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Inthe70s · 12-14-2015, 08:36 PM

Hi all and happy holidays! One of my toys is a '79 25' class c motor home built on a Ford E350 chassis with the 460 4 bbl carb and C-6 tranny. It has 46K original miles and is in excellent original condition. It started this problem where after it is driven about 3 or so miles it goes from running smooth and like a top to suddenly dropping in RPMs and wanting to die while at idle, unless I give it some gas. It used to have a hesitation at stops but after the carb was rebuilt, that was resolved. I changed all of the fuel lines and filters and cleaned out both tanks. It has a stock mechanical fuel pump as well as a Carter elec fuel pump. It does it regardless if it's 90 degrees outside or 9 degrees. It's not running hot. It runs exceptionally smooth, starts on the first turn and idles very smoothly.....but then suddenly drops after a few miles. I'm going to change the spark plugs, spark wires, coil, distributor cap and ignition module since they can probably stand to be changed. Any other thoughts? Thank you in advance for your help!

Chinewalker · 12-14-2015, 09:00 PM

Vapor lock?

Maybe the fuel line running too close to the exhaust manifold causing fuel to vaporize at low flow?

I used to have a '80 C class with a 460 and remember wedging a folded piece of galvanized tin between the header and the fuel line.

Inthe70s · 12-14-2015, 09:17 PM

Quote:

Originally Posted by Chinewalker

Vapor lock?

Maybe the fuel line running too close to the exhaust manifold causing fuel to vaporize at low flow?

I used to have a '80 C class with a 460 and remember wedging a folded piece of galvanized tin between the header and the fuel line.

I thought that too but this doesn't quite have the same feel as vapor lock and trust me, I know vapor lock after having several older cars. I had also insulated all of the fuel lines and re routed them as far away from heat sources as possible. Plus it seems strange that it would do this when its 10 degrees outside and it's barely warmed up at 3 miles?

dennisthe · 12-14-2015, 09:28 PM

It sounds like it's going lean when the choke comes off. Have you blocked off the egr valve? This was a common problem back in the day when i was working at a ford dealer. Loosen up the bolts and slide a small piece of cardboard between the valve and where it mounts and run it, if it runs fine there might be a piece of carbon holding it open. There is a gasket with a screen in it that will keep it from happening again. If it still does it remove the cardboard and look for a vacuum leak. You can spray carb cleaner around it will pick up rpm's if you hit the leak area.

Chinewalker · 12-14-2015, 09:37 PM

Heat riser valve? (if one is installed)

Automatic choke operating correctly?

Just a couple of thoughts. Here's hoping you get things sorted out.

Inthe70s · 12-14-2015, 10:11 PM

Those are all good suggestions and I'll have to research them. I did just replace the choke pull off as that was not operating properly but didn't seem to make any difference. I don't remember if I also mentioned that I did check the fuel psi and it seems appropriate per factory specs. There's a fine line between enjoying a good challenge and being driven nuts!

TQ60 · 12-15-2015, 08:45 AM

Get a vacuum gage and place it on the center hump and see what it tells you.

Those years were when many odd things were attempted for smog that were not real good ideas but before efi.

Sounds like a temperature monitor is indicating engine is warmed up and to come out of open loop (not really but close) then go into closed loop where the smog controls start acting.

Get the Haynes or Chilton manual that covers your chassis and it should be able to guide you.

E350 van may be close enough but try to get as close as possible.

You should complete your "full tune up" as it is warranted but may not cure what ails your engine.

Paint thinner in a squirt can or water in a spray bottle do same for vacuum leaks.

Good luck

77Travco · 12-15-2015, 08:59 AM

EGR valve.

Mr Plymouth · 12-15-2015, 09:09 AM

Sounds to me like it,s running out of fuel,the pickup tube in the gas tank could be blocked,just my 2 cents

Mr Plymouth · 12-15-2015, 03:53 PM

Or it can be a very weak fuel pump.

Inthe70s · 12-16-2015, 07:34 AM

Thank you to everyone that responded as they are all good suggestions. I think I will do my maintenance, see how it runs but now I'm leaning towards the EGR issue. That seems to make the most sense although it could be anything. Whatever the culprit is, it seems to act up after it is well warmed up and relatively soon after it has warmed up so something is reacting to the temperature and timing. If you like tinkering and restoring older vehicles like I do, then it's what I always say..."its always something"

Randalpho · 12-16-2015, 10:24 AM

The 460 engine was known for intake manifold leaks at the heads. When it's warm and running bad, pour some water (I use to use an empty anti-freeze jug)along the sides of the intake manifold where it's bolted to the heads. It will really run bad or stall and you may see steam come out the tail pipe if it's leaking. I had one where you could see a bathtub drain swirl at the point of the leak.

You can also use the water trick by pouring it slowly over different items, when the water is sucked in at the source of the vacuum leak you'll know it and can better pinpoint it. Works better than carb cleaner and won't ignite..........

Remove the vacuum hose from the EGR valve to eliminate the control source (could be mixed up vacuum hoses or bad thermal vacuum control switch).

Also remove the gas cap to see if it's a tank venting issue.

Mr.Geritol · 12-16-2015, 11:52 AM

I know you say you changed the filters. On the 84 Ford 460 we had there was a filter in the carb under the fuel line attach fitting ( I am currently having a senior moment and trying to remember what it is called" It was a 100 micron filter. we had the same problem you describe.
You need a 1 inch wrench to get the fitting off the carb. After I changed it, problem gone... After that I carried a spare one because some you ng parts guys gave me a blank stare when I asked for a filter that goes in the carb.

Jigger · 12-16-2015, 05:23 PM

..........I'd bypass the factory fuel pump and hook the electric pump directly to the carb and drive it . I'd also put some gas in a clean 1 gallon container , insert the suction hose for the electric pump into the container and see how well it runs , again with the factory pump bypassed . You may need to drop the fuel tank and examine the intank screen filter pickup for blockages . I hope you solve your problem . , santex