1998 GMC Sierra transmission slip

JOHNNY U

MEMBER
1998 GMC SIERRA
V8
4WD
AUTOMATIC
255,000 MILES

I have A 1998 gmc SIERRA. I got it second hand and the tranny was changes out about 4 years ago. I have has the vehicle 2.5 years with no trouble. I just had th truck in for a rad and manifold gasket change out. I now notice that the once the truck reaches about 100 km /hr and the enginge settles down into overdrive teh transmission slips a bit. And does not do it again until I go to 100 km/hr and let the overdrive kick in. The guys at the shop say the tranny fluid is topped up to spec. I still don't know what is going on. Any advice is appreciated.

1 Reply

PEAR69

MECHANIC
1,482 POSTS

If your transmission did not act like this before you took it in for the intake manifold gasket change ( I am assuming that it was a intake manifold change) Then the timing is probably off a little.
In order to change the intake manifold gasket, the distributer must be removed from the engine. It is almost impossible to put the distributer back in--in the exact same spot--right on the money. Your distributer is probably off a few degrees of timing. Unfortunatly, the timing on these engines CANNOT be set with a timing light. Why?-- You may ask -- The computer automaticaly adjusts the advance and retard on the timing in these engines; there for the timing must be set with a scan tool that is capable of doing this. The setting is + or - 2 or 3 degrees from T.D.C. A scan tool that is capable of this is very expensive, so this is a job for a dealers' service dept.

Also, when the timing is off, even a little, the transmission will act this way. It is easily mis-diagnosed as a transmission problem when in fact it is an engine timing problem. Let me know how you make out.