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Essex Primer Leaking Through?

snopercod

Well Known Member
I have the traditional "Essex" type pump primer, and I suspect it's leaking fuel into the #1 cylinder (I only have the primer line plumbed into the #1). The symptoms are: #1 CHT and EGT are both slightly low. The EGT is 100 deg lower and the CHT is 20 deg lower than the other cylinders on climbout. When I prime before startup, the primer is "flat" and takes several pumps until the primer has fuel in it. The spring seems to be OK since I have to push against it to lock the plunger. It sure seems like the outlet valve is leaking through. I changed the O-rings but the "rebuild" kits don't include the seal on the end of the plunger. Does anybody have any advice, please?
 
I would imagine that the primer pump is in a position with it's height above the tank and assume it is upstream of the mech fuel pump. I don't see how fuel could flow against gravity to get to the primer pump and flow to the cylinder. If the pump is sitting below the fuel line on the suction side of the fuel pump, it is possible that gravity will allow it to flow to the pump. However, only gravity can get it to the cylinder. If any part if the line between the primer pump and the nozzle is above the the primer pump, the fuel cannot flow without assistance.

I have a .01 GPH Gami spread and the spread from my lowest to highest EGT at cruise is about 70-80* (this has more to do with placement of the sensor relative to the exhaust valve). My CHT spread is about 20* from hottest to coldest and my #1 is the coldest. In the RV, I would expect the #1 cylinder to be the coolest.

Assuming the primer pump is not being fed from a pressurized line, I don't see how that circuit could flow enough fuel to make a meaningfull difference in the total flow to that cylinder at cruise.

That pump should have two check valves. The fact that one is bad (intake), doesn't mean the other (outflow) is, though it is a decent possibility. In order for fuel to flow through freely, both check valves would have to be bad. Further, I would also expect the design to prohibit flow when in the locked position, but have never seen a design diagram for one of these. This is a safety issue that I can't imagine was overlooked in the design of a TSO'ed part.

EDIT: I just looked a picture of one of these primers. It looks like the check valvles are are ball/spring and those don't typically wear; They are typically only frozen closed due to gum/varnish or forced open due to debris. Not likely that both would fail open, allowing free flow through the chamber, unless you had a lot of debris flowing through it. As long as one check valve is working, fuel can't flow through it, at least without pressure greater than gravity. Failed o-rings on the piston will not allow flow from inlet to outlet.
 
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Thanks for your thoughts, Larry. My primer is located below my header tank, so it has a small fuel pressure on it at all times. It's also located below the level of the cylinder but, with the engine running, the vacuum in the intake would suck the fuel uphill if there were a leak in the outlet check valve. I guess I'm going to have to remove the unit from the panel (a real PITA) to inspect/repair the check valves. (I changed the O-rings by just removing the plunger.) I'm sure as heck not going to buy a new primer for $550.
 
Thanks for your thoughts, Larry. My primer is located below my header tank, so it has a small fuel pressure on it at all times. It's also located below the level of the cylinder but, with the engine running, the vacuum in the intake would suck the fuel uphill if there were a leak in the outlet check valve. I guess I'm going to have to remove the unit from the panel (a real PITA) to inspect/repair the check valves. (I changed the O-rings by just removing the plunger.) I'm sure as heck not going to buy a new primer for $550.

Clearly gravity alone is not pulling fuel through the primer and into your intake chamber or you would know about it, as your fuel would eventually move from your header tank to your sump, floor, etc. overnight. I suggest pulling the line off the nozzle at the cylinder and put 5-10" of vacuum on it (about what you'll see at cruise power) and looking for fuel flow. This will tell you whether or not your #1 is drawing excess fuel through the primer circuit. You can borrow a vacuum tool from autozone.
 
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the vacuum in the intake would suck the fuel uphill if there were a leak in the outlet check valve.

Both check valves would have to be leaking to allow the engines vacuum to pull fuel from the tank and through the primer. Gravity alone will not allow fuel to flow into the primer chamber. It has to be pulled in via a vacuum if the inlet check valve has not failed. Given the application, the check valves must be designed to require more vacuum than the engine produces to break or they would all allow fuel to flow while the engine is running.

Larry
 
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I suggest pulling the line off the nozzle at the cylinder and put 5-10" of vacuum on it (about what you'll see at cruise power) and looking for fuel flow.
That's what I need to do before ripping into the primer. I tried to do something similar with a 60cc syringe, but never could come up with a good way to adapt the 1/8" copper tubing to the vacuum source. I guess I need to work on that.
 
That's what I need to do before ripping into the primer. I tried to do something similar with a 60cc syringe, but never could come up with a good way to adapt the 1/8" copper tubing to the vacuum source. I guess I need to work on that.

Pull the fitting out of cylinder (it is 1/8" NPT) and re-attach it to line. You should be able to get a 1/8" NPTF to 1/4" barb fitting at HOME depot. Vac line should fit without too much persuasion on the 1/4" barb. They may sell a 3/16" barb and that would be preferred.

Larry
 
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Brake vacuum pump.

Get some one to let you use one of the brake system hand vacuum pumps.
The kind with the little plastic bottle. Just pull a low vacuum and see if fuel comes out, you don't need 28", 10-14" would tell you the story.
Hope this helps. Yours, R.E.A. III #80888
 
Clearly gravity alone is not pulling fuel through the primer and into your intake chamber or you would know about it, as your fuel would eventually move from your header tank to your sump, floor, etc. overnight. I suggest pulling the line off the nozzle at the cylinder and put 5-10" of vacuum on it (about what you'll see at cruise power) and looking for fuel flow. This will tell you whether or not your #1 is drawing excess fuel through the primer circuit. You can borrow a vacuum tool from autozone.

An excellent idea. Thanks!

What were the results of the test?
 
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Why not just disconnect at the cylinder and plug both connections and run the engine to see if the symptoms change? Leaking primers leak the most fuel into the induction system at idle and the least at full power due to the changes in suction in the intake port as MP increases. Normally, leaking primers do not show symptoms in cruise or above power settings but will cause havoc at idle power settings. at least that has been my experience.
Good Luck,
Mahlon
 
a great mystery

Why not just disconnect at the cylinder and plug both connections and run the engine to see if the symptoms change?
Today I plugged both connections at the outlet of the primer (for another reason), and went flying. I compared the EGTs at idle from that flight with previous flights where the primer was hooked up. I couldn't see any difference at all in cylinder #1. So my gut instinct is to say, "The primer isn't leaking", but where is all the fuel going? After the plane sits and I go to use the primer, I have to pump it three or four times to pull fuel into the primer. Shouldn't it stay full all the time if the outlet check valve isn't leaking?
 
Today I plugged both connections at the outlet of the primer (for another reason), and went flying. I compared the EGTs at idle from that flight with previous flights where the primer was hooked up. I couldn't see any difference at all in cylinder #1. So my gut instinct is to say, "The primer isn't leaking", but where is all the fuel going? After the plane sits and I go to use the primer, I have to pump it three or four times to pull fuel into the primer. Shouldn't it stay full all the time if the outlet check valve isn't leaking?

My thoughts:

Primer locks closed, so chamber empty. I will guess that the inlet tupb goes upward for just a bit then mostly downhill to the tank/fuel line. A perfect check valve seal would force the fuel to stay in the tube. Even a tiny air leak in the seal/check valve would allow gravity to do it's thing and slowly allow the fuel in the tube to drain back. You'll need a few pumps to get the fuel back up through the piping. It's easy to leak enough to allow the drain back over time, but still be good enough to not be noticeable when using the primer.

Larry
 
Actually, my primer is below the fuel tank, so it should stay charged at all times unless the outlet check valve it leaking. In that case, the vacuum in the intake manifold at idle would suck all the fuel out of the line and the primer cylinder itself (and maybe even the fuel tank?), which seems to be what's happening. One thing I need to check - which I didn't on shutdown today - is whether the primer is "hard" when I pump it. It should be with the outlet capped off. This may be a situation where I'll just have to live with it. If it's not messing up my mixture, why should I care?

Thanks for your thoughts.
 
OK, my head was "in and locked" when I asked "Where is the fuel going?" With the primer in and locked, there is no fuel in it. Duh. So yesterday I capped off the outlet and slowly pulled out the plunger. There was a lot of resistance, but it would suck fuel into the chamber. The plunger wouldn't go back in, though, as it shouldn't. This told me that 1. The inlet check valve was allowing fuel to enter, but not to go back up into the tank. 2. The internal O-rings weren't leaking past.

Just to be sure, I removed the plunger, and inspected and lubed the O-rings (with DC-4) anyway.

After removing the outlet cap and restoring the system back to normal, I pumped the primer again and got the same old thing: I had to pump it 4 times before there was any resistance on the plunger in either direction. The only thing my (alleged) brain can come up with is that retracting the plunger is sucking air into the chamber through the outlet.

I had thought that would mean the outlet ball check valve was bad until I realized that the ball itself is on the end of the plunger, so when you unlock the primer, there is no check valve. The ball is on the end of a spring-loaded rod which is connected to the plunger. This must be why you're supposed to check "primer in and locked" if your engine is running rough. Another "Duh" moment...

That being the case, how does pulling the plunger out manage to suck fuel from the tank when it can just as easily suck air/fuel from the primer line? All I can think of is that this design depends on the resistance of the small orifice at the primer fitting in the cylinder. I know that there's resistance there because when the primer chamber is full of fuel and I push on the plunger, it goes in slowly as the fuel is injected into the intake.

Which brings up another question. My primer line is probably 50" long in total from the panel to the #1 cylinder. With the engine running, does the vacuum in the intake vaporize the fuel in the primer line so that it's basically empty the next time I go to start up? That would explain the 3-4 pumps required to prime the primer. I guess I could check the next time the cowling is off, but I already feel like I'm over-thinking this.

It's really embarrassing that I'm having trouble understanding how this supposedly simple system is supposed to work.
 
OK, my head was "in and locked" when I asked "Where is the fuel going?" With the primer in and locked, there is no fuel in it. Duh. So yesterday I capped off the outlet and slowly pulled out the plunger. There was a lot of resistance, but it would suck fuel into the chamber. The plunger wouldn't go back in, though, as it shouldn't. This told me that 1. The inlet check valve was allowing fuel to enter, but not to go back up into the tank. 2. The internal O-rings weren't leaking past.

Just to be sure, I removed the plunger, and inspected and lubed the O-rings (with DC-4) anyway.

After removing the outlet cap and restoring the system back to normal, I pumped the primer again and got the same old thing: I had to pump it 4 times before there was any resistance on the plunger in either direction. The only thing my (alleged) brain can come up with is that retracting the plunger is sucking air into the chamber through the outlet.

I had thought that would mean the outlet ball check valve was bad until I realized that the ball itself is on the end of the plunger, so when you unlock the primer, there is no check valve. The ball is on the end of a spring-loaded rod which is connected to the plunger. This must be why you're supposed to check "primer in and locked" if your engine is running rough. Another "Duh" moment...

That being the case, how does pulling the plunger out manage to suck fuel from the tank when it can just as easily suck air/fuel from the primer line? All I can think of is that this design depends on the resistance of the small orifice at the primer fitting in the cylinder. I know that there's resistance there because when the primer chamber is full of fuel and I push on the plunger, it goes in slowly as the fuel is injected into the intake.

Which brings up another question. My primer line is probably 50" long in total from the panel to the #1 cylinder. With the engine running, does the vacuum in the intake vaporize the fuel in the primer line so that it's basically empty the next time I go to start up? That would explain the 3-4 pumps required to prime the primer. I guess I could check the next time the cowling is off, but I already feel like I'm over-thinking this.

It's really embarrassing that I'm having trouble understanding how this supposedly simple system is supposed to work.

I looked at a diagram on the web when composing a post for you a week or two ago. It showed a check ball in the outlet area and not on the plunger. That seems like an odd design as you decribed it. I am not sure what you are seeing on the plunger is the primary check valve ball, but a secondary safety of some sort. I suspect you have some crud in the check valve that is preventing it from sealing properly. I would be pretty confident there is a removeable piece on the oultlet that will allow the ball/spring to come out and be able to clean it up.

Larry
 
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Two varieties of Essex primers

I am not sure what you are seeing on the plunger is the primary check valve ball, but a secondary safety of some sort.
Sigh...In doing further research, you may be right. OTOH, it seems that there are two kinds of Exxex primers - short ones and long ones like mine. The plunger on mine has a nice, shiny, spring loaded ball on the end, but I found a post over on the Shortwing Piper forum that describes the rebuilding process in detail and includes links to photos. The plunger on the short version just has a pointy end, not a spring-loaded ball like mine. This photo shows the replacement balls, seats, and springs, which are supposedly available directly from Essex. Even though the primer in that post is different than mine, at least I now know what's under that slotted screw on the tail end of my unit. I should remove and inspect the unit, but it's really hard to get to.

Thanks for taking an interest in this problem, Larry.
 
Thanks for sharing the links above - very useful info.

I'll add a couple of anecdotes which might help. I hope they don't just add confusion!

1) Essex long-body primer, header tank feeding downhill to firewall-mounted gascolator, then uphill to panel-mounted primer pump, then downhill to the one primer nozzle. This system always takes a couple of strokes on the pump before pumping resistance is felt.

2) Essex long-body primer, low-wing with electric fuel pump, no gascolator, nozzles on 4 cylinders. Never needs any suction strokes to build full pressure.

3) This happened to me just last night, and it's a good lesson learned on a brand new airplane. First engine start was very "lumpy" to get going, so for this second start I opted to prime the engine (Essex short body primer) before start and leave the primer pulled out, "charged" and ready to feed additional fuel to more quickly smooth out the engine. BAD IDEA. Of course the engine caught on the 3rd blade and no more priming was necessary, but I failed to push the primer in and lock it. The net result? Horribly rich mixture on the 3 cylinders with primer nozzles as the 12" MAP associated with 1000RPM idle sucked fuel through the primer pump, thanks also in part to the engine-driven pump pressurizing the inlet line to the primer pump. Pulling the mixture out made #1,2 & 4 run better but 3 went lean. #4 cylinder has by far the shortest length line to the primer pump, while #1 & #2 have longer lines. EGT on #4 was quite a bit cooler than #1 & 2. Idle Cut Off would not shut down the engine, so there was a lot of fuel going through the priming system. Lesson learned - checklist has been modified to highlight PRIMER IN AND LOCKED.
 
Thanks for the additional information, JOY. I can use all the anecdotes I can get :) I think we can agree that the vacuum in the intake at idle will suck the fuel out of the primer line (or the primer itself if it's not locked). I've already mentioned that after a normal shutdown, the next time I go to start up I have to pump the primer 3 or 4 times before I feel any resistance. I tried an experiment the other day and after shutdown, I pumped the primer 3 times until I felt resistance, and then locked it. When I went to start up the next day, I felt resistance almost immediately when pumping the primer. What does this tell me? I think it tells me that the primer line was still full due to the engine not running.

One thing unique about my system is that I have 30" of -3 flexhose running from my primer to the firewall. That has a larger ID than the 1/8" tubing I was using before. So maybe there's more volume to refill on startup?
 
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