Fuel Flow Issue

[Mikey]

Hello all.

 

I had a problem with my fuel flow yesterday. ('82 M20K/231) I took off from Paso Robles, CA around 10a headed for Santa Rosa, CA. It was about 82 degrees OAT. Fuel flow was normal on take off and climb out (about 25GPH) until about 7000 feet. The fuel flow started dropping off TIT started increasing and fuel flow would only max out at about 15GPH. Leveled out at 8000 and ran it LOP. Everything was normal for LOP operations. Did some troubleshooting in the air (switched tanks, low boost pump, high boost pump) Nothing changed during LOP so felt it was OK to continue. We landed at Santa Rosa and spent some time with a mechanic on the ground. He couldn't figure out the problem either. Did a high speed taxi down the runway and fuel flow was normal. Took it up in the pattern and again everything was normal. We did fuel up before we left KPRB and I didn't sump the tanks for water. Maybe there was water in the fuel?  Any other ideas?? Thanks!

 

[DVA]

Hi,

 

Two questions - 1) did toggling on/off the electric boost pump change the fuel pressure/flow at altitude? 2) was the fuel flow drop off gradual as you climbed, or did it drop off quickly at from normal at 7000'?

 

[Mikey]

Neither low or high seemed to make a difference. It didn't drop off all at once. I noticed TIT climbing around 6k and checked fuel flow and it was dropping off fairly quickly. 

[jackn]

There is a hose that connects the manifold to the fuel pump. I would check to see if it's become disconnected or failed. Sounds like your fuel pump is sensing ambient pressure and adjusting to it. 

[Mikey]

17 minutes ago, jackn said:

There is a hose that connects the manifold to the fuel pump. I would check to see if it's become disconnected or failed. Sounds like your fuel pump is sensing ambient pressure and adjusting to it. 

Do you think it only fails at altitude?

[jackn]

Well, the differential pressure would certainly increase with altitude, thus exacerbating any loose or cracked connection. 

[peevee]

6 hours ago, jackn said:

Well, the differential pressure would certainly increase with altitude, thus exacerbating any loose or cracked connection. 

If it was reading ambient assuming he takes off at 30some inches of mp it would be reading much lower on the ground. Wouldn't it exhibit symptoms on the ground also?

Have you checked your boost pumps on the ground to ensure they function?

[Guest]

An open upper deck reference line to the engine fuel pump should cause low fuel flow at take off as well.  The fuel pump on the "high" setting should drown the engine in fuel, as long as the engine pump is correctly set and functing.

Clarence

[Mikey]

Thanks for the help. Since we landed and checked everything we cannot duplicate the problem so I have no idea what to do next. We've put about 6 hours on the plane since flying around the Northwest. I'll keep ya posted when we get it figured out.