scott

i have a 1991 jeep cherokee base mdl. 2whl drv.,date of manf. 9/90 Just bought it. I need to know if I have a 90 or 91 4.0 eng. inline 6/I am replacing spark plugs The sticker in the eng. comp. says use rc12lyc/ the ones i removed are rcy9c for a 90 mdl/rc12lyc 91 plugs are about a quarter inch longer/than the 90 mdl ones i pulled out ryc9c. The book says 90& down will have the letter L in the vin #/91&92 have the letter S problem is both #s are in the v.i.n... Its the orignal eng. & it has the high output emblem on the re ar,last owner did not do much prvnt. mant. for the 30000 miles he had it v.i.n.# is 1J4FT28S1ML530642 The motor barely ran when i got it,overall bad mant.

jeeptech

Tenth position is year code - yours is m. m=1991

scott

thanks, so the eng is a 91? use the longer plugs not the short ones that were in it?

scott

th eres no chance it has a 90 eng in it 90plugs and 91 plugs are different I know anything made after the 6th month is considered the next year maybe put 90 eng in it? even though the spec decal under hood says use 91 plugs