IO360 LCO problem Precision Servo
 

Good morning/afternoon
I am just wondering if anyone might have come across the following problem ? The one year old engine will not give a clean cut off (LCO) when hot (runs on for 5-10 secs). It has two P mags installed. The engine has done 110 hrs but the problem has been there some time. Superior stated that the normal problem with these symptoms is a scored plate (mixture control rotating plate). Engineering at my airfield (UK) have plenty of experience of servos (mine is a Precision) so I worked with them for a couple of hours to try and find and fix the problem. We checked to see if any fuel was leaving the servo outlet to go to the spider with the Andair pump on (+/- 35 psi) at LCO and there may have been a hint of fuel but minimal. We took out the plate and although the slightest marks could be seen the chief engineer was one hundred percent sure that it did not amount to scoring or could be the cause. We took the last filter (servo) out and cleaned and inspected (perhaps the smallest debris from manufacture but hardly noticeable) and flushed fuel through before reassembly. Took each injector nozzles out to ensure air bleed holes clear. We reassembled everything and tested for fuel getting past the servo at LCO ? none. Test flew two circuits and LCO fine. Next longer flight, and on the ground, taxying back for several mins (meaning hot everywhere under the cowlings) and LCO was the same problem. It is also the same whether LCO is attempted at 1000 rpm or 700. LCO is only a problem when very hot under the cowlings (eg after a long taxi on landing) and that?s only ever been the circumstances when the problem occurs. In addition, my RV7 has always been particularly difficult to start, hot, trying all normal, ?Injected Engine? hot start procedures (cold she starts perfectly). I am wondering whether the cause of the difficult hot starts could be associated with whatever is the cause of the poor LCO (I realize the hot start difficulty may be nothing to do with the LCO problem). As an aside, it is a vertical air intake servo.
Any similar experiences and solutions would be much appreciated.
Regards
David Robbins

Your fuel in the line between the injection servo and the fuel divider on top of the engine is getting heat-soaked and reaching the boiling point. When you pull the mixture to lean cut-off, no more fuel is leaving the servo but the fuel already in the line between the servo and the divider is boiling - providing just enough pressure to keep pushing some fuel through the injectors and into the engine, which is causing your problem.

You can try insulating that fuel line with a double-layer of firesleeve, or using some radiant heat shields or a combination of those - but heat-soaked fuel in that line is the root of the problem.

Insulated
 

Thanks Greg

The line servo to divider is insulated, as per the photo (if my link works). The FT60/Red Cube in the line is not insulated but, I assume, would not cause the problem?



Regards

David

photo try again
 

Oh yes it could - anything that can absorb heat from the surroundings and transfer it to the fuel is going to contribute. I have my FT-60 mounted on the cool side of the firewall.

David, is the servo-to-red cube line running close to an exhaust pipe?

I agree with Greg; the likely culprit is heating of the hose and cube feeding the divider. At idle, flow rate is low, so the fuel temperature rises more than it does when flowing at a cruise power rate.

Although the fuel may be reaching 140F+ in the aforementioned line, the fuel can also be picking up heat on its way to the servo.

So, look hard at all lines, those routed near pipes in particular. Firesleeve is a reasonable insulator, but distance is king. And try wrapping the lines with a shiny foil, like aluminum tape, right over the top of the firesleeve.

Agree with others. If this really bothers you, i would install the 4# spring in the spider. This raises the low volume fuel pressure a bit on the post servo side and will raise the boiling point a few degrees. It helped during my remediation efforts for hot running roughness. I also insulated linesand installedheat shields on exhaust in some areaas.

Don at AFP has them.

Larry

That will work, to a degree - what worked a LOT better for me was to change the restrictor orifice size on the injectors to .022". This keeps the line pressure a bit higher between the servo and divider at low-flow conditions, but also requires higher fuel pressure to be able to run full-flow at full power settings. I raised my fuel pump regulator to run about 42 psig to cover that.

Larry and others - "Should it bother me?"
 

Hi Larry

You say "..if it really bothers me .." which it doesn't particularly. I am just assuming that it is not doing anything much good and best I fix it? e.g. the engine fires very roughly and so jerks engine mounts etc during those 5-10 secs? Two possible work arounds? -1) Both mags to off - as I was used to when shutting down Rotax 912 engines? Someone else suggested, with care, go to WOT - would that help?

Regards

David

Kill the mags after going ICO. Pretty much every other engine in use shuts down by killing the ignition. Nothing wrong with doing that here. Our method was put in place years ago to protect people hand propping planes. With electric starters, not much risk from a hot mag and therefore no need to confirm on every shut dowm.