Fuel problems

[stumplord]

I have a 1986 Holiday Rambler Presidential! Will not stay running , I have replaced plugs and wires and distributor cap and rotor button both fuel filters and rebuilt carb,cleaned fuel tank replaced electrical fuel pump and cleaned out tank, replaced the sock! When I went to get the new fuel pump they couldn't cross reference the part number and gave me one that looked similar! Problem is when idling it runs fine when put under load it doesn't get gas and shuts off! I also replaced the front fuel pump that goes on motor! It has a 454! I think electronic fuel pump is to small!

[enblethen]

It could be fuel starvation if the electric pump is not pushing fuel toward the mechanical pump.
OP: you could check the fuel pump relay with a good ear. Turn on the igntion to on, listen for fuel pump relay to drop out in 8-10 seconds. That would indicate that the fuel system should be pressurized.
Inspect the oil pressure sending unit. Look at the connector for any signs of melted areas or oil leak. Located on front right side of engine. My book does not go back far enough to verify wire colors
Oldest model shows orange wire should have 12 volts DC with ignition in on position. The tan/white wire should come out of oil pressure sending unit and output from the relay to power fuel pump. If you have a elper you cn disconnect at oil pressure sending unit. connect volt meter to tan/white wire at connector. You should get 12 volts while relay is energized.
You can jumper between orange and tan?white wire to see if you can hear pump run.

[ScottG]

JRscooby wrote:
Could the issue be clogged exhaust? Been sitting, could a nest be built in exhaust? That would let it start and idle, maybe rev for short burst. But the longer it runs build pressure in exhaust, have no vacuum to pull fuel in. Set for a bit, the pressure bleed, start and run again. Hook up vacuum gauge, hold a steady 1200-1500 RPM. If the gauge starts to drop, you have issue.

That's a really good thought. Especially true if the 02 sensor has aged out and the cat is plugged.

[JRscooby]

Could the issue be clogged exhaust? Been sitting, could a nest be built in exhaust? That would let it start and idle, maybe rev for short burst. But the longer it runs build pressure in exhaust, have no vacuum to pull fuel in. Set for a bit, the pressure bleed, start and run again. Hook up vacuum gauge, hold a steady 1200-1500 RPM. If the gauge starts to drop, you have issue.

[wa8yxm]

enblethen wrote:
MDKMDK:
That is another good reason not to go to dealers. They do not have old school mechanics. If it cannot be diagnosed with a scan tool, they don't know what to do.

On my 1992 Lumina APV the dealership noted excessive brake pedal travel. So did I. they looked at it like 3 times and only minor, and temporary improvement.

Finally I put my vintage brain to work on the problem. and Next day hauled into Belle Tire (my tire place) and gave one special instruction to the consultant (Mic the drums) drove out with brakes like new.. of course the rear brake drums were new (front disc).

Yup. the Factory Trained Brake technician did not understand the older brake system. Not the only screw up they pulled.

[ScottG]

enblethen wrote:
Yes! I am not saying not to take it to a shop, but not dealers.
Many after market shops can do a much better job then most dealers on older model vehicles.

X2. Sad but oh so true. Especially true when it comes to something older.

BTW, the fuel pump relay was a common failure item in that era. Most mechanics replace the relay as a precaution when they replace the pump.

I would also blow through the line and check for a restriction. Could be full of rust or the line could be damaged somewhere.
Also, ethanol based fuel will eat the rubber lines of that time period. You need to replace them all (you could be sucking air through a cracked line).

[MDKMDK]

Assuming there are no mechanics at any dealerships that have seen or can work on a carburetor any more? That's a pretty broad assumption. You just don't like dealerships. OK, fine, but don't try to discourage others from trying them. I've had nothing but good luck with them. Like I said, they built it, and they can still service it. If they can't they'll tell you up front. Simple?
What have they done to you to make you dislike them?

[enblethen]

MDKMDK:
That is another good reason not to go to dealers. They do not have old school mechanics. If it cannot be diagnosed with a scan tool, they don't know what to do.

[MDKMDK]

Iraqvet05 wrote:
GM built it but when was the last time one of their service technicians have seen a carburetor? Sometimes it pays to seek older mechanics with experience.

Does GM discriminate against older mechanics? They don't have any at any of their locations? Scandalous!!! Most of the shops I've used have a couple of them in staff. They know there are still classic cars out there that have carbs.
None of the younger mechanics own older classic cars that they can't work on, because they've never seen a carburetor? C'mon, if they've ever pushed a lawn mower or worked on an RV generator, they've seen carburetors. They aren't as scarce as you imply.

[Iraqvet05]

GM built it but when was the last time one of their service technicians have seen a carburetor? Sometimes it pays to seek older mechanics with experience.

[352]

From experience. Try the fuel pressure regulator. Good luck

[Chum_lee]

I'm not sure if the fuel tank vent is in the fuel filler cap or on the fuel tank itself but, if the fuel tank vent is fully or partially plugged, that could give you the exact symptoms you currently experience.

Try driving it with the gas cap removed. Fuel pumps don't pump well with anything other than ambient pressure at the inlet side. (inside the tank)

"You need a fuel pressure gauge to quantify the issue."

I agree with the above. Spend your time doing some diagnosis. Stop throwing parts at the problem. (it's much cheaper in the long run) Does this engine/exhaust have a functional catalytic converter and/or an EGR valve/system?

Chum lee

[4x4van]

Are you sure that ALL of the fuel filters have been found/changed? My last RV (Ford, but...) had 3; a sock on the in-tank fuel pump, a filter on the chassis about midship, and finally a brass filter in the carb itself at the fuel input.

[enblethen]

Fuel pump relay is on the left side of the engine compartment mounted to a brace. It only operates for a 5-10 seconds to pressurize the system.
The fuel pump power operates through the oil pressure sending unit. It is possible that the circuit in the sending unit has failed.

[MDKMDK]

enblethen wrote:
Yes! I am not saying not to take it to a shop, but not dealers.
Many after market shops can do a much better job then most dealers on older model vehicles.

I've only ever been ripped off by "good 'ol Joe at the corner garage".
GM has the specs, diagrams, schematics, tools, and expertise to trouble shoot stuff like this. They built it, and they can service it. They almost always solved my problems with my 2002 Chevy Express van (Roadtrek) in 24 hours or less. One puzzler took all day, but they finally found it. The OBDII codes seeemed to point to a fuel system, or cap pressure problem, but it turned out to be a single corroded sensor wire buried in the bowels of the engine compartment. Found, fixed, and moved on. Yes, they're more expensive than good 'ol Joe, but they fix it and warrant it. I've never needed to take advantage of the warranty, either.