I would start out by replacing the gauge. Assuming you are talking about a regular mechanical gauge with 1/8" NPT male pipe thread sticking out the back side, you could get this gauge from STM for $20. Last I knew this was a Marshall gauge, which is a good brand. Anyway, whatever they send you, I think it will be ok. If you wanted to for sure get a Marshall gauge, you could email STM to ask about it, or you could shop for a "Marshall Instruments CW00100 Liquid Filled Pressure Gauge" which is the white face w black lettering version of this gauge. The Amazon price on this CW00100 gauge has doubled (to $40) since I bought one in July of 2023. But other places sell them for $25 to $30.

Anyway, it seems most likely to me that your old gauge now has a broken mechanism inside, maybe even a leaky Bourdon tube (but I think that would cause a visible external fuel leak), so it's shot, and I'd even consider it to be possibly hazardous since it's fuel we are talking about. I'd replace it with a pipe plug.

As far as the Fuelab AFPR is concerned, it's probably just fine. Mine is old, it's been on the car since 2009. It still works fine and I never have to readjust it. I've never replaced the diaphragm. Every time I check it, it's still running at the same pressure. I don't leave the gauge in mine all the time. For normal driving I have that port closed off with a pipe plug.

We're on Boost said:

I would start out by replacing the gauge. Assuming you are talking about a regular mechanical gauge with 1/8" NPT male pipe thread sticking out the back side, you could get this gauge from STM for $20. Last I knew this was a Marshall gauge, which is a good brand. Anyway, whatever they send you, I think it will be ok. If you wanted to for sure get a Marshall gauge, you could email STM to ask about it, or you could shop for a "Marshall Instruments CW00100 Liquid Filled Pressure Gauge" which is the white face w black lettering version of this gauge. The Amazon price on this CW00100 gauge has doubled (to $40) since I bought one in July of 2023. But other places sell them for $25 to $30.

Anyway, it seems most likely to me that your old gauge now has a broken mechanism inside, maybe even a leaky Bourdon tube (but I think that would cause a visible external fuel leak), so it's shot, and I'd even consider it to be possibly hazardous since it's fuel we are talking about. I'd replace it with a pipe plug.

As far as the Fuelab AFPR is concerned, it's probably just fine. Mine is old, it's been on the car since 2009. It still works fine and I never have to readjust it. I've never replaced the diaphragm. Every time I check it, it's still running at the same pressure. I don't leave the gauge in mine all the time. For normal driving I have that port closed off with a pipe plug.

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Thank you very much! It seems like an odd problem to have a directly coupled gauge go bad.

No, there is no external leaking, but the car isn’t being driven for a couple more weeks pending some maintenance. Plenty of time to get a new gauge installed.

It will be interesting to how accurate the old gauge was when it appeared to be working following the fuel pump replacement and rewire.

We will report back and mark this Resolved when we know for sure it was the gauge and/or APR.

Keep in mind that it's impossible for the car to "idle and drive well" with 0 fuel pressure. That's why I think the gauge is bad. You can check out the regulator when there's a good gauge on it.

I was maybe a little unclear about the possibility of the gauge being a leak. If the bourdon tube is cracked or popped, then I think you'd have a leak. But there could be some other type of mechanical failure in there that wouldn't cause a leak.

Also, if you take your gauge off and look at the ID of the 1/8" NPT fitting on the back, you'll probably see that the ID is tiny. It will either be literally a pinhole, or it will have a screw in it that looks like it is blocking off the hole. An opening like this would get clogged pretty easily. So you could look at that, but I doubt it would ever be clogged thoroughly enough to make the gauge just read 0 all the time.

DogWhistle said:

It seems like an odd problem to have a directly coupled gauge go bad.

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Yeah, it’s probably not very common unless you are talking about water systems, like especially swimming pool pumping systems. Gauges in those systems sometimes go from corrosion. Probably because of the chlorinated water. Here’s kind of a fun video by a person taking care of their swimming pool, showing replacement of a leaky gauge. It’s a mess . Now I'm sure your gauge doesn't look like that! But the point is these things can fail and they are not very fail-safe.

Just for fun, I googled “Did aircraft in the WW1 and WW2 era sometimes have to emergency land because of a popped bourdon tube in their oil pressure gauge, which let oil leak out?"

I got a pretty cool ai overview which among other things told me why the orifice at the inlet of the gauge is so small. It’s what I thought. It might also reduce fluttering when there are pressure pulsations. The “liquid filled” reduces pulsations from whatever cause (like heavy vibration).

This is the gauge I have in my car. Works perfectly. That is definitely a bad gauge as stated.

diambo4life said:

This is the gauge I have in my car. Works perfectly.

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Yes, a Marshall, and only $26 from Amazon. Good!

We're on Boost said:

Also, if you take your gauge off and look at the ID of the 1/8" NPT fitting on the back, you'll probably see that the ID is tiny. It will either be literally a pinhole, or it will have a screw in it that looks like it is blocking off the hole. An opening like this would get clogged pretty easily. So you could look at that, but I doubt it would ever be clogged thoroughly enough to make the gauge just read 0 all the time.

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Thank you for the caution of the gauge opening being so small. New Marshall gauge is on the way and we will definitely make sure the pinhole opening is clear prior to the installation.

Update: New Marshall gauge arrived and installed. These are very nice gauges.

Made sure the narrow passage was clean and clear and installed it on the regulator.

When we fired it up, the pressure jumped to 62psi! Apparently the old gauge had been faulty for some time.

Dialed it back to an idling 42 and it idles and revs so smooth. Probably the best it has in 2 years. Pressure stays steady no matter the engine temp.

Test drive soon to verify road manners. The fuel pump rewire is so nice to have compared to the double and variable relays of a stock 4G64.