Yesterday I am climbing from sea level to 8,500ft I am at full throttle and 2,500 rpms during the climb, usually at cruise the fuel pressure is at 6, at climbs usually at 4.
Usually my climbs are just to 4,500 ft or less.
On this climb to 8,500ft when I am at about 7,000ft I see the fuel pressure slowly dropping from 4 to 3 (bottom of green arch) then it continued dropping to 2 , at that time not knowing if it will keep dropping, I decided to run the electric pump ON, and that took the fuel pressure back to 6.
Once I leveled out I turned fuel pump OFF and the fuel pressure stayed at 6 all the way to my descend and landing.
Maximum fuel pressure for this engine is 8, and the red bottom number is 0.5, so at 2 was still above the bottom red line, but is this normal to see the fuel pressure drop during a long climb?
Engine was running smooth at all times and the fuel flow was not changing when the fuel pressure was dropping.
We had 40 gallons at the time.
O360 equipped and Del Air fuel tank extensions.
This happened at full throttle, if I back off the throttle a bit , the fuel pressure returns back to the green arch.

Why is the 170B equipped with an electric fuel pump and not just gravity fed like the 182?
Also why the need to switch tanks at cruise above 5,000ft?

The engine driven and electric backup fuel pumps on most of the 170 to O-360 engine conversions are required to provide sufficient fuel to the (more powerful) O-360 engine.

Thirty five (or more) more HP requires more fuel. So, the easy fix for the modifiers was to add an engine driven fuel pump to meet certification requirements in all configurations and attitudes.

It turns out the limiting factor was the diameter of the fuel line between the fuel selector and the gascolator. A mechanic who taught at UAF’s A & P school had a 172 with an O360 conversion, which had the pumps. He did some research and found that some later 172s with O-360s didn’t have the pumps. The difference: larger diameter fuel line from selector to gascolator.

So, he changed that section of line to a larger diameter and removed both pumps. This was field approved. He also got field approvals for this mod on a number of local C-170s. He offered to do mine, but I never saw a disadvantage to the pumps....at least not enough to remove them.

But that diameter fuel line is what requires the pumps. Pumps increase pressure, moving more fuel than just gravity.

As to what you’re seeing in climb....don’t worry about it. I suppose it’s possible your engine driven pump is a bit worn, or?? But as you noted, it’s still keeping sufficient fuel flow, so no worries. Understand that the engine driven pump ASSISTS gravity in maintaining fuel flow. In an extended climb at wide open throttle is worst case scenario for fuel flows. Try backing off WOT to get past the auto enrich feature of the carb. That’ll reduce fuel flow. Monitor CHTs, and if they don’t get too high in climb, that’ll save some gas. But WOT in climb is generally a good thing.

Short response: Don’t sweat it.

MTV

I have never seen that "Switch to single tank" before in a 170, converted or not. No mention of it in the Delair STC.

https://rgl.faa.gov/Regulatory_and_Guid ... enDocument

This AD is specific to 172s, but I've heard people say the 170 fuel system is identical. I had a T-41B with the same placard, so they may have expanded the scope of the AD with some other action.

jcadwell wrote:https://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/AOCADSearch/2C7B32477D2A88A286256A340063355D?OpenDocument

This AD is specific to 172s, but I've heard people say the 170 fuel system is identical. I had a T-41B with the same placard, so they may have expanded the scope of the AD with some other action.

First, that is a Service Letter, NOT an Airworthiness Directive. Service Letters are "suggestions" under Part 91 operations. Mandatory if you're operating under Part 135.

Also, it doesn't apply to the 170, though, as noted above, the fuel systems are pretty similar. That said, please read the Service Letter completely.....they tried to duplicate the "condition" and couldn't do so without actually injecting additional air into the tanks!!! Good grief.

Back then, Cessna was being sued right and left, often because of their fuel tank design, which can make it difficult to get all the water out of a fuel tank. That caused a number of accidents, and resulted in the new 172s having something like 16 sump drains in the fuel system.....

Anyway, I sure wouldn't attach that placard to an airplane other than the 172.

More to the point, fuel quantities, which have been scratched off of that selector, are required placards. I realize that was done because bigger tanks have been added. Nevertheless, accurate placards are required.

MTV

https://support.cessna.com/custsupt/con ... s_id=36949

Above is a service letter associated with the vent.

How can you tell 72-07-02 isn't an AD?

Even more to the point it looks like someone has been beating on that selector and shaft with a freek'in ballpeen hammer, yikes.

jcadwell wrote:https://support.cessna.com/custsupt/contacts/pubs/ourpdf.pdf?as_id=36949

Above is a service letter associated with the vent.

How can you tell 72-07-02 isn't an AD?

Sorry, you’re right, that IS an AD, NOT a service letter. I was looking at the service letter.

But in any case, it doesn’t apply to the Cessna 170, or, so far at least, any other Cessnas. I don’t know if the fuel system in the 170 is exactly the same as the early 172s, but it had to be close. The early 170Bs had a single vent mounted amidships on the roof of the cabin. Later Bs had a different vent system.

MTV

MTV

I don't think that's normal or correct. My fuel pressure is always 4-6 psi.

Could be a blocked vent, clogged filter in the electric pump, or bad engine-driven pump?

The AC-type lever pumps are pretty reliable, but I had a Rapco overhaul unit fail at 250 hours. It was poorly assembled with inadequate screw torque, allowing the rubber seals to migrate.

If you hate money, buy and install a new engine driven fuel pump.

motoadve wrote:Yesterday I am climbing from sea level to 8,500ft I am at full throttle and 2,500 rpms during the climb, usually at cruise the fuel pressure is at 6, at climbs usually at 4.
Usually my climbs are just to 4,500 ft or less.
On this climb to 8,500ft when I am at about 7,000ft I see the fuel pressure slowly dropping from 4 to 3 (bottom of green arch) then it continued dropping to 2 , at that time not knowing if it will keep dropping, I decided to run the electric pump ON, and that took the fuel pressure back to 6.
Once I leveled out I turned fuel pump OFF and the fuel pressure stayed at 6 all the way to my descend and landing.
Maximum fuel pressure for this engine is 8, and the red bottom number is 0.5, so at 2 was still above the bottom red line, but is this normal to see the fuel pressure drop during a long climb?
Engine was running smooth at all times and the fuel flow was not changing when the fuel pressure was dropping.
We had 40 gallons at the time.
O360 equipped and Del Air fuel tank extensions.
This happened at full throttle, if I back off the throttle a bit , the fuel pressure returns back to the green arch.

Why is the 170B equipped with an electric fuel pump and not just gravity fed like the 182?
Also why the need to switch tanks at cruise above 5,000ft?

I have seen this several times on my 0360 c170. At first I would turn on the electric pump but now I just watch temps and pull back a hair on the throttle. Maybe mine and yours are the only two that do this, but report if you find something out.

mtv wrote:The engine driven and electric backup fuel pumps on most of the 170 to O-360 engine conversions are required to provide sufficient fuel to the (more powerful) O-360 engine.

Thirty five (or more) more HP requires more fuel. So, the easy fix for the modifiers was to add an engine driven fuel pump to meet certification requirements in all configurations and attitudes.

It turns out the limiting factor was the diameter of the fuel line between the fuel selector and the gascolator. A mechanic who taught at UAF’s A & P school had a 172 with an O360 conversion, which had the pumps. He did some research and found that some later 172s with O-360s didn’t have the pumps. The difference: larger diameter fuel line from selector to gascolator.

So, he changed that section of line to a larger diameter and removed both pumps. This was field approved. He also got field approvals for this mod on a number of local C-170s. He offered to do mine, but I never saw a disadvantage to the pumps....at least not enough to remove them.

But that diameter fuel line is what requires the pumps. Pumps increase pressure, moving more fuel than just gravity.

As to what you’re seeing in climb....don’t worry about it. I suppose it’s possible your engine driven pump is a bit worn, or?? But as you noted, it’s still keeping sufficient fuel flow, so no worries. Understand that the engine driven pump ASSISTS gravity in maintaining fuel flow. In an extended climb at wide open throttle is worst case scenario for fuel flows. Try backing off WOT to get past the auto enrich feature of the carb. That’ll reduce fuel flow. Monitor CHTs, and if they don’t get too high in climb, that’ll save some gas. But WOT in climb is generally a good thing.

Short response: Don’t sweat it.

MTV

Hey Mike, any chance you have or know where a could find a document that specifies the diameter require the fuel line that would allow for the electric pump to be eliminated.

Thank you in advance,

I'm not Mike but start your research with FAR 23.955 paragraph (b) and (c). Must flow 150% of that required for max power with gravity and only 125% with fuel pump installed in most adverse attitude (nose up). The backup pump requirement is related to protection in the case of a failure of the engine driven pump. I am not sure what the size of the line is but it would be safe to say that the size installed in a 182 with 230 hp would be adequate. Specific fuel requirements are spelled out elsewhere in the FAR's, but the 25% reduction in required flow with the installation of the pump covers the 25% increase in required fuel flow do to the 180 hp engine. That makes the fuel system part of the STC certification and installation much easier (less costly). Hope this helps.

Tim

bat443 wrote:I'm not Mike but start your research with FAR 23.955 paragraph (b) and (c). Must flow 150% of that required for max power with gravity and only 125% with fuel pump installed in most adverse attitude (nose up). The backup pump requirement is related to protection in the case of a failure of the engine driven pump. I am not sure what the size of the line is but it would be safe to say that the size installed in a 182 with 230 hp would be adequate. Specific fuel requirements are spelled out elsewhere in the FAR's, but the 25% reduction in required flow with the installation of the pump covers the 25% increase in required fuel flow do to the 180 hp engine. That makes the fuel system part of the STC certification and installation much easier (less costly). Hope this helps.

Tim

Great intel, thanks

So after some research and poking my head into a few cowlings I found at two 0-360 172 with no EDP or electric FP. The common element was 1/2' fuel lines from the fuel selector valve to the gascolator and onward to the carb, so that's my plan as well, wish me luck.