I have an 89 with a 3.9 and an auto trans. Today it developed an interesting issue. Whenever it kicks down a gear if I'm going up a hill or something, the whole truck jerks as it seems to stall for a split second before getting going. On the way to work it was only as I was going up hills and I was low on fuel so I thought maybe it was just really low and not getting any at that moment, but it was worse on the way home when I had a full tank. It was even doing it on straight stretches of road and a lot more frequently.
First, do you guys think it's the tranny or a fuel issue? Could it be a fuel pump if the only problems I have are in those instances? The fuel filter's new so I'd rule that out, but anything else is possible so I'm trying to get an idea of where to even start looking for problems. Thanks for any help you can provide.

 

It could be anything. I would say a good tune up would help. Do you have any check engine light codes?

 

The light hasn't come on at all. Today it got much worse and isn't only occuring while shifting, so I'm pretty sure it's not the tranny but is probably spark/fuel. If it was a bad injector that would only shut down the one cylinder right? It definitely seems like it's a total shut down for the split second that it happens. I'm starting to lean toward fuel pump.

Can someone tell me what pressure I'm supposed to have at the rail and pump during startup, idle, and while revving?

 

Fuel pressure should be approximately 241-310 kPa (35-45 psi) at idle.

If operating pressure is above 45 psi, electric fuel pump is OK, but fuel filter/fuel pressure regulator if defective. Replace fuel filter/fuel pressure regulator assembly.

With the engine turned off, note that the pressure should not fall below 24 psi for five minutes.