Help - Knock, LTFT, and low mpg (stock cal) - 3400

[86FieroSEv6]

Double check the eng. to body ground and the harness grounds where they attach to the engine?

[chri0029]

Double check the eng. to body ground and the harness grounds where they attach to the engine

The engine to chassis ground seems to be good. I am not sure about the harness grounds (too many to find/check).

Please elaborate. What exactly are you thinking? I would think I would be throwing a code or a sensor reading would be open or N/A if I had a bad ground.

All the sensors for the most are reading 'normal', except the MAP seems a little weird when the engine is running (reads correct baro when the engine is not on). I will verify the MAP sensor reading vs. a vacuum gage in the near future.

I think the low mpg has to be some mechanical issue.

[86FieroSEv6]

Check the MAP reading with the O2 sensor removed, that should indicate whether you have an exhaust obstruction. Poor harness grounds can shift the sensor reporting enough to cause grief, but you said the MAP input was correct key on engine off.

[chri0029]

I originally thought it was an exhaust obstruction so I disconnected the exhaust pipe at the manifold and watched MAP. I even took out the air filter along with the pipe disconnected and took a log at idle and 2500 rpm. The MAP only improved very, very slightly. That's when I took a few other cars for a test drive and noticed their map readings were very similar to mine (either they all had a consistent issue or the 3400 in general has an intake system which produces that type of MAP). Any .hpl or .xls 3400 logs (with a stock cam) anyone can send my way would help. The more data, the better.

Even though the MAP shows correct baro pressure at Key On Engine Off, I wonder if it's skewed at only low pressure (giving me false high engine load). I guess anything is possible. So my next step is to check my MAP read-out vs a vacuum gauge with the engine running. Then I will check the timing chain slop. (when it gets a little warmer outside of course)

[86FieroSEv6]

I have seen timing chains with a fair bit of wear run just fine. Usually if they are worn to the point of performance deficit, then there will be accompanying noise from the timing drive end of the engine. The next 3x00 I get in the shop I'll try to remember to hook up and observe the MAP values. What does the troubleshooting guide say the normal idle and load MAP values should be?