What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Problems Idling While Hot

iamtheari

Well Known Member
I’ve been noticing that my IO-390 doesn’t like to idle smoothly when it’s hot. I adjusted the idle mixture as well as I could. Everything works fine when the engine is cold. But after a flight the idle is rough. If I throttle back to 800 rpm it tends to die. Idling are 1000 rpm works better but it seems to hiccup every few seconds, dropping rpm and coming back promptly. The EGT for cylinders 1-3 is consistent but #4 is significantly lower than the rest. I can lean the engine for an increase in idle rpm but it still has the hiccup every few seconds.

In flight, I have good power. The engine runs fine up to about 40F lean of peak.

Is there anything specific I should be looking at?
 
Have you adjusted the idle mixture? My engine came from the factory set very rich. It took 2 full turns of the adjuster wheel to get it to where my rpm increased properly when moving the mixture to idle cutoff. There is a Lycoming procedure on how to do it.
 
The EGT for cylinders 1-3 is consistent but #4 is significantly lower than the rest. I can lean the engine for an increase in idle rpm but it still has the hiccup every few seconds.


Is there anything specific I should be looking at?

I would first confirm that #4 is LOP at idle (further leaning drops the EGT) and not ROP (further leaning increases the EGT). If the former, check for an intake leak on #4. If no leak found and EGTs on #4 are consistent at higher power settings, may want to check the spider for blokage in the lower portion of the #4 V slot. Then find the procedure for adjusting your idle mixture. Be sure to do it with the engine at full, normal operating temps.

Larry
 
Last edited:
Vaper in the fuel lines?

I have similar issues on hot days after a flight. I think it is just the fuel boiling in the lines creating air in the lines? Seems to be a little bit of an issue with lots of injected motors on different aircraft....
 
I have same issue with my IO390, simple fix when engine is running after hot start is I add 1-2 seconds of boost to help clear the lines.
 
I have seen weak "sniffle valves" opening and closing at idle, causing slight surging and rough running at an idle. I threw mine away, as it is a tailwheel bird, any pooled fuel in the sump would flow to the aft and not drain out the sniffle valve. I plugged the hole with a standard 1/8" pipe plug, and problem solved.

DAR Gary
 
I have seen weak "sniffle valves" opening and closing at idle, causing slight surging and rough running at an idle. I threw mine away, as it is a tailwheel bird, any pooled fuel in the sump would flow to the aft and not drain out the sniffle valve. I plugged the hole with a standard 1/8" pipe plug, and problem solved.

DAR Gary
I hadn’t even thought of the sniffle valve getting involved. My plane, engine, and sniffle valve are all new, but new things can fail. Did yours behave differently when hot vs cold?
 
I hadn’t even thought of the sniffle valve getting involved. My plane, engine, and sniffle valve are all new, but new things can fail. Did yours behave differently when hot vs cold?
Not sure what sniffle valve you have, but mine would not flow enough air to make a difference, plus it's in the intake system and if it were leaking it would just lean the mixture slightly across all cylinders. I think that's a red herring.
 
I can confirm the same behaviour with the FlightChops RV-14 390-EXP119 when it is idling hot on a hot day.

I've done the servo idle mixture adjustment and it improved it some. It still lopes between 800 and 1100rpm and comes on strong when clearing through 1300rpm. We have a 3 blade composite prop, I figure the light weight is probably not helping matters down in the lower RPMs.

I fly an IO360 powered RV7 as well and it also has a slight miss when really hot on the ground at lower RPM, but it's not as bad as the 14 with the 390.

For our 14 its Cyl 1 running cooler than the other 3, leaning increases EGT on #1 so its not Lean of Peak. I generally lean until all 3 are up and closer together on the ground, which gives the smoothest running condition.

I suspect when the fall / winter weather hits we will have a much smoother running engine at low speed on the ground.
 
Some photos of your fuel line routing under the cowling might help us spot issues.
I plumbed it according to the RV-14 kit plans. It’s such a luxury to have that planned out for me. I don’t have pictures handy but, from memory, the routing is from the lower center firewall up to the engine-driven pump, then through a fuel flow servo clamped to the left intake tubes with heat shielding wrapped around it, then from there to the fuel servo on the front of the sump, and from there up around to the spider on top of the engine.
 
I plumbed it according to the RV-14 kit plans. It’s such a luxury to have that planned out for me. I don’t have pictures handy but, from memory, the routing is from the lower center firewall up to the engine-driven pump, then through a fuel flow servo clamped to the left intake tubes with heat shielding wrapped around it, then from there to the fuel servo on the front of the sump, and from there up around to the spider on top of the engine.

So how is the fuel line insulated as it passes the exhaust pipes on the way to the servo?
And, maybe just as important, how is it insulated from the servo to the spider as it runs near exhausts and between hot cylinders?
 
I didn’t notice any improvement.

The symptoms suggest fuel heating and vapor generation. Although we tend to use 140F as a threshold value for vapor, the truth is that avgas is a mix of many volatile constituents, some of which have much lower vapor points.

No improvement with boost pump on says vapor formation is probably not happening prior to the engine driven pump.

Which brings us to Bryan's note above. The standard RV-14 firewall forward runs the servo feed line and fuel flow sender very close to the left side exhaust head pipes, in particular #2 (see attached). Personally I think that line should be shortened by an inch or more. Even if not a fuel heater, it would add some lifetime to the hose. For sure the pipes need heat shields.

In addition, there is an exhaust crossover under the rear of the servo. Do you have a heat shield on the pipe there?

BTW, the heat shields can be improved without much effort. A bare shield reflects some radiant energy. It absorbs the rest, and in effect divides it up. Some is carried away by convection, while some is carried to the other side by conduction, where it is re-radiated. Some of the re-radiated energy is reaching the servo. To reduce it, simply add an insulator to the servo side. Cut a patch of fiberfrax felt to match, and cover it with a wrap of aluminum tape. Done.
 

Attachments

  • Hose and Pipe 1.jpg
    Hose and Pipe 1.jpg
    81.7 KB · Views: 380
  • Heat Shield.jpg
    Heat Shield.jpg
    52.3 KB · Views: 380
Last edited:
Thanks for those tips. I did install the heat shields per the RV-14 plans, including the one on the cross-over. I'll consider trying to improve them the next time I am working with the cowl off, or at my first condition inspection this winter.

Is my #4 cylinder running much lower EGT at idle a red herring? I've watched it carefully and it is right in the middle of the pack in flight whether I am running ROP or LOP.
 
Is my #4 cylinder running much lower EGT at idle a red herring? I've watched it carefully and it is right in the middle of the pack in flight whether I am running ROP or LOP.

When you lean at idle, does it go up or down?

Remember, the numerical value isn't relevant. The points of interest are up-or-down with leaning, and does it peak at about the same mixture as the others.
 
When you lean at idle, does it go up or down?

Remember, the numerical value isn't relevant. The points of interest are up-or-down with leaning, and does it peak at about the same mixture as the others.
When leaning at idle, it tracks the others, it's just markedly cooler at idle. When I get the mixture lean enough, the engine does run smoothly. It's really lean, pulled back about 2/3 of the way between full rich and idle cutoff. I probably just need to re-do my idle mixture adjustment, with a hot engine on a cold day. I did it with a not-so-hot engine on a moderate day this spring. I don't want to get it too lean by adjusting it in the summer and then not be able to start the engine in the winter.
 
Do the symptoms improve when you turn on the electric boost pump?

Negative. Pressure is solid at 25-27 without the boost and about 29 to 30 with the boost and it has no effect on the idling condition of the engine when hot.
 
I’ve been noticing that my IO-390 doesn’t like to idle smoothly when it’s hot. ...
Is there anything specific I should be looking at?

I used to have same problem. Fuel pump cooling shroud was no help. What fixed the problem (very significant improvement) was installing the new spider spring (4psi) called out in Lycoming SI 1539 and Lycoming SI 1489C. Part number 2577011. It makes a huge difference.

As I recall, I bought the new spring from Airflow Performance. I think the shipping was more than the spring.
 
Any update from the OP? Have you solved the rough idling on the IO-390?
It isn't solved, yet. I think the new fuel spider spring recommended above is the next step, which I will do either this fall when the plane is back home (I have to drive over an hour to its temporary home while my home airport is under construction) or next spring when the condition inspection is due. I have some other squawks to address at those two times, anyhow.

I have continued to adjust the idle mixture and idle speed, which I am finding easier to do a few clicks at a time when the cowl is off for oil changes or other reasons. For now, I accept that the RPM will drop every few seconds when it's hot and idling.

I'll definitely update this thread if it gets worse or if I get it fixed.
 
It isn't solved, yet. I think the new fuel spider spring recommended above is the next step, which I will do either this fall when the plane is back home (I have to drive over an hour to its temporary home while my home airport is under construction) or next spring when the condition inspection is due. I have some other squawks to address at those two times, anyhow.

I have continued to adjust the idle mixture and idle speed, which I am finding easier to do a few clicks at a time when the cowl is off for oil changes or other reasons. For now, I accept that the RPM will drop every few seconds when it's hot and idling.

I'll definitely update this thread if it gets worse or if I get it fixed.

Are there specific fuel spider springs for the IO-390?
 
I have the same idle issue with the IO-360B1B on my -7A when the weather is warm and the engine is hot but was never too concerned as the stumble disappears as soon as the throttle is opened. After reading this thread, I got curious and emailed AFP to see if they thought changing the spring in the fuel divider on my FM-200 FI system would help. Kyle at AFP responded as follows:

"We do not make a spring that will move our flow divider opening pressure to 4 psi. We do sell that Precision part number spring, but it will not increase the opening pressure of our flow divider for its spring rate is already the same as ours. The diaphragms areas are different, so a 4psi spring to them is a 2 psi in our F/D. They do that to try to increase the back pressure to raise the vapor point. They do not make smaller nozzles so that is their only way to try to help the situation of vapor. We change to a smaller restrictor size in the nozzle so it will raise the vapor point from the nozzle back to the fuel control vs the flow divider back to the fuel control. Sometimes that can help as well as looking at the install to see if it can be cleaned up(shorter hose routing, no pressure cowls, no gascolators, transducers before the pumps or on the firewall etc). You can drop the restrictors to .024's in all the nozzles to see if it helps enough."

Restrictors https://airflowperformance.com/index.php/product/special-restrictors/

As a side note, Kyle also stated that flow dividers and fuel controllers should be overhauled every 8 and 12 years respectively. I didnt know this and mine are greatly overdue, although still seem to be functioning just fine. Same deal for Bendix/Precision systems.

Overhaul period https://airflowperformance.com/wp-content/uploads/2016/05/Overhaul-Period.pdf
Overhaul forms https://airflowperformance.com/wp-c...r-form-for-FM-Overhauls-Rev-November-2019.pdf

Erich
 
I have the same idle issue with the IO-360B1B on my -7A when the weather is warm and the engine is hot but was never too concerned as the stumble disappears as soon as the throttle is opened. After reading this thread, I got curious and emailed AFP to see if they thought changing the spring in the fuel divider on my FM-200 FI system would help. Kyle at AFP responded as follows:

"We do not make a spring that will move our flow divider opening pressure to 4 psi. We do sell that Precision part number spring, but it will not increase the opening pressure of our flow divider for its spring rate is already the same as ours. The diaphragms areas are different, so a 4psi spring to them is a 2 psi in our F/D. They do that to try to increase the back pressure to raise the vapor point. They do not make smaller nozzles so that is their only way to try to help the situation of vapor. We change to a smaller restrictor size in the nozzle so it will raise the vapor point from the nozzle back to the fuel control vs the flow divider back to the fuel control. Sometimes that can help as well as looking at the install to see if it can be cleaned up(shorter hose routing, no pressure cowls, no gascolators, transducers before the pumps or on the firewall etc). You can drop the restrictors to .024's in all the nozzles to see if it helps enough."

Restrictors https://airflowperformance.com/index.php/product/special-restrictors/

As a side note, Kyle also stated that flow dividers and fuel controllers should be overhauled every 8 and 12 years respectively. I didnt know this and mine are greatly overdue, although still seem to be functioning just fine. Same deal for Bendix/Precision systems.

Overhaul period https://airflowperformance.com/wp-content/uploads/2016/05/Overhaul-Period.pdf
Overhaul forms https://airflowperformance.com/wp-c...r-form-for-FM-Overhauls-Rev-November-2019.pdf

Erich

Did you notice the stumbling throughout the life of your engine, or only recently as the overhauls you mentioned became overdue?
 
Did you notice the stumbling throughout the life of your engine, or only recently as the overhauls you mentioned became overdue?

Engine has consistently stumbled during hot idle when weather is also hot. Goes away as soon as I apply power. This seems to be a pretty common thing with fuel injected RVs. High temps, no cooling air, and low fuel flow through the tiny spider lines results in vaporization.

Erich
 
Back
Top