Problems after overheat

stalls
van
chevrolet

#1

New owner of a 1978 Chevrolet g30 350 5.7l with an edelbrock carb. Idles rough in all gears including park and neutral when cold. After it warms up it will idle fine in park and neutral but not in reverse or drive. Acceleration is fine. All this happened after an overheat (temp light never came on). No signs of oil/coolant mixing. No moisture in tail pipe.
Checked spark plugs they all look good. Checked under distributor cap, all dry.
Furthermore, temp light does not come on when key is in on position. I grounded the wire and the light comes on. Changed coolant temp sending unit and still the light doesn’t come on.
There is no pcv valve and the evaporative emission control canister is completely disconected. Is it safe to drive Lucky Number Two (that’s the van’s name, long story)?


#2

Assuming there are no air leaks and no mechanical issues with the engine, it’s possible that the problems may be normal considering the Edelbrock carburetor.

A stock Quadrajet has a choke-pulloff diaphragm which is what prevents the engine from being overchoked on an initial cold start. The Edelbrock does not have this and I’m assuming the carb you have is one with the electric choke rather than the manual choke version.

The rough idle could be because the idle speed is dropping too low once shifted into gear. Many stock carburetors compensated for this with a dashpot of sorts and again, the Edelbrock does not have this feature either.
If you hold the brake pedal with one foot and very lightly bring the idle speed up, does the vibratin go away?

That’s not meant to reflect badly on the Edelbrocks; just the way it is. I’ve got several Edelbrocks in use around here; great, simple carburetors.

I’ll have to take a look later at a schematic in regards to the sending unit. Offhand, sounds like a sender for a gauge has been used.


#3

When my early 70’s Ford truck is running fine, then suddenly, overnight, starts to exhibit this symptom, I always suspect some kind of vacuum leak first. Excess air is entering the engine that isn’t supposed to be. Where the leak is varies. Sometimes a vacuum hose has come off it’s port. Or it has cracked at the connector. Or a seam in the vacuum hose has split. Or one of the vacuum controlled devices is leaking internally. On the latter, on my truck anyway, the vacuum device that fails is usually a vacuum controlled flap device located in the contraption that holds the air filter. When the engine is cold that flap is supposed cause air heated by the exhaust manifold to go to the carb, and when warm that flap swaps position and the engine intakes air from the normal cold air intake from outside. The diagnosis is simple. I use a hand held vacuum pump to see if that flap device holds vacuum and operates when vacuum in applied.


#4

Have you done a compression test? Vacuum test? If this occurred suddenly immediately after the overheating, you may get some abnormal readings.


#5

My best guess is you have to adjust the choke control. If it is lke similar carbs of the eras it is controlled by a heat sensitive spring, The housing of the choke control allows one to adjust it on the fly. Basic starting point is when closed and cold, adjust the choke so it just barely closes.


#6

Planned to do a compression test but wrongfully assumed if no signs of oil/coolant mixing and no acceleration issues there would be no gasket damage. Plan to do the compression test tomorrow. I should also explain that before the overheat, it had slight idling issues, not nearly as bad as it is now. One more bit, there are vacuum lines disconnected and completely missing all over, been reading through haynes manual to track down where lines should go and which are missing. Im on a tight budget, so I plan to do a compression test before spending any more money. But my question still remains. Am I at risk of causing further engine damage driving with these symptoms unresolved.


#7

If all you have is a bit of a rough idle and the oil & coolant are both up to level and reasonably clean I wouldn’t worry about driving it around while you finish getting it sorted out. If you start blowing a lot of smoke and the oil/coolant situation changes, then park it.


#8

Also the Edlelbrock carb is shiny and new looking and it has an electric choke that is also disconnected. I was given the van by a stranger so can’t complain, which is part of the long story explaining the name (Lucky number two). My girlfriend thinks the name is to blame.


#9

“and it has an electric choke that is also disconnected.”

I think right there is your problem. If you go to Edelbrock’s website, you should be able to find how to properly connect and adjust the choke. You can also find instructions to adjust the idle and mixture screws. That may just solve your problems.


#10
there are vacuum lines disconnected and completely missing all over

It’s important to try to eliminate any air getting into the engine except the path it is supposed to go, through the carb. When the engine is idling, on each of the vacuum hoses that are disconnected, test for suction at the disconnected end. If any suction is noted, that means air is going through that hose into the engine, so plug the end of that hose. That will prevent air from entering the intake manifold though the hose and leaning out the mixture. You may or may not notice an improvement in the idle doing this, depending on whether there are other ports to the intake manifold with no hose on them at all. But its worth a try.


#11

Sorry, I’ve been without internet and living like a caveman. turns out the idle was just too high.