1987 Volvo 780 with V6 idles from about 500 rpm’s and then goes up to about 1200 rpm’s and then repeats the cycle every 3 seconds. When it idles down it stalls out at times. Any ideas?
Check for any vacuum leaks, a plugged PCV valve or a clogged air filter.
Worn camshaft lobes or burned valves.
Faulty emissions or dirty injectors.
Check the IAC valve too.