HELP - I have a rough idle when engine is hot

I need help with a rough idle problem. It only happens when it is hot after a flight. The cold idle is very smooth. The problem really rears its ugly head after a flight when I touch down and get pretty slow. About the time I am ready to turn off the runway and the tower is telling me to contact ground. Normally I would be idling at about 800 RPM, however the engine would die if I was tried that. I have to keep the engine at about 1200 RPM to keep it running. It will run for a few seconds and then cough and almost die and then it catches and run for a few second and coughs and almost dies again. I have tried different things with the mixture but no relief. As a bit of additional information, when I pull the mixture to kill the engine I get about a 50 rpm increase in engine speed.

Let me describe my airplane/engine set up.

I am flying an RV-9 with an IO360 engine, composite C/S prop, Airflow Performance fuel injection system and dual P-mags. I have a plenum cover and to combat the heat I installed a mechanical fuel pump shroud with a blast tube from the rear engine baffle wall. The only fuel lines in front of the fire wall that are not covered with fire sleeve are the lines from the spider to the injection nozzles.

I talked to Airflow Performance and they thought it was boiling fuel problem maybe at the mechanical fuel pump.

I have talked to others and they suggested an air filter issue. I am flying a K&N filter with a Rod Bower ram air system. I have opened the valve to bypass the air filter and it still idles rough.

Has anyone had a similar problem? Does anyone think it is fuel boiling in the fuel injection spider?

Any thoughts?

Thanks for your help.

Are you burning 100LL or something else?

Cheers, David

Welcome the Lycoming injected stumble, we're a fairly large community.

At idle, the low fuel pressure after the servo allows the fuel to boil at low heat levels and this causes the problems you are experiencing. I have done several things, but the two that made the biggest difference for me was the swap from a 2# -> 4# spring in the spider and the creation of a fuel return from the servo (entrance side) back to the fuel tank (it is metered by an .035" orifice to insure that full flow is available even if it is left open). When I forget to open the fuel return, I will start getting roughness much earlier. With it open, I don't notice rough running until the RPM's get down below 700 near the end of my roll out (I am FP and like the lower RPMs to improve speed reduction on short final). I also believe that my move from >028 -> .022 injectors helped as well. Not sure you could get enough flow from these with a 360 (I have a 320).

I believe that the bulk of the heat is coming from the servo, but I can't prove it. After shut down, I can hear the fuel bubbling away in the servo. My next step is to add a phenolic spacer between the servo and the sump. I have been avoiding it, as I will have to tweak my FAB to maintain alignment.

Good luck and let us know what works and what doesn't. Don at AFP is a great resource for ideas, suggestions and thy parts you may need. Remember, the heat absorption is cumulative, so anywhere that you can reduce the heat absorbed by the fuel, the better you will do. Also, many good threads on the subject here. Do some searching.

Larry

Do you have a fuel flow metering device in the engine area? If so, is it covered with an insulating cover too?

Bevan

We get the same but usually only after filling up when hot.

Ironically, I made 3/4" spacer between the servo and the motor during build but that was to avoid all the sillyness of the cutout in the FAB. That's probably why we don't get it too badly.

We just accept it as a characteristic !

Does it make any difference if you turn your boost pump on?

Jesse said:

Does it make any difference if you turn your boost pump on?

Click to expand...

+1 here. Jesse raises a good point. Many people find that running the boost pump during these conditions provides some relief (it did for me before installing the fuel return) and you should definitely try it.

Larry

Thanks for the help...

Thanks for the comments and questions. I am running 100LL and idling with the boost pump doesn?t seem to make any difference. Also, my fuel flow transducer in behind the firewall.

I was hoping to find a simple answer but there must not be one. How about insulating the spider and lines to the injectors? Has anyone tried that?

I did talk to a guy that is supposed to be good at these things and he, like Larry, thought it was the servo. He suggested wrapping the exhaust headers. I?d have to talk to Larry Vetterman, but I thought I read something about that being hard on your exhaust system.

Larry. I have a fuel bypass valve installed at the spider. I would not want to do this except in front of the hanger, but I could open the bypass a little and see if it makes a difference. I could watch my fuel flow and take it from 2.5 gpm to 5 gpm. Is the goal in your bypass to move fuel faster ahead of the servo to get cooler fuel to that part of the system?

Again thanks,

Tumper said:

I was hoping to find a simple answer but there must not be one. How about insulating the spider and lines to the injectors? Has anyone tried that?

Click to expand...

Been there, done that. Not any real difference.



I've since taken this stuff off.

I have all of your same basic symptoms. IO-320 with Airflow Performance FI and purge valve.

I just know that when it is hot out, and the engine is fully warmed up after a flight, I have to keep my rev's up to about 1000 or risk the engine sputtering and quitting.

I would like to try the higher pressure spring in the divider. Is this a task that is easy to do, or is it best to send it off to Don to retrofit?

Tumper said:

I was hoping to find a simple answer but there must not be one. How about insulating the spider and lines to the injectors? Has anyone tried that?


Again thanks,

Click to expand...

I found that mixture matters.

By the time I taxi to park on a hot day (IO540), the idle is not smooth...not alarming, but, just not smooth.

I found that leaning the mixture helps or completely eliminates the issue, not to mention that ground leaning keeps your plugs cleaner.

Boost pump makes no difference in my case.

Like Scott said, aggressively lean at all times when on the ground. Lean to the point that the engine bogs down when adding throttle. Your plugs and exhaust valves will thank you.

Carl

Tumper said:

Thanks for the comments and questions. I am running 100LL and idling with the boost pump doesn’t seem to make any difference. Also, my fuel flow transducer in behind the firewall.

I was hoping to find a simple answer but there must not be one. How about insulating the spider and lines to the injectors? Has anyone tried that?

I did talk to a guy that is supposed to be good at these things and he, like Larry, thought it was the servo. He suggested wrapping the exhaust headers. I’d have to talk to Larry Vetterman, but I thought I read something about that being hard on your exhaust system.

Larry. I have a fuel bypass valve installed at the spider. I would not want to do this except in front of the hanger, but I could open the bypass a little and see if it makes a difference. I could watch my fuel flow and take it from 2.5 gpm to 5 gpm. Is the goal in your bypass to move fuel faster ahead of the servo to get cooler fuel to that part of the system?

Again thanks,

Click to expand...

I have insulated the injector lines with no noticeable improvement.

The bypass needs to be pre-servo, not post-servo. The servo meters the fuel based upon airflow and the fuel flow out of the servo should not be altered. I tapped the other side of the filter chamber on the servo and ran it to a block with the FP sender on the firewall. I added another 1/8" NPT port on that block and ran that to a valve in the cabin that further flows to the tank return.

Larry

bruceh said:

Been there, done that. Not any real difference.

I've since taken this stuff off.

I have all of your same basic symptoms. IO-320 with Airflow Performance FI and purge valve.

I just know that when it is hot out, and the engine is fully warmed up after a flight, I have to keep my rev's up to about 1000 or risk the engine sputtering and quitting.

I would like to try the higher pressure spring in the divider. Is this a task that is easy to do, or is it best to send it off to Don to retrofit?

Click to expand...

A 15 minute job. For a Bendix unit, take out the four screws holding the top plate on (not the small access cover on the bottom)(the top is furthest away from the injector line ports). Leave the diaphragm in place and do not change the clocking of the diaphragm. The spring sits between the diaphragm and the cover. Don sells the spring.

Larry

I had a significant similar problem early-on. It was reduced to a minor problem/annoyance by: a) Don Rivera assisting with reduced FI nozzles (.028 to .022), and b) running aggressively lean on the ground at all times. Now in the hottest wx it evidences something that sounds like minor "missing", but is never in any danger of dying or quitting.
I'm not sure why the lean-running should help with a boiling fuel problem - if anything, you'd think it would worsen it. Nevertheless it does seem to help.

I've got a very similar setup in my plane, 9A with IO360 and CS prop, Bendix injection, except I deleted my engine-driven fuel pump and I use dual electrics instead, and I still had the same problem you have found. I finally ended up adding a second layer of firesleeve around all my FWF fuel lines up to the spider (not the stainless lines after the spider) just for some insulation and that did the trick, no more "Lycoming Hot Stumble". Note that this will not help heat-soak on shutdown, just while it's still running.

Hot starts can be interesting with all the fuel boiling in the lines, I intend to replace the spider diaphragm bias spring as others have done, and switch the orifice size from stock .028" to .024" for the 360. I believe the .022" is intended only for the 320, you need .024 for the 360.

Reducing the restrictor diameter to .024" at the suggestion of Don Rivera worked for me in the RV-8. I live at high altitude, so I never needed the higher flow injectors anyway.

Guy

Sorry to revive this thread but it's getting warm in the midwest and the stumble is rearing its ugly head more than usual for me.

I see hints that some have resolved this issue in the thread. I'm curious to learn of the configuration for any of you who don't have this problem. Could you post the following details to help the rest of us?

- What FI system you have
- What engine do you have?
- Where is your red cube? pic?
- Are you cooling your mechanical fuel pump?
- Anything you specifically did to fix this? Or did you never have it?
- Pics of your fuel hose routings would be excellent!

Please consider helping us, I think together we can figure out which setup works and which ones don't and hopefully have a path to stumble-freedom for all of us.

Thanks!

I have the exact same problem. Engine tries to die after landing. No amount of leaning / boost pump seem to help. I am also thankful for this thread in trying to find an answer.

Reducing injector size is a good idea, but you first need to know what your current ones are. Then discuss with Don Rivera as others have noted.

Also, be sure after putting the smaller nozzles in that you get full fuel flow at WOT at or near sea level (or whatever is normal for your engine/field elevation). Too small nozzles and you might not get the proper fuel flow. The servo can only accommodate a limited range.

I had the same problem last fall when I started flight testing my aircraft. I tried various idle mixture settings, cleaned and checked the injectors with no success. I called Precision Airmotive, who suggested putting heat shield on the injector lines. I did so this spring, using AB Tech S-AS1072-M006-04-OR 500'F/260'C Aerospace Marine Firesleeve-1/4/0.25 Oxide Red. It took 7 feet and the price was very reasonable.
It has made a difference in the hot engine idling. The engine will still chug a bit occasionally, but leaning the mixture a bit helps. I have also noticed a big difference in hot engine starting. I generally starts right up with the mixture in idle cutoff moved slowly to rich with the throttle cracked. So it has made a noticable difference with my engine.


Frank Huber
RV-7A
Aero Sport IO-360 with a Precision Silver Hawk system

Has anyone found a difference by trying to cool the mechanical fuel pump? I see that Vans has a shroud kit for this. I’m struggling to understand why the fuel pressure should drop if the issue is located after the fuel pressure sending unit. I could see the pump just working ineffectively if there was a big glob of vapour inside.

I had a similar problem in my last RV. I read somewhere that insulating the tiny fuel injector lines is a big no no. My problem was solved by fire sleeving everything including the fuel flow cube, adding a blast tube to the mechanical fuel pump, and most importantly by installing heat shields everywhere the fuel lines get close to any exhaust. I previously tried wrapping part of the exhaust but ended up with a big crack after about 50 hours.

My engine IO-320D1A idles erratically when hot too. Not an issue while in the air...fuel pressure remains steady and the engine runs fine. But a long taxi after landing is often erratic on the way back to the hangar, and is associated with variability in fuel pressure. That's manageable, and was improved with some exhaust shielding and some insulation of the lines from the fuel pum. But more annoying is the difficult restart on short turnarounds. Multiple Young Eagle flights in a row, or a short fuel stop is where this is most problematic for me.

I made a simple fan out of a high-volume 12v muffin fan that I put into the oil cooler door when I shut down on a short turn-around. I did this after noting at RV fly-ins that everyone leaves their oil door open. I figured if a little is good, more is better. It's amazing to me how hot that under-cowl exhausted air is initially, and that it cools down fairly quickly. Was cheap to build from the fan, some Kydex and a molex pigtail.



..