O470K Fuel Pressure Problem

N

nshetter

New member
I am getting no fuel pressure after starting with the wobble pump and primer. Things look normal and then the presure drops to zero and engine dies. Engine runs fine if I use wobble pump. Plane has been sitting for 45 days. Pressure was normal on last flight. Fuel pump was overhauled 5o hours ago. Is there a pressure relief valve in the fuel pump that I could have set off with the wobble pump? My understanding is that this pump is a vaccume pump on one side and low pressure pump on the other. Could there be some sort of air lock or loss of "prime" on the suction side. At one time I had a bad fuel line fitting and found that the pump was sucking air into the system causing erratic flow. Any comments or suggestions are welcome. North
 
North,
I had a similar problem when I bought mine but it would run with very low pressure, and no pressure at full power. Soon as the power was backed off the pressure came up to the minimum. I ferried it that way because the seller had flown it that way for years, but it was sucking air the whole time. I believe it was bad gaskets at the strainer. I replaced more than one thing at a time, and the pressure problem went away. So I had to pick the one that looked like it was the problem and it appeared to be the strainer gaskets.
Have you double checked the fuel selector vavles? (Aux's off so you cant suck air ) that subject was discussed lately also. Hope you find it too be easy. Brian
 
While we're on this subject, I loose fuel pressure on the second start with a hot engine, especially on warmer days. Unless I turn on the the fuel pump once for 5-10 seconds (or formally used the wobble pump) to regain fuel pressure the engine would quit. Once fuel pressure is regained operations are normal. I always assumed this was normal and caused by a vapor lock. I've gotten use to it. I never thought to ask if this was a common experience with this fuel system? If so has anyone found a solution?
 
Glenn,
Never have had that problem.Hot or cold starts are normal and it has never lost pressure or quit. I have a stock wobble and stock system as best as I can tell. But I do always establish some fuel pressure with the wobble before any start. Just the way I do things, maybe that has bypassed the issue you guys are speaking of. I also used to use the accelerator pump for hot starts, until a previous forum discussed the dangers of that. Brian
 
Hello All - My problem is not related to starts is is TOTAL lack of fuel pressure after starting. Engine starts normally with pressrue from wobble pump but then dies unless "boosted" with wobble. My guess is an air leak but am not sure where to look or how to trouble shoot . - North
 
North, have you tried the pressure test by using someone else to apply continuous wobble pump pressure to the system ( which is normally under suction while running) and you look under the cowl for leaks. That will at least test everything from the wobble to the engine driven pump. Then check to see if you probelm changes on different tank selections. Brian
 
Thanks to all for your input. i will not be able to spend time on trouble shooting until after the Valenintes day weekend. I will keep you posted on what I find. I also suspect some sort of air leak but find it hard to understand what "changed" by a 30 day layover. North
 
Thanks for reminding me to reply. i did use the positive pressure test approach, first with wobble pump and then with air pressure. I found two fuel lines with tiny pin hole leaks. there was no evidence of fuel leakange . One line was from firewall to gascollator, one from cascollator to carb. These lines had been replaced about 8 years ago by previous owner but not noted in logs. I have now replaced ALL fuel lines firewall forward. I replaced all the lined from takns to firewall when I bought the plane.

Botom line for me is when fuel flow is not normal look for a fuel line leak. I made up a simple purssue test device. Remove line in question. Cap one end. Attach low pressure air line via plumbing fittings and wipe with soap/water solution. Leaks were obvious even thogh line looked like it was brand new.

Kind regards to all - North
 
Thanks for the answer. I think for the test to work with the wobble pump, the presure has to be constant for a while, requiring two people. I guess that it would take a while for the fuel to make a wet spot big enough to see when the holes are pinholes. glad you found them. Brian
 
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