Topic Author
Posts: 2579
Joined: Sun Jun 17, 2001 3:29 am

Vapor Lock?

Tue Nov 02, 2004 9:03 am

My understanding of vapor lock is the following:

In a fuel injected aircraft, a hot engine or nearby exhaust manifold can cause the liquid fuel being pumped through the fuel lines to vaporize, or as most people say, boil over. Thus, two things happen: one, the fuel pumps aren't designed to efficiently pump fuel vapors, and two, the pressure in the fuel lines as a result of the vapor attempting to occupy more space can cause the fuel not to flow through the injector and into the engine. Also, this can be caused by enough air being trapped in the fuel line either by improper venting or running a tank dry before switching.

One question, can vapor lock occur when an engine is running? If so, what's the appropriate procedure to take? Run the aux fuel pump? I know it can occur before start-up, and I'm assuming that it then takes quite some time to restart the engine as the vaporized fuel is forced out of the line. Please, if there's any flaws in what I'm stating, correct me! That's the entire purpose of this post, I need education when it comes to vapor lock. Thanks!

[Edited 2004-11-02 01:03:29]
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Joined: Fri Oct 24, 2003 11:14 pm

RE: Vapor Lock?

Tue Nov 02, 2004 10:20 am

I've always wondered that too. This past summer a Piper Arrow crashed at a local airport and the NTSB says it was vaporlock, but the engine quit just after takeoff, while they were airborne. Any ideas?

Here is the link.
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RE: Vapor Lock?

Tue Nov 02, 2004 11:37 am


One that I know of is an issue with early model 172's without vented fuel caps. They are placarded for single fuel tank operation in cruise above 3000'. They were having venting problems with the tanks, and this was the "fix". Later models received vented fuel caps on the right tank in addition to the main vent in the left tank.

Generally though things like that are ironed out before an aircraft is certified, so it is not all that common
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RE: Vapor Lock?

Tue Nov 02, 2004 11:56 am

Here is a good quick reference to "hot starts" with injected engines.

You have to read a few paragraphs down before getting into starting techniques.

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Joined: Mon Nov 12, 2001 10:34 am

RE: Vapor Lock?

Wed Nov 03, 2004 10:26 am

The only large radial engines used on US civil aircraft that were fuel injected (CurtisWright turbo-compound series) did indeed have fuel vapor problems early on in their development.
This was solved by using a higher output (flow) engine driven fuel pump.
The TCM fuel injected series engines used on twin Cessna aircraft (including the GTSIO series used on my aircraft) have an engine driven fuel pump that delivers substantially more fuel than the engine uses...the excess is returned to the respective tank, from the fuel divider on top of the engine.
This helps prevent vapor associated problems when the engine is running.
However, TCM recommends that aux (electric) fuel pumps be used at higher altitudes (on LOW)...CE421 and AeroCommander 685 aircraft in particular.

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