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Induction Leak

KatanaPilot

Well Known Member
I am chasing an induction leak on my new IO-540 equipped with SDS EFII. I started with idle manifold pressure around 17" at 800 RPM. Found one leak around the threads of the Borla fuel pressure regulator. Took care of that and got the MP down to around 13.5" at 800 RPM. Still seems high, as I am used to 10-11" on my RV-7.

I had the SDS no-leak o-rings installed, but took them out thinking I might have a pinched o-ring. There were none, but I installed the OEM intake rings with new gaskets just to eliminate that as a possible source. I cleaned the intake tubes and also installed constant tension clamps on the rubber hoses between the BPE sump and the intake tubes.

I have three different sources of MP fed by two independent MP taps on opposite sides of the engine. I've tested these lines under vacuum and they hold steady. So I do not think it is an indication problem.

Next step is to spray some water at the rubber hoses on the intake tubes with the engine at idle.

Any other good suggestions as to where to look?
 
Next step is to spray some water at the rubber hoses on the intake tubes with the engine at idle.

Any other good suggestions as to where to look?

I've always using something a bit more flammable than water, carb cleaner for example works well.
 
Thanks everyone.

Spraying carb cleaner around the exhaust just aft of spinning propeller sounds like a heck of a lot of fun. And yes, I do recall tricks like that from my high school days fixing cars.

Thanks Vic. Maybe I'm chasing a problem that no longer exists.
 
We recommend taking vacuum signals from the ports provided on the throttle body. These are orificed and provide a much smoother MAP signal than taking the vacuum source off only one intake runner (which we don't recommend) The latter results in a sine wave signal to the sensors and fuel pressure regulator as the intake valve opens and closes.

There is averaging in the software to minimize this but garbage in = garbage out as they say. It's not best practice in this realm.

I generally seen 10-12 inches at an 800 rpm idle, this varies slightly with hot or cold engine, propeller type and fine pitch setting.
 
We recommend taking vacuum signals from the ports provided on the throttle body. These are orificed and provide a much smoother MAP signal than taking the vacuum source off only one intake runner (which we don't recommend) The latter results in a sine wave signal to the sensors and fuel pressure regulator as the intake valve opens and closes.

There is averaging in the software to minimize this but garbage in = garbage out as they say. It's not best practice in this realm.

I generally seen 10-12 inches at an 800 rpm idle, this varies slightly with hot or cold engine, propeller type and fine pitch setting.

Thanks Ross. I do have the Garmin recommended restrictors in both MP lines which I thought would provide adequate damping. Using the ports on the throttle body created other issues due to location - which is why we went with the cylinder ports. The indication is steady now with the orifices installed, but I’m just trying to verify that 13.5” is normal on this engine. Sounds like it might be just fine.
 
While very small orifices may be ok for display purposes on the Garmin, they may affect response time of the MAP sensor to rapid throttle movement and cause running issues there. If the orifices are smaller than .030 and you have a lot of line volume, I'd be wary.

We had both straight and 90 deg vacuum fittings available for the throttle bodies however our source for the 90s became crazy expensive with many month lead times so we've discontinued those and had to re-position the fittings to the TB side to use straight fittings recently.

If the engine is pulling 13.5 inches, measured off one intake tube and running smoothly without a huge EGT spread, you may have no issue at all.
 
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I’ve already said this, it I assure you it is fine. I perform run ups on about 200-250 engines per year. It is normal to see 10” on 4 cylinders, and 13-14” on 6 cylinder engines. This is AFTER the condition inspections where we check for leaks and replace all of the gaskets and hoses, if needed.

Vic
 
OK, but why?

Thanks for the information.

The engine runs smoothly at low power and the AFR is in a good range - so I'm not too concerned now about any additional induction leaks.

What nobody has been able to explain though, is why a 6 cylinder engine would have higher (numerical) manifold pressure at idle versus a 4 cylinder. It seems like it should be the opposite, since the 6 cylinder should be a more efficient or higher capacity "pump" . The 540 has a higher compression ratio than the IO-360 in the RV-7. Both have identical instrumentation (G3X, GEA, same Kavlico sensor).

Any engine engineers out there that can explain the theory behind this phenomenon? Inquiring minds would like to know.
 
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...

What nobody has been able to explain though, is why a 6 cylinder engine would have higher (numerical) manifold pressure at idle versus a 4 cylinder. It seems like it should be the opposite, since the 6 cylinder should be a more efficient or higher capacity "pump" . ...

When your engine is off, the MP is 28.5" (at least mine was on my last flight, looking at my savvy data) which is ambient air pressure. When running, the difference between ambient air pressure and your MP is how much the engine is sucking, and how much the throttle plate is restricting the airflow.

With a larger engine, which needs more air, it seems reasonable that the throttle will be open a bit more, less airflow restriction, higher MP. At least that's my guess - apologies if this is totally wrong.

BTW, my io-360 at 850 RPM is around 12.5". Taking off at around 2500 RPM my MP is 27.7" which tells me that my induction system has about 0.8" of inefficiency, if I understand this correctly.
 
Take the Lycoming engine class and you'll find out why.
All the 540's these days have roller lifters. Your older 360 probably did not have roller lifters.
The roller lifter cam has a different profile than the non-roller, thus the difference.
 
Thanks for the information.

The engine runs smoothly at low power and the AFR is in a good range - so I'm not too concerned now about any additional induction leaks.

What nobody has been able to explain though, is why a 6 cylinder engine would have higher (numerical) manifold pressure at idle versus a 4 cylinder. It seems like it should be the opposite, since the 6 cylinder should be a more efficient or higher capacity "pump" . The 540 has a higher compression ratio than the IO-360 in the RV-7. Both have identical instrumentation (G3X, GEA, same Kavlico sensor).

Any engine engineers out there that can explain the theory behind this phenomenon? Inquiring minds would like to know.

The amount of vacuum seen at idle is based upon many factors, but compression ratio is not one of them, though valve overlap (time that both In and Ex valves are both open) is. Mixture has a big impact on vacuum also, which is why I use it to set an optimum idle mixture. An optimum mixture will create the highest vacuum for an given RPM. In theory, the vacuum seen at any individual cyl intake chamber should be the same as that seen in the entire plenum. However, every 720* that local vacuum drops a bit when the valve opens and this creates transient changes in vacuum levels that the instrument has to deal with. Readings taken closer to the throttle body will be more stable. I would work hard to follow Ross' suggestion to tap MAP at the TB. Your fuel delivery levels are highly dependent upon MAP, so extra effort is warranted here.

When I set idle mixture, I usually end up reducing MAP by an inch or two, so suggest you tweak the 750 RPM/13" MAP setting in your fuel table and see if the MAP drops. I can't remember where my 540 sits at idle, but I vaguely remember it as 11.x" (tapped at the #5 primer port). When doing it this way, the RPM must be reset to the base line after each adjustment. You are looking for the lowest MAP at a specific RPM.

Also, at idle levels, MAP varies a good bit with RPM. So if you get 13" at 800, you can't compare it to 11" that someone else got when at 700.

The 6 vs 4 theory about a bigger pump does not apply. Each cylinder needs a certain amount of air to work and the clearance between the throttle blade and the bore will vary accordingly to allow the appropriate amount of air for the appropriate amount of cylinders. That said having 3 cycles vs 2 in each revolution will create some variances, as the vacuum level is constantly cycling and you are really just seeing averages. The impact of these variances is also impacted by intake plenum volume.

Larry
 
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Take the Lycoming engine class and you'll find out why.
All the 540's these days have roller lifters. Your older 360 probably did not have roller lifters.
The roller lifter cam has a different profile than the non-roller, thus the difference.

Both statements are correct.

I guess valve timing/cam profile/overlap, etc. has a strong influence on manifold pressure. Should have been an ME instead of an AE. :(
 
Both statements are correct.

I guess valve timing/cam profile/overlap, etc. has a strong influence on manifold pressure. Should have been an ME instead of an AE. :(

While the profile has to be different to accommodate roller lifters vs flat tappets, I doubt that actual meaningful events baked into the profile (IVC, IVO, EVC, EVO, overlap, etc) changed from the earlier cams. Those changes have profound effects and seriously doubt that Lyc would change them on an engine that is otherwise considered to be the same as the earlier non roller variants.

When you go to roller tappets, without changing the profile, the opening and closing acceleration would be too high and stronger valve springs would be required to deal with it. The interface between the lobe and the lifter is VERY different between roller and flat and therefore different profiles are required to meet the same objective.
 
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My IO-540 exhibits about 10" MAP at idle

What nobody has been able to explain though, is why a 6 cylinder engine would have higher (numerical) manifold pressure at idle versus a 4 cylinder.

That's an interesting question. FWIW, my RV-10's IO-540-C4B5 (narrow deck, mechanical fuel injection with MAP taken from cylinder #5 primer port) exhibits about 10" at idle. Savvy screen shot from yesterday attached.
 

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That's an interesting question. FWIW, my RV-10's IO-540-C4B5 (narrow deck, mechanical fuel injection with MAP taken from cylinder #5 primer port) exhibits about 10" at idle. Savvy screen shot from yesterday attached.

It is interesting. I have had confirmation now from 3 RV-10 owners that their cold MAP is around 13.5" at idle. One of the three had hot MAP at 11". Engine builder confirmed that MAP was 13" in the test cell.

I'm satisfied with where my engine is now, pretty confident there are no leaks and have plans to rework the pressure taps in line with Ross Farnham's suggestions.
 
Cold MAP will always be a bit higher than hot MAP. The friction of the thick oil on bearings/ shafts/ gears and especially the pistons and rings, means the throttle blade has to be cracked open slightly more to achieve the same rpm as when hot. More throttle angle means high MAP.

It's important to know if we are talking hot or cold engine here.
 
Just did a post oil change leak check on my Rocket yesterday - I saw 9.5 inches at 850 RPM. This is SDS EFI, SDS "no leak" induction tube seals, Pressure tap is at the throttle body, and three individual MAP sensors are in agreement.

Plan to fly today, so I'll see what it looks like hot.

While mixture plays a big role (as Larry points out), so does ignition timing. I happen to run a whole bunch of advance at idle and it significantly smoothes things out. My engine will idle down to 350 RPM (not that such a condition is any kind of goal).

If you have SDS ignition, try punch up your timing at idle until it seems happiest (smoothest and highest RPM, usually). Pretty good chance you will need to adjust the throttle stop to get your idle down to normal, and thats going to drive lower MAP.
 
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Good point Mike, timing makes a significant difference in idle MAP.

The 500 rpm ignition timing shouldn't be set over 10 deg as this slot is used for cranking and we want to avoid any chance of kickback. There is 15 deg of cranking retard as default below about 330 rpm which should fire the plugs at around 5 ATDC.

We recommend a minimum of 20 deg at 750 and 1000 rpm and this could be set even higher as Mike does.
 
That's an interesting question. FWIW, my RV-10's IO-540-C4B5 (narrow deck, mechanical fuel injection with MAP taken from cylinder #5 primer port) exhibits about 10" at idle. Savvy screen shot from yesterday attached.

I came across this thread while monkeying with injector balancing.
I’ve been battling with it a little bit and have induction leaks in #2 and #5.
5 turned out to be a loose barb for the EI MP takeoff on the firewall.
#2 has me stumped though.
After battling for years with the standard gaskets in my RV7, I used the SDS oring setup in these intakes so I’m confident it’s not that.
That only leaves the sump end of the intake tube on #2 I guess. Or a loose primer port plug (I’ve checked this tho).
Any other recommendations?

Tim’s egt plot looks very similar to mine. EGT rise on a few cylinders on throttle reduction.

Curious as to whether you investigated this any further Tim?

Cheers.
 
...
I’ve been battling with it a little bit and have induction leaks in #2 and #5.
5 turned out to be a loose barb for the EI MP takeoff on the firewall.
#2 has me stumped though.
After battling for years with the standard gaskets in my RV7, I used the SDS oring setup in these intakes so I’m confident it’s not that.
That only leaves the sump end of the intake tube on #2 I guess. Or a loose primer port plug (I’ve checked this tho).
Any other recommendations? ...
Have you tried the "reverse shop vac, pressurize the system, soap bubbles" method? I'm surprised that a loose MP sensor connection could actually move enough air to make a difference.
 
Have you tried the "reverse shop vac, pressurize the system, soap bubbles" method? I'm surprised that a loose MP sensor connection could actually move enough air to make a difference.

Yeah I was surprised at #5 too but the data don’t lie. It’s fine now.
Sealing up the whole shebang sounds tricky. How would you do that? Plug the exhausts and pull off the filter housings and somehow adapt the vac onto the throttle body?
Maybe I give that a go.
 
I
That only leaves the sump end of the intake tube on #2 I guess. Or a loose primer port plug (I’ve checked this tho).
Any other recommendations?
.

Definately worth the time to pull the rubber coupler to check for signs of problens and re-install. Some sumps have o-rings on the tube stud and others are pressed in. Worth checking these for looseness. Also cyl heads can develop cracks and a crack around the intake chamber can cause this.

Larry
 
Definately worth the time to pull the rubber coupler to check for signs of problens and re-install. Some sumps have o-rings on the tube stud and others are pressed in. Worth checking these for looseness. Also cyl heads can develop cracks and a crack around the intake chamber can cause this.

Larry

Thanks Larry
I’ll attempt pressuring ant soapy water. But will definitely pull the whole tube and check the sump end either way.
Cracked cyl head dosent sound great :eek:
 
I hooked up the shop vac in reverse to the intake this morning.
It worked quite well pressurizing everything.
I squirted the whole induction system (and exhaust for good measure) there were zero bubbles anywhere. All the sump tubes and head gaskets all airtight.
The culprit is the injector body in #2. You can feel the air rushing out of the mesh and it makes a big bubbly show.
None of the other injector bodies do that. This injector body also exhibits far more fuel staining than the rest. (The rest have very little at all)

I must confess I don’t fully understand how these bodies work. I understand that they take some air in to help atomize the fuel stream, but this must also leak air into the induction chamber - which I guess is fine as long as they are all the same and it’s a finely calibrated setup.
I’ll email Don and Co at Airflow and see what they say.


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I hooked up the shop vac in reverse to the intake this morning.
It worked quite well pressurizing everything.
I squirted the whole induction system (and exhaust for good measure)
The culprit is the injector body in #2.

I'd have never guessed.

Please post what Don says. Should be interesting.
 
Check with Don but if you have that cylinder on the fuel intact stroke I believe the injector is working normally, since the injector has an air inlet. See attached image.
 

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Check with Don but if you have that cylinder on the fuel intact stroke I believe the injector is working normally, since the injector has an air inlet. See attached image.

Yep I considered this and rotated the propeller through intake and exhaust on all cyls. Didn’t change it.
I’ve emailed Don. I’ll report back
 
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