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  #1  
Old 08-09-2018, 11:09 PM
bifft bifft is offline
 
Join Date: Feb 2012
Location: Utah
Posts: 75
Default Does this sound like vapor lock?

You might remember me from my previous engine problem thread: http://www.vansairforce.com/communit...d.php?t=153150

Have a O-360 with Rotec TBI. With the flow straightener, was able to fly with all winter/spring. However once summer rolled around have been having problems. It would run up fine at the hangar, but after taxiing out to the run up area (about 1/2 mile away) it wouldn't run at full power. Lower power settings work fine. Taxi back to the hangar no problem. Back at hangar, no full throttle available. Anything over about 2100 RPM and it just quits.

At first, I suspect it is something related to the TBI mixture problem I was having before. I never was quite happy with the way that I had to advance the throttle and mixture together, didn't feel safe for a go around.

Based on this previous thread: http://www.vansairforce.com/communit...8&postcount=68

Take off the TBI, drill out five holes at the full throttle end to #79 (they were already bigger than #80). Put it back on, spend a morning resetting the idle mixture setting. As I'm doing it alone this means lots of short runs with the cowling off. Oil temps get high (but in the green - around 190F) but everything else is fine. Really like how it runs with the bigger holes, the same mixture setting will run the engine from idle to full. Everything running great.

So, put the cowling on the now hot engine, do a preflight and go to fly. No full throttle available. Fuel pressure looks fine (5 psi). Start thinking about vapor lock with that hot engine. It ran fine when uncowled so the TBI was in full air blast, but now cowled up runs poorly. Mess with the mixture trying to get it to run. It just keeps getting worse. Eventually can't get it to run over 1500 RPM or so. After shutdown fuel pressure goes high (up to 15 or so).

Try again another morning, when it should be cooler, and the enigne has cooled down. Not quite as early as I had planned, ATIS as 84F. This time, run up in the run up area just fine. Actually take off. About the time I hit pattern altitude the RPMS come down just a little. Once around the pattern and land. Taxi back to the hangar and no full throttle available. Oil temp about 170F.

Ran an air blast tube from the reat of the baffle to the TBI. Tried test runups this evening. Outside air temperature was 90F per the ATIS. Trying to make vapor lock happen do a long low thottle setting runs with occasional full throttle (can't run full for long as CHTs get high). I don't have the forest of sensors that the glass panel guys run, so using oil temperature as my guess for under cowl temps. Runs great up to 140f. At 140F get a little stutter if I advance the trottle quickly, but fine if I run slow. At 180F oil temperature the problem is back. Max RPM available 2100. At 190F max it will run is 1900.

Highest in-flight oil temp I've seen so far is only 160, but don't consider this fix enough.

So, my question. Does this sound like vapor lock? I want to make sure I've got the right diagnosis before I do anything else.
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  #2  
Old 08-10-2018, 06:20 AM
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snopercod snopercod is offline
 
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Location: Asheville, NC
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I went through a similar troubleshooting process with my carbureted O-290-D2. Like you, I had suspected vapor lock, so I instrumented my fuel line, gascolator, fuel pump, and carburetor. The only temp that ever got up into the danger zone for vapor lock was the carburetor. To make a months-long story short, the heat from the hot oil sump was heat soaking the carburetor and causing the fuel to boil in the bowl on a hot day or after a long wait in a que for takeoff. My fix was to install a 1/16" phenolic "gasket" - with fiber gaskets on either side - between the oil sump and the carburetor. This acted as a thermal break and my problem (mostly) went away. I also added more air to my oil cooler, but blocked that off last winter and haven't put it back yet. So when my engine overheats, I still get a little over-rich coughing and sputtering when pushing the throttle forward for takeoff but, if that happens, I just lean the mixture a bit and it goes away. I can't say whether this is your problem, but it's something to think about. You can buy really cheap thermocouple temperature readouts on Amazon.
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  #3  
Old 08-10-2018, 08:10 AM
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Sam Buchanan Sam Buchanan is offline
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Location: North Alabama
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Earlier this year I had an identical event with my carbed O-320. Temps were unseasonable high at mid-day after a lunch break, and there must have been winter-blend auto gas in the tanks. Fuel pressure was lower than normal during taxi and the engine quit when full throttle was applied for take-off. After getting my heart rate back in the green I added 100LL to the tanks and flew home with no issues.

I assume this was fuel boiling in the carb. I've run auto gas for several years with no problems but this may have been the first time I've experienced ambient temps in the mid-90's in early May.
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  #4  
Old 08-10-2018, 08:56 AM
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rocketbob rocketbob is offline
 
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The TBI will become heat soaked quickly regardless of ambient temps unless a FR4 or similar material spacer is used between the unit and the sump. I fabricated mine out of 0.250" thick FR4. Once I did that I never had any vapor issues. Fuel flowing thru the TBI is more than adequate to keep the unit cool as long as it is thermally isolated from the sump.
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  #5  
Old 08-10-2018, 10:18 AM
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Carl Froehlich Carl Froehlich is offline
 
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Quote:
Originally Posted by rocketbob View Post
The TBI will become heat soaked quickly regardless of ambient temps unless a FR4 or similar material spacer is used between the unit and the sump. I fabricated mine out of 0.250" thick FR4. Once I did that I never had any vapor issues. Fuel flowing thru the TBI is more than adequate to keep the unit cool as long as it is thermally isolated from the sump.
Bob - any details on this spacer? Material part numbers?

Carl
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  #6  
Old 08-10-2018, 09:51 PM
VA Maule VA Maule is offline
 
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Do you have a vapor return line running back to the tanks from the TBI ? I believe this is the recommended vapor lock prevention method recommend by Rotec. If the vapors can't go back to the tank they can only go out the spray bar causing an excessively lean condition.
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  #7  
Old 08-11-2018, 12:08 PM
bifft bifft is offline
 
Join Date: Feb 2012
Location: Utah
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I'm not hearing any dispute on the diagnosis, so on to the cure.

I like the idea of insulation between the sump and TBI, I had thought of that but wasn't sure what material to use. Looking up FR-4 online, (https://en.wikipedia.org/wiki/FR-4 for Carl) it is flame redardant epoxy fiberglass sheets. I could order some of that, or I could use the practice flat fiberglass epoxy sheet I made when first practicing fiberglass work before starting on the fairings. Take two sheets of that, glue them together with microballoon filled epoxy to fill the weave and add more insulation. Wouldn't be as fire resistant, should hold up to gas/oil Ok. Temperatures don't get over boiling water, so probably Ok for that as well. Will order some FR-4 as it is pretty cheap, but can build up my "micro sandwich" this weekend.

I don't have the vapor return line. They weren't yet recommending it when I first got the TBI (part of being a very slow builder) so I don't have the lines to the tank. When I sent it back to Rotec before they modified the pressure regulator for a return line but as I didn't have the rest of the plumbing set up I just plugged it.

If the insulation isn't enough vapor return line is my next planned step. The outgoing port on the pressure regulator is a nipple for 1/8" ID hose. I haven't found a fitting that will let me convert that to AN compression fittings to run normal aircraft hose. Have been thinking of using reinforced rubber lines from the car parts store with hose clamps to hold it on, then firesleeve over that. (That is how it is plugged right now.) I have a small lathe, so could turn up a bulkead fitting to run it through the firewall, and then another for into the tank. Draining the tank, removing the access plate and drilling a hole in it is something I've been holding off on.

That does leave me with the possibility of pumping fuel overboard by mistake if I leave it on the other tank too long, so might be better to get a duplex valve and run to both tanks? Lots more money and work.

Other things that might help: air blast to fuel pump, fuel pump shroud. Remove gascolator from firewall.
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  #8  
Old 08-11-2018, 01:46 PM
Mark33 Mark33 is offline
 
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Location: Baton Rouge, La.
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Default Vapor lock

I previously posted the following on another thread:

My buddy had basically the same problem with fuel getting hot and vapor locking. He ended up installing a small 1/8" return line to his left tank. He "T'd" in to the fuel line just prior to the red cube so that even when he has the "sweep" line/valve open, it doesn't interfere with the calculation of the fuel count. With it being such a small "metered" 1/8" line, even if he forgets it open it really doesn't affect his fuel pressure....(or very little). Even though it only sweeps out about 1 cup of fuel per minute, it completely 100% cured his vapor lock issues. He actually even installed a second red cube on that little return line and he gets an annunciation on the G3x that reminds him that the sweep line is open. You can hear the electric pump change sounds as it starts to pick up the cold fresh fuel and get rid of those hot vapors. It really did make it a whole new airplane and he has never once had to deal with vapor lock issues again. The only thing that he has to even slightly be aware of is that if both tanks are completely full he just makes sure that he has the left tank selected...because that's the tank that the little return line is going to. Even in that situation with full tanks, if he were to forget to select the left tank, the worst case scenario is that he'd loose one or two cups of fuel through the overflow. Once again, for him, it was the absolute cure to all of his hot fuel issues.

Mark
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  #9  
Old 08-11-2018, 10:55 PM
crabandy crabandy is offline
 
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Default

Quote:
Originally Posted by Mark33 View Post
I previously posted the following on another thread:

My buddy had basically the same problem with fuel getting hot and vapor locking. He ended up installing a small 1/8" return line to his left tank. He "T'd" in to the fuel line just prior to the red cube so that even when he has the "sweep" line/valve open, it doesn't interfere with the calculation of the fuel count. With it being such a small "metered" 1/8" line, even if he forgets it open it really doesn't affect his fuel pressure....(or very little). Even though it only sweeps out about 1 cup of fuel per minute, it completely 100% cured his vapor lock issues. He actually even installed a second red cube on that little return line and he gets an annunciation on the G3x that reminds him that the sweep line is open. You can hear the electric pump change sounds as it starts to pick up the cold fresh fuel and get rid of those hot vapors. It really did make it a whole new airplane and he has never once had to deal with vapor lock issues again. The only thing that he has to even slightly be aware of is that if both tanks are completely full he just makes sure that he has the left tank selected...because that's the tank that the little return line is going to. Even in that situation with full tanks, if he were to forget to select the left tank, the worst case scenario is that he'd loose one or two cups of fuel through the overflow. Once again, for him, it was the absolute cure to all of his hot fuel issues.

Mark
Mark,
Was this on a carb or injected engine?
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  #10  
Old 08-11-2018, 11:15 PM
Mark33 Mark33 is offline
 
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Location: Baton Rouge, La.
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He has the Ellison throttle body on his -7 but I’m sure the same thing could be done even if you had a carburetor or injected setup. The key is to purge that hot fuel out so that your fuel system....whatever type of fuel system that may be...can do its job. It’s amazing what a difference that moving just that small amount of hot fuel can have on the cranking and running of your engine.

Mark

Last edited by Mark33 : 08-12-2018 at 10:51 AM. Reason: Add content
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