Hi, this is a followup to another post under 92 F-series.
I just bought a 92 F-150 with EFI 5.0L on straight propane. It has spark but will not fire. When cranked the evaporator does not provide propane to the mixer unless I provide a source of vacuum other than the engine to the lockoff. Basically as soon as I apply vacuum to the lockoff with engine off propane releases from the evaporator to mixer hose. The mixer however also will not open and allow that propane into the throttle body even when cranking. What makes the bellows part of the mixer to move and allow flow? engine vacuum?. Also the engine turns over slowly even with a freshly charged battery ( tried 2 brand new ones). there appears to be no vacuum at the vacuum tree on the intake manifold when cranking. Could all the problem be is the engine not cranking over fast enough?. A new starter or battery cables should fix this.... Valve train is moving and compression is 60 psi cold (330,000 Km) spark is good. Tried running a BBQ tank direct into throttle body with no effect. Also there is a small component on the side of the evaporator that is wired to the ignition/start post of the solenoid and to ground and has a vacuum connection to the intake manifold. What is this? there is a vacuum port on this component, one on the mixer to throttle-body-hoses adapter and one on the lockoff. what should be connected to what vacuum port or source? Tommorrow I will blank off all other vacuum ports such as brakes and cruise control to try and provide more to the propane system. I have also disassembled and cleaned the mixer (very oily and dirty) Anyone have any suggestions or ideas? I am propane illiterate even though it seems very simple. What am I missing?
Thanks in advance
007BRONCO

 

your compreesion seams to be way too low ; if i remember correctly for a v8 should be between 95 to 120 psi. 330k ml is too hi (if its not being overhaul)

 

Sounds like it is/was dual fuel, that vacuum-electric device might be or probably is the vacuum lock for when it is running on gas. Are you sure there's no switch for alternate fuel somewhere on the dash? Have a look and make sure it's switched all the way to one side or the other. Have you ever seen it run? Can you talk to the previous owner? If you're only going to run propane make sure that the lockoff vacuum hose is connected directly to the manifold and disconnect and plug any hoses going to that vacuum electric solenoid.

That compression does sound low, what were the results with the wet test?

Edited 11:15

 

just did a quick wet test with wd-40. 4 of the cylinders almost doubled, 4 stayed mostly the same, those were paired cylinders. 1&2 and 7&8. So I'm thinking bad head gaskets and worn rings. I have seen the truck run. It is propane only fueled. And also the timing chain and gears are good.
compression wet and cold was:
1. 15
2. 15
3. 90
4. 110
5. 80
6. 85
7. 30
8. 30
I'm pulling the motor for a rebuild.
Thanks for you reply
007BRONCO