Question for Paul McBride, the General Aviation News engines expert: I have an interesting problem with my 1974 Piper PA-28-235 with a Lycoming O-540-B4B5 engine that has my IA stumped.
I have owned the airplane for over 20 years, hangared in Central California, and flown by me only 70 to 80 hours a year. TT 2,400, with a major engine overhaul five to six years ago. The issue began shortly after the overhaul. The engine pump was replaced during the overhaul.
Under setting conditions the fuel pressure gauge in the cockpit will read zero. Fuel flow as reported by an EI FP5 gauge always shows proper flow.
Let me further explain the “when”:
- Always during a climb and not from takeoff, but after power reduction to 25/25 normally passing through 4/6 AGL. The pressure without electric boost pump is in the middle of the gauge, 4-5 PSI.
- By simply lowering the pitch attitude from climb to level flight, the gauge reading returns from zero to midrange — no power change, just lowering the nose.
- At NO time does the engine stutter to starve from fuel. Pressures reads at or near zero, fuel flow at normal.
- The above is with the mechanical fuel pump only. I have found if I turn on the boost pump fuel pressure, reading goes back to normal. This is during climb out.
- The airplane has four fuel tanks — this happens on any tank with any quantity, summer or winter operations.
Here a snapshot of what my IA has done:
- Replaced the engine-driven pump
- Blown out full supply line to fuel pressure gauge
- Replaced and checked all fuel filters
- Verified tank vents are clear.
I took some photographs:
In Climb
Showing climb profile:
Nose lowered, same power settings — this is the normal location of fuel pressure with engine driven pump operations:
I have “just lived” with this issue, but it needs to be corrected. Any help would be greatly appreciated.
Rodney Voumard
Paul’s Answer: Rodney, I’ll begin by saying that you certainly have an unusual opportunity here, so let’s see if we can offer some direction.
First, as your PA-28-235 is an older aircraft that, in itself, may be part of the contributing factor here.
As I recently mentioned in another one of my articles, the accuracy of the instrumentation always comes into play when we encounter situations such as yours.
After reading and rereading your problem, I honestly don’t know where to begin.
My first thought was that the engine driven fuel pump was just getting tired, but since you’ve installed a new pump, we can eliminate that.
After reviewing the Lycoming Operator’s Manual for the O-540-B4B5 regarding engine fuel pressure, I noted that the pressure should be between .05 minimum and 8.0 PSI maximum. I’d consider this a pretty generous spread and after reviewing the pictures you enclosed, I’m wondering if we may not just have a gauge issue. Why it seems to be induced by lowering the nose of the aircraft is a real mystery for me.
Maybe we’d better start off in a different direction. Have you tried to measure the outlet pressure right at the fuel pump with a known accurate gauge in order to eliminate the aircraft system? This would confirm the actual pressure coming out of the fuel pump.
I hate to admit this Rodney, but right at this moment I’m drawing a complete blank as to what to suggest next.
Hopefully, with what you have already learned from troubleshooting and the few minor suggestions I’ve made, you’ll be able to figure out what’s going on.
I think all of our readers would appreciate learning exactly what the problem was, and should you find out and be willing to share what corrected the situation, I’m certain it would be appreciated.
And if any of our readers has some suggestions for Rodney, please include them in the comments below.
Its an indication problem. Is the FP gauge wet or electric? if it is wet plumb in a know good gauge in the cockpit. Perhaps the line to the inst is obstructed. If it is electric i would suspect the diersonval (sp?) inside the instrument. You have already spent all the money might as well replace the FP instrument.
Two experiences to relate:
Mooney with O-360, after carb was R&Red, get occasional low pressure in climbout. Never sputtered or quit. Split in line, or seal/gasket sounds like possibility.
Owned a Triumph Spitfire and lived in dessert climate. Side draft carburetor was right above exhaust manifold. I think originally came from factory in ’79 with some heat shields in between. Engine would dog out on hot days at highway speeds. Shop diagnosed as vapor lock. Installed electric fuel boost pump/removed mechanical pump.
It would be interesting to see what the pressure does in climb if you reduced power. Pressure is the inverse of flow in this scenario and during a full power climb the engine is going to need more fuel hence lower pressure. If the powers was reduced pressure should rise. I don’t know why pushing the nose over would change the pressure other than an increase in RPM. If he is pulling the power back to maintain the same RPM as in climb when he levels off then that would reduce the fuel flow necessary and increase presume. I also believe Klaus’ theory on the check valve is worth looking into.
looks like it’s at .5 to me, w /in specs.
The Piper Tomahawk I owned years ago did the same, particularly in warmer weather. In fact, the warmer the weather, the lower the fuel pressure during climb once the engine was already warm. Like yours, it never did this on the initial climb out from departure. I always assumed the heat generated during climb was causing some vaporization of the fuel.in the engine driven fuel pump causing some cavitation in the pump. Unless the carb runs out of fuel, the engine isn’t going to stumble. One way to disprove this guess would be to supply some additional cooling air to the mechanical fuel pump and add firesleeve to the lines to shield the fuel lines from the heated air coming off the cylinders. Additionally, you might note that during climb, your pressure gauge is actually still in the green at .5 psi, as was mine on my old Tomahawk. The engine never stumbled. The electric pump was there if the pressure reading made me nervous, and it did this for the decade that I owned the plane. It was never a problem.
Funny I recently sold my Cherokee 140 with a 0-360 that had the same symptoms. On climb out with the electric pump on the pressure was in the middle of the green. Shut the pump at 1000 AGL while climbing out and the gauge would immediately go to near 0.
Put the electric pump back on and the gauge would show normal again.
There was no change in power.
In level flight and electric pump off with a slight power change and the gauge would read normal….
I wonder if the gauge was the problem?
A note here is the Cherokee 140 180HP STC requires the fuel pump to be change to the bigger electric pump……
It was strange but caused no problem?
I have flown every Cherokee model and have never seen this problem except in this 140/180?
Just a simple observation here…
From the photos provided, the low pressure indication is not zero and in fact is well above zero. This is a rather crudely-marked gauge so it’s difficult to tell where on the gauge the zero condition might be, but it’s definitely to the left of the position of the indicating needle in that photo.
Adding to what has already been said, I would ensure the engine driven fuel pump overboard vent line is clear and not positioned to introduce ram or vacuum air pressure as a result of pitch change. If nothing appears evident, remove left magneto and inspect the engine driven fuel pump drive pin and concentric cam. It’s part of the magneto drive gear.
The electric fuel pump check valve is worn out. The spring load on the flapper is not correct. The best fix is to have the electric fuel pump assembly overhauled or replaced.
If the pressure change is real and not a gauge issue, I would look for a leak upstream of any pump involved. Leaks on the suction side may not leak fuel out but let air in when the pump is on. The leak is thus effected by the head of the fuel vs suction. That could explain the pressure change with attitude.
We think alike on this.!!
Reattaching an old hose can cause it to crack/split.
If a separate fuel pressure gauge shows the same problem, I’d look for a leaky fuel hose to the carb.
A crack/split can allow air to be drawn in during a climb, with the fuel tanks lower than the carb., and negative pressure in the inlet hose.
Leveling, in cruise the fuel tanks are close to the level of the carb inlet, causing slight positive pressure, and no air leak.
I had a vehicle , that when parked on a slope to the right, It would not start. I found a split in the inlet hose on the right side of the engine.
So, when parked, the engine would roll enough to flex the hose and cause air to be drawn in.
This is a long shot but a fitting might be lose and pulling apart as the wings flex with the different angles of attack.