I have a 1994 s10 blazer w code cpi engine. For the last few months I have been experiencing what feels like a misfire or like the engine is stalling under part throttle. Usually this happens around 1700-2500 rpms. I have a 5spd, and most of my driving is in town around 25-35 mph so I just shift early usually about when it starts to stall. The other method is that if I can keep the rpms above the 2500 mark the power magically returns and the truck surges forward. I have had this happen when cold or hot. Sometimes it seems to be fine directly after a cold start for the first block or two, but just tonight I had it happen right after starting, though the truck was only sitting for a few hours so maybe it only happens when in a closed loop state? I'm not sure. This initially happened as a slight miss around cruising speed that would sometimes happen, like a sudden jerk, but then it would clear up.

Some of the work done within the last few years:

Spider Replaced
Fuel Tank, Sender, Pump
Distributor, spark plugs and wires
thermostat
ecm
exhaust replaced from y pipe back with new O2 sensor
Throttle Position Sensor
Temp Sensor

I have an OBD1 to usb cable and I can read codes / track ECM data. It appears that the computer is receiving a smooth range of input from the TPS and it is reading the engine temp correctly. I have set the timing at zero degrees with the timing plug disconnected. The weird thing is that when I replaced the distributor it wouldn't run if I pointed the rotor directly at the number one spark plug pickup. At TDC it sits sort of a little bit before that at it ran for the most part okay for the 400 mile trip from my parents house to where I go to college.

I should note that the truck did originally come with a 4l60e and this summer a nv3500 was swapped into place, but it still has the manual programming in the ecm. It ran fine for about six months though, so I wouldn't think that this would be the problem. It hasn't thrown any codes and if you look at the computer it thinks it is in 2nd gear which is probably the default?

One thing that I have been thinking of lately is that I noticed while setting the timing the harmonic balancer looked like it was wobbling. I cannot move it with my hand, but the truck has always had a weird vibration at about the rpms that it is experiencing the issues so i don't know if that is related or not?

I have spent so much time and energy pondering this issue the last few months that I really just tired of it. Any Help would be appreciated. I was wondering could this type of problem be caused by a mechanical failure like a gasket or valve or something. At idle I swear I hear a slight leak or noise coming from the passenger side exhaust area. You can hear it from inside the truck it sounds like a small exhaust leak. It has made this noise from as far back as I can remember. I've had it about 4 or 5 years.

If you guys have any ideas or suggestions I would appreciate it. Unfortunately I am at college and I only have so many tools with me, but I am trying to do my best.

Thanks,
Doug

 

Turned out it was my EGR.