Cold start knock 92 Deville 4.9L

letsgo · #1 (**permalink**) 10-16-08, 08:06 PM

I bought this 92 Deville 6 months ago. After 2 months, every 3 or 4 weeks the engine had a loud metallic knock when starting up in the morning. After about 5 minutes warm up, the knock would be gone. This means it is not an oil pressure issue which would be over after several seconds. The knock would go away after pouring Sea Foam down the throttle, then come back after a few weeks. Because of this I believed it was a carbon deposit, causing piston slap. I decided that the problem was made worse by using a high ash oil. I just changed from that oil. The knock won't go away now. It never happens when the engine is warm. It sounds like it is coming from the #6 cylinder. I am guessing a deposit causing piston slap, or piston pin wear, or bent connecting rod from hydro-lock (coolant disappears with overheating). Also, the egr valve doesn't work, causing spark knock (non metallic) when engine is hot. Maybe this causes deposit formation? Thank you.

**Ranger** · #2 (**permalink**) 10-16-08, 08:11 PM

Remove the air filter, hold the throttle plate wide open and look down there. See the two EGR tubes staring back at you? Are they carboned closed? If so, rod them out and see if that gets the EGR working and has any effect on the knocking. 4.9's are more known for "main bearing thump" than piston slap due to carbon build up like the Northstar.

letsgo · #3 (**permalink**) 10-16-08, 11:59 PM

Thanks for the suggestion. I cleaned out the EGR carbon build up when I bought the thing. It's not bearing thump. the knock comes from the area of cylinders #6 and #4. It has to be something that the many cleanings with the Sea Foam down the throttle temporarily fixed.

**Ranger** · #4 (**permalink**) 10-17-08, 12:37 AM

Why not try the Northstar method. What is that stuff? Techron or something like that (GM top engine cleaner). Pull the plugs, fill the cylinders and let it set for 2-3 hours. Suck it out, change the oil and run it, then change the oil again.

letsgo · #5 (**permalink**) 10-23-08, 05:39 PM

I've waited several morning start-ups just to be sure the cure was real. Never before has the knock simply gone away without Sea foam down the throttle. I am sure it is permanently gone now. The problem was the high ash oil developing deposits in the combustion chamber. With new low ash oil (Mobil1 ext. perf.) in the engine, and just the usual injector cleaner and gas treatment, the knock went away. This engine, even though not the newer Northstar, still is susceptible to combustion chamber deposits. There should be no problem with the lower ash sm oils. I had the synthetic Rotella 5w-40 in it (the old SL/CI4+formula), which has now changed its formula to the lower ash CJ4 and SM ratings, so even it should no longer be a problem.