I just found this site last night and read as many threads as my eyes would let me. I see you guys have discussed this code many times but still did not seem to help me in this situation. The truck I am looking for some help with is a 1994 K1500 code S. It has a few problems, hopefully all related but probably more than one, here goes. As I mentioned one is a code 78 that keeps coming on at slight to heavy accel. There is a lot of black smoke even at no load. When the outside temp starts to warm up the truck will start and then stall after a few seconds and then rolls many times before restart with lots of smoke when it does start. The smoking has come on gradually in the last year and is now bad enough that it won't pass emitions test unless the boost pressure sensor is unplugged while test is performed. The IP has been changed twice, the last one in fall of 2004 with a new GM pump. Dealers have changed these pumps, as well as set timing many times, replaced many wastegate solenoids. The truck is original with 330 thousand kms. Vacum has been verified and ok at 20* at wastegate at idle. Not sure of boost pressure yet? By reading many threads I am sure I finally found someone who cane help. Thank you in advance.

There has been three threads just like this. I'll link them.
http://dieselplace.com/forum/showthread.php?t=30963
http://dieselplace.com/forum/showthread.php?t=30878

Ben, where are you in ON? Close to Toronto?
If you can bring it by the shop I can give you a hand.
1015 Industry St.
Oakville, ON

No I am in Arnprior Ont., about 45kms west of Ottawa. I did read the threads that you directed me to and I am sure will cure the smoking proplem but do you think they will help me with stall/no start and the code 78 problems??

code 78 is the waste gate. either the wg solenoid is sticky (you said it may be new) you have an intermittent leak in the vac line (dang plastic line or a loose boot) or your connection is intermittent on the solenoid (suspect this if GM put on new connectors to the solenoids, they use un-sodered buttend connectors).

The stalling is either fuel starvation or electrical and both of those can be many things.

If you take the hose off the wastegate actuator, does it move freely and is it turning the shaft going into the wastegate? (some people have had the weld break loose on the rod so it looked like it was turning but it was just turning on the shaft)

I have watched the wastegate actuator move freely by itself but honestly did not look any further to see if actually moving the wastegate or if broken weld as you say. I am trying to get the truck back to check on some of this stuff you guys have mentioned.

At idle, your vacuum actuator for the wastegate should be sucked in. You should not be able to pull it out without much, much pulling. If it loosely pulls out from the actuator, you don't have enough vacuum to hold the WG closed.

If your getting 20" vac at the turbo canister and it's not holding tight you should hear a big sucking sound.

No sucking sound there and yes the actuator is fully sucked in, as you throttle on it the actuator moves out easily.I know you will loose a little power but if the actuator were adjusted would that give me a little less boost, and stop giving code or will the pcm just close wastegate more to allow boost it wants to see?

http://dieselplace.com/forum/attachment.php?attachmentid=3906

Something like this will keep it closed if nothing else will.
This is the stock WG actuator slightly modified, total cost $3 for the hardware.

Do you have vac at the baro sensor on the firewall?

Yes there is vac at baro sensor, if this wre my own truck I would most definately remove vacum actuator but this fellow is more the type to say why can't you just fix it to be the way it was (if you know what I mean).

If that actuator moves that easily, sounds like not enough vacuum. It should be rock hard steady-immovable almost. It may suck in at idle, but the least bit of boost may blow it open if you don't have enough vacuum. (That's if I understood you correctly)
Check for :
vacuum leaks
bad vacuum pump
bad WG silinoid.

Sorry, first thing in the morning, didn't see "second" page on this thread. QM's right on with a money saving tip. Since this is a 1500, looks like you've got to fight the battle with EGR vacuum stuff too. If you have a bad pump, other things may be showing up too in the EGR department too having problems.

I will recheck the vacum and all lines but if I have 20" at wastegate solenoid would that not tell me that I an not loosing any vacum anywhere and that pump is good? Should there be more than 20" at idle? When you say bad pump do you mean under load while driving. Vacum stayed at 20" wether at idle or wot.Sorry guys I don't mean to be dragging this on. I whish I could get the vehicle back to check more.

If you connect a boost gauge, you should be able to figure out the problem much easier.