Prolonged Shut Down

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buzzard86

Pre-takeoff checklist
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Warrington, PA
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Jim R
Hey gang -

My Cherokee (O320) has been strange to shut down lately. Initial symptom was that it would quit and then hit a blade or two backwards. Had it looked it, everything checked out and it seemed to be OK for a little while. It's acting up again intermittently, now it just coughs and sputters for 5-10 seconds when the mixture is pulled. Coincidentally, my fuel pressure has started running near the red line and occasionally fluctuating noticeably in cruise but without any perceptible engine changes. My initial suspicion was vapor lock, but pressure spiked this week on initial start of a cold engine so that theory is gone. Now I'm suspecting a bad mechanical fuel pump.

It's headed back to the shop for an expert inspection, but before then I'm just curious. Would/could high fuel pressures make it difficult to shut down? Wondering if the two are related or if I'm chasing two separate issues.
 
eman1200 said:
that's not fuel injected, correct?
Click to expand...
Nope, just a plane jane carbureted O-320. Mixture cutoff was inspected and working properly, yet the problem persists. The high fuel pressure seems more than coincidental, but I don't have enough mechanical experience to make the correlation.
 
Magman said:
You might want to assure the Mixture is hitting the stop in cutoff.

A Primer leaking internally can also mess up shut down.
Click to expand...
Thanks. We did already inspect mixture once and it was hitting full stop in cutoff. Good call on the primer, I hadn't thought about that.

Mechanic will be looking at it regardless. I'm just trying to understand potential causes for my own education.
 
Check the carburetor accelerator pump seal. The seal may be leather and well-dried out. If it is leaking, a bit of fuel is pulled into the carb throat. You likely have a Marvel-Schebler MA-4-SPA.

www.dropbox.com

00.Marvel-Schebler.Carb.Manual.pdf

Shared with Dropbox
www.dropbox.com www.dropbox.com
 
Domenick said:
Check the carburetor accelerator pump seal. The seal may be leather and well-dried out. If it is leaking, a bit of fuel is pulled into the carb throat. You likely have a Marvel-Schebler MA-4-SPA.

www.dropbox.com

00.Marvel-Schebler.Carb.Manual.pdf

Shared with Dropbox
www.dropbox.com www.dropbox.com
Click to expand...
Thanks Domenick. I believe I do have the MA-4-SPA. Engine was just OH'd last summer and the carb was rebuilt at that time, so I wouldn't expect the seal to be dried out. However, the seal failing in some other fashion is probably work investigating. Something seems to be proving a least a trickle of fuel after the mixture shutoff.
 
With those symptoms on mine it was the primer.
 
Primer is a good guess. And it's an easy, cheap fix.
Also, take a quick look at the carb bowl. I know it was overhauled, but there is a tendency (backed up by an MSB) for the bolt stays to loosen and the carb bowl to loosen. I found the condition on my Warrior many years ago.

www.dropbox.com

Marvel.Carb.MSB-14.Loose.Carb.Bowl.pdf

Shared with Dropbox
www.dropbox.com www.dropbox.com
 
Domenick said:
Primer is a good guess. And it's an easy, cheap fix.
Also, take a quick look at the carb bowl. I know it was overhauled, but there is a tendency (backed up by an MSB) for the bolt stays to loosen and the carb bowl to loosen. I found the condition on my Warrior many years ago.

www.dropbox.com

Marvel.Carb.MSB-14.Loose.Carb.Bowl.pdf

Shared with Dropbox
www.dropbox.com www.dropbox.com
Click to expand...
Thanks. We are definitely going to focus on the primer for the shut down issue. Seems to be the most likely culprit.

Still trying to figure out the fuel pressure issue. Starts in middle of normal range, climbs to just shy of red line in cruise, and then spikes beyond red line at shutdown. Pressure will occasionally oscillate in cruise, but engine runs flawlessly. I'm baffled.
 
buzzard86 said:
Still trying to figure out the fuel pressure issue. Starts in middle of normal range, climbs to just shy of red line in cruise, and then spikes beyond red line at shutdown. Pressure will occasionally oscillate in cruise, but engine runs flawlessly. I'm baffled.
Click to expand...
Wet gauge or electric sender?
 
buzzard86 said:
Good question. Pretty sure it's electric but am honestly not 100% sure. It's a stock '70 Cherokee 140.
Click to expand...
If stock then most likely wet gauge. Easy enough to find out. Can you see the backside of the gauge? If it has a stainless tube, it is wet. If just wires, it's electric.

Wet gauges can be cantankerous. That line has a small ID, so is susceptible to clogs, air bubbles, etc.

Also, does running the boost pump affect the reading?
 
Ed Haywood said:
If stock then most likely wet gauge. Easy enough to find out. Can you see the backside of the gauge? If it has a stainless tube, it is wet. If just wires, it's electric.

Wet gauges can be cantankerous. That line has a small ID, so is susceptible to clogs, air bubbles, etc.

Also, does running the boost pump affect the reading?
Click to expand...
I'm going to take a look behind and see... can't get it into the shop until next week.

Boost pump does settle out the oscillations, but has no effect on the excess pressure indication. I'd assume is was a bad gauge, except that the indications are pretty consistent by phase of flight and it was somewhat of an insidious problem that, on hindsight, started with it indicating fuel pressure even after shutdown (would normally go to zero after I shut everything down).
 
buzzard86 said:
Hey gang -

My Cherokee (O320) has been strange to shut down lately. Initial symptom was that it would quit and then hit a blade or two backwards. Had it looked it, everything checked out and it seemed to be OK for a little while. It's acting up again intermittently, now it just coughs and sputters for 5-10 seconds when the mixture is pulled. Coincidentally, my fuel pressure has started running near the red line and occasionally fluctuating noticeably in cruise but without any perceptible engine changes. My initial suspicion was vapor lock, but pressure spiked this week on initial start of a cold engine so that theory is gone. Now I'm suspecting a bad mechanical fuel pump.

It's headed back to the shop for an expert inspection, but before then I'm just curious. Would/could high fuel pressures make it difficult to shut down? Wondering if the two are related or if I'm chasing two separate issues.
Click to expand...
I must not be as smart as some of these people round here cause cough sputter spit for a bit sounds normal to me.
 
4RNB said:
I must not be as smart as some of these people round here cause cough sputter spit for a bit sounds normal to me.
Click to expand...
This isn't the usual second or two as it runs out of gas. The motor tries to keep running and sometimes will quit and then kick back a few blades. It's definitely getting a little bit of fuel somehow even at mixture cutoff.
 
buzzard86 said:
This isn't the usual second or two as it runs out of gas. The motor tries to keep running and sometimes will quit and then kick back a few blades. It's definitely getting a little bit of fuel somehow even at mixture cutoff.
Click to expand...
Ok. I spent a minute to look at C172 shutdowns, saw a 3 sec one and a 5 sec one.
Like I said, not so bright here.
Video of yours?
 
buzzard86 said:
This isn't the usual second or two as it runs out of gas. The motor tries to keep running and sometimes will quit and then kick back a few blades. It's definitely getting a little bit of fuel somehow even at mixture cutoff.
Click to expand...

That can happen if the octane in the fuel is too low or the engine timing isn't correct. Cutting off with the mixture might prevent it ...
 
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