Strange results on induced misfire test on 4.3 MPI

[Mischief Managed]

Helping a friend troubleshoot his 4.3 MPI engine that is not making full power. My TechMate scanner shows no fault codes, the TPS, air temp, MAP, coolant temp all work normally. The IAC is no good, but that just affects idle speed and I don't think it is causing the lower power on plane. Tried doing an induced misfire test with really strange results. IF I induce a misfire on cylinder 1, RPM drops from 1600 to less than 600 and the engine sounds like it's going to die. If I induce a misfire on cylinder 2, the RPM just drops a little. If I try the test on any other cylinders, the TechMate tool outputs a "function not supported by this ECM message". Any idea what this test is really doing on the ECM555 equipped engine? The manual says it's supposed to shut of the injector of the selected cylinder, but I cannot image why I can't test cylinders 3 through 6.

 

[achris]

The 4.3 fires odd and even injectors, not individual. When you shut off 1, you're killing 3 cylinders. When you shut off 2, you're killing the other 3 cylinders. 3,4,5,6 are not connected to anything.

A dead IAC will cause 2 things. First, it does limit power. Second if it's shorted, it can burn out the driver inside the 555, so disconnect it immediately, or replace it. The $45 ones for a Ford from Summit or O'Reilly is the same.

Chris....

 

[alldodge]

Fuel pressure ok?
Look at timing advance to see how much your getting
Has the distributor cap been changed?

 

[Mischief Managed]

Fuel pressure ok?
Look at timing advance to see how much your getting
Has the distributor cap been changed?

Have not tested fuel pressure (my gauge did not work...). Never tested advance, it's computer controlled and there were no codes, so never felt the need. Cap, rotor, wires and plugs are all brand new. Spark verified on all 6 cylinders. Will be testing injectors next.

 

[alldodge]

Understood, but looking at what your total advance is (scanner) might give a clue, in degrees and how much BTDC
See what the scanner shows for MAP and ECT once warmed up

 

[Mischief Managed]

The 4.3 fires odd and even injectors, not individual. When you shut off 1, you're killing 3 cylinders. When you shut off 2, you're killing the other 3 cylinders. 3,4,5,6 are not connected to anything.

A dead IAC will cause 2 things. First, it does limit power. Second if it's shorted, it can burn out the driver inside the 555, so disconnect it immediately, or replace it. The $45 ones for a Ford from Summit or O'Reilly is the same.

Chris....

Thanks for the info. Am I right to think there's an issue on cylinder 2, 4 or 6?

For review:

With the engine running at 1600 RPM in neutral, when I induce a misfire on "cylinder 1", the RPM drops to around 600 and the engine sounds like it's going to stall.

With the engine running at 1600 RPM in neutral, when I induce a misfire on "cylinder 2", the RPM drops to around 1400 and the engine seems to run OK.

 

[alldodge]

Injectors 1,3,5 are turned ON/OFF by a single Lt Green/Purple wire.
Injectors 2,4,6 are turned ON/OFF by a single Green/White wire

Since your loosing more rpm when turning OFF Cyl 1, I would agree there are some issues with injectors 2,4,6 or a bad connection on those injectors