Cold idle advance

[Jcode]

Is it normal for the RPM's to vary and surge when the cold idle advance is engaged? Not long after I bought my truck I hooked the CIA up to a switch so I could cut it off when it revs up so high (I hate hearing a cold eng scream). The CIA usually will not engage until I press the throttle a little, then, when it activates, sometimes the RPM's will just keep climbing until the eng is screaming. At other times when the CIA comes on it acts normal, the RPM's raise slightly, and cuts off when the engine reaches the correct temp. Shouldn't the RPM's stay steady (steadily elevated at the same increased RPM, until it cuts off) kind of like a automatic choke operates on a gasser?

[DmaxMaverick]

You have 2 gremlins at work during your cold startup. The fast idle (which you have noticed), and the HPCA (Housing Pressure Cold Advance, which advances timing when cold). The fast idle is adjustable, but the HPCA is internal.

If you get some air in the fuel system before a cold start, it can play havoc with the timing, and engine speed when on fast idle (like you describe). It shouldn't ramp up/down if all is normal. Could also be an indicator of a failing injection pump.

[Jcode]

Thanks for the help. Is there any way air could be getting into the fuel other than at filter changes?

[DmaxMaverick]

Sure....

Leaky, old, tired injectors.
Failed fuel filler cap (allows vacuum to build as fuel cools).
Any poor coupling/connection can allow air in during vacuum, but not leak fuel out under pressure.
Fuel filter bleed/drain/enclosure seals.

The best way to know if it is air getting in, is install a clear tubing on the pump return line, with a loop in it to trap air. Do this immediately after a warm/hot shutdown, then run the engine until the tubing is free of any air. Check it before starting cold, then watch it during/after the cold start. Any air leaking in before the pump will show up here.

On the 1985 models, the fuel filter housing can crack and leak. Check for fuel seepage anywhere near it. The seals on the bleed and drain valves can also harden or crack, allowing slight leakage. 25 years of service is a long time, so don't rule out anything.

[Robyn]

Normally if you are getting air into the system the rig will be a tad balky to start after sitting over night or a day or two.

If the truck starts right up and seems OK other than the wild RPM swings, I am suspect that the Injector pump has gone to greener pastures.

There are a few parts in these Older pumps that wear out over time and can cause the isues you describe.

Try installing a piece of clear hose from the upper fitting on the IP (fuel return line) and then back to the steel line. Start the truck when its cold and watch for a stream of air bubbles.

Aftr removing the line and installing the clear one you will get some air in the system but if you see a stream of bubbles, it indicates an air leak between the tank and the lift pump.

Fuel is under pressure from the lift pump (RH front of engine near the pan rail)

Any leaks here will "usually" result in a fuel leak.

A slight leak from the tank to the lift pump will allow air to enter.

As maverick mentioned, 25 years of service is a long time and anything is possible.

There are at least 2 soft line connections aft the engine bay, one about mid span on the RH frame rail and the other at the tank connections

If the rig has dual tanks the situation can get even more interesting as the valve system used to switch the tank draw can and does get hinky over time.

My first suspect though with the "screaming" when the Cold advance is turned on is that the IP is worn out.
The throttle shaft seals on the IP's wear out too over time and will let air seep back into the system while sitting.

This along with a worn out pump will cause some strange behavior.
A worn out advance piston can add to the mix too.

It is possible that your pump is one that was never refitted to the newer governor assembly.

The early pumps used what was called a flex ring inside the thing that was part of the governor assembly.
This part is/was a soft plyable plastic ring that controled engine speed.

The flex ring would fail in due time and all sorts of hinky things would happen.

Not exactly sure when these were discontinued in the factory pumps but many of these could still be out there on the road.

When any pumps were rebuilt after about 1984 or so a new style assembly was installed that replaced that failure prone component.

Keep us posted.


Missy

[Jcode]

Wow- great info guys. Will keep you posted. If I do end up having to get a new (rebuilt) IP, where would be the best (not necessarily cheapest) place to get one? Who rebuilds NAPA IP's? Thanks.