I've got an off idle stumble I'm trying to fix on my Carter AVS 383 auto & I put a fuel pressure gauge on it to see what it's getting. It shows about 4.5 psi on cold start, but then slowly drops down to about 2 psi by the time the engine is warm. Revving the engine makes no difference. It's one of those Jegs brand gauges & I've never used one before. Any ideas? Can a mechanical fuel pump actually lose pressure as the engine warms?

couple of things it could be.
1. Worn fuel pump pushrod
2. Do you have a vented gas cap or adequate venting from the tank?
3. Todays crappy gas with Ethanol could start to boil

First any pump that's delivering 4.5 lbs should be in the trash can, second check the length of the fuel pump rod for wear, and as long as you removed it to compare to a new one, install the new one..peace of mind, plus elimination

I'll try loosening the fuel cap to see what happens to the pressure. That's about the only reason I can see for the pressure to drop as the car warms up. I'd expect a worn pushrod to have the same reduced pressure whatever the operating temperature.

If you are using a liquid filled gauge, the gauge may be reading lower as it warms up. This is common with liquid filled gauges. The actual pressure may not be dropping.

Yes, it's a liquid-filled gauge. So, these gauges are only accurate when the engine is first started, then, right?

Why would they do that? I assumed that the liquid is only to keep the needle from fluttering.

In theory, the liquid reduces needle vibration. In reality, it doesn't make a big difference. In fact, most pressure gauges use a restriction on the inlet to reduce the needle vibration. This does a lot more than the liquid.

To keep the liquid from leaking out of the gauge, the gauge is sealed with a plug. As the gauge warms up, the heat increases the pressure inside the gauge. As the pressure builds, it makes the gauge read lower. To test this theory when the gauge is warm and reading low, "burp" the gauge by cracking open the rubber plug to let the pressure out. If the gauge then reads higher, you've found the problem. You can then solve it by removing the plug and draining the fluid. Don't put the plug back in.

Thanks for the explanation! I'll try messing around w/it once the temp in the garage gets below 100 degrees.