16 replies to this topic

[Johno]

Novice mechanic here. Holden 355 red motor.

I have a mechanical fuel pump and want to check fuel pressure.

I have a vac guage that will check it but how do I go about it?

[axistr]

The vacuum gauge will only be useful to check the suction side. Disconnect the fuel hose coming from the tank and pump fitting and secure the vacuum gauge to the pump inlet fitting. Crank the engine and watch the vacuum gauge. The gauge should quickly indicate a suction pressure, probably around 2-3"mg. If it can't hold this negative pressure over 10 seconds the pump valves could be faulty or you have a leak at the gasket or diaphram. It's generally better to check the pressure side to the carby with a pressure gauge. Disconnect the pressure hose and attach the gauge. Again crank the engine and the gauge should read around 8-12 psi and hold pressure for 30 seconds as a minimum.   

[Johno]

Thanks Alister, I thought it should be no more than 6psi on a stock fuel pump?

I’m attempting to rule out fuel pressure too high causing a rich condition.

[Bruiser]

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[Johno]

It’s running a 600 edelbrock so 6 is about max.

[Johno]

So I just take off the outlet hose on the fuel pump and attatch the fuel pressure guage hose to the outlet. Crank the engine over.

Will it pump fuel into the pressure guage hose?

I assume I need to leave the fuel pump inlet hose on to maintain pressure?

Do I need to disconnect the coil while doing this?

[axistr]

Thanks Alister, I thought it should be no more than 6psi on a stock fuel pump?

I’m attempting to rule out fuel pressure too high causing a rich condition.

 

Sorry Johno, yes my mistake I was thinking of the figures I used to get checking the pressure on early diesel engine lift pumps.

 

Pressure gauges are generally constructed of brass inside so fuel won't damage them and will quickly vaporize off anyway. Yes just remove the pressure side pipe, connect the gauge and crank the engine over. After around 3-4 engine rotations you should get the maximum pressure reading and should hold its pressure after you stop cranking. There were readily available adjustable pressure reducing valves back in the eighties. They were fitted between the pump and carby in the pressure pipe. From memory (and that getting worse by the day) they were adjustable from 2-6 psi. Needle and seats generally don't like much over 6-8psi so figures around 3-4psi would be what your looking for. 

[LXCHEV]

Hey John,

 

You could also consider installing a permanent fuel pressure gauge into the line. Cheap and readily available. Assuming you have rubber fuel line, just cut it and install a simple brass T piece, and then screw in a small gauge into the T. You can get gauges from as little as $30. They look quite neat... and then you have a permanent troubleshooting tool always there. I've had a low pressure gauge on the Torana (carby) forever to serve this purpose, and a high pressure one on the wife's EH (EFI) for the same reason. Her car used to suffer from a lot of fuel pressure issues before we resolved some lift pump issues etc... so it was very handy to monitor what was going on.

 

If you want something a little more elegant, then go for A/N style fittings etc.... 

Edited by LXCHEV, 06 September 2023 - 10:07 AM.

[axistr]

Just a quick word of advice, I also fitted a cheap permanent inline pressure gauge on the pressure pipe when I was running a Holley carby. After half an hour from initial fitment I smelt fuel. Pulled over and the glass was full of fuel and leaking. I took it back to Repco and they gave me a replacement which lasted around six months which did the same thing. Round three fitted a quality brand which was on it until I removed it to fit EFI. I was just lucky the car didn't catch on fire.

[LXCHEV]

Repco brand or other?

 

I think the one on my car is a Holley - and touch wood - still going strong after a decade.

 

With the EFI one on the EH, think that was Speco from memory - and liquid filled.

Edited by LXCHEV, 06 September 2023 - 03:19 PM.

[axistr]

It was twenty years ago now but they may have been speed pro. I went to my local hydraulic parts sales shop and picked up better quality gauge Aeroflow

probably all made in China.

[Johno]

Excellent advice, thanks all.

The inline fuel guage is on the wishlist but just looking to confirm the pressure at the moment to help get rid a rich condition in the engine.

[axistr]

I don't know to much about your setup (engine). However the first basic checks I would be looking at is, carby float level, low vacuum (leaks or large duration cam). faulty or incorrect carby power valve. Internal carby gasket leaks or incorrectly assembled, incorrectly tuned engine.  At what engine RPM range are you having problems running rich?

[Johno]

It’s running rich just idling at 900.

Carby is off, I’ve cleaned it, will put a kit through it and set float levels.

The carby had some crap in it around one needle and seat but expected to something more significant.

[Johno]

Checked fuel pressure just before the fuel filter. Couldn’t get tester into the fuel pump outlet. It read about 6.2 PSI so that’s fine.

Carby is cleaned and rebuilt, will put it back on and see what happens.

[ozyozyozy]

If it’s running rich at idle, prob needs the idle mixture adjusted, which is quite common with most carbs.

[Johno]

Adjusting idle mixture screws doesn’t solve the problem