Strange stumble problem on a 2000 Blazer 4.3

[crashandburn]

I got this blazer with a blown transmission. Swapped in a good one and road tested it. It idles smooth and runs great up to 3000 rpm. Above that it falls on its face and seems to be running out of fuel. Well the gas gauge didn't work either, so I put in a new fuel pump/sender and a filter.
No change. I pulled the MAF sensor connector to force it into closed loop as I had a similar symptom caused by a bad MAF on a Suburban once. No change.

Here's the really strange part... If I stay in the gas and struggle it up to like 4000, it clears out and pulls strong as it should.
No SES light, and the OBDII port does not seem to be working.

Any ideas?

 

[ZooMad75]

Take a peak into the intake with the snorkle off of the throttle body. Look for evidence of fuel on the floor of the plenum below the throttle blade. Pretty common to have the injector spider to leak. It will lead to all kinds of driveability issues. Since you already have a fresh pump I'm going to assume the pressure isn't falling off. It's worth watching what the pressure does when you bring the RPM up. If the pressure dips and you see fuel on the floor of the plenum I'd get the updated injector spider. This new part is completely different in that it moves the injectors down to the intake ports where they should have been in the first place. It's a little more expensive but worth it.

 

[crashandburn]

I'll have a look at that. I've seen those injectors fail and just cause a dead cylinder before. If I had time today I would have drove it with a pressure gauge hooked up. I have one with a long hose.

 

[TreeFarm]

Check that fuel filter too. Its cheap part and its easy to get to.

 

[crashandburn]

Check that fuel filter too. Its cheap part and its easy to get to.

Swapped that at the same time as the pump. Before I realized the power would come back after 4000 it really just seemed like a simple fuel starvation problem.

 

[crashandburn]

FIXED!
It was the crank position sensor.
I found that the cigarette lighter fuse was missing, and it also powers the OBD2 port. Tossed one in there and and got the CPS code. Swapped and it runs like new.

 

[imiceman44]

FIXED!
It was the crank position sensor.
I found that the cigarette lighter fuse was missing, and it also powers the OBD2 port. Tossed one in there and and got the CPS code. Swapped and it runs like new.

I was going to say cam position sensor, I know someone was having trouble with these and was getting limited in rpms.