Strange idle problem

[ankarback]

I swapped the quadrajet from my old blazer to the "new" suburban and now I´m having some problem getting it to idle ok.

The carb ran just fine on the blazer. Now it runs fine with the trans. in park but once you put it in drive or reverse the engine stalls and dies. The engines are both 350's and the only difference is that the suburbans engine has an EGR-valve but it´s not conencted to anything.

Could it be a vaccum leak in the modulator on the trans? It's a TH350.

Really need some help with this one. I can't go wheeling with it without idle.

 

[diesel4me]

check for vacuum leaks..

You likely have a vacuum leak--the modulator is a possibility,but usually they suck tranny fluid into the motor and burn it more than they leak enough vacuum to cause an idle problem..Check the base gasket good by spraying carb cleaner around it,and any suspected gaskets or hoses while its idling(might have to turn the idle speed up more to keep it running while testing)..make sure the secondary butterflies (on the bottom rear of the carb) are not sticking open slightly..

Dont overlook the PCV valve,its hose and the port its hooked too--it needs good vaccuum to work right..also the EGR valve might be sticking open slightly(if it has one)--even if its unplugged,it can still be stuck partly open and kill the idle..hose and fittings to the power brake booster,or the booster itself can have leaks that will kill idle too..if the carb has an "idle stop solenoid",it might not be getting power to energize the solenoid on this truck,and it did on the other one!--it sets the idle speed with the solenoid rather than the idle speed screw..has to work for it to idle!--(or you can crank up the idle speed screw and leave it unplugged,but it might "diesel"when you shut it off
if you do that!)..good luck!

 

[rusty79beast]

I agree with diesel4me. I had the same exact problem with it running in park but put it in reverse or drive and it would stall! I found out that the carb had come loose and there was a leak between the intake manifold and the carb base! Hope this helps.

 

[ankarback]

I found the problem. It was the small hose attached to the vacuummodulator that was leaking. i replaced it with the one from the blazer and know it works just fine again Thanks for the help

It's interesting what you say about the idle stop solenoid. It´s not connected and it wasn´t connected on the blazer either. The engine never diesels but a couple of times the idle has got stuck very high (around 3000rpm). It's getting normal again by tapping on the carb with a hammer....
Could this have to do with the solenoid? And if so how should it be connected?

 

[diesel4me]

No

If the solenoid is disabled,it has no effect on the idle anymore,and you have to turn up the idle speed screw on the carb linkage to compensate for it no longer being used..since the throttle butterflies are now "open" a bit,and no longer close completely now that the solenoid is disabled,it could cause the engine to "diesel" or "run on" when you shut it off..setting the idle on the low side helps prevent that,or you can shut it off in "drive",then shift into park.

I kind of doubt having just the modulator hose leaking alone would be enough to cause stalling by itself--I'd check everything twice,and make sure nothing else is leaking too...usually takes a pretty substantial vacuum leak to stall a motor..having the modulator hose off on my 305 made it skip, and idle a bit faster,but it wouldn't stall it..

As for the ocaisional fast idle/sticking problem,its probably the fast idle cam for the choke engauging when it shouldn't..once the choke is fully open,it should not move back to the fast idle portion of the cam...maybe the choke isn't opening all the way,or staying open once it does..sticking secondary butterflies will cause it to rev at idle,and tapping with the hammer might temporarily free them up--they could be getting hung up on the base gasket--seen that happen a few times,had to loosen the carb bolts and shift it around slighty till the no longer touched,then re tightened them...

Sometimes silt and rust build up in the bowl of the carb,and moving and tilting it while the carb is off can cause it to migrate into the jets,and other internals..your float could be sticking if thats the case,but it usually floods or you can see gas pouring down the barrels of the carb if thats the case..