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Old 08-05-2000, 01:25 PM
450 SLC 5.0
Posts: n/a
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!
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Old 08-05-2000, 03:16 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Hard to give a pat answer when your post basically says nothing is wrong.

Obviously something is wrong. What you need, instead of a bunch of parts, is an evaluation of whats is happening. Since its not likely for compression to be intermittant, its likely to be fuel or ignition. My first step would be to evaluate the ignition on an ignition scope and while on the analyzer I would vary the mixture to see if mixture was a problem.

You said the lambda control system was working. If you are right the mixture is being held to an average of .5% CO (if measured by an exhaust gas analyzer ahead of the cat). By average I mean that the richest cylinders are held to that mixture. If you have cylinders that are recieving say 10% less fuel those cyliders will exhibit lean misfire.

The way to evaluate this simply is to press down on the plate of the airflow meter. This instantly increases the mixture; if the car instantly smooths it means that you have a lean misfire condition. Solving the problem isn't as easy. BTW if your lambda system is working a small mixture change (by placing a finger on the plate)will be almost instantly corrected by the system; so the idle improvement will also be momentary. If you do this in small increments you will eventually exceed the ability to compensate and a rich mixture can be maintained.

The answer from here is tough as the 450's were kinda rough idling cars anyway. The 380/500 cars with basically the same system K-Jet w/lambda do a much better job running lean because the injectors are mounted to a swirl chamber on top of a separate idle air manifold system. The premixing and idle control smoothed the engine out at the lowest mixtures. (cars were never meant to run that lean)

If mixture doesn't change the idle I would verify and monitor timing. It should be steady far enough advanced.

The key to finding the answer will be in indetifying whats different between times of good idle from the rest of the time. Simple huh (bg)?

Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician
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Old 08-05-2000, 08:52 PM
Posts: n/a
Maybe your rough idle is from your engine "drinking" 2 quarts every oil change. check your plugs every time you add a quart. your problem maybe there.
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