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Rough running engine at high power

panhandler1956

panhandler1956

Well Known Member
Ok, looking for some advice. I searched the archives, but haven't seen anything that matches my situation, unless I missed it.

I have an IO-360-A1A 250 SMOH.
Airframe has 70 hours.
Bendix injection and mags
Ran fine for the last year and suddenly developed this problem.

It starts and runs fine, but when you apply takeoff power it spits and sputters badly. It initially would smooth out, but now it's really bad and won't make power - really rough.

Had one cylinder egt go abnormally high and had an intermittently bad left mag check.

So far I have:
Checked the timing - it's good
Replaced the plugs (two were weak)
Replaced the left mag harness (found a weak plug wire on the cylinder that had high egt)
Cleaned the injectors
Cleaned the fuel filter and screen
No water in fuel
Boost pump on or off- no change
Isolating mags - no change
Cables all go to the stops

This has fixed the egt and mag check issue, but the bad stumble remains.

I pulled the flow divider off and cleaned it, but haven't tested it yet.

I was thinking next steps, if the cleaning doesn't work, is to send the servo and divider out for overhaul.

Any other ideas?
 
Rough Runner

Brent: Have you tried running it up to t/o power on each mag individually? When it's running rough what are the egt's doing? Check the intake pipe gaskets and seals/hoses. Remove airfilter and check the intake duct for obstructions. Try a runup with the airfilter removed. Flow check the fuel nozzles with boost pump on, wide open throttle, mixture rich(good time to clean them while they are out). Swap the plugs from the left mag to right and right to left. If nothing turns up I have couple more ideas before sending off the servo. I'm airlining back from the UK tomorrow and will be avail after 2300z. If you wish send me a contact # on the co BB and I'll call.

Don Broussard
RV9 Rebuild in Progress
 
Thanks guys!

I dont think its a single cylinder issue because now that I fixed the bad plug wire all 4 CHTs and EGTs are matched. If it were a leaking intake tube, plugged injector, or a valve/ring issue wouldn't that show up as a hot or cold cylinder?

It runs the same on left mag only, right mag only, or both, boost on or off.

Seems like something central to all 4 cylinders that leads me to the either an air or fuel issue in the injection system. I have one of those purge valves on the bottom of the sump, can that stick and cause problems?

Don,
Thanks for the help. I'll give you a shout this week.
 
Brent - your situation sounds eerily similar to that of an RV6A owner a couple of hangars down from me. His is a carb'd O-360. It would run well enough but as soon as he opened the throttle it would cough and sputter and belch black smoke. He troubleshot until he was blue in the face. Mags, plugs, harnesses, mag timing etc etc. Sent his carb out for overhaul, no difference.

What resolved his problem? Draining the fuel and installing a new load of fuel. He had simply managed to get bad gas. I don't want to count the number of man hours spent troubleshooting a problem that had such a simple solution. Think about when this problem started relative to your fueling of the aircraft. There might be a link!
 
Canadian_JOY said:
Brent - your situation sounds eerily similar to that of an RV6A owner a couple of hangars down from me. His is a carb'd O-360. It would run well enough but as soon as he opened the throttle it would cough and sputter and belch black smoke. He troubleshot until he was blue in the face. Mags, plugs, harnesses, mag timing etc etc. Sent his carb out for overhaul, no difference.

What resolved his problem? Draining the fuel and installing a new load of fuel. He had simply managed to get bad gas. I don't want to count the number of man hours spent troubleshooting a problem that had such a simple solution. Think about when this problem started relative to your fueling of the aircraft. There might be a link!
Click to expand...
Always ask yourself, what did I change, maybe you got gas. Good thinking!
 
other ideas

Bad gas is a reasonable possibility.

Probably not the sniffle valve on the bottom. It would cause a lean mixture at idle and run poorly at idle, but it can't leak enough air to significantly alter the mixture at WOT. Thats probably true for intake pipe seals too.

Visually confirm that the throttle plate is wide open when the throttle is. Not just the lever on the stop. (the lever could turn on the throttle shaft)
Visually confirm there is not a dead mouse in the intake manifold. Sort of kidding, but could be some obstruction in there. Make sure the servo is tightly bolted onto the manifold - that could leak enough to affect WOT mixture. Any other plugs or ports or cover plates that could leak?

Ugly possibility - a bad cam gear, teeth mangled enough that the valve timing has changed. Seems like someone a couple of years ago found a bad cam gear that was a counterfeit of some kind in a fairly new engine, and it was nearly completely stripped. Look for metal in the oil filter and take an oil sample for analysis. If a tooth suddenly gave way, this would come on fairly suddenly like this.
 
Not really an engine man but

Often we get too far afield in troubleshooting - so, how about the possibility of a weak battery/alternator combo? Run a load test on the battery and check the alternator. Simple tests that cost you nothing. Hope that you find a simple and inexpensive fix. Best of skill to you.
 
Alternator and battery have nothing to do with engine, when ignition is mags.
Is it possible that both mags are weak? It would be very unlucky to have both failing at the same time, but it fits the symptoms.
 
Thanks for all the helpful advice!
I will expore the bad gas since its cheap and relatively easy.

David,
I'll send you the download this weekend when I go back out to the airport. Thanks for offering that.
 
Sounds like a fuel issue, I would have the servo and fuel distribution manifold flow checked.
 
Hi Brent,
I have a good friend who had exactly the same issue as your's but after a carburetor float replacement.
He replaced his float and did some ground testings to see if it was working.
Everything was fine but engine ran very rough at high power settings. He spent months troubleshooting the problem.
He finally found that because he did the testing w/o the FAB and filter, there were terrible turbulences in the intake (due to propeller airflow) that causes the problem.
If you have remove the air filter to clean it and did not put it back in place because the problem wasn't solved that would explain the problem you are seeing. (I believe your engine is injected and you have the snorkel).

I hope you will have the problem solve soon.
 
Brent, did you check the Service Bulletins on the Fuel Servo?

I saw one regarding the brass hex plug could loosen up and cause engine issue. It also states if it falls out of course, complete engine loss.

I have a AvStar fuel servo and there was an SB for the diaphragm, luckily at the time my new engine wasn't affected.

Just another thought.

Good luck.
 
Bastien said:
Hi Brent,
I have a good friend who had exactly the same issue as your's but after a carburetor float replacement.
He replaced his float and did some ground testings to see if it was working.
Everything was fine but engine ran very rough at high power settings. He spent months troubleshooting the problem.
He finally found that because he did the testing w/o the FAB and filter, there were terrible turbulences in the intake (due to propeller airflow) that causes the problem.
If you have remove the air filter to clean it and did not put it back in place because the problem wasn't solved that would explain the problem you are seeing. (I believe your engine is injected and you have the snorkel).

I hope you will have the problem solve soon.
Click to expand...

Thanks! I have the air filter assembly still on. Good idea though.
 
SmilingJack said:
Brent, did you check the Service Bulletins on the Fuel Servo?

I saw one regarding the brass hex plug could loosen up and cause engine issue. It also states if it falls out of course, complete engine loss.

I have a AvStar fuel servo and there was an SB for the diaphragm, luckily at the time my new engine wasn't affected.

Just another thought.

Good luck.
Click to expand...

My engine is predates the SB affectively, but thanks! That one certainly sounds bad.
 
Not yet

Waiting on a new fuel line from Bonaco. To replace the old crddy looking one that goes btween the throttle body and the flow divider. Hopefully I
It comes in this week.
I'll report back.
 
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