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Rebuilt engine, '86 Toyota Landcruiser

After the first 1K miles on a new rebuild of my original '86 Toyota 2F engine, I pulled the sparkplugs to look for obvious signs of oil burning (the reason for the rebuild in the first place). Five of the six sparkplugs are pristine; however, one plug has a semi-sticky blackish deposit on it. I showed the plug to the mechanic who did the rebuild; he speculated that the original plug may not have ignited properly and replaced the offending plug. Fast-forward another 2K miles after the rebuild, again, pulled the plugs only to find similar conditions in the same cylinder with the now-replacement plug (both times I reinstalled the plugs in their original cylinders, all others are those provided with the rebuild and are clean). It’s now got about 8K miles remaining before the 12K mile warranty expires. Any speculation on what’s happening? (Aside from the possibility that the mechanic is delaying an actual fix until after the warranty expires.)

If the rebuilder won’t investigate you can take the car to another mechanic. A valve might not be working and sealing properly, or gasses could be blowing by the rings. A compression test comparing the readings for the “suspect” cylinder to several of the “good” might reveal something.

A good tailpipe emissions test might also show a problem. On the old air cooled VW’s one cylinder always seemed to run hot due to inadequate air flow to cool it. You might have a cylinder that isn’t getting all the flow of coolant needed showing up in a different look to the plug.

Hard to say if this is a real problem, but it is a clue that should be investigated.

First off, I would never have gone to that amount of work on that vehicle. Secondly, although some here will argue, I would only put a rebuilt in that was done in a factory setting and not by a local mechanic. Just too complex.

Could be a seating problem with the rings-bad install, bad cross hatching of the cyl, etc. Oil is getting into the combustion chamber.

First step is a compression check and you may need to have this done by someone other than the person who rebuilt the engine because that person has a vested interest in the results.

There’s nothing wrong with a shop doing a rebuild. The only question would be if they’re doing it correctly or not and that means inspecting and performing all of the minutae involved with a proper overhaul.
Throwing a set of bearings and rings into a worn block and having a valve job performed is not an overhaul; just in case that is what happened here.

Much info needs to be known about this overhaul; mileage on the engine, block bored, crankshaft turned, etc, etc, etc.

A semi-sticky black deposit usually points to an oil problem and that could be due to a valve guide/seal or cylinder/ring problem; or both. If a problem exists on one cylinder then I would suspect others could follow in the future.

Check the compression on the suspect cylinder and compare the readings with those on a couple of the “good” cylinders. There could be a valve not functioning properly, or some compression lost blowing by the rings.

Have you replaced that plug wire?