Ask Paul: The trouble with troubleshooting

Q: The problem I’ve been trying to troubleshoot is with a Lycoming IO-360-C4P engine installed in an owner maintained airplane.

First, the engine would not idle well. It ran rough and the owner lived with it idling at 1,000 rpm or above. There were also some issues with EGT at power and running rough.

The fuel system was checked and we found pieces of rubber in the distributor and a couple of injectors. New fuel hoses were obtained, flushed, and installed and the fuel distribution system cleaned.

But the #1 and 2 cylinders are still acting up. We pulled the #2 cylinder intake tube and found heavy fuel staining in the cylinder head intake side.

A fuel flow test from the nozzles, at full throttle, full rich showed even amounts of fuel. A fuel flow test at idle and full rich seemed to indicate a high fuel flow, greater than 3 gph.

The servo and distributor were sent to a shop, checked, adjusted, returned, and installed.

But we still had the same issue with the #2 cylinder.

A burp check was accomplished on all cylinders with no faults found. We also soap checked the cylinders for a crack, none were found.

The timing and plugs were also checked.

Now the MAP (manifold pressure) is high at idle, and drops when the throttle is bumped to 1,000 rpm.

Of other note is the #2 EGT drops at idle (less than 700 rpm) — like it just quits. It comes back to life, but lean, at 1,000 rpms.

Just wondering if you have any insight.

Dave Mulholland

A: Dave, I’ve done a quick review of the data you provided and think we still have a problem in the # 2 cylinder area.

I can’t find any fault with the troubleshooting you’ve completed and would have thought you would have discovered the problem. Obviously, that’s not the case, since you still have high manifold pressure at idle. I’ve gone over the graphs and can still see the higher manifold pressure and lower EGT on the #2 cylinder at idle.

The only thing that comes to mind now is do you think there may be a possibility of an induction leak originating from the intake tube where it’s swagged into the sump? I know you mentioned soap checking the cylinder, but I’m not certain if you did the area around the sump.

The other question I have is does this show up when the engine is first started or only when the engine is at normal operating temperatures?

Is there any possibility that the #2 intake pipe has a slight crack at the flange, which is usually a result of uneven tightening of the intake pipe bolts? Because they are hidden by the flange itself, cracks sometimes go unnoticed.

One more thing: Have you readjusted the engine idle rpm since the fuel servo has come back from the shop? How about checking the rpm rise at idle (650-700 rpm) and leaning the mixture control to the point where the engine quits? Just before it quits, you should see a 25 to 50 rpm rise. If not, then you may need to do more adjusting in that area. 

Dave responds after reading Paul’s answer:

This is a long response, but this issue has been going on since 2018.

It happens in a cold or hot engine.

Numerous folks have stated they thought it was an intake leak. Problem is, I don’t see how an intake leak would affect a fuel-injected engine. The servo trickles idle fuel into the system and does not start modulating fuel pressure until somewhere after 1,200 rpm. In theory, you could remove all the intake tubes and the engine should idle, although it may be messy. Remember, in theory.

All intake tubes are tight, crack free, new gaskets and sump tube swages are tight.

I previously played with the idle mixture some six to eight clicks without any change in the performance or indications.

Never did get an rpm rise on shutdown.

We pulled the #2 cylinder to check it out and found no issues. Valves and seats were done, honed, and new rings installed. 

Checked the cam, tappet body faces, plungers, everything we could see with the cylinder removed. No issues.

The engine behaved the same after re-installation. We pulled the #2 cylinder intake tube and found it soaked in oil. We borescoped the sump and found nothing abnormal. We ran it again and no oil this time.

Just for fun, we turned the fuel distributor 180° and reconnected it, temporarily. It worked. The #2 cylinder came to life and EGTs were even at idle. We put it back in its original orientation and the #2 cylinder died at idle, again.

We opened the fuel distributor valve again (probably the third time) checking for debris, but none found.

Cranked the idle mixture 13 clicks lean and #2 came to life and engine ran smoothly, but the other three cylinder EGTs rose dramatically to the point that very little leaning with the mixture control caused the engine to quit without an rpm rise.

Next, we rotated the fuel distributor valve and mounted it permanently. We went six clicks back rich on the idle mixture, reset the idle speed and now get a 20 to 30 rpm rise on shutdown. 700 on the idle and MAP was 10.5.

It would appear that flow out of the flow divider is not even.

Contacted the local shop that tested the divider. They stated that the dividers are not cylinder specific and that all they do is a total fuel flow check at 4 psi. Their tech states that flow from each port is adjusted until all ports flow the same on new builds.

The bottom line is that we may have been chasing an issue caused by an unbalanced flow divider. The #2 was so rich it would not run at idle, hence the high MAP. Since both the servo and the flow divider had been to the shop, we trusted they were in good working order. The next flight data will tell the story.