Just did head gaskets on my 93 4.3 vortec (cpi) It runs good when cold but just when you first start it up. anywhere between 100 and 210 it runs irradically..sounds like a cammed 350 really. But once the thermostat opens it runs perfectly smooth and is driveable again. Also for the first few mins after the thermostat cracks it will hold 35mph without toutching the gas. Have switched many sensors and egr and still nothing. Am I searching for a vaccum leak and if so what may be the best way to find it? Have been around and worked on many vortech engines but just cant seem to get this one right. Any help would be appreciated. Thanks

 

Best way to find a vac leak is to use ether/starter fluid spray. Spray ether on/around the various vacuum lines and engine seals one at a time. When you hit the right spot, you should hear the RPMs increase by a noticeable amount (as it burns off the ether). If the RPMs stay the same, you don't have a leak in that area....move to a new spot.

 

I actually have done that also but cant seem to find where its coming from. Im almost 100% sure its vaccum..just cant find where. Yesturday i switched to a 180 thermostat and changed the temp sensor and it still does the same thing. As soon as it hits 180 the thermostat cracks and it runs perfectly fine.

 

Hmm... wonder.. does the air out of your heater warm gradually or blow cold till it get hot?

 

If you have a scan tool check to see if it is in Open/Closed loop operation. During warm up it should be in open loop operation. With the scan tool you can also check to see what the ECM thinks is the engine temperature is while you are warming up. If you have a faulty temp sensor this could also cause the sypmtoms that you describe. A Vacumn leak should not come an go with engine temp. A leaking EGR will cause a rough idle even after warmup. At idle the EGR should be closed.

 

It gets warm gradually. And blows nice hot air once its warm. I was told the heater core was replaced last year. I dont have a scan tool but thats definatly a good idea. Ill see what i can come up with. Egr is brand new with new gasket. Like i said right at cold start it runs and drives normal but after just a few minutes it starts to "lope" at idel and is almost not drivable..almost like a miss or its loading up. The exact second the thermostat cracks, its perfectly fine again..im thinking vaccum because if im correct the thermostat cracking will cause vaccum in the engine and then make up for the leak..

 

The engine temp does not cause vac. Vac is present at all times. What it could be is there are Vac switches that will open or close with engine temp. You could pull the VAC line off of the EGR valve so it can not open. EGR Solonoid controls when vac. is applied to the EGR valve. You could remove and cap any vac line at either the intake or base of the TBI to see if it improves your engine performance.

 

Does the vehicle run abnormally if you let it sit and warm up without driving it, or is it only when you drive it?

 

Right..but when the thermostat opens it does create vac. Ill pull the line off the egr and mess with plugging the lines to see if it makes and diff and let you know. It runs abnormally at all times until the exact second the thermostat cracks..then its fine. Doesnt matter if i just let it sit and run until its warm or attempt to drive it until its warm, but it doesnt like that much. Also when its running bad, if i put it to the floor its fine..goes like a bat outta hell. but anywhere from a little throttle to about 3/4 it seems like it misses. I really appreciate the help! Hope i can come to a conclusion soon.

 

I would also check your coolant temperature sensor, see if the wiring is faulty. And if you have a multimeter you can check the resistance of the sensor at different temperatures. This sensor can effect the performance of the engine because it is an input for the PCM or ECM to determine how the fuel delivery system should work when the engine is hot or cold. Might be an explanation for the misfire you're experiencing.