Hello,

I am chasing an issue on my 1990 5.7, 1228746 ECU. Yesterday it was acting like it had a vacuum leak, idling about 1850. Today it started and idled normally except the RPM was bouncing from 550 to 950 or so. Vacuum is stable at about 19 in low idle, up to 21 at high idle. Injectors look fine and have an even spray pattern. Fuel pressure is steady at about 14 psi.

I decided to check the timing. Unplugged the plug for setting base timing and took a look. The timing is all over the place, bouncing 15 to 20 degrees total, moving both retarded to advanced.

I pulled the cap (cap and rotor look great), the rotor does rotate a little bit (maybe 1/16" to 1/8") as well as a little more when riding up the gear teeth. I don't know how much movement is considered normal. Anyone?

The ignition module is a Delco and has about 2,000 miles on it. The coil has about the same. I replaced both at the same time along with the cap and rotor. The spark control module is original.

Before I go do any further testing and troubleshooting I wanted to get some advice. Suggestions?

Thanks