Finally fixed rotten rough idle

[brucepick]

Rough idle has been my nemesis for months and months. But the last couple weeks I've been getting daily CELS with code P0302, and occasionally P0301/P0304. Meaning: misfire in cyl. #2 (or 1 or 4). Which would be related to a rough idle, you think??

Car burns oil so I figured the plugs were oil-fouled. Figured the #2 cyl had a bad case of oil getting in it. BUT I finally spotted that the #2 ignition wire wasn't snapping firmly into place on the plug tip. So I put in all new wires, and after one more CEL immediately after the change and then a 60 mile drive home, it has a much smoother idle, no more CELs (knock on wood), better power (butt dyno of course) and the jury is still out but I think better mpg.

I've replaced plug wires before on other cars, hoping for improvement of various ills, with never any noticeable improvement. But it does happen, apparently.

[Old Tele man]

...from your description, it sure sounds like you've diagnosed (and fixed) the problem.

[Old Mechanic]

Might even reduce your oil consumption.

regards
Mech

[brucepick]

Thanks guys but sorry, lousy rough idle is back. And the CELs. Seems to be burning oil too. Still getting decent FE as long as I'm not fighting snow, rain or wind.

I've been using Marvel Mystery Oil (MMO) in the crankcase. I read some discussion (on bobistheoilguy.com??) where this thin stuff caused faster consumption but it dropped after discontinuing MMO. The Civic is due for an oil change. This time I'll not add MMO and I might give it fresh plugs (again!) or at least maybe clean an old set with emery cloth.

[comptiger5000]

I'd say try an OCI without the MMO this time, and pull the plugs, and wipe them clean. See what happens from there.

[brucepick]

The CELs became more and more frequent, had to be cleared multiple times on ScanGauge on each 55 mile commute. Did some reading and was reminded that it had been about 50K miles since adjusting the valves.

So I did that. Some were way too tight, some way too loose. Better now but still a few CELs sometimes.

At same time, I did a double oil change - drain + fill, idle till fully warm, then drain and change filter and fill. So all the MMO is out of the system. And I did clean the plugs, using brake cleaner and a T-shirt rag.

[Old Mechanic]

If your car has a vacuum controlled fuel pressure regulator, it might be worthwhile to check to see that it does not have a bad diaphragm. If it does it would be leaking fuel into the manifold and possibly to the cylinder that is misfiring.

Pull a vacuum on the regulator and see if it holds pressure. It's a simple quick test that might be the problem.

The regulator would be at the point in the fuel rail where the excess fuel is returned to the tank after passing the injectors.

A missing cylinder can increase consumption as well as the additional fuel diluting the oil and causing increases consumption. One quick test if you don't have a vacuum tester is to just smell the oil on the dipstick and see it it has any residual fuel smell to it.

If the vacuum supply hose from the regulator is close to the cylinder that is misfiring it would be evidence that the regulator might have a bad diaphragm. OBD will not pick up a bad vacuum operated pressure regulator, just the resultant symptoms.

My brother had that exact problem with a Chevy truck before 50k miles. The dealer kept resetting the CEL and could not figure out what was wrong. The bad reg was flooding the number 5 cylinder and causing a misfire, and diluting the oil so the engine started using oil at 50 k miles on a truck he bought new.

When we finally figured out what was going on, the oil consumption stopped completely. Not saying that is your case but it might be something worth checking out.

regards
Mech