Hi. I have a 86 z28, 4BBL Quad. jet carb w/ elect. choke, V8 305. auto trans. I have a huge hesitation that I am noticing, mostly starting from a dead stop. The RPMs go up as I accelerate, but the car doesn't want to move forward, almost like it''s not getting enough gas, but then as I pick up a little spped, I feel the throttle kick down and I can pick up speed fast, like whatever was holding it back has been released. The other major symptoms are very rough idle/ very smelly exhaust when cold, my idle and timing are inconsistant (& change on their own). So far, I have replaced the 02 sensor, the fuel pump, the accelerator pump, rebuilt the carb, changed the fuel filter, & checked my wires and plugs (HEI). Has anyone else had a problem like this? Oh, I can hear a flapping down in my engine on my passenger side, that I have pretty much determined tp be my EFE valve, (it's located in the exhaust) which would maybe explain the rough idle when cold, but would it cause the hesitation when accelerating, which I notice more when the engine has warmed up? HELP!!

 

If you suspect the efe valve is the problem make sure that it is opening when the car heats up. It is controlled by a thermal vacuum switch on the thermostat housing, it has vacuum when cold and none when hot. Possibly the valve is stuck.

 

I believe the diaphram inside the valve is bad, becasue when I guess the vacuum turns on, it starts making a flapping noise, like all the air is pushing through it. I notice this more once the car is warmed up. Would a failing EFE valve cause the major hesitation when I accelerate?

 

When that valve is closed it directs the flow of exhaust through the intake manifold and out the other exhaust manifold , so if it was stuck closed I think the heat in the manfold would cause a hesitation. Try wiring it open and see if that helps.

 

I would check the vaccum lines off of th O2 sensor and the vaccum line that runs to the exhaust. It seems like you have something blocked open or inoperative.

 

The check engine light goes on and off intermitently, and I coded it out and it's a 45, which according to my books, means it's running to rich. In response to checking the vacuum lines to my o2 sensor, I don't have any vacuum lines going to my 02 sensor-Just a plug. What is weird is that the problem is constantly changing. One trip it will idle high, the next very low. The hesitation issue will be real bad to where the car goes form 0 to 10mph in a full 2 minutes, with the accelerator floored, to all of a sudden clearing up for a few miles. I wish i could figure out if it is a throttle issue or a vacuum/sensor issue? I don't understand how it can be running rich when it acts like it's starving for gas!

 

i have a similar problem except i get a code 44 - lean. mines also weird cos my plugs have been fouling black quickly like its getting too much gas! it has hesitation from a stand still start and a rough idle once the car warms up. just about everything has been checked and as a last resort my mechanic has put in a new memcal which may have fixed it. im still waiting to here back from him today after he has tested it thoroughly.

 

Did you rebuild the carb? and did you readjust it when it was put back on? Are you getting good dwell readings?

Is your timing truly changing at idle-have you put a light to it?

For the EFE, disconnect the vaccuum line to it and cap it, then see if you notice a change.

Are the engine rpm's really going up without the car moving? What do they go up to? and at what speed?