Browncbr1 Posted August 25, 2021 Report Share Posted August 25, 2021 Perhaps someone may have had this symptom before and can shed a little light. recently, on start up, I experience stumbling and abnormally high fuel flow. Fuel pressure is normal. Flow is about double normal @ 4.5gph. If I lean, FF does not change and engine will eventually act like it’s about to die from starvation. After a normal runup, the issue goes away. all new intake gaskets, clean injectors, clean plugs with good fire. Fuel screens clean. ??? Quote Link to comment Share on other sites More sharing options...
ShuRugal Posted August 25, 2021 Report Share Posted August 25, 2021 Morning sickness can be a sign of unhealthy valves.Not sure how the high fuel flow fits into that. Pull the cowls, start it and run it for a minute, then shut it off and see what's wet?Sent from my Pixel 3a using Tapatalk Quote Link to comment Share on other sites More sharing options...
Shadrach Posted August 25, 2021 Report Share Posted August 25, 2021 (edited) 12 minutes ago, Browncbr1 said: Perhaps someone may have had this symptom before and can shed a little light. recently, on start up, I experience stumbling and abnormally high fuel flow. Fuel pressure is normal. Flow is about double normal @ 4.5gph. If I lean, FF does not change and engine will eventually act like it’s about to die from starvation. After a normal runup, the issue goes away. all new intake gaskets, clean injectors, clean plugs with good fire. Fuel screens clean. ??? Go out to the airport and try again but this time cover up you FF gauge. If everything else acts acts normally, I would bet that you're getting a faulty reading. If the engine stumbles when you lean but the FF reading remains constant why would you believe the reading? Edited August 25, 2021 by Shadrach 1 Quote Link to comment Share on other sites More sharing options...
Browncbr1 Posted August 25, 2021 Author Report Share Posted August 25, 2021 7 minutes ago, Shadrach said: Go out to the airport and try again but this time cover up you FF gauge. If everything else acts acts normally, I would bet that you're getting a faulty reading. If the engine stumbles when you lean but the FF reading remains constant why would you believe the reading? That could be…. Maybe I should clean my ff transducer. I pulled the #1 jug because I thought it could be a sticking intake valve, but everything looked perfect. Lapped the seats and reassembled…. Same condition persists. Quote Link to comment Share on other sites More sharing options...
Browncbr1 Posted August 25, 2021 Author Report Share Posted August 25, 2021 13 minutes ago, ShuRugal said: Morning sickness can be a sign of unhealthy valves. Not sure how the high fuel flow fits into that. Pull the cowls, start it and run it for a minute, then shut it off and see what's wet? Sent from my Pixel 3a using Tapatalk No signs of fuel leaks externally. I’m wondering if the fuel servo or spider isn’t distributing correctly? Quote Link to comment Share on other sites More sharing options...
ShuRugal Posted August 25, 2021 Report Share Posted August 25, 2021 I don't know how the internals of the fuel servo work, but if it has moving parts which are sticking while cold, that would probably present the symptoms you are describing.Sent from my Pixel 3a using Tapatalk Quote Link to comment Share on other sites More sharing options...
Shadrach Posted August 25, 2021 Report Share Posted August 25, 2021 I'll say it again. If the engine is responding properly to the physical movement of the mixture control but the FF gauge does not, it's probably best to start with the gauge. Tearing into a servo that appears to be working correctly seems like a poor idea. If your tachometer remained at 600rpm for the first few seconds of start up and taxi would it be prudent to tear into the engine and fuel system? 3 Quote Link to comment Share on other sites More sharing options...
Browncbr1 Posted August 25, 2021 Author Report Share Posted August 25, 2021 (edited) 36 minutes ago, Shadrach said: I'll say it again. If the engine is responding properly to the physical movement of the mixture control but the FF gauge does not, it's probably best to start with the gauge. Tearing into a servo that appears to be working correctly seems like a poor idea. If your tachometer remained at 600rpm for the first few seconds of start up and taxi would it be prudent to tear into the engine and fuel system? I agree, but the problem is that the engine doesn’t start as easily and doesn’t run normal until runup. It misses every few seconds about 40F egt drop. Still scratching my head. I think next time I start it, I’ll run it initially only on the surefly, then try only on the slick. Like I said, runup is normal 30-40rpm drop with normal egt rises on all cylinders. Edited August 25, 2021 by Browncbr1 Quote Link to comment Share on other sites More sharing options...
Shadrach Posted August 25, 2021 Report Share Posted August 25, 2021 2 minutes ago, Browncbr1 said: I agree, but the problem is that the engine doesn’t start as easily and doesn’t run normal until runup. It misses every few seconds about 40F egt drop. Still scratching my head. This issue started right after maintenance was performed on the intake, injectors and plugs? Quote Link to comment Share on other sites More sharing options...
Browncbr1 Posted August 25, 2021 Author Report Share Posted August 25, 2021 5 minutes ago, Shadrach said: This issue started right after maintenance was performed on the intake, injectors and plugs? No, I have attempted to remedy the issue by checking plugs, rotating plugs, replacing intake gaskets, cleaning all fuel screens, checking valve seats and guides. Fuel servo, spider, and boost pump were overhauled in 2015. The mechanical fuel pump was replaced in 2018…. everything after runup is normal. Flight is normal. No wobbly fuel pressure or flow. Quote Link to comment Share on other sites More sharing options...
Browncbr1 Posted August 25, 2021 Author Report Share Posted August 25, 2021 Also, I kinda started to notice the missing condition after having leaned the low speed mixture a couple clicks due to the hot summer weather.. I had always had it a little too rich anyhow, (too much rpm drop when full rich) but I moved it back to the original setting after noticing the idling missing. The high FF didn’t start happening until more recently. The severity of the missing coo relates to the higher FF. If FF reads normal, engine idles smoothly. Quote Link to comment Share on other sites More sharing options...
Browncbr1 Posted August 25, 2021 Author Report Share Posted August 25, 2021 wondering if the servo is the culprit... following this thread, i see similar symptoms.. Quote Link to comment Share on other sites More sharing options...
Shadrach Posted August 25, 2021 Report Share Posted August 25, 2021 14 minutes ago, Browncbr1 said: Also, I kinda started to notice the missing condition after having leaned the low speed mixture a couple clicks due to the hot summer weather.. I had always had it a little too rich anyhow, (too much rpm drop when full rich) but I moved it back to the original setting after noticing the idling missing. The high FF didn’t start happening until more recently. The severity of the missing coo relates to the higher FF. If FF reads normal, engine idles smoothly. When you said "die form starvation" in the OP I understood that to be occurring while leaning. Now you seem to be suggesting that the engine is flooding out from excess fuel flow. If indeed it is flowing at 4.5gph at idle, the plugs should be black after idling that way for any period of time. If your Idle EGTs are in the 700s it's likely really rich. Are you sure the idle mixture was properly adjusted? It reads like it was set to rich. 1 Quote Link to comment Share on other sites More sharing options...
Browncbr1 Posted August 25, 2021 Author Report Share Posted August 25, 2021 3 minutes ago, Shadrach said: When you said "die form starvation" in the OP I understood that to be occurring while leaning. this is correct, but the high FF reading doesn't change while doing this. yes, the plugs look like they are overly rich after ground run. but, like i said, everything goes to normal after run up. EGTs struggle to get above 1000 unless upping rpm to 1200 while warming oil up.. #1 cylinder stays in 900's and each miss, EGT drops about 40 Quote Link to comment Share on other sites More sharing options...
Browncbr1 Posted August 25, 2021 Author Report Share Posted August 25, 2021 After speaking with two fuel system experts, they both said the flow divider meters the low speed fuel, which is confusing because the mixture screw is on the servo. It seems that the mixture screw on the servo actually meters air. They said it sounds like a sticking piston in the spider. Will have that looked at a report back. 2 Quote Link to comment Share on other sites More sharing options...
Guest Posted August 25, 2021 Report Share Posted August 25, 2021 5 hours ago, Browncbr1 said: After speaking with two fuel system experts, they both said the flow divider meters the low speed fuel, which is confusing because the mixture screw is on the servo. It seems that the mixture screw on the servo actually meters air. They said it sounds like a sticking piston in the spider. Will have that looked at a report back. With the cowl off, cut the safety wire and removed the four screws on the top of the flow divider. Gently remove the top, carefully loosen the diaphragm and gently lift it and the plunger/piston out of the body. It’s a very tight fit, but shouldn’t be stuck. Clarence Quote Link to comment Share on other sites More sharing options...
Yetti Posted August 25, 2021 Report Share Posted August 25, 2021 Baby jar test. What is the DA? Is it flooding stumbling? When was the fuel servo last rebuilt? Quote Link to comment Share on other sites More sharing options...
Browncbr1 Posted August 25, 2021 Author Report Share Posted August 25, 2021 4 minutes ago, M20Doc said: With the cowl off, cut the safety wire and removed the four screws on the top of the flow divider. Gently remove the top, carefully loosen the diaphragm and gently lift it and the plunger/piston out of the body. It’s a very tight fit, but shouldn’t be stuck. Clarence Do we look for debris, or irregular wear on the piston, or a brittle diaphragm? I couldn’t find any part list online or a source for any parts. Quote Link to comment Share on other sites More sharing options...
Browncbr1 Posted August 25, 2021 Author Report Share Posted August 25, 2021 1 minute ago, Yetti said: 2 minutes ago, Yetti said: Baby jar test. What is the DA? Is it flooding stumbling? When was the fuel servo last rebuilt? I did the baby jar test. clean expected levels DA is about 2500 servo and spider were overhauled in 2015 Quote Link to comment Share on other sites More sharing options...
Guest Posted August 25, 2021 Report Share Posted August 25, 2021 1 minute ago, Browncbr1 said: Do we look for debris, or irregular wear on the piston, or a brittle diaphragm? I couldn’t find any part list online or a source for any parts. If the shops you’ve spoken with are suggesting a sticky piston taking the cap off is the only way to know. Clarence Quote Link to comment Share on other sites More sharing options...
Shadrach Posted August 25, 2021 Report Share Posted August 25, 2021 12 minutes ago, Browncbr1 said: Do we look for debris, or irregular wear on the piston, or a brittle diaphragm? I couldn’t find any part list online or a source for any parts. 13 minutes ago, Browncbr1 said: Do we look for debris, or irregular wear on the piston, or a brittle diaphragm? I couldn’t find any part list online or a source for any parts. https://precisionairmotive.com/wp-content/uploads/2019/06/15-338e.pdf 1 Quote Link to comment Share on other sites More sharing options...
Shadrach Posted August 25, 2021 Report Share Posted August 25, 2021 (edited) 12 hours ago, Shadrach said: https://precisionairmotive.com/wp-content/uploads/2019/06/15-338e.pdf After thinking through your posts and reviewing the literature linked above, it seems to me that a stuck piston is very plausible, given my laymans understanding of the system. If the piston is not fully closing, it is not restricting flow as it should at the idle pressure supplied by the servo. As soon as the throttle is opened for flight operations, the fuel pressure supplied by the servo is sufficient to move the piston to a less restrictive position for high power and everything acts as it should. How is the plane responding to idle cut off at shut down? The piston in the flow divider should close when the fuel pressure from the servo drops fuel pressure at idle cut off. Fuel pressure from the servo acting against the sprung piston is how fuel is metered. If the piston is not moving freely across its entire range of motion it is not going to meter fuel correctly at some pressures. Edited August 26, 2021 by Shadrach Quote Link to comment Share on other sites More sharing options...
carusoam Posted August 26, 2021 Report Share Posted August 26, 2021 Other things to look for while you are chasing fuel metering…. 1) There is a pseudo normal increase in FF…. As measured by the gauge… If air is being drawn into the system… 2) Air has a funny way of being separated at the fuel injectors… so a bunch of air will raise the apparent FF, and not get noticed by the engine… 3) Too much air getting into the system is bound to show up somewhere, somehow… 4) Fortunately, when the leak is big enough to allow that much air in… there are usually blue stains to indicate what needs to be fixed… 5) A common, air leak occurs at the fuel selector valve… air leaks in, and fuel leaks out and stains the valve when the engine isn’t running…. 6) When performing the baby jar test… using a clear tube…. A bunch of air bubbles will be seen passing down the tube… Its really easy if you see blue stains anywhere during your inspection….. PP thoughts only not a mechanic… Best regards, -a- Quote Link to comment Share on other sites More sharing options...
Browncbr1 Posted August 26, 2021 Author Report Share Posted August 26, 2021 11 hours ago, Shadrach said: After thinking through your posts and reviewing the literature linked above, it seems to me that a stuck piston is very plausible. Given my laymans understanding of the system. If the piston is not fully closing, it is not restricting flow as it should at the idle pressure supplied by the servo. As soon as the throttle is opened for flight operations the fuel pressure supplied by the servo is sufficient to move the piston to a less restrictive position for high power everything acts as it should at higher power settings. How is the plane responding to idle cut off at shut down? The piston in the flow divider should close when the fuel pressure from the servo drops fuel pressure at idle cut off. Fuel pressure from the servo acting against the sprung piston is how fuel is metered. If the piston is not moving freely across its entire range of motion it is not going to meter fuel correctly at some pressures. This is my understanding also and would explain the symptoms. The engine does shut down ok, but of course, I’ve only shut it down after warm up and run up. Given the amount of junk I found in my servo screen, I think it’s possible that some made it past and maybe got lodged in the piston guide. I had never heard of this kind of condition before, which led me to think morning sickness, so I mistakenly spent a lot of labor cost r&r my #1 cylinder, which looked perfect actually. Everyone was shocked it has 1100hrs.. the engine shop said it looks like a few hundred hours. I guess LOP, Avblend, and 25hr oil changes really help. We will be opening it up today or tomorrow. Will report back 1 Quote Link to comment Share on other sites More sharing options...
Browncbr1 Posted August 26, 2021 Author Report Share Posted August 26, 2021 5 hours ago, carusoam said: Other things to look for while you are chasing fuel metering…. 1) There is a pseudo normal increase in FF…. As measured by the gauge… If air is being drawn into the system… 2) Air has a funny way of being separated at the fuel injectors… so a bunch of air will raise the apparent FF, and not get noticed by the engine… 3) Too much air getting into the system is bound to show up somewhere, somehow… 4) Fortunately, when the leak is big enough to allow that much air in… there are usually blue stains to indicate what needs to be fixed… 5) A common, air leak occurs at the fuel selector valve… air leaks in, and fuel leaks out and stains the valve when the engine isn’t running…. 6) When performing the baby jar test… using a clear tube…. A bunch of air bubbles will be seen passing down the tube… Its really easy if you see blue stains anywhere during your inspection….. PP thoughts only not a mechanic… Best regards, -a- Thanks for your ideas, fuel and air were the first things I checked. No staining anywhere no leaks under pressure. I hope it turns out to be the divider. I have not thought of clear tubes on the baby jar test though, good idea 1 Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.