauge) are cold and not yet at operating temperature.

 

[Hemikiller]

Liquid filled gauges need to be vented to compensate for underhood heat. I have the Aeromotive gauge, works just fine.


https://www.summitracing.com/parts/aei-15632

 

[mrgmhale]

Well the gauge i'm currently using is not electrical so I thought the simplicity of the set up would make it even easier to troubleshoot. The gauge is screwed into an NPT connection tee that is installed in the rubber fuel line. No sending units or electrical connections so it sounds almost like the Holley 750 set up you are using.
What manufacturer is the gauge you are using underhood?

I ordered my gauge from Holley. Small dial. Once back home I will send a photo via followup post. I looked at various videos and photos while remotely accessing our drive server - nothing clear. Drag.

 

[mrgmhale]

Okay, I have a photo that I annotated I hope you find helpful. There is also a link showing a fuel line and gauge set similar to what I have:

https://www.cjponyparts.com/mr-gasket-fuel-line-chrome-with-pressure-gauge-for-holley-4150-carburetors/p/FL02/?year=1961&gclid=CjwKCAjwvdajBhBEEiwAeMh1UzjfpEIz4XEzWEhSCZIIQFqZAWjt0WgnfXw_4fJuh8VHNmC5mT9XBhoCcmIQAvD_BwE