Possible vapor lock or ??? 454 / 496 GM motor

[jeff york]

I had an odd incident last night. I pulled my 1977 Airstream Argosy 454 / 496 GM powered coach out of storage. I ran it about 10 minutes before leaving and then drove about 25 miles and stopped for gas. I pumped about 22 gallons of regular gas into it. It sat maybe 20 more minutes running while I pumped the gas. Then, right after I pulled out, the motor shut down. I got it to restart and then as soon as I got through the intersection it shut down again. I could hit the key and it would sputter and fire up and quickly shut down. I had to pump the heck out of it to try and keep it running. But, it would quit. I took some starting fluid and got it to start again and it quickly shut down. On the second try with starting fluid it started and I drove it about a block to a Walmart parking lot. I shut it off I think. After a few minutes, I started it back up and decided to try and get it home about 4 miles away. It ran just fine the whole way. So, now I am wondering what happened. 

Back ground. I recently replaced my mechanical fuel pump with an electric fuel pump. I test drove it quite a bit before taking it out on the road so I thought everything was ok. I think after it shut off last night I checked the fuel pressure control valve and it may have been set as low as 2-2.5 PSI. It was dark so I am not sure. I pushed it up to 4 PSI and thats when I started it and it ran the rest of the way home. However, it ran fine for the first 20 plus miles and probably an hour. Its a Rochester Quadrajet carb. The fuel lines at the motor I changed to flexible neoprine ( I think) and on the mechanical pump it was a hard line from the mechanical to the carb. AS per electric fuel pump installation, I placed a pre filter in front of the pump to filter large particles and a new filter to filter small particles at the carb. 

There is no return line on the electric pump. 

So, did I get vapor lock ? Odd this happened right after re-fueling. Was my fuel pressure set to low ? Do I need to add something to insulate the gas lines from heat to prevent vapor lock ? Do I need to add stand offs on the fuel line to get them farther from the engine to stop heat soak ? The fuel line sits pretty close to the intake manifold and the valve cover. Sorry for the lenght. Just trying to throw out all the details.  

[jeff york]

Jack, There is a large particle filter between the gas tank and the electric pump. There is a new filter in front of and mounted to the Quadrajet carb. This is a GM 454 / now 496 motor. The two times this has happened, it happened after leaving it sitting and idling after driving at highway speeds for about 20 miles. The fuel pressure regulator is currently at 4 PSI. Its wide open throttle that more pressure is needed. The current Mr. Gasket electric pump I am using is a facet style pump just like the ones I use in my racing plane. Not the same brand but probably the same manufacture. I have never had a fuel pump failure even at over 200 mph or when idling for long periods of time. however, the pumps in my planes are used for transfer from tank to tank and for take off and landing to maintain positive fuel pressure. So, I would think that the Mr. Gasket is capable of handling 496 cubic inches because they handle 530 cubic inches in one of my planes. Positive and negative Gs that my Airstream motor home does not have to live in. 

I can put the old mechanical pump on. I think It stopped working because it could not prime due to a cracked fuel line was causing it to suck air.

Jin Stoltz.  I will try to find your GM diagram and look up that loop 

[Rich Thomas]

Hi Jeff, brain fart never thought about the 20 min idle. was only thinking of things that would shut you down. sorry for the inconvenience..

[Jim Stoltz]

That's what the manufacturer of my pump told me - the crankcase could theoretically fill with fuel if the diaphragm on the stock pump fails. Not a likely scenario, but possible. Even though that electric pump doesn't have a return provision one can be constructed. The pages I put up from the GM manual has a diagram. Not too hard to create a return loop.

[Jack Wasmuth]

It is not advisable to run both as if the diaphragm goes out in the mechanical pump it is possible to fill the crank case with gas. If he install the correct metal fuel filter after the pump it has a return nipple, These were common on all Dodge big blocks. The electric pump needs a filter before it and the carb needs one after the pump. The filter after the pump acts as a damper to slow down the pulse of the pump to stop float bounce.

[Jim Stoltz]

I agree Jack. I think all he needs is the stock fuel pump back in place to act as a return.

[Jack Wasmuth]

Jeff take it from that you do not need a Holly pump. They are over rated and not necessary. Your Rochester carb only needs about 5 psi at wide open throttle. the float can handle about 6 psi. Do you understand the difference between pressure and volume? Most electric fuel pumps will produce more pressure and volume that is needed. That is why a return lime is need with ALL electric fuel pumps.If you would like to discuss this further and easier contact me at the live chat here at GORVS.

 

[jeff york]

Im will plug the old return line from the old mechanical pump. Then replace the generator fuel line. I think my original fuel pump was fine by the way. After putting a brand new mechanical pump on I could not get it to prime. Then when I put on an electric fuel pump ( push) I ended up seeing a leak that the push pump pushed the fuel to. Where as a mechanical suck pump never showed the leak because it would not prime. I was advised by a mechanic, to replace my new Mr. Gasket electric pump with a Holley blue electric pump. Re install my mechanical pump and run my fuel to the mechanical pump as a way of using the old pump as a pressure regulator. 

Rich, as far as my mains sticking close maybe you could explain that a bit more as I am not sure what you mean. If you mean the float sticking closed, I am not sure how letting it sit idling for about 20 minutes would make a difference verses running down the highway. Electric fuel pump pressure should be consistent regardless or actually lower at speed possibly. 

[Rich Thomas]

Is it possible at all that your mains are sticking closed due to a float problem. 

[Jim Stoltz]

I would plug off that return line. It runs back to the tank.

With regard to the "T" for the generator, I would fix that as well. Since you're pushing fuel from the tank rather than pulling it with the stock pump it should matter to the engine if you're sucking air. It will matter to the generator, though, which is trying to draw it from the tank.

Not sure if either is related to your problem, but you never know.

[jeff york]

Update. I recently took the Argosy out on a short trip to go camping. I decided to top off the tank with fresh gas and added about 20 gallons of premium. Plus a short trip to the grocery to pick up a few last minute items. Then I drove about 20 miles to Fort Boonesboro. All went well on the highway. I then stopped to check in to the campground. This took about 15-20 minutes and I left the Argosy running. After checking in, I began driving back into the campground and once again, went about 70 yards and once again the engine suddenly stopped running. So, since someone here suggested the last time this happened to remove the gas cap and then put it back on, I got out, removed the gas cap and put it back on. Then I turned the ignition and the motor instantly started. I then drove around to my site, parked . Two days later, I started the Argosy back up, drove 20 plus miles back home and didn't have a single problem. I did notice that the carb or engine had a slight stumble at low rpm when I would press the gas pedal. I only noticed this initially when the motor was not yet fully up to temp. I had a few traffic lights that I had to sit at when I got close to home. So, I sat for a few minutes idling wondering if the engine would stop after pulling away. Now, this was not sitting for 20 minutes as in the twp previous events but, it was a few minutes of sitting idling after driving 20 miles at freeway speeds. So, I am still wondering what is causing the engine to shut off ? The commonality is, letting it sit at idle for several minutes after driving it at least 20 miles ? Few points to remind, I have installed a Mr. Gasket electric fuel pump back at the gas tank. this replaced the stock mechanical pump. The mechanical pump is completely removed from the engine and a block off plate has been put on whee the mechanical pump was factory installed on the 454 engine. All new fuel filters were installed. dry gas was also put in the tank to remove any water that may have accumulated in the tank.

Notes: A, the original small return line that was on the mechanical pump was left open at the bottom of the engine at the site of the original pump. I am not sure if this line is a fuel return line or a tank vent line. Its not lugged. I left it open.

B, the Mr Gasket 12s electric fuel pump has no vent line. It simply has a fuel in and a fuel out. Mr. Gasket claims their pumps prevent vapor lock.

C, I noticed my generator will not run well and the governor keeps pushing the throttle back and forth. Upon looking at the generator I noticed the fuel line on the generator is badly cracked and I believe it is sucking air and causing fuel starvation to the generator. Not sure, just thinking. That fuel line T's off of the same fuel line feeding the 454. So, could it be causing a air sucking into the fuel pumping. But, if thats the case then I would think I would have an issue on the highway when maximum fuel pressure is needed and maximum fuel flow. 

Any thoughts ?