Stuttering 360

[bjohnson1234]

I have a Mattituck XP-360 that has a stuttering issue. It's either not noticeable at cruise RPM or it doesn't happen, but I have definitely noticed it in flight and on the ground. Every 3 or 4 seconds it very briefly loses some power, maybe a drop of 50 RPM or so and then spins back up. It's both audible and visible in the prop. I have sent my engine data to savvyanalysis and everything came back looking good, so I'm pretty sure it's not ignition, intake leak or sticking valve. It also seems to happen more frequently when the engine is hot (maybe always?). I checked the timing recently and savvyanalysis confirmed that it's correct. I have a fixed pitch prop, so it's not a governor issue. All my fuel lines have firesleeve and I cleaned out the fuel filter in the fuel injection servo. I also checked to make sure there are no broken springs on the valves. Everyone I've spoken to is stumped. Has anyone else seen anything like this? Any ideas what it might be?

[N427EF]

I am not sure I can help but I am sure someone will be able to point you in the right direction.
Can you tell us a little more about the history of your engine/airplane?
XP-360 I assume its a carbureted version right?
How many hours, did you build it, when did this start, has any maintenance been done prior to this RPM anomaly.
Ignition set up, mag EI or combination, what kind?
Engine instrumentation? and I see you have recording capability.
What kind of fixed pitch prop?
Anything else you can think of, even things that might seem unrelated.
We love to help you.

[bjohnson1234]

I bought it. It has about 180 hours on it, I've put about 100 of those on it and the problem has always been there. Been through annual and has good compressions.

It's fuel injected.

LASAR ignition system, but the problem is the same with the breaker pulled and running on either mag and fuel pump on/off makes no difference

AFS EFIS with engine monitor

Catto 3 blade prop

I have seen other people talk about a similar issue that turned out to be fuel vaporizing in the lines, but I have fire sleeve on all my fuel lines. The only unprotected lines are the ones leaving the distribution block.

Crossover exhaust, maybe too much heat on the gascolator?

I tried multiple times to replicate the problem with the cowl off and was unable. After putting the cowl back on, the problem quickly returned and then stayed after I took it back off again. Tomorrow morning I'll try starting it cold with the cowl on and see what happens.

I live in Orlando and it's really hot and humid.

Originally Posted by N427EF

I am not sure I can help but I am sure someone will be able to point you in the right direction.
Can you tell us a little more about the history of your engine/airplane?
XP-360 I assume its a carbureted version right?
How many hours, did you build it, when did this start, has any maintenance been done prior to this RPM anomaly.
Ignition set up, mag EI or combination, what kind?
Engine instrumentation? and I see you have recording capability.
What kind of fixed pitch prop?
Anything else you can think of, even things that might seem unrelated.
We love to help you.

[Russ McCutcheon]

I have never had this happen in the air but mine (IO-360-A1A) runs poorly on the ground hot, its fuel vaporizing in the injector lines, once you add throttle to take off the pressure in the injector lines increases to a level where the fuel remains liquid and the engine runs normally, it does not do it with the cowl off because you?re not trapping the heat. This behavior on the ground is totally normal, nothing to worry about.

Whatever is happening in the air is probably something else because the presser in the injector lines is up and the temp is lower, could not say what?s causing the inflight issue.

[Norman CYYJ]

If your engine is stuttering maybe it needs to take some therapy to find the root cause of it.

[N427EF]

You have a Mattituck XP IO-360.
More questions:
My guess is like Russ pointed out, a fuel vapor related problem.
The fact that you cannot duplicate the problem with the cowl off, somewhat confirms my suspicion.
The problem is finding the location where your fuel lines are heated to the point
where vaporization occurs and your fuel delivery becomes sporadic and partially intermittent. As Russ pointed out this occurs most often during idling after long taxiing on hot days and with low octane fuel when fuel flow is nearly stagnant. Well built engine installations do not do this and the fact that it happens to you in flight, tells me that perhaps your exhaust pipes are too close to some part of your fuel system. (Gascolator?)
What is your fuel pressure in flight?
It should be between 20 and 25psi. (wrong fuel Pump maybe?)
Have you tried the electric boost pump on to see if the problem persists?
The boost pump will not help if the problem exists after the fuel controller.
There is only one hose going from the fuel controller to the spider and fire sleeved as you pointed out. Usually routed up between the cylinders and not a problem in other installations.
Do you have a Bendix or AFP fuel injection or other?
Are your injector lines installed with stand offs and adel clamps to keep them away from cylinder heat? If Mattituck built it, I am sure it was done right.
What is your oil temperature on average when this problem occurs?
EGTs and CHTs?
What is your engine's compression ratio and are you using 100LL for fuel?
Do you have any pictures to post of your engine installation?
Specifically fuel system installation.
I think we are getting somewhere.

[Mike S]

When the stuttering happens, are you running LOP???

If so, how well are your EGTs balanced?

[Gloster Air Parts]

If you are losing 50 rpm only, then could it be an ignition problem with one may only? When it happens try an in flight may check.
Pete

[cfiidon]

I have an ECI XIO-360 and I used to stutter on the ground. Sometimes it would just quit altogether while taxiing. It was exacerbated by leaning for ground ops.

I diagnosed it as fuel vaporization. It occurred on a day that was 75, but not the next day when it was 65, or when I left the oil access door open. I noticed that the fuel pressure would decrease exponentially just before it stumbled and died. If prepared, a quick hit of boost pump would allow recovery.

Although infrequent, I did notice it one day while flying. It was intermittent.

I installed a blast tube on the fuel pump and so far the issue has nearly disappeared. If I lean during ground ops it will stumble more than when full rich, but it is not bad and has not died on me like before.

It is a simple one inch blast tube from the left baffle that points at the aft section of the fuel pump. You are correct, it just is too dang hot here in Florida.

You might try some tests to confirm this... Note temperature, open oil access door, perhaps while on the ground block open your rear baffling to allow some cooling air to circulate, watch fuel pressure closely and see if you can correlate the events.

Let us know what you find out.

Don

[bjohnson1234]

Wow, that's for all the feedback! Let me try to answer all the questions:

Fuel is 100LL and it's less than a week old

Crossover exhaust does come close to the gascolator, but turning the fuel pump on doesn't help

Fuel pressure is 22-24

The line going from the injector servo to the spider is routed behind the engine. I moved it yesterday when I cleaned the injector fuel filter and briefly I thought it was fixed, but the problem returned. There are no standoffs, but it doesn't go near the cylinders until it goes through the baffle. Could it be happening in the lines from the spider to the cylinders? They don't have any heat protection on them.

Precision fuel injector servo RSA-5AD1 part# 2576536-2

I have seen it happen both when the oil temp is high (in flight 180 or so) and at start on the ground (100ish)

#3 CHT runs 20-30 degrees hotter than the rest, otherwise EGT/CHT look normal as confirmed by savvyanalysis

Happens regardless of mixture settings

I have done an in-flight mag test, sent the results to savvyanalysis and it came back with no issues

I have some blast tubes going to the mags and LASAR controller, but nothing to the fuel pump.

The symptoms in the air are identical to the ones on the ground, could it still be the same issue? I have specifically noted it at 2000 RPM, but I will take it out today and see what point it stops at. I'll try to post some pictures as well. Thanks again!