Mechanical Fuel Pump increasing pressure

Put a gauge on it and run her up to WOT.

 

I put a gauge in the line today. I didn't run it up at speed however though. At idle after a few seconds, the pressure quickly increased and was bouncing between 12 & 16 PSI with the pulses. I removed the pump and pulled it apart. All looks in place however I am not all that experienced with what to look for in this instance. Is there supposed to be any kind of relief valve in these? Or is the thought that the float will open relatively quickly and fuel pressure would then be relieved when the needle opens?

 

A typical, relatively modest tune MK IV should run 5 - 8 PSI, carbureted . This wouldn't be the first time I've installed a new part, to find it wasn't working right. I think you're right and should consider the pump suspect.

I'd reinstall the regulator and run 5 PSI, knowing it'll starve at higher RPM's then jack the pressure up until you get it, without causing flooding at idle. Did you look at the needle with a magnifying glass?

 

Yeah, I did look at the needle, all looks fine. It's clean, even, and looks perfect. The problem I have it this regulator goes up to 6 PSI max. I've run it with the regulator maxed out at 6PSI and it still falls off at 3400 RPMs. I can't get the pressure to increase any more with this regulator. The weird thing about this is that the pump is about 6 seasons old, why or how can the pressure increase?

 

The pump is sounding more and more like the culprit.

 

yep... I agree. new pump on it's way. I'll be sure to update all when I get it installed. So much for boating 4th of July weekend... not the end of the world, too many yahoos out there anyway.

 

Well, this is pretty common, but I'm of the belief that PWC are to be run down at every opportunity. In fact, I'm fairly sure it's permissible in Florida.

 

New pump installed last night... All is well. So to answer the original question, yes a mechanical fuel pump can increase pressure over time. Thank you all for the contribution as always.

 

hard to understand how the spring got firmer?

 

There has to be more to it then that.

 

fuel pump was likely for throttle body injection(TBI) application.

 

No, the 812454A1 Mercury part doesn't produce the pressures need for typical TBI systems. This particular pump can't even manage 8 PSI and though lower pressure TBI systems exist, they need about twice this pressure to function.

 

Par's correct. That was the mercruiser spec 1991 7.4L 4 barrel pump. It worked fine for 6 years, then the damn thing decided to start putting out double it's rated fuel pressure. I'm guessing that the pressure had to have been increasing gradually over the past few years. This year, the pressure finally became too much for the needle to hold.

 

If the pump had some gunk around the edge of the diaphram on top that effectively reduced its diameter the same spring underneath it would create more pressure and less volume
Possible or not?

 

That's what I was thinking, the displaced volume under the diaphragm decreased over time with deposits, but the stroke and power provided the pump remained the same. It's compression ratio increased slowly, until it was pumping way more than necessary. I'm surprised the needle tolerated the pressures it did. I'll bet it was one of those rubber tipped needles.