525SC Miss/Backfire
 

I have a 1999 525SC in a 47 Fountain that has intermittent miss or backfire above 2500 RPM under load. If run this motor at 2000 RPM and other 2 at 3000 RPM, no miss. Have swapped coil, distributor module, etc. no change. Any ideas before I swap knock sensor and rev limiter modules? Thanks

Hows the cap and rotor look?

There is no knock sensor on a 525SC and the issue won't be in the rev limiter box.
Do a complete tune up with plugs, cap and rotor. Check all the ignition wiring connections for corrosion.

No knock sensor on a 525SC? Thought there was. Plugs, cap and rotor look good but I'll change them out as much easier to get to than the other modules. Thanks

I had the same type of problems with my 525sc engine a few years ago,it was being caused by a lean condition through the base of the carburator,sent the carbs for rebuid through Diamond Performance and replaced base gaskets and the problems went away,also as recomended on another post do a complete tune up with wires.Good luck.

Almost all of the 525SC's used a TB IV ignition which does not have a knock sensor/timing retard function.

The last run of 525SC's, which you may have, does use a TB V ignition and may actually have a knock sensor.

Well, I did a cold compression test today, and found that the number no 8 cylinder intake valve is stuck in the open position. Anyone know of a way to free it up short of pulling the head and reworking the valve guide? All other cylinders are 135 to 140 PSI. Engine has 420 hrs. Could the CMI headers have an internal leak that has allowed moisture to enter the cylinder and corrode the valve stem? Thanks for any info.

mrfixxall
 

Time to pull it and fresh in it up,its a ticking bomb and here is a sign..It could get worse and eventually cost you a whole engine.

Pull off the rocker and see if that is inhibiting in any way. if not it could be a bent valve