Originally Posted by mrmagic
So will the timing chain cause back pressure problems??? B/c there is a lot of smoke coming from the oil dipstick & from the oil cap while the truck is running.
Yes the late cam timing places the burn cycle regardless of ignition timing off in regard to the crankshaft this causes the exhaust to open late which holds cylinder pressure till the piston is well down the bore, this increases blow by. But I'm suggesting the timing set as one of several possible causes but being a 91 it probbaly has a lot of miles on it and these factory timing sets don't last much beyond a 100,000 miles as an average you see some die at 60-70 thousand and others go 200,000 so there is an element of crap shoot in this.
A sign of excess wear of the timing set is the need to reset the ignition timing always in the direction of advance so it catches up to the crank. A test you can do with a timing light is to disconnect the tan and black wire going into the distributor this takes the computer's advance calculation out of the equation. This is something you did when setting the timing? Then with the timing light hooked up run the engine speed up a few thousand RPM and suddenly close the throttle while observing the TDC mark on the damper. It should if the timing set is good not have deviated from TDC since the computer is out of the loop. If the timing wanders around more than a couple degrees the timing cahin and gears need replacement. However another culprit that can give the same test results is wear of the distributor gear but with the flat tappet cam in these engine that usually isn't a problem.
You can also run a vacuum test; the TBI engines should hold 17 to 21 inches of mercury at idle, probably more toward 19 to 21 since the cam is very mild. Late cam timing will produce a steady reading of 10 to 15 inches depending on how late the cam is running to the crank.
If you can confirm or eliminate that we can move on. Lots of guys the write into this forum with questions frequently do so after spending good sums of money on parts like the throttle position sensor, manifold absolute pressure sensor and other sensors only to find it's the cam timing set. So if we can rule that out for little expense we're getting ahead of the game. If that's the cause then you are putting your hard earned money into the problem rather than around it.