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Large hesitation on & off idle when hot

GyroF-16

GyroF-16

Well Known Member
Patron
Hello all-

I’ve been trying to address the following problem for over a month now…

IO-540 w/ Airflow Performance fuel injection and FM-200
F-1 Rocket

Starts easily and runs well during taxi out, runup and in flight*
After landing on taxi back (5 minutes), idle is slightly irregular (+/-50 rpm) and varies without throttle movements.
Oil temp in these situations is about 210°.
Most concerning - On the ground when hot after flight, with any but very slow throttle applications, the engine stumbles and hesitates before accelerating. Or stumbles, then quits. Quick moves to idle induce large RPM sags to just over 600 rpm before eventually recovering to 750. Various mixture lever settings do not reduce the behavior (extreme rich and lean makes it worse).

I’ve played with idle mixture, and have it set so that it idles well at 700-740 with the mixture lever at mid-range, and rpm rises approximately 60 rpm when moving mixture to idle cutoff. I’m deliberately setting idle mixture a little rich because my home field is at 6,900 MSL, with DAs regularly at 9k and above on warm days. I want a setting that won’t have me too lean when traveling to lower elevations. I’ve tried readjusting the idle mixture after flight with the engine hot, and couldn’t reduce the adverse behavior.

Of note, I recently had the fuel controller and fuel divider overhauled by Airflow Performance. This behavior after flying is new since the overhaul of those parts.
All my flights prior to the overhaul of those components were on cooler days (35-65°), and it always ran well on the ground. Now I’m flying on 85° days and experiencing the issues above.

Of note - even since the parts were overhauled, the engine responds well in flight and in the pattern, even during touch-and-goes.

I’m wondering if there’s a maintenance-induced problem stemming from the overhauled fuel control or fuel divider?
Or fuel boiling in the lines because it’s hot under the cowl after flight? (No reports of this from builder/previous owner, though).

Thoughts?

* In flight, the only oddity is that cyl #3 EGT drops to 200° below the other EGTs (e.g. 1079 vs 1261-1288) at anything below about 20 gal/hr fuel flow. On takeoff and initial climb, when running rich to keep CHTs cooler, #3 CHT is in the range with all the others. Once I lean (even ROP) below 19 gal/hr, it drops, but still trends up and down with all the other EGTs. Once I’m on the ground at/near idle power, #3 EGT is right back in the middle of the other EGTs. I’m guessing that #3 is running rich for some reason, but haven’t investigated this issue further (yet). Only mentioned in case it provides a clue for the “poor idle when hot on the ground” problem.


Edited to add:
Ignition:
Slick mag on Left
Lightspeed II on Right
 
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6900 is pretty high. in the cases where you have hesitation on throttle advance, how much are you leaned?. have you experimented with different degrees of leaning while advancing the throttle, trying to see how it affects it? Best to experiment both richer and leaner than you typically use. probably worth the effort to help narrow down the issue. if you can address it with leaning, it is possible they messed with the jets during overhaul and got it too rich or too lean. or maybe just different than before and now requires a different level of manual leaning.
 
Also, if problem includes both idle performance AND transition off idle, the idle mixture becomes more likely of a suspect. May be worth optimizing idle mixture for your altitude and see if the problem goes away or lessens. I am a fan of the lowest MAP approach to idle mixture setting. Easier and more precise.
 
Yes, it’s definitely an idle and especially a transition to/from idle issue.
But only happens when the engine is hot (oil 210°) after flying - never on runup or testing up to about 180° oil temp.

I’ve tried leaning aggressively with the mixture lever when idling hot - it does not improve the transition to/from idle. It stays the same, and at some point (really lean) becomes worse. Same for mixture lever moved richer.

I’m just puzzled that it happens after flying. In the past, I’ve known engines to hesitate before they reached full operating temp.
This one is now doing it AT operating temp (but only on the ground near idle).
 
GyroF-16 said:
Hello all-

I’ve been trying to address the following problem for over a month now…

IO-540 w/ Airflow Performance fuel injection and FM-200
F-1 Rocket

Starts easily and runs well during taxi out, runup and in flight*
After landing on taxi back (5 minutes), idle is slightly irregular (+/-50 rpm) and varies without throttle movements.
Oil temp in these situations is about 210°.
Most concerning - On the ground when hot after flight, with any but very slow throttle applications, the engine stumbles and hesitates before accelerating. Or stumbles, then quits. Quick moves to idle induce large RPM sags to just over 600 rpm before eventually recovering to 750. Various mixture lever settings do not reduce the behavior (extreme rich and lean makes it worse).

I’ve played with idle mixture, and have it set so that it idles well at 700-740 with the mixture lever at mid-range, and rpm rises approximately 60 rpm when moving mixture to idle cutoff. I’m deliberately setting idle mixture a little rich because my home field is at 6,900 MSL, with DAs regularly at 9k and above on warm days. I want a setting that won’t have me too lean when traveling to lower elevations. I’ve tried readjusting the idle mixture after flight with the engine hot, and couldn’t reduce the adverse behavior.

Of note, I recently had the fuel controller and fuel divider overhauled by Airflow Performance. This behavior after flying is new since the overhaul of those parts.
All my flights prior to the overhaul of those components were on cooler days (35-65°), and it always ran well on the ground. Now I’m flying on 85° days and experiencing the issues above.

Of note - even since the parts were overhauled, the engine responds well in flight and in the pattern, even during touch-and-goes.

I’m wondering if there’s a maintenance-induced problem stemming from the overhauled fuel control or fuel divider?
Or fuel boiling in the lines because it’s hot under the cowl after flight? (No reports of this from builder/previous owner, though).

Thoughts?

* In flight, the only oddity is that cyl #3 EGT drops to 200° below the other EGTs (e.g. 1079 vs 1261-1288) at anything below about 20 gal/hr fuel flow. On takeoff and initial climb, when running rich to keep CHTs cooler, #3 CHT is in the range with all the others. Once I lean (even ROP) below 19 gal/hr, it drops, but still trends up and down with all the other EGTs. Once I’m on the ground at/near idle power, #3 EGT is right back in the middle of the other EGTs. I’m guessing that #3 is running rich for some reason, but haven’t investigated this issue further (yet). Only mentioned in case it provides a clue for the “poor idle when hot on the ground” problem.
Click to expand...
What mags do you have?
 
Same symptoms I experienced with an IO360 in a Mooney. 0-3000 ft airports were no issue with moderate OAT. Smooth idle and no stumbling. But above 3000 ft and hot would have a terrible idle fluctuating and a stumble. Even low altitude airports when it was over 85 and heat soaked had issues. Years of troubleshooting, overhauled fuel servo, manifold valve, new mags, wires, plugs, engine overhaul, induction leak checks, idle mixture adjustments, the behavior stayed the exact same. Multiple reputable Mooney shops stated the issue was the heated fuel in the injection system (fuel vaporization due to heat soaking) and that there was not much that could be done. Hitting the boost pump didn't seem to help.

The only thing that seemed to help quite a bit was changing to GAMI injectors that we went three rounds to balance out to a <0.1 spread. Once this was done it still happened but not as significant as before and only when hot. I could always find a sweet spot with the mixture where it was better but still rough. Heat soaked idle at high density altitude it would idle poorly but otherwise ran great. I just learned to live with it and work around it.

Good luck - lots of good advice here.
 
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GyroF-16 said:
Yes, it’s definitely an idle and especially a transition to/from idle issue.
But only happens when the engine is hot (oil 210°) after flying - never on runup or testing up to about 180° oil temp.

I’ve tried leaning aggressively with the mixture lever when idling hot - it does not improve the transition to/from idle. It stays the same, and at some point (really lean) becomes worse. Same for mixture lever moved richer.

I’m just puzzled that it happens after flying. In the past, I’ve known engines to hesitate before they reached full operating temp.
This one is now doing it AT operating temp (but only on the ground near idle).
Click to expand...
do you know what injector size you are running? you can consider going from the 2# spring to the 4# in the flow divider to help reduce boiling in the SS lines at low rpms.. i had no transition issues, but that helped my idle quality when heat soaked.
 
Multiple reputable Mooney shops stated the issue was the heated fuel in the injection system (fuel vaporization due to heat soaking) and that there was not much that could be done.
Click to expand...
Not much can be done to a certified airplane but much can be done to an experimental airplane.
Good suggestions offered above but none addresses the elephant in the room, namely vaporizing fuel in the fuel system during hot operating conditions.
What you describe tends to happen anytime during or after prolonged low fuel flow in the system, including after descent and heat soaking FWF fuel system components. This tends to also happen especially during taxi and waiting on the ground when fuel vaporization can quickly turn into vapor lock.
I am not sure of your fuel system setup but I am willing to bet that you have a red cube somewhere between the cylinders and possibly even a gascolator somewhere FWF next to an exhaust pipe?
Your overhaul of the Fuel controller may have something to do with your issues but as you described, you are now operating in a much hotter environment and a heat related problem seems more likely.
Whatever you can do to improve cooling and removing any unnecessary "fuel boilers" FWF will greatly help in reducing or eliminating your idle hesitation.
 
lr172 said:
do you know what injector size you are running? you can consider going from the 2# spring to the 4# in the flow divider to help reduce boiling in the SS lines at low rpms.. i had no transition issues, but that helped my idle quality when heat soaked.
Click to expand...
Good question!
When the fuel control and divider were overhauled, I also changed from 0.028 restrictors to 0.025.
A call to Airflow Performance verified that the fuel divided has a 2 psi spring. Duane said that using a stronger spring would keep the fuel divider active at higher flow rates, and would not help reduce fuel vapor issues.
 
N427EF said:
Not much can be done to a certified airplane but much can be done to an experimental airplane.
Good suggestions offered above but none addresses the elephant in the room, namely vaporizing fuel in the fuel system during hot operating conditions.
What you describe tends to happen anytime during or after prolonged low fuel flow in the system, including after descent and heat soaking FWF fuel system components. This tends to also happen especially during taxi and waiting on the ground when fuel vaporization can quickly turn into vapor lock.
I am not sure of your fuel system setup but I am willing to bet that you have a red cube somewhere between the cylinders and possibly even a gascolator somewhere FWF next to an exhaust pipe?
Your overhaul of the Fuel controller may have something to do with your issues but as you described, you are now operating in a much hotter environment and a heat related problem seems more likely.
Whatever you can do to improve cooling and removing any unnecessary "fuel boilers" FWF will greatly help in reducing or eliminating your idle hesitation.
Click to expand...
Yes, I have a Red Cube located just to the right and aft of the alternator. No gascolator.
 
whats your gami spread? you may want to move the #3EGT probe to another exhaust and see if the temp change moves or stays with #3 (similarly with CHT)
 
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