Spark Knock,78' Cadillac

Hi,78' Eldorado,Has spark ping, repaired vacuum leaks and replaced distributor vacuum advance,and set timing correctly.Still getting spark knock when accelerating.Also replaced spark plugs.Do 1978 425's need premium fuel?. This car sat for awhile,don't know for how long,but it does run very good except for the spark rattle.Any idea's?? Diesel1962

Make sure the egr system works. I assume you are checking the timing with the dist. vacuum is disconnected, but an inop egr could be the issue, usually this would only affect the engine when fully warmed up.

Hi Max,Yes I set timing with vacuum disconnected,and underhood decal says to plug vacuum at EGR valve.Is there a way to check that EGR valve is working properly? Can it be removed from the intake and lubercated to make sure it is not froze up? It has been awhile that I had the pleasure to work on something this old.Had to dig up alot of old shop manuals that I saved over the years!!. Do you think that it could also be alot of carbon built up on the pistons??.Or do you think it could be the timing chain and gears?Hope to here back from you....Diesel1962

If so designed pull up on ther egr diaphragm from below the egr housing ,the engine should stall or run rough, if not it is bad or the passages are clogged, if it does, check the vacuum circuitry to the egr.

Thanks again Max,The EGR valve cannot be pulled up from the bottom.I guess I will remove it from the engine and try cleaning it.It is getting good vacuum to the valve.And I have been doing the timeing adjustments with the engine at operating temp.Will let you know what happens...........Diesel1962

Hey Max,I removed EGR valve and found it was stuck half way open.Cleaned and lubed valve and it don't work.Vacuum is shot on it.But the engine does idle allot better and the spark ping is about gone.Have to save my nickels and dimes to buy a new valve! LOL!!.Any way thank's for the info...Diesel1962