O-320 fuel pressure?

What should the fuel pressure be on a carb'd O-320? We fired up the engine and have Vision micro guages and am indicating around 8 psi, this was the redline and seems too high.

Buz

buz said:

What should the fuel pressure be on a carb'd O-320? We fired up the engine and have Vision micro guages and am indicating around 8 psi, this was the redline and seems too high.

Buz

Click to expand...

It IS too high, at least if your carb is the same as mine. I rebuilt the carb because it would occasionally leak fuel. That didn't help so took a look at the specs and the top limit was 6.0psi and the fuel pump would shoot up (at times) to 8.3. I replaced the fuel pump with a rebuilt one from Aircraft Spruce--problem solved. If your carb overflows, it is a definate safety issue. A rebuilt is less than $200.

Bob Kelly

My fuel pressure (Marvel-Shebler MA4-SPA) runs about 3.5-4.5 on engine pump. 5-6 with boost.

2-9 comes to mind as the allowable range. But I can't find my Marvel-Shebler book right now. The O-320 Type Certificate Data Sheet might have the range.

That being said, I agree that you'd want to operate somewhere in the lower part of the range if possible.

The O-360 uses 2-9 I believe. Don't know about the O-320.

Spruce to the rescue

Per ACS, their replacement for the LW15472 fuel pump (standard on the O-320) is rated at 4-6 psi. Their number is 41271. I bought a rebuilt one of these and my FP shows about 5.6. I think the carb is rated for 6.0 max input pressure, although some were rated for 8.0 max (actually 0.5-8.0 psi.) You can pull this up on Spruce's catalog.

Bob Kelly

This plane never had a fuel pressure guage until now, we installed a new panel. The engine had not been run in awhile. Does the fuel pump have an internal regulator, when it fails does it fail to higher pressure?
The engine ran fine on the ground for 15 minutes or so.
I have a feeling its always been this way but didn't know because of no guage.

Buz

FP reg

Buz,

My fuel pressure problem came after the engine had set for about a year. When I finally got around to changing the pump, I pulled the old one apart, just to see what was inside. Our local A&P said he had never seen the inside of one as there are no repair kits; it is strictly a replacement item. If there is any regulator inside, I couldn't find it. The pressure seems to be regulated by the spring pressure on top of the diaphram. I could be way off on this, but that's how it looked to me.

Bob Kelly

We are first gonna verify the Vision reading with a mechanical guage, and go from there. I wonder how many planes with no guages are flying around with 8 psi pressure?

Buz

Very high probability it is your Vision Micro gauges and/or sender and quite likely some grounding issue. My Vision Micro EPI-800 fuel pressure gauge has a mind of its own. Works fine about 70% of the time and the rest of the time it is pegged. I've spent time chasing it down and it is either an intermittent sender or also there is a battery chip in the main brain unit that needs to be replaced every five years (I think that is what VM recommends but don't hold me to that) and mine has been in there 10 yrs. I spoke with a tech rep at Lycoming about 5 yrs ago and was told the fuel pump will not fail high due to a pressure relief valve. Designed that way so the fuel pump won't fail high and flood the carb. All this is with regards to my standard carbed O-360. A Lancair on the field with the same equipment has had similar symptoms with his VM gauges.

Failing high

Yup, I had four A&P's tell me they couldn't fail high, but I KNOW they do! We did the external FP guage test and it confirmed it. Haven't had a fuel overflow or high reading since I swapped out the pump. One common factor here (as I understand it) is that both of our engines had set for a year. Mine had set prop end up for a month or so with the fuel pump full of oil above the diaphram, which may have been a factor.

Bob Kelly

Yes the plane did set for awhile, but it was on the plane in the "normal" attitude. Won't get to mess with it until next week, but I will let you know the verdict.

Buz

Buz, I was searching the older Matronics RV lists and found the following regarding high fuel pressure:

"We've seen this in Grummans in the past. A blocked breather will cause high fuel pressure because it biases the mechanical fuel pump diaphram. "

Maybe one more thing to look at.

resurrecting old thread

Relevant to my interests

This breather - where is it on the pump? Same as the overflow tube that drains fuel if the diaphragm leaks?