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Engine Problems Lycoming IO360 - Robin DR 500

We are at one‘s wits‘ end…

Aircraft: Robin DR500-200
Engine: IO-360 A1B6
Propeller: MT- Muehlbauer three-blade variable-pitch

When the engine is cold it creates full power and has a fuelflow of 18 gallons p.h. at 2700 RPM.

During flight, engine performance decreases slowly but steadily … after some time it barely reaches 100 kts.

If you try to reproduce this on the ground, it is as follows:
When the engine is cold everything runs well …
Switching engine off and wait for 15 minutes it seems temperatures rise underneath the cowling …

If you restart the engine after those 15 min the engine runs rough and uneven and consumes only 16 gallons p.h. and rotates with 60-100 revolutions less …

If we repeat the whole thing without cowling the engine runs well. The performance is still good!

We have been at several shops with the plane and have replaced everything in front of the firewall: brand new IO360 from Lycoming, fuel lines, exhaust system, everything has been renewed and stil the engine dose not work.

Today the spider was turned 180 degrees “upsidedown" again…slight improvement ..

can someone help us and has an Idea?


Do you have an accurate fuel flowmeter (fuel totaliser) and engine instrumentation (EGT measurement)?

If you are flying at cruise power, peak EGT, and the IAS drops off that means engine power is reducing. Are you still at peak EGT then? What is the fuel flow doing? If that is dropping off too then I would check the inlet duct for obstructions and if it is clean get the fuel servo overhauled.

How can it be rotating with 60-100rpm less when it has a CS prop which should adjust the blade pitch to maintain a constant rpm?

You replaced the whole engine and it still does it?

The aircraft cannot be safely flown with this issue.

Shoreham EGKA, United Kingdom

What happens if you turn the electric fuel pump on when the engine is runnng hot ?

What happens if you turn the electric fuel pump on when the engine is runnng hot ?

- no changes, stil just a fuelflow of 16 gal.


I don’t understand this bit:

“have replaced everything in front of the firewall: brand new IO360 from Lycoming, fuel lines, exhaust system, everything has been renewed and stil the engine dose not work.”

[my bold ]

Has the fuel servo and the governor been changed / overhauled too?

So unless this is another “Robin project with teething troubles” which was bought by 3 people of whom 2 are not on the internet, something very big is not right here…

EDIT: this bit makes no sense too

Today the spider was turned 180 degrees “upsidedown" again…slight improvement ..

There is only one possible orientation for the fuel distributor.

Shoreham EGKA, United Kingdom

Today the spider was turned 180 degrees “upsidedown" again…
we are so desperate now – It was worth a try:

The fuel servo is okay, the fuelsystem is okay – we used a gasoline canister to bypass the whole system – no changes
There are no significant differences in the EGT
The governor went to overhoul today… it is the only part which is not new yet…
Even the propeller has been overhauled, as the plane was rolled into a taxiwayamp by the maintenance staff…This happened during one of the many attempts of an maintenance shop to solve the problem.

We are happy about any suggestion, because we can not think of anything any more


Hmm, I am not sure I understand the symptoms.
Regardless, the good thing is you can reproduce the issue on the ground.
Let’s assume there is nothing wrong with the engine and the problem is thus with the airframe side of the engine installation .
You said the issue only happens with engine cowling on, and then only when hot. Is this 100% consistent (ie 100% of the time)?

I maintained and flew behind the IO-360-A1B6 engine in our 177RG for about 800 hrs so was quite familiar with the engine. Max FF should be 18-19GPH , but 16GPH by itself should only lower available power a bit , perhaps from 200 to 190 hp..this will have a barely noticeable effect on speed and definitely nothing close to the 30-40 KIAS loss reported.The engine will end up overheating and perhaps damaged unless the situation is corrected soon, but 16GPH in itself, while maintaining MP and propeller setting, will not make you lose a lot of power. The reduced FF may be a consequence rather than the cause of the problem. Working on that assumption(and also, you bypassed a big part of the aircraft fuel system by feeding it directly without changes) causes could be:

A) CS propeller
On the ground, the CS prop, if properly set, and with the control full forward, should be at fine pitch all the time and behaving like fixed pitch. If for some malfunction, while ground running at full power, it is being governed out of fine pitch, you should be able to tell. If FF and MP stay normal, a loss of power due solely to a malfunctioning propeller can be told by the equal or higher MP and a change in prop noise..Instead, when you reduce RPM sightly by using the throttle, you will see a decrease in MP and will not notice a change in propeller noise.

B) Throttle/induction system
When you see the loss of power, the FF is reduced, but is MP changed? If there is nothing wrong with the throttle system, and the fault is with the propeller or fuel systems, then MP should slightly INCREASE, if at all, when you experience the power loss. If you have an EGT indicator then you should see mostly constant (prop issue) or increased (ff issue) EGT. If there is a decrease in MP then something is wrong in your induction: perhaps a problem with your throttle control (you need to be sure the control reaches AND maintains the full-throttle stop), or your induction air box/air filter/alternate air system.

C) Exhaust system
is there anything loose inside the exhaust that could be blocking exhaust airflow? Symptoms would also be a similar or higher MP when power is lost. If you have an EGT indicator, then you should see reduced EGT upon power loss.

If the above fails to show an issue, perhaps the indications are inaccurate. Do you have a FF meter other than the standard spider-pressure-driven indicator? If so, do they match? Are you sure there are no obstructions in your fuel hoses or the injector body inlet screen? Your mechanical pump cannot be at fault since use of the electric pump does not change symptoms. Also, make sure mixture control reaches and maintains the rich stop, even with the cowling on. You can use a phone or some cheap camera to tell.

Please give it some thought and come back with comments.

Good luck

Last Edited by Antonio at 19 Nov 21:21
LESB, Spain

The URL in post #6 is duff and I can’t anything on google.

My earlier questions still stand. We possibly don’t know the fuel flow accurately. A 30kt loss is a massive power loss. There are likely to be instrumentation issues compounding this.

Shoreham EGKA, United Kingdom

OK; yes, this reduces the heat transfer from the crankcase to the flow divider.

This heat transfer is not usually an issue in flight, due to the massive airflow in that area. Also the ~30kt speed loss cannot be explained while the fuel flow is still indicated at 16 USG/hr.

What engine instrumentation is available?

Shoreham EGKA, United Kingdom
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