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Need help with engine adjustments

S

SuperCubDriver

Well Known Member
I just completed my RV-8 - posted in another thread.
The airplane has a AerosportPower IO-375M1S low compression with 2 P-Mags, Superior sump and Airflow Performance FM-150 Fuel Injection System with purge valve.

I?m having two issues and looking for help and advise:

1. Idle quality is lousy, RPM is fluctuating. When advancing the throttle engine smoothes out at about 1400 RPM. Leaned the idle with the adjuster screw on the throttle body with no help. mixture lever is almost unresponsive. Checked for intake leaks visually, all bolts are secure and no visible gap. Boost pump on or off doesn?t matter, fuel pressure without boost pump is around 15-27 psi, with boost pump on 35 psi.
Is there a good and simple way to check for intake leaks?
I did not touch the ignition yet but left, right or both mags is just the same.

2. When advancing the throttle without the boost pump at around 2000 RPM the fuel pressure is going down to 5 psi followed by decreasing fuel flow, so the engine does not power up with only the engine driven pump. It accelerates fine with good power for a very short time til the fuel flow drops.
With boost pump on it accelerates normal with normal power output (FF 16 GPH). I once disconnected the engine driven pump and operated the engine with only the boost pump. Idle problem is still present then and the engine is powering up.

So if anyone had a similar experience or ideas how to approach these problems I am very interested in getting directions.

Thanks - Hermann
 
SuperCubDriver said:
I just completed my RV-8 - posted in another thread.
The airplane has a AerosportPower IO-375M1S low compression with 2 P-Mags, Superior sump and Airflow Performance FM-150 Fuel Injection System with purge valve.

I?m having two issues and looking for help and advise:

1. Idle quality is lousy, RPM is fluctuating. When advancing the throttle engine smoothes out at about 1400 RPM. Leaned the idle with the adjuster screw on the throttle body with no help. mixture lever is almost unresponsive. Checked for intake leaks visually, all bolts are secure and no visible gap. Boost pump on or off doesn?t matter, fuel pressure without boost pump is around 15-27 psi, with boost pump on 35 psi.
Is there a good and simple way to check for intake leaks?
I did not touch the ignition yet but left, right or both mags is just the same.

2. When advancing the throttle without the boost pump at around 2000 RPM the fuel pressure is going down to 5 psi followed by decreasing fuel flow, so the engine does not power up with only the engine driven pump. It accelerates fine with good power for a very short time til the fuel flow drops.
With boost pump on it accelerates normal with normal power output (FF 16 GPH). I once disconnected the engine driven pump and operated the engine with only the boost pump. Idle problem is still present then and the engine is powering up.

So if anyone had a similar experience or ideas how to approach these problems I am very interested in getting directions.

Thanks - Hermann
Click to expand...

Without doing research on what you have, the engine pump should provide a lot more than 5 psi for the FI system. Look at the specs, it will tell you the minimum fuel pressure. If the pump does not provide that, then it is defective, or it is an incorrect pump (maybe for a carb engine).

Once that is addressed, then it will be appropriate to look at the idle symptoms.

BTW - you should be wary of flying this airplane with a bad mechanical pump.
 
Fuel Pressure

If you have the standard AC style engine driven fuel pump, what kind of fittings do you have at the pump.
Many Pipers and IIRC all factory built Pitts aircraft use an AN bulkhead fitting, with jam nut and a special o ring. There have been a number of cases on RV's and Rockets where these fittings were improperly installed and/or improperly torqued. The o ring style fitting is very critical, if under torqued it will leak, if over torqued the o ring may be damaged. What is the history of the fuel hoses? If they were not fabricated and tested in a hose shop there is always the possibility of a defect in the hose inner lining, usually a flap of rubber that is not easily seen when looking at the hose visually.
Was the engine test stand run by the engine builder? If it has some test stand time you could probably get away with aggressively leaning the engine with mixture control for all ground operations. My experience is only with the Bendix system but these engines tend to run very erratically on the ground at low r/m. Leaning aggressively for all ground operations helps.
The fuel pressure/fuel flow issue does need to be addressed before flight.
 
Fuel Pump Fittings

The fittings as used on Pitts and many Pipers with Lycoming engine is AN833-6 elbow with long end screwed into pump. AN 924-6 jam nut. O ring that I have always used is a Piper part number.
Vans may call for something different.
 
No way I fly without engine running on the engine driven pump only.
Pump is LW-15473 high pressure diaphragm pump, at least on the papers, how can I check the pumpo is the right one?
It has the fittings with the o-rings and I did a flow test with the boost pump. At least with the boost pump is more than 200% flow of maximum flow required (through the engine driven pump). The hoses are teflon/new and I doubt there are things like rubber flaps inside, I blowed them through prior installation but will check again. Up to the firewall I?m having aluminum tubing.
Note that the engine driven pump is providing around 25psi pressure but not with the higher fuel flow. I am also thinking of some restriction before the pump but I also think this is unlikely. I checked the fuel filter which was very clean, also the inlet filter in the FM-150 is clean.

The engine was test run by the engine builder but I?m not sure they used the now installed equipment for their runs. So for example the oil pressure was way low and on the test sheet in the upper half of specifications. I also checked the oil pressure with a second gauge. That makes me feel that I have to start from the very beginning.
 
With the engine idling, what does the rpm indication do as you slowly pull the mixture to idle cut off? You should see 50-100 rpm increase (running a tad rich).
 
IO375 timing is different

did you retime the Pmags. The timing is different on the IO375 from the usual 25 degrees. We had the same issue on a re-install after upset landing prop strike cause a change from IO360 180 to IO 375. Once thePmags were timed right the engine ran great.
Just a suggestion something to check
Tyler
A&P, EAA Tech advisor
RV7A Builder
 
Pump

LW15473 Is the correct pump. The pictures in Spruce catalog show the part number stamped close to the mounting flange on the left side, the part number is shown vertical with the LW at the top. You can view this at aircraftspruce.com, search Lycoming fuel pump, click on the image to enlarge it.
Another remote possibility if the fuel pump was removed and reinstalled after the engine runs is the possibility that the actuator rod is not properly aligned and making contact with the arm of the fuel pump. Very unlikely but it has happened.
I have no experience with the purge valve system but an easy check would be to simply cap off the line to the purge valve and do a test run. An alternative is to disconnect the line between the purge valve and the tank, stick the line in a can and run the boost pump.
You could also run a line from a gas can direct to the engine driven pump for testing. Starting would require cranking the engine a bit with the mixture full rich to prime, then normal starting procedure.
Most of the above very unlikely but possible. Verify the pump part number first.
 
Pumps

Just as a reference point the majority of the early Pitts S1 aircraft including the first few factory S1's used the Bendix PS5C carb. This was supported by a medium pressure engine driven pump which is no longer available new. Backup pump was a hand wobble pump. These airplanes flew thousands of hours on just the engine driven pump. The wobble pump was used for priming and as a backup in case of engine driven pump failure.
Your engine should run perfectly on just the engine driven pump, although you will have the typical low power roughness associated with the fuel injection.
I have 1000 hours plus on just the engine driven pump, never the slightest problem unless I ran a tank dry.
 
The low pressure pump would not be able to produce 25 psi at low rpm... But to verify which pump is installed is an easy visual. The high pressure pump has a wave like ring washer around the bottom of the pump between the screw heads that hold the pump together and the pump body and the low pressure pump has no wave washer between the screws that hold it together and the pump.

If you are a member of the yahoo Lycoming group at the bottom of the files section there is a picture of the two pumps side by side.
https://groups.yahoo.com/neo/groups/lycoming/files

It does sound as though there may be a blockage or a leak causing the FP to deteriorate as demand increases.

Good Luck,
Mahlon

jrs14855 said:
LW15473 Is the correct pump. The pictures in Spruce catalog show the part number stamped close to the mounting flange on the left side, the part number is shown vertical with the LW at the top. You can view this at aircraftspruce.com, search Lycoming fuel pump, click on the image to enlarge it.
Another remote possibility if the fuel pump was removed and reinstalled after the engine runs is the possibility that the actuator rod is not properly aligned and making contact with the arm of the fuel pump. Very unlikely but it has happened.
I have no experience with the purge valve system but an easy check would be to simply cap off the line to the purge valve and do a test run. An alternative is to disconnect the line between the purge valve and the tank, stick the line in a can and run the boost pump.
You could also run a line from a gas can direct to the engine driven pump for testing. Starting would require cranking the engine a bit with the mixture full rich to prime, then normal starting procedure.
Most of the above very unlikely but possible. Verify the pump part number first.
Click to expand...
 
Today I had another look at the pump. There is no part number on the pump but it has the wave like ring washer at the bottom. I prepared an aux tank which will eventually go in the rear baggage area today and I will connect the tank to the inlet of the engine driven pump and will see how the pressure develops. That way the pump doesn´t have to suck through the boost pump.
Mixture control is unresponsive til cutoff.

I´m interested what timing numbers are best for the IO-375, and yes I adjusted to 25 deg. On my -7 with an IO-375 this worked out ok.
 
Last edited:
Update - problem solved

I connected the aux tank to the engine driven pump inlet and the engine was running on full power with 25 psi and 17 GPH. Then were is the problem behind the firewall? Well I removed the boost pump and made a connection between the fittings and the engine was running fine again. So the boost pump is faulty and restrictive, blowing into the inlet proves clearly the restriction.

Idle is much better now. I had to enrichen the idle mixture quite a bit and it started to smooth out.
 
SuperCubDriver said:
I connected the aux tank to the engine driven pump inlet and the engine was running on full power with 25 psi and 17 GPH. Then were is the problem behind the firewall? Well I removed the boost pump and made a connection between the fittings and the engine was running fine again. So the boost pump is faulty and restrictive, blowing into the inlet proves clearly the restriction.

Idle is much better now. I had to enrichen the idle mixture quite a bit and it started to smooth out.
Click to expand...

Herman, it does not sound like the problem is solved, but the diagnostics have isolated the problem. You might have a suction leak. A pressure test of the section of the fuel system between the firewall and wing root is suggested. If you have access to 30 psi air, then carefully purge that section, then block it at one end. Then pressurize at the other with a gage and use a valve to shut off the air supply. If the pressure drops more than 1 psi in a few hours, then you have a leak. You can find it with a soapy solution that will make bubbles at the leak. A friend and I have each found a leak this way. It still maybe a restriction, but eliminating a suction leak first will allow the more difficult task of quantifying the cause of a pressure drop.
 
Good point with the suction leak. I might check but I really think the boost pump is the problem. Two reasons: I should smell fuel with the boost pump running - ok only if there is a leak downstream of the pump. The other reason should be the final answer: I replaced the boost pump with a piece of hose and the engine is running fine; and blowing into the boost pump inlet shows the flow restrictioin.
 
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