Kevin,

 

Consider that at idle the hot engine driven pump gets no cooling at all.  There is no return line so that the boost pump is only marginally effective in raising the fuel pressure - thus reducing vapor "lock".  Consider that the exhaust system may be radiating infrared heating to the pump - one way to reduce that effect is to put a SS clamped (at tabs) piece of raised SS blocking radiation from reaching the pump.  Like a motorcycle exhaust shield.   Heat is your enemy.

 

Scott krueger

 

Hello fellow Lacairians,

I've been dealing with a rough idle problem on a Lycoming IO-360-B1B in a Lancair 360 for way too long. It only happens when the engine is hot, so I'm wondering if anyone else has had this problem and what, if anything, can be done about it.  The engine runs very strong in flight, and the problem only occurs on the ground when hot and at low RPM.  Turning on the boost pump has no effect. The previous owner said he had the same problem so this has been going on for a very long time.

After having 4 mechanics look into it and spending thousands of dollars, the problem remains. So far, we have put on a new fuel servo, new injectors, new plugs, and checked the timing and ignition system and found no problems. I've also subscribed to Savvy Analysts and they can't find anything wrong in the engine data. The next couple of things we are going to look at (again, but more thoroughly) are both fuel pumps (mechanical and electrical), and the fuel flow divider. We are also thinking that there might be an air leak somewhere in the fuel system because the boost pump won't prime until the engine is cranked on the first start of the day, but then it boosts just fine.  We are also thinking that we could have a vapor lock problem since it only happens when hot, and these are obviously tightly-cowled engines. I've heard from other Lancair 360 owners that many of these planes tend to idle rough when they're hot, so perhaps we've been chasing our tails this whole time and this is something we just need to live with?

One last anomaly that might be worth noting since it has now happened twice. During climb out on two hot days, fuel pressure began fluctuating (+/- 5 psi), but then stabilized in level flight at higher airspeeds. Once again, turning on the boost pump had no effect. Perhaps another indicator of a possible vapor lock issue? 

Any advice would be greatly appreciated!

Thanks,
Kevin



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