Rough cold start idle
 

My 1972 4.5 starts quickly when cold, and runs fine as soon as I step on the gas, but idles roughly (as if too rich?) when first started. Idles fine when warm.
What should I be looking at in the D-jet cold start mechanisms?

Check the operation of your aux air valve. It sounds as if it is stuck in the open position.

It is right up front, just behind where the upper radiator hose enters the engine. It has two large air hoses running to it.

If it was stuck in the open position it would not Idle fine when Warm I don't think .

Perhaps it IS Sticky like they get and as well the mounting gasket may be leaking . As well the Hoses ( Black Rubber ) become cracked and leak Air needing replacement

If you remove the Valve you can check it on the stove with a temperature Gauge
As well and perhaps more important would be to clean it up or replace it . Check again before installing . You can adjust them when all apart but its tricky some videos on You Tube

My 75 450 does the Same thing Runs way Rich and Lumpy when cold but fine once warmed up

The reason for the aux valve is to smooth out the idle when it is warm. They are fully closed when the engine is hot. If they stick in the closed position you will get a rough start and then, when the engine is warm and the aux valve should be closed, they smooth out.

I test these by first removing it from the engine. A bit of form-a-gasket or gasket sealer will be needed to replaced it. Or a new gasket.

Then bring a pan of water to a boil and drop the valve in. It should fully close as in 100% with no leaking of air from one port to the other.

The valve should be in the open position when cold. If it is in the closed position when you remove it then you know right away it is bad.

Anyway, take a small flashlight and, after using a pair of pliers to lift and hold the valve from the boiling water, shine the light into one port and look into the other. You should not see any light at all passing through.

You can also attach a hose to one port and try to blow through it. If you can blow any at all it needs help. You should not be able to blow any air through it when it is hot and sealed off.

An old trick is to pinch down the bulb in order to squeeze the rubber compound inside the valve. This rarely works well but it is worth a try.

And.....

Air flow to the engine is tricky on these D-Jets. A good first step is to make sure all the hoses are sound and not cracked from age and exposure to heat.

And...

The best way I hove found to test a cold start valve is to remove it but still leave it hooked up. Place it in an empty tin can and try to start the engine. If the engine is stone cold it should squirt a bit of gasoline out. So all you have to do is crank the engine and see if there is any gas in the can after a few cranks.

If not then the valve is bad or the thermotime switch is. The thermotime switch is the temperature sensor looking thing in the front of the engine, on the top and to the left as you look into the engine bay, the one with two electrical connections.

Testing these is complicated. You need a ohm meter and a 12v power source, like your car's battery. They pass current when cold at a range of 25 to 65 ohms. The closer to 25 the better. If they pass zero ohms they are shot and must be replaced.

But they have internal heaters. After they heat up, and they heat up electrically, they will drop to zero ohms so they must be tested cold.

By the way.... These are not cheap. The current rate for a working on is around $200, higher if they are right at 25 ohms.

Thanks to all of you for the insights.

Those are Great Post's thanks I learned something ! So the Valve should be all the way Closed when the motor is warmed up completely. I did not know this . this enplanes some issues I have as well



Just how is the easiest way to dissemble this Valve / clean it / should it be lucubrated when reassembled perhaps ?


Can the Valve be heated in water to disassemble ?

Thanks for the Great Post . I do Note that in a wet cold environment the

Valve suffers from Galvanic Corrosion ( along with all the other Aluminum parts under the hood ) and this must contribute to Valve Failure I expect