Just replaced the 318 in my son's 98 Grand Cherokee with a 318 that was supposed to have been pulled from a 97 Ram. But the motor has a 4-1993 casting date. Anyway the only noticeable differences were the oil pan and exhaust manifolds which I switched. Also the replacement motor has egr but the original didn't. I didn't switch intakes but did use the Jeep's injectors, rails and throttle body. I also didn't switch flex plates, and noticed after the motor was set in I could only line up 3 of the four converter bolts. I was told all magnum converter bolt patterns are symmetrical, now I know they aren't. Also the flex plate must have been damaged because it contacts the crank sensor at one point in rotation. (I know I should have taken the motor back out at this point but I figured I'd take a chance, and I lost.) It runs but it won't idle steady. Even when you speed up the rpms the power seems to come and go. And the tach will fluctuate wildly, but not with the engines rpms. Does anyone know if the tach reads from the crank sensor or the hall effect switch in the distributor? I'm thinking the crank sensor isn't reading every notch on the flex plate and causing my problem. I've checked part #s for flex plates for a 98 Grand Cherokee and a 94 Ram and the #s are different. Any help appreciated before I slide the trans back and switch flex plates (which I should have done to start with.)

96 and up mag motors used a symertical torque converter bolt pattern. All previous did not.

Thanks!

and if you kissed the crank position sensor, it's damaged. that sensor is what controls spark timing, which could explain the irregular idle.

does it throw any codes? going from memory, I think P0320 is the CPS code.

I changed the flex plates and the problem is gone. 70Cuda383 is right, when the raised places on the old flex plate were touching the sensor it must have grounded it out and caused the problem. Just wanted to say thanks again and maybe help someone avoid the same problem in the future.