I have an 07 4.2L 5 speed. With NO cel codes.

I have a miss at idle and my rpms on my codes scanners shows the RPMS are anywhere varying back and forth in this range 690 to 850 at idle after the engine is warm.

I also have spark knocking when accelerating.

I have changed air filter, plugs, wires and fuel filter. I have also cleaned the MAF sensor and everything is still just like it was before I changed anything.

Pressure at fuel rail is 40psi

Any suggestion?

Thanks

 

I haven't had any problem with mine but I'll throw out a few possibilities. This is a pushrod engine with iron block and heads, so you don't get into the broken spark plugs and cam driver issues of the V8s. The key here I believe is the spark knock. There should be none under any circumstances. Possibly if you can find the cause for that you may cure the idle. Right off the bat I suspect a bad intake manifold gasket, resulting in a lean condition which can cause pinging and rough idle. I would try spraying the gasket area with WD-40 to see if it momentarily smooths out, which would prove the gasket theory. Push rod engines sometimes develop sticking lifters or valve stems which could cause the idle problem but probably not the pinging. You could add some Marvel Mystery Oil to the crankcase which usually takes care of the sticking.
What does the coolant look like? Should be yellow/gold-colored and pretty clear. If you have intake gasket trouble, which has been noted with the 4.2L, you could get coolant loss and oil/coolant mixing, which is not good.

 

INTAKE AIR LEAK SOMEWHERE.

Watch the IAC or (TPS V on a throttle by wire system) on the scantool and see if it moves a whole bunch, if it does you probably have a leak, as at idle the vacuum is the highest. I agree the spark knock is the dead give away. The MAF is measuring airflow thru the zip tube, but not thru the engine. So if you have a leak at the intake to head mating surface the MAF will never know about it.

With the engine cold, get the idle as stable as possible by having all accessories off. If you have a cable throttle, disconnect the IAC plug and use paper to keep the throttle stop open just enough for it to idle. Spray starting fluid around the perimeter of the manifold and listen for a change is speed. Please use cation with this, it is dangerous.

If you have an electronic throttle, ther is not a whole lot you can do but you can still hear a difference.