I've got an issue with my 05 6 speed that I tried searching for a solution with no luck. When cruising on the highway between 4 and 4.5k rpm(speed limit on my commute is 80-85mph) and I add a small amount of throttle for say a small incline the engine will hesitate and buck slightly.

All the usual suspects have been gone through in the 6 weeks I've owned the car-
New LS coils
New Spark plugs
New Fuel Pump
All intake valve solenoids replaced, new vacuum hoses
All intake valves operate freely
MAF cleaned
All grounds cleaned

Only mod is an AEM intake. Any ideas where to look next?

Thanks

 

Did you ensure that:
- air throttle body screen was re-inserted
- reset the ECM/PCM

 

There are two air screens forward of the MAF.

Did the 20 stomp brake reset and have unhooked the battery for several hours multiple times also. There was no change in the behavior.

Also thought I should note that motor was replaced by Mazda 40k miles ago.

 

I hooked up my OBD reader on the way home tonight. The cause of the stumbling was pretty clear. The AFR is 11.3. At 3,900 rpm it's cycling back and forth around 14.7. As soon as it goes over 4,000 it drops to the 11.3.

I guess I need to figure out why it goes so rich at that point.

 

It is leaving closed loop into open loop and probably has a large positive long term fuel trim .... have a look .

 

Long term fuel trim is -22.66%. This was at idle. Does the OBD output of this value change with RPM. Not sure if it's a global correction or RPM dependent.

 

Answered my own question with a drive at lunch. LTFT @ the RPM's that are giving me trouble are -10%

 

Could be the AEM intake is that is causing your issues . They do tend to make our engines run rich . Never seen one with a trim that high at idle though . Check your intake screens for damage .

 

Turned out to be the MAF being dirty. I had cleaned it when I got the car but it somehow got gunked up again. We just had a bunch rain in the last few weeks and it hadn't rained since I got the car prior to that. The water must have dislodged some dirt etc from the filter. That's my best guess at least.