This I/O-320 is driving me nuts. I could really use your advice if you have a second to read what I've done so far. Here's where I'm at:

PROBLEM: L/H engine misses (and occasionally backfires) under 1500 rpm when hot. Above 1500 rpm- no issues; engine runs great. It only misses when the engine is warm with the cowling on. It runs perfect cold/cool or with the cowling off. Run the engine for a little while to warm the cylinders and oil with the cowling on, then it'll start missing when you bring it back to idle. The miss can be heard, and can be seen by an intermittent 100 rpm drop accompanied by a 1" MP increase immediately following the rpm drop. Let the engine cool a bit, and back to normal operation. You can watch the tach and MP gauge bounce up and down consistantly once the miss begins. I even have a video of the MP/Tach if you want it.

1. Mag check acceptable. Acceptable mag check at 1000 rpm increments above 1500 rpm. No increase in miss frequency under 1500 rpm running on either single mag. Plugs and wires should be good based on that. Changed plugs from "good" R/H to "bad" L/H engine anyways in case we had multiple bad plugs- no change. Suspected mag coil at first due to heat soak, but there is literally no difference between L/H and R/H mag operations.

2. Fuel injectors were cleaned and swapped between engines. No effect, and R/H engine runs good with the "bad engine" fuel injectors. Fuel pressure is normal.

3. Fuel divider (spider) swapped between engines (suspected sticking valve). No effect.

4. Mixture control has no effect on frequency of miss.

5. Idle mixture setting (on fuel servo) has no effect on frequency of miss.

6. Boost pump has no effect on frequency of miss (suggesting fuel servo is ok).

7. Input fuel pressure to the fuel servo is 22 psi at idle (above the 20 psi minimum for the RSA-5).

8. Fuel Servo and Fuel Selector fuel filters are clean.

9. Ten ounces of fuel was pumped through each fuel injector through the system- no water observed.

10. Inspected for induction leaks. No noticeable leaks. All intake gaskets are new. All induction hoses new. No noticeable cracks in the intake pipes.

11. Have hit the exhaust manifolds and cylinders with an infrared thermometer. Can't see any real appreciable difference in temp immediately after shutdown. I'm sure real-time CHT/EGTs would be very beneficial here though!

12. Performed another compression test. All cylinders in the mid 70s/80.

13. Did not perform valve wobble test, but the odds of a sticking valve are very slim. The miss only occurs with a hot engine, and this engine has 450 SMOH with new Superior cylinders, which rarely have valve issues.

14. Detected a hint of tapping in the engine. Pulled all lifters, disassembled and tested. One plunger was suspect (it separated when pulling it out of the tappet- never have had that happen). Cleaned and checked the plunger and it held pressure fine. Replaced the plunger anyways. Performed dry tappet clearance test. Found one intake valve with excessive clearance (0.084"). Adjusted with longer push rod. Measured valve depression once the lifters were bled off. All valves depressed the same amount (suggesting my cam lobes are ok). The exhaust valve on the #3 cylinder wasn't pressing down quite as much as the others (0.020" less) immediately after shutdown. That's the lifter that was replaced.

15. Checked sniffle valves. They're closing fine.

I ran the engine last night and thought the problem was fixed at first. You can't get the problem to present itself with the top cowling off. Something is staying cool enough with the cowling off, even when cylinders hit 350 deg F and oil is warm. Replace the cowling, and you can get the problem to start fairly quickly. Mag checks are again fine.

Maybe I'm getting an induction leak when the engines warm only. Maybe vapor lock. I hear Cessna installs their spiders upside down now to prevent vapor lock on the ground. I can't figure it out though.

Any other ideas?!?! I'm going nuts here trying to get this aircraft back in the air and am incredibly frustrated.

Have a great 4th!
David

David,

No advice - just encouragement - it will feel great to fly.

David, Did you try running the boost pump when this miss starts? Sounds like you may be getting vapour in the fuel lines as the engine is idling on the ground. How is the hose from the upper baffle to the shroud over the engine driven fuel pump? Maybe with the lower volume of fuel going through the pump, the fuel is starting to vaporize.
With everything you have done for trouble shooting, I would say you have eliminated any contamination/ leaks/ mechanical problems.
So check that out first, then you might want to check your fuel servo for an internal fuel leak past the seal. Given your hot only symptoms, thats a long shot but maybe worth looking into.

Good luck,
Gary

I am also wondering if the engine-driven fuel pump itself might be the problem.

Thanks gang. No change with the boost pump on. I suspected the fuel pump, but fuel pressure is good at the input to the fuel servo (as determined by external gauge). I suppose it could be generating some air pockets possibly at high temp. I can try swapping mechanical fuel pumps, though I'd expect the problem to go away with the boost pump on if the mechanical pump was having any issues. Plus would be surprised that it didn't cause issues above 1500 rpm.

I'll try anything at this point though! Keep the ideas flowing! Thanks!

I bet you have a bad fuel injector line to one of the cyl. Take a good look under all of the B nuts and makes ure you dont have a crack in the flare/flange area. If it heats up and opens the slightest bit, a slightly reduced pressure will cause intermitant vapor "lock" on one cyl, which will then miss with the subsequent excessive rich next intake stroke causing an afterfire.
-Zach

David, Ref Zachs suggestion about a cracked injector line, If you don't see a crack in a line, it might be too small to see when cold.
Try swapping the right Eng injector lines with the left eng lines. If the miss follows then you nailed it, if not its back to the drawing board.
Let us know what you find.

Gary

Ended up being vapor lock, caused in part from simply heating the engine too much/long on the hot Florida ramp in summer, and also due to a fuel line with a fairly tight bend radius. The hoses were replaced by the last owner in 2012. They made one line too long and made quite a tight bend radius between the firewall and fuel pump. I cut the line shorter and re-ran it. It helped a lot. The engine will still miss a little under 1500 rpm (very low fuel flow) due to vapor lock eventually after you excessively heat it at high power on the ground, but it has to be heated excessively. No issues over 1500 rpm and no issues under normal ops. Thank you all for your inputs!!

Got one more flight off. Had my R/H engine oil temp spike before landing. Found the vermatherm to be bad. Will have a new one Monday. R/H prop also decided to start surging UP in rpm. I overhauled both governors! Obviously something between the pilot valve, fly weights, or spring in the governor (nitrogen in the prop was fine). Prop shop took it back under warrantee to tear down. Ohhh, the joys of aircraft restoration. Spent 8 months on the airframe, never thought I'd be spending this much time on 400smoh engines overhauled in 2006!

David - so glad to hear you got to FLY!

Sorry, for a reply to an older discussion, but I've had a similar problem and the thing that seemed to help the most was tightening the exhaust slip joint. I had one that was leaking enough to coat the underside of the engine. So, of course I addressed it at annual, used exhaust type swedging tool to make it a tighter fit and low and behold, the miss went away, the MP evened up as well. I don't know how to explain the improvement though.

Had similar issue. Mag checks were good at higher RPM. At the low end one mag would start cutting out. Turned out to be a bad condenser in the mag. Not sure what style the twinco has.