Engine Primer Problem

[Greg Bockelman]

Tom, or anyone with an idea,

When my cabin gets hot, my primer does not work well. The hotter it gets, the less it works. It got to the point where it wouldn't work at all. This is in a Cessna 195. I am not the only one who has the problem. But not everyone has the problem. The big issue here is that the engine will absolutely not start unless it gets primed. I wrapped an ice cold wet towel around the primer and it helped just a little bit.

Has anyone had the same problem and if so, is there a solution?

 

[Tom-D]

Do you have the old Lukinhimer(sp) with the large aluminum knob?

 

[Greg Bockelman]

Do you have the old Lukinhimer(sp) with the large aluminum knob?

I think so, Tom. But I am not sure. It is the second one that has been in there. I have replaced the O-Ring and Fuel Lubed it, but that didn't help.

 

[Tom-D]

I think so, Tom. But I am not sure. It is the second one that has been in there. I have replaced the O-Ring and Fuel Lubed it, but that didn't help.

My best guess is that the aluminum knob acts as a heat sink, and swells the piston in the pump cylinder.

cooling the cylinder (pump body) won't help. pull the pump out, and cool the piston and see if that helps. IF so, have a machinist remove a .001" off the pump piston, re-polish, and re-lube.

 

[Greg Bockelman]

Thanks Tom, I will try that. I am not sure how a "swollen" piston will cause it not to draw. It seems that it would work better because the seal is tighter. But I know stranger things happen.

 

[Tom-D]

Thanks Tom, I will try that. I am not sure how a "swollen" piston will cause it not to draw. It seems that it would work better because the seal is tighter. But I know stranger things happen.

Are you saying that you can't pull it out, when it is hot? it may simply be binding in the bore of the cylinder.

 

[Tom-D]

Any restriction in the fuel lines would be there all the time. and can be trouble shot by disconnecting the lines. I'm hoping that this has been done before changing two pumps.

The body of the primer pump is attached to structure which conducts heat away from th body(cylinder) the plunger is isolated from any method of disapating heat so it swells and binds.

easy method of proving this is, the next time it occures, place an ice cube on the aluminum knob and wait. see if it frees the piston.

 

[gismo]

Any restriction in the fuel lines would be there all the time. and can be trouble shot by disconnecting the lines. I'm hoping that this has been done before changing two pumps.

The body of the primer pump is attached to structure which conducts heat away from th body(cylinder) the plunger is isolated from any method of disapating heat so it swells and binds.

easy method of proving this is, the next time it occures, place an ice cube on the aluminum knob and wait. see if it frees the piston.

Tom, AFaIK Greg's problem (as he described it to me last weekend) isn't that the piston gets stuck, but that there is no resistance to the movement of the knob. That suggests that either there is no liquid fuel in the primer or (less likely) one or both check valves are stuck open, or the fuel is getting past the piston.

My guess is that the fuel is being boiled leaving nothing but gas vapor in the primer body when Greg pulls it out. I also pointed out to Greg that if there's any restriction on the intake side of the primer, the fuel will have a lower boiling point when he pulls on the knob and thus lowers the pressure in the line coming into the pump. Steve's suggestion to examine the amount of "lift" (distance that the fuel supply is below the primer pump) and or look for a restriction in the line supplying fuel to the primer is right on if my guess is close.

Greg has said that the problem seems to be related to cabin temp, but I suspect that it has more to do with the ambient air temp ahead of the firewall assuming that the fuel to the primer comes from a gascolater in the engine compartment. He also said that he's replaced the primer at least once but as we know, it's entirely possible that the replacements were as defective as the original.

One thought on isolating the problem would be to temporarily rig a separate supply of fuel to the primer such as a can in the cabin with the primer supply line stuck through the cap. With that Greg could determine whether the temp of the fuel and/or the relative heights of the reservoir and the primer were part of the problem.

 

[Tom-D]

The last I knew the 190/195 was a high wing and had its tanks in the wing. This would give gravity pressure to all components of the fuel system. including the gascolater, and primer pump inlet.

If you have a blockage in the inlet line to the pump, you can pull the primer piston out part way, but it will snap right back in when you let go of it, due to the vacume caused by pulling it out.

If you have a blockage in the outlet line such as dirt in the nozzle, you can pull the primer out, but you can't push it in, because you can't over come the pressure built up in the outlet line.

To have a pump that is free to move in and out, that will not pump, you must have an open check valve. and temp should have no effect on that.

Greg, Can the pump be operated freely?

 

[Greg Bockelman]

OK, let me try to answer some questions here. Tom, the primer plunger moves freely in both directions. It is not binding at all. Either cold or hot. When it is cold, you can feel the resistance when you pull, because it is drawing fuel. When it is hot, there is no resistance because it is not drawing fuel. Maybe it is drawing or trying to draw vapor. I just don't know.

As you all know, the 195 is a high wing airplane with fuel in the wings, so the primer is below the fuel source. The fuel runs from the wing to the fuel selector which is on the sidewall in the cabin. From there it goes to the gascolator on the bottom of the firewall. That is where the primer line comes from. The primer line is a pretty small diameter copper tube that runs up the outside of the firewall. It goes through the firewall and past my oil tank. The oil tank is 5 gallons of 160 to 180 degree oil. The primer itself is about 4 or 5 inches from that oil tank. The primer line then returns to the firewall about the same way it came in. Then out to the spider valve, or whatever you call that distribution block, then to three cylinders.

When I had my inner cowl off, the primer line on the outside of the firewall was not hot to the touch. It was probably close to ambient temp. But the primer still did not work well. What I did was run ice cold water down the primer line, put an ice cold wet rag around the gascolator, and one also around the primer body. The result of that was that I got about an eighth to a quarter stroke of good prime, but not a full stroke. That was enough to get me started. Seems to me if it was a vapor problem, I would not be able to get even that much prime.

I am going to do an oil change this week. When the engine is swung, I intend to take each component apart one at a time and see if there is any blockage. Oher than that, I am open to more suggestions.

Bottom line is that it is kind of a PITA to fly in the summer with this problem.

 

[Tom-D]

When the primer pumps with no fluid flow, there must be some thing wrong with one of the check valves in the pump it's self.

They can be removed and inspected by removing a screw plug on the foward end of the pump body.

Next thing I would inspect is the gascolater, see if it has a bubble of air in the top, where the fuel pick up for the primer line is.

If the primer pump is working properly it should pull any air in the system thru in a few quick strokes of the pump. these single action pumps are as happy pumping air as they are pumping fuel. because they are of the positive displacement type, each time the pump is stroked some thing must move.

next time this happens try stroking the pump quickly a few times see if it will pick up fuel.

or try pulling the pump piston out and see if you get fuel to feed from gravity into the pump.

 

[Greg Bockelman]

When the primer pumps with no fluid flow, there must be some thing wrong with one of the check valves in the pump it's self.

They can be removed and inspected by removing a screw plug on the foward end of the pump body.

I've had this apart. No apparent problem. But then again, I am not sure what I am looking for.

Next thing I would inspect is the gascolater, see if it has a bubble of air in the top, where the fuel pick up for the primer line is.

OK. I can do thatl

next time this happens try stroking the pump quickly a few times see if it will pick up fuel.

or try pulling the pump piston out and see if you get fuel to feed from gravity into the pump.

I've done both of those, and it doesn't help.

Thanks for your help, Tom.

 

[Tom-D]

Figure out why the fuel will not flow from the tank to the pump and you'll fix the problem.

remove the primer line from the gascolater and see if you get a flow out of the gascolator,

 

[gismo]

I've done both of those, and it doesn't help.

Greg, are you saying that when you remove the primer piston/plunger, no fuel comes out of the primer body (when hot/always)? It shpuld flow quite freely unless there's enough tension on the input check valve to keep it closed. In any case fuel should flow out the line from the gascolater if you disconnect the primer end.

 

[Greg Bockelman]

Greg, are you saying that when you remove the primer piston/plunger, no fuel comes out of the primer body (when hot/always)? It shpuld flow quite freely unless there's enough tension on the input check valve to keep it closed. In any case fuel should flow out the line from the gascolater if you disconnect the primer end.

Well, maybe I misunderstood what Tom said. I have not physically removed the plunger from the primer body yet. I HAVE pulled it out to its limit and left it there for a time to see if it would fill up. It didn't.

I have a lot of things to try. Maybe I will get to it this next week. Bottom line is that I will probably start disassembling things from the gascolator on up to see where the problem might lie. Didn't want to have to pull the panel, but if that is what it takes...

 

[Richard]

After all that has been said, it sounds like a plumbing problem? Either air or something solid blocking the inlet? Maybe a decomposed o-ring?


Notice the ? marks. I'm following along on this interesting problem and am not yet certain of the true nature of the problem. I'm gonna' search for a schematic of the 195 fuel system.

 

[Richard]

Greg, I read your query on the 195 board. I noticed you said you know it's not an o-ring problem because you replace with new parts. On occassion, I have replaced suspect parts with new parts only to find out those new parts were themselves defective. Just a thought.

 

[Greg Bockelman]

Greg, I read your query on the 195 board. I noticed you said you know it's not an o-ring problem because you replace with new parts. On occassion, I have replaced suspect parts with new parts only to find out those new parts were themselves defective. Just a thought.

Yeah, I thought about that too. This is the second primer that has been in the airplane. Same problem. I will begin a serious trouble shoot sometime this week. I will let you all know what I find, if anything.

 

[SCCutler]

I favor Tom's check valve theory, but only because of the easy motion in both directions coupled with no apparent fuel pumped. Not satisfied with that explanation, especially since it has happened two pumps in a row. Something like this, you just have to take it down piece-by-piece, eliminate culprits and you'll find the troublemaker.

 

[Greg Bockelman]

I favor Tom's check valve theory, but only because of the easy motion in both directions coupled with no apparent fuel pumped. Not satisfied with that explanation, especially since it has happened two pumps in a row.

Except that if it was drawing fuel, I would be able to feel it. And I would feel resistance going back into the intake line. The only difference would be that the fuel would be going back into the intake instead of out the output side. That is why I don't think it is the checkvalves. But I am not dismissing the idea yet.

Something like this, you just have to take it down piece-by-piece, eliminate culprits and you'll find the troublemaker.

That is exactly what I intend to do starting this afternoon.