Engine consistently stumbled post oil change between 2200 and 2400 RPM; now it seems fine

[FloridaMan]

Performed oil change Tuesday. Cut open filter and inspected for debris. No debris. Full power run-up and leak check good.

 

I decided to take it up for a quick flight. My hangar is very close the runway that's most often used so I usually warm up and run up before taxiing out and leave the fuel pump on, mixture rich, take off flaps, et cetera. I had to spent a significantly more time idling around on the ground at full rich than usual. 

 

Went to take off. Applied full power. Engine stumbled. Taxied off runway. Performed run-up. Went to take off again. Same stumble. Taxied back to my hangar and did some more run-ups and checks. It seems to always occur between about 2250 and 2400 RPM and is a slight stumble. It would always go to full power, but sometimes would just stumble a bit and sometimes it would hang for a couple seconds at 2400 RPM before progressing. All engine instruments appear normal. The stumble is similar to what you might feel if you tried to taxi/takeoff with the mixture too far out. It felt like a misfire. Cycling the prop and then running up made the issue go away, so did trying it a little bit leaner, but neither was consistent. 

 

Two notes: I changed the oil and filter with the engine cold; I usually do it hot, but decided to try it in a way that I didn't risk burning my hands and spilling hot oil. I did not sump the ring by the fuel selector; I had flown since previously fueled and the plane stays in a private T-hangar.

 

Yesterday, I performed the fuel servo check with the ram air open (so I could look inside), mixture idle cut-off, throttle open and fuel pump on. No fuel leaks observed. The fuel pump did sound a bit anemic. I sumped the hell out of the tanks and pulled the ring under my feet for several seconds until I got a decent puddle. I let the plane sit for a couple hours and then pulled it out, fired it up and taxied around for about half an hour doing every combination of fuel-pump on/off, run ups from idle to full power, from 1900 to full power, et cetera. The stumble did not occur a single time, but it concerns me that I don't know what caused it. I guess it could've been fouled plugs that didn't show up at my 1900 RPM mag check, vapor lock or possibly air in the prop governor, but the fact that I don't know what caused it is cause for concern as it was fairly consistent the day it occurred and now is no more. 

 

Also, last month after spending an hour in IMC, I opened the ram air in VMC and noticed the airplane surging, so I closed it and as was good. I assumed that perhaps some water had loaded up in the port in front of the gate. I tried it again the next day; the surging started. I enrichened the mixture and the issue went away. There has been no significant change in fuel burn. I do not have an engine monitor. 

 

There are two suggestions that I've heard. One is that there could be air in the prop governor and the other is that it may be the fuel servo. The engine is 1700SFRM/1994. Last annual was completed by Phil at Avon Park (reputable Mooney tech in Florida) in August. Cylinder #4 showed 56/80 so it was removed and overhauled; the piston was replaced. Propellor was removed, inspected and resealed. Cylinder #1 was replaced a few years ago.  I have put 55 hours on the tach in the month following annual. 

 

The thought of possible blockage or contamination within the fuel lines is also something I'd want to get checked. Is there a screen or filter that I should request an A&P inspect? 

 

I found this document: http://www.precisionairmotive.com/Publications/15-810_b.pdf Page 16 states that hesitation between 2200 and 2400 RPM is often a propellor issue. 

[N601RX]

Is it possible you didn't have the prop control all the way in? I tried to do a runup one day with it out some and the engine sounded very loaded down and would not go past 1700 rpms.

[FloridaMan]

Prop was full. The misfire would hold for just long enough for me to notice the hesitation and then the rpm would increase to 2700. It never lasted for more than a second. Typically, it sounded like "rrrrrrrrrrrrrrbumpbumpbumprrrrrrrrrrrrrrrrrrrr"

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[Skywarrior]

You mentioned that you spent a lot of time idling at full-rich mixture.

 

Try leaning aggressively while idling, taxiing, etc. If you still get stumbling, do a lean run-up.

 

You may have just had gunk on your spark plug tips.

 

This same thing happened to me when I was transitioning to my plane, and my instructor had me do those steps.

 

Took care of it.

 

I now lean aggressively while on the ground.

[rockydoc]

I would suspect foiled plugs do to, "...more time idling around on the ground at full rich."   I would try aggressive leaning on the ground and during the run up.  You could pull the plugs just to visually check them for fouling.

[FloridaMan]

It seems ok now, but I wanted to run the issue by you guys.

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[Bartman]

I had similar thing happen on Monday. On taxi from the ramp to the runway at hilton head I neglected to lean and left it full rich. On runup I had a fouled plug on one mag, (first time in about 5 years as a matter of fact). It took 3 or 4 tries at a 2,400 RPM runup that was VERY agressively leaned to clear the plug. I attribute it to the long full rich taxi and warmup. However, I have had the aircraft for about 650 hours now and I know it has the massive electrode plugs. I'm thinking that it may be time to go with the fine wirre Tempest.

[FloridaMan]

Can a single fouled plug cause an engine to stumble when adding in full power? I pulled my upper and lower cowlings today, inspected for leaks around all the fuel lines and fuel servo. I really need to go ahead and bite the bullet and buy an engine monitor. It would have at least helped me figure out where to start looking if it were a single cylinder, or fuel flow issue, et cetera. 

[Hank]

I taxi around so lean that giving extra throttle going into the grass by the ramp made it stumble and threaten to choke out, maybe 2/3 back towards cutoff.

I also don't "warm up" the engine. My Owners Manual says that if it will make full power and run smooth, it's warm enough to take off. My coldest departure has been 8F, I taxied a total of about 1/2 mile hangar to departure end, and did my normal 1700 RPM run up. I think I was 2nd in line, call it 5 minutes, leaned out the whole time except for full rich runup. Now I'm on a warmer climate and my hangar is a longer taxi, so it's officially not a concern, cooling on the ground has superseded it.

[MB65E]

Yes, plugs are funny sometimes. Guessing bad sparkplug ...Could be an injector with some junk in it. (Doesn't take much) that's also about the same time/rpm that the fuel servo transitions from an idle function to full power.

Don't think the prop will ever make a sound like you mentioned. Also, I try and wait untill 200cht and 100oil temp before a run up. It's treated me well so far. But I'd check the injectors, timing, plugs, and the little finger screen in the fuel servo if it's injected. (I think you were injected right?)

Good luck,

Matt

[ArtVandelay]

Can a single fouled plug cause an engine to stumble when adding in full power? I pulled my upper and lower cowlings today, inspected for leaks around all the fuel lines and fuel servo. I really need to go ahead and bite the bullet and buy an engine monitor. It would have at least helped me figure out where to start looking if it were a single cylinder, or fuel flow issue, et cetera.

This is where an engine monitor pays off, you would be to narrow it down to which cylinder and which plug, also I lean aggressively, even during run up because being full rich can hide problems, like partially clogged injectors or in my case a few months ago, a completely clogged one

[FloridaMan]

Update. I went through the fuel system with a mechanic and inspected the servo filter and other components. I did not pull the injectors, ran the engine up again and the problem had returned with a vengeance. This time the problem was there and stayed there at 2400 RPM. While it was occurring, I noticed that pulling power back got me past 2400 RPM. I also did some work with my primitive EGT which has a 4-cylinder selector, but it's not marked as to which cylinder is which. I noticed at 2400 RPM, one EGT was cold, and upon backing the RPM to 1900 it was significantly higher than the other cylinders. 

 

It turns out there was a piece of metal partially clogging one of the injectors. The issue seems to be completely fixed now. 

[carusoam]

Next step...

JPI, EI or Insight?

Checkout the latest from insight...

http://www.insightavionics.com

Any idea where the debris particle came from?

I'm glad you found that issue without further surprises.

Best regards,

-a-

[MB65E]

A-

Isn't is crazy that a small piece like that can cause so much ruckus!!

I like the G1 and G2. Have the G1 in the "E"

I worked on a G1 install in an S35. The older gentleman who was the owner had a hard time seeing the little numbers for the CHT/EGT. It was mounted just above the switches on the lower panel. I'd Recommend mounting it so ones not looking at it at an angle...

-Matt

[FloridaMan]

The IA on the field thought it might've come from the gascolator, but, in being "downstream" from the fuel servo and, from looking at the screen in the servo, I don't think that piece could've made it through the screen. I'm not completely familiar with the arrangement of the system, but one of the hoses that goes to the firewall was replaced with a braided stainless hose. I'm going to guess that the little piece of metal came from the assembly of that new hose.