Rough Idle

[WayneT]

I have an 84 6.2 4x4 that I've been working on. I finally it got started today
and with a little pedal it runs fine but when I release the pedal it has a rough idle even when the engine is "Warmed up". it will idle roughly and die sometimes.
My temp gauge reads about 200 and climbing after the engine warms up. The oil pressure reads between 15 and 30 with a litle pedal and will go up to 30 and stay there with the engine revved. I installed a new oil pressure sending unit, a used but working vaccum pump and a diesel water separator filter.

Any ideas on what I can do to improve my idle?


Wayne

[john8662]

Hey Wayne,

The post got moved from the 6.5 forum to the 6.2 forum.

As for the rough idle, the most common cause is from air in the fuel system. Check all the rubber hose connections in the engine compartment connecting to the fuel lift pump, injection pump, and fuel filter housing.

When was the last time the fuel filter was changed?

[WayneT]

Hello John,

I removed the rectangular filter unit that was on the firewall and replaced it with
a International fuel water separator unit. There was several electric wires that hooked up to the one I took off and I zip tied them out of the way as I don't think that it will make any differance if they are not hooked up.

The hose that goes to the IP is very hard to get to without removing the intake. It looks OK. The guy that had the truck before me installed a electric fuel pump. Does that mean I don't need a lift pump or do they work in tandem?


Wayne

[john8662]

Why remove the original fuel filter housing?

Anyways, you're correct that the wires being disconnected and tied out of the way won't have any affect like this.

Having two lift pumps also won't hurt if installed correctly and are both working.

I guess the next task will be to establish whether both of the lift pumps are indeed working and not acting as a restriction. The electric lift pump if not working can block fuel (some designs). Also verify that the hose connections are correct and tight on the lift pump, I recently had a truck that had one installed by a prev owner that was leaking air into the system, causing similar symptoms.

Do you have an air bleed valve on the fuel filter housing you're using now? If so, you can use it to verify that your lift pump(s) are working one by one.

Disconnect the larger red wire off the injection pump to keep the engine from firing.

Disconnect the power source to the electric lift pump.

Then open your air bleed valve on your filter housing (connect a hose if you can and place in a bottle) and crank over the engine, verify that fuel is being pushed out of the hose rapidly. If so, good mechanical lift pump, if not, bad mech. lift pump, time to replace.

To test the electric lift pump, you can supply power to the pump w/o cranking the engine and see if you get fuel pressure/flow, and physically observe if you hear any noise from the pump (to verify that it at least works).

Oh, what's the deal with the low oil pressure? Is it only making 30 psi cold? If so, you've got a more serious condition on your hands. I'd suggest just getting a low-cost mechanical oil pressure gauge from the auto parts store and temporarily installing it in place of the electric sender to verify what the oil psi condition really is.

[DmaxMaverick]

Where in the circuit is the electric fuel pump, and why was it installed? If the mechanical pump has failed, and an electric is between it and the IP, it will suck air in through the mechanical pump.

I prefer the mechanical pumps on the older engines that came with them for many reasons. Did an OPS (Oil Pressure Switch), or other failsafe get installed in the electric pump circuit? If not, it isn't legal, or safe. what is to cut off fuel in the event of a collision? Mecanical pumps don't have issues keeping up with fuel demands, as they put out volume on the same curve as the engine RPM.

replacing the fuel filter is fine, as long as the filter you installed has at least the same or better efficiency as the OEM. The wires are no big deal. they are for the filter heater and WIF(Water In Fuel) sensor. You won't have these features enabled, but they won't effect engine operation.

A simple cause of rough idle can be blocked fuel return lines. A restricted or blocked return line at the IP will cause issues with internal pump pressures, and restricted or blocked lines at the injectors will prevent them from popping correctly.

Also, If you have EGR, it may be effecting your idle. How does it run, off idle? Smoke? Power? Rattle? Other noises? If it has EGR, try disconnecting it by removing and blocking the vacuum line that goes into the air cleaner. If this effects idle, you may have it mis-routed, or an issues with the vacuum circuit or valve. Be sure the actuator is closing completely with vacuum removed.

What truck configuration do you have? 1500, 2500, 3500, etc. What modifications have been made? How old is the IP and injectors?