I have an older lycoming 0235 which has 900 hours on it. Ever since I bought it one of the cylinders has had lower compression than the others. At my last anual I had that cylinder pulled and discovered I had a burnt intake valve. Yep, intake valve, not exhaust valve.

Approximately 100 hours later I'm doing my pre-flight and while spinning the prop to check compression I find that one of my cylinders (not sure which one) has zero compression. I mean nothing...like a three cylinder engine. I spin the prop about five times through and it's the same story...one of the four cylinders has abolutely no resistance.

On the advice of a local mechanic (I'm away from my home field) I started the engine and it ran fine. I shut it down and spun the prop and the compression seemed normal in all four cylinders. I flew it home and all seemed normal, though now one of the cylinders is ddefinitelysoft.

I'm waiting for my mechanic to have a look, but in the mean time I wonder if this rings a bell with anyone. I have to assume that one of my valves was stuck open, then closed up when I started the engine. I just don't understand why.

I'm told this engine doesn't like 100ll because it's a low compression engine designed to run on 82 octane. It collects a lot of lead fouling, which may be causing my valves to stick. 100ll is the only game in town around here, and I'm not going to ccarryauto gas to the airport every day.

Anyone out there have similar problems, and if so did you find a fix?

I've been told that higher octane fuel burns hotter, and that if your using 100 when 82 or 87 is required, it could cause damage to your engine... I'm not a mechanic so I don't know if it's true, but it sounded logical to me...

Let us know what you find out...

Your low cylinder may have been caused by some carbon holding the valve slightly open. I would get some TCP and use it every time you have to use 100LL. Also, lean the engine at all times on the ground, to the point that it will not even run above about 1200 RPM, this will prevent you from ever attempting takeoff with it leaned way out. If fouling is really bad, you may have a stuck valve, causing "morning sickness" in the winter particularly. This is a problem that TCP will probably not improve because it's in the valve guide.

Good luck,

Rocky

100 LL is approved for every 80 octane engine. It still has a lot of lead, though, and lead fouling may in fact cause problems, even in engines that were designed to run on 100.

It is possible that you had a valve stick, as you suspect. One of the places that tend to lead foul is the valve train.

It is also possible that the rings were simply lined up on that cylinder, causing low compression. The rings rotate some and if they happen to all get lined up with the gaps together, you'll get no compression.

Have a good engine guy take a look at it, and see what he thinks. There are some additives that Lycoming recommended to put in your fuel to reduce lead fouling. Some of that stuff may be an answer as well.

But, I'd definitely get it checked out.

MTV

For a very interesting story about valve sticking in Lycoming engines and "morning sickness" read accident report "SEA05FA097". about a PA-28 whos engine seized over the Idaho mountains. Aircraft number N8805W.

DEGJR

Interesting that I was just reading a FAA Advisory Circular ( 7/18/84 AC No: 91-33A) concerning engines designed to run on 80/87 Octane which now must burn 100LL due to 80 & 87 being discontinued.

Looks as if 80 Octane ( red ) avgas had 0.5 ml/gal of TEL and 100LL ( blue ) has up to 2.0 ml/gal.

The circular refers to lead fouling due to the increase amount of TEL in 100LL. This possibly the source of your problem.

Is there an Auto gas STC for your engine? Maybe burning Auto gas and adding some TEL or burning some 100LL on some schedule to keep everything lubed would be the answer.

TD

sector15 wrote:I've been told that higher octane fuel burns hotter, and that if your using 100 when 82 or 87 is required, it could cause damage to your engine... I'm not a mechanic so I don't know if it's true, but it sounded logical to me...

Let us know what you find out...

Higher octane fuels burn cooler not hotter. The higher the octane the slower the flame front is, this allows the surrounding surfaces in the engine to absorb the heat better and transfer it to the cooling fins, oil etc. A lower octane fuel burns faster and thus the heat is harder to shed.

The Lyc O-235 has always had a problem of lead fouling. This usually happends in the sparkplugs. But can also get on the valve seat; keeping the valve open.

Something you can do in the field if you have a cylinder with no compression...

Take out one sparkplug, put your finger over the hole and find the compression stroke for that cylinder. Now look at the cylinder numbers and firing order and find which cylinder has no compression.

Take off the valve cover (make sure you have reusable silicone valve cover seals) and use a wood dowel (not a stick that will splinter) and hammer to hit the valve stem/retainer to open and close the valves and try to dislodge the lead from the valve seat. This is communally called "staking a valve".

Lyc. has a service instruction about doing this and the next step of how to clear a valve without removing the cylinder.

Now this is obviously something that an A&P should be doing. But if you find that fixing your plane is better than walking out I would get with your local A&P and get a copy of the service instruction and have the A&P show you how to stake a valve.

Just FYI...

The O-235 is my favorite lowwer power engine even with the lead fouling problems. TCP additive works somewhat but proper leaning is a better solution. What we have done in the past is always lean the engine. Right after startup lean the engine. Keep it leaned until the mag check which should be done at full rich. When you pull back the power after level-off again lean the engine. Also keep the engine leaned during landing and taxi back. This is because you don't want a cool(ing) engine with a rich mixture with this engine. This keeps the lead deposits to a minimum. But also remember to go back to full rich before going to full power (as in go around).

Hope this helps...

Todd Giencke A&P, IA, Avionics

Find some one with a bore scope and use that to look at the seats, and the cylinder side of the valves. The deposits should be round on the face of the valve, not a figure eight or egg shaped. If they are not round, the valve is burnt.....found like four burning vales this way myself.

Car gas & mystery oil, that's the stuff.

AV gas has a very high octane rating it is also a very slow burning fuel, it is designed to run in low compression engines at a constant rpm, the reason for the higher octane is the altitude. Running AV gas in a car engine will result in a very slow burn or flame front, power will be low, and overheating will often result from the delayed ignition.

To obtain the maximum energy from the gasoline, the compressed fuel-air mixture inside the combustion chamber needs to burn evenly, propagating out from the spark plug until all the fuel is consumed. This would deliver an optimum power stroke. In real life, a series of pre-flame reactions will occur in the unburnt "end gases" in the combustion chamber before the flame front arrives. If these reactions form molecules or species that can autoignite before the flame front arrives, knock will occur.

Simply put, the octane rating of the fuel reflects the ability of the unburnt end gases to resist spontaneous autoignition under the engine test conditions used. If autoignition occurs, it results in an extremely rapid pressure rise, as both the desired spark-initiated flame front, and the undesired autoignited end gas flames are expanding. The combined pressure peak arrives slightly ahead of the normal operating pressure peak, leading to a loss of power and eventual overheating. The end gas pressure waves are superimposed on the main pressure wave, leading to a sawtooth pattern of pressure oscillations that create the "knocking" sound.

The combination of intense pressure waves and overheating can induce piston failure in a few minutes. Knock and preignition are both favoured by high temperatures, so one may lead to the other. Under high-speed conditions knock can lead to preignition, which then accelerates engine destruction...

Aviation gasolines were all highly leaded and graded using two numbers, with common grades being 80/87, 100/130, and 115/145 [109,110]. The first number is the Aviation rating ( aka Lean Mixture rating ), and the second number is the Supercharge rating ( aka Rich Mixture rating ). In the 1970s a new grade, 100LL ( low lead = 0.53mlTEL/L instead of 1.06mlTEL/L) was introduced to replace the 80/87 and 100/130. Soon after the introduction, there was a spate of plug fouling, and high cylinder head temperatures resulting in cracked cylinder heads [110]. The old 80/87 grade was reintroduced on a limited scale. The Aviation Rating is determined using the automotive Motor Octane test procedure, and then converted to an Aviation Number using a table in the method. Aviation Numbers below 100 are Octane numbers, while numbers above 100 are Performance numbers. There is usually only 1 - 2 Octane units different to the Motor value up to 100, but Performance numbers varies significantly above that eg 110 MON = 128 Performance number.

The second Avgas number is the Rich Mixture method Performance Number ( PN - they are not commonly called octane numbers when they are above 100 ), and is determined on a supercharged version of the CFR engine which has a fixed compression ratio. The method determines the dependence of the highest permissible power ( in terms of indicated mean effective pressure ) on mixture strength and boost for a specific light knocking setting. The Performance Number indicates the maximum knock-free power obtainable from a fuel compared to iso-octane = 100. Thus, a PN = 150 indicates that an engine designed to utilise the fuel can obtain 150% of the knock-limited power of iso-octane at the same mixture ratio. This is an arbitrary scale based on iso-octane + varying amounts of TEL, derived from a survey of engines performed decades ago. Aviation gasoline PNs are rated using variations of mixture strength to obtain the maximum knock-limited power in a supercharged engine. This can be extended to provide mixture response curves which define the maximum boost ( rich - about 11:1 stoichiometry ) and minimum boost ( weak about 16:1 stoichiometry ) before knock.

I'm with zero.one.victor. My Dad had a Luscombe with a C-65 in it, and on a trip to Oshkosh burning 100LL we had valve sticking problems.

A mechanic ran us into town to pick up some marvil mystery oil to which we would add a little to the 100LL, and we didn't have a problem after that.

Some time after that trip, Dad switched to auto gas.

you can check for a stuck/sticky valve without pulling the cylinders. there is a lycoming service bulletin on this and it specifies to do this inspection every 400 hrs of service or sooner if symptoms occur. this can easily be done with a compressed air source available.
if you have a sticky valve- have the cylinders pulled and the guides and valve stems cleaned! this is also a good time to clean the combustion chambers and lap the valves if needed.
i had a customer's o-320 spit a lifter and pushrod out because a valve stuck with allegedly no warning. TSMOH: 550 hrs (new millenium cyls)
it cost him a cam and case inspection, some new valvetrain components, and labor to split the case and reassemble everything.
FYI: he ran auto gas 90% of the time.

Not to add to the confusion, but lead may not be the issue in California. I've been told (by a cylinder shop that paid for spectrograhic analysis of samples of California-refined 100ll) that California-refined 100ll substitutes toluene and other compounds for lead, and that 100ll from California actually has very little lead in it.

I'm not a chemist, but this is consistent with what my A&P and I observed. We used to have lead deposits on the plugs when I could get 80/87. When 80/87 disappeared from California in 1997, we stopped seeing lead deposits. I also started burning valves in my Continental O470. I began running richer and cooler, and also started adding Marvel Mystery Oil to the fuel. I haven't burned a valve since (knock on wood).

I do remember having lead fouling back in the '80s when I ran 80/87. I even used to put in an additive to help dissolve the excess lead to prevent fouling. I haven't had any fouled plugs and haven't seen any lead deposits on plugs for at least 10 years, and I buy 90% of my avgas in CA. If there was a lot of lead in California 100ll, I would expect it to show up somewhere.

Just a thought.

CAVU

Interesting thread.

My franklin 165 was designed for zero lead fuel. The engine has stellite (sp?) valves and seats that can handle this. When I use 100ll I get sticky valves and I have had to stake them to get them seating properly again.

The marvel mystery oil debate rages every where pilots get together and hangar fly. I don't know if it works or not, but it does not seem to hurt either.

TCP is an additive that normally comes up about now and I am considering using in my plane to see if I can scavenge the lead when using 100ll.

I do know that the franklin is prone to carb ice and it seems to pick it up more often when I use auto fuel, it seems to be related to the vapor pressure difference.

I do know that I would like to find a cure that I can count on and use when going xc. During an xc I am normally forced to burn 100ll only due to the lack of auto available, and this is when I have had sticky valves.

I would be very interested if the 100ll in CA is different than other places. I would think that may make the engine manufactures a bit edgy and void some warranties?

It's possible to fix a sticky/stuck valve without puliing the cylinder. Google up "the rope trick". There was some discussion about this on the 170 Assn website a while back. I'm too slow a typist to 'splain it all here, but it's quick& easy and it works. I'm not much of a mechanic and I fixed a stuck valve in a couple hours, first time out. Next time (if/when) it'll go a lot quicker. Beside the normal tools, you will need an appropriately-sized reamer for reaming the guide. I used a .437" reamer on my C-145 (O-300) cylinder. Look up the spec's on your engine's valve stem OD and valve guide ID and tolerances thereof.

Eric

Higgy, I'll vouch for TCP use in 100LL. I run mogas 90% of the time in my Franklin 165 (with MMO), but as you know when you go on the road mogas is scarce and our engines really don't like or need the lead in the 100LL. When I am forced to use 100LL I always add TCP and so far have not had any problems.

I may be making a move to northern California within the year and I'm not excited at the prospect of the unavailability of alcohol free mogas. The Franklin is economical to run when mogas is available. Running 100LL is frustrating because it is more expensive by the gallon and then you have to add the additional cost of the TCP (marginal I know, but annoying nonetheless).

Shane, is the mogas on the Tahoe side alcohol free? Maybe I'll just get a tank in the bed of my truck and drive over to fill up.

Sorry, meant Zane, not Shane

I wasn't sure, so I just called the guy at the Reno Costco gas station and he said that ethanol is added during the winter months.

Just came across 98 octane, no lead, no ethanol at our local Shell station here in Brainerd. About $2.40-2.35 a gallon. I wonder why this wouldn't make a good 100LL substitute for the majority of 4 and 6 banger aircraft engines.