I hope that someone out there can help me out with this – maybe I’ve missed something? This has really got me stumped. It is my U.S. spec 1980 450 (M117-985 engine) It is the first year of oxygen sensor & 3 way catalyst. This car has been fanatically maintained by me for the last 8 years, (I am the second owner) I have always used synthetic oil, and the internals are immaculate (not even any cam lobe wear) It burns oil at the rate of about one quart every 1200-1500 miles. It has about 240,000 miles on it.
Problem: Really Rough idle, but occasionally is O.K. (maybe 5% of time).
ALL ignition components are either new (plugs, wires, cap & rotor) or have been swapped out with parts from a good running car – (coil, ballast resistors, ignition control box).
1. Compression is good across all cylinders.
2. NO vacuum leaks.
3. Lambda control system is functioning properly & mixture is set properly.
4. Ignition timing is correct, and total advance is correct.
5. Warm up regulator seems to function properly, but I haven’t checked the actual pressures. Since the problem occurs at any temperature, I don’t believe that it is likely at fault.
6. All injectors were replaced 3 months ago with new brass ones.
7. At the same time, I took the center plunger out of the fuel distributor and cleaned it with carb cleaner. I did not take apart the individual pressure regulators (duhhh). The car ran beautifully for 3 months since then, so I didn’t screw that up.
8. I have run several types of “top end” cleaners such as Lubro Moly’s Ventil Sauber, Red Line’s cleaner, Techron, and I even used the GM / Delco X-66 that was recommended by one of the regular readers. (AMAZING stuff, by the way!) Therefore the intake valves are as clean as they are going to get unless I take the heads off, which I really don’t want to do!
Being that it occaionally idles smoothly, it seems like an intermittent electrical problem but I have checked everything. I know that I’m probably REALLY getting desperate to even think of this – but is it possible that the timing chain tensioner could be at fault, intermittently causing a shift in cam timing? Both the chain & tensioner have about 70,000 miles on them. I can’t think of anything else – do any of you have ANY ideas? Or is it just time for me to bite the bullet and pull the heads off? It seems like I shouldn’t have to, considering that the mechanicals are good – or do these engines just need a valve job at this mileage in order to clean the carbon off the intake valves?
Desperately Hoping that someone can steer me in the right direction!
THANKS IN ADVANCE!