Civic VX miss under load

[oh95vx]

I'm back again with more VX problems - sorry to say.

Right after I changed the L1H1 oxygen sensor about 6 weeks ago, she ran great - for a while. A couple weeks ago, I started to feel a bit of initial stumbling on acceleration. That's worsened to where it's a persistent miss.

Under even moderate acceleration I can feel frequent misfiring. The little girl just ain't got much go. Even in neutral, I can hear the miss in the exhaust note when I punch her throttle.

I have no CEL codes.

The exhaust smells odd. I'm not sure how to describe it.

Plugs (also ~6 weeks old) have some loose black carbon on the bases, just above the threads, but not much on the tips.

O2S also has a bit of carbon at its base, but the tip looks good (I think).

Ordinarily I'd suspect fuel delivery problems, and to be honest I don't recall when I last changed the fuel filter. However, the missing happens only after she's warmed up. When cold (and in open loop), she seems to run fine, suggesting something in the engine sensors or control is at fault. (Or am I wrong about that?)

ICM and coil were new a couple of months ago, just before the O2S. I didn't install them, however, so who knows what went in there?

New cap and rotor just after that.

TPS is smooth over its entire travel, no bumps or jumps, with resistance ranging from something like 2-3k to about 4.5k.

I just changed the MAP sensor. That was $75 for nothing.

Timing light seems to show the distributor advance working, though I haven't tested it in gear with the wheels off the ground yet.

Could the replacement ICM be causing this? Any other possible culprits you can think of?

I appreciate the coaching. Thanks guys.

[GasSavers_James]

Wish I could help, but I am new to the VX. It sounds like you have replaced all of the usual culprits. Did you check the resistance of the plug wires, and dielectric grease the ends? You probably did this but did you set the right spark plug gap?
Mine does a similar thing. It only seems to miss under load once it has warmed up. However the difference is that it idles perfectly, and works perfectly once out of the lean burn mode. high rpm and low are fine if you put the throttle down and get it out of lean burn. It is worst at 1,700rpm under moderate lean burn load. I am hoping it is just the o2 sensor.

[GasSavers_James]

They are a sensitive automobile. Hopefully we can work together to diagnose problems and keep em running efficiently.
Just a note on how sensitive they are, mine idles beautifully at 550 rpm when all accessories are off, but turn the fan switch to 1 and that jumps to 800-900, probably burning quite a bit more fuel.

[kwtorbe]

I suggest a hotter spark plug if your plugs have carbon on them (they aren't oily are they?). I might guess at fuel injectors too. Have you checked the cap and rotor? The coil sometimes gets corrosion on it....The exhaust smell--is there any smoke?

My HX feels like it misses on the highway but a fellow HX owner and I determined it is the lean burn kicking on...

[thisisntjared]

50/50 shot its the fuel filter or dirty plugs. it could also be the distributor. how many miles are on the beast?

in the future change the cheaper parts first. MAP sensors do not readily go bad.

[oh95vx]

Cap and rotor are new.

"Could also be the distributor" - Hmmm. The ICM and coil are also new, about the only thing left in the distributor is the bearing, right? Or is there something else? How would I test it?

Plug wires are only about 2 years old if that, but I'll check 'em. No dielectric grease, but I'd expect the miss to be worse in wet weather if that were the problem, and wet or dry makes no difference.

Plugs are gapped right, or were when I put them in 6 weeks ago.

James, your VX sounds like mine was 2 months ago. She was smooth as butter at idle and under acceleration, but at any steady speed she missed and bucked. A new oxygen sensor fixed that, but now this.

BTW, 174,000 miles on her.

[mrmad]

Might not be a bad idea to get a compression/leakdown test just to ensure the rings/valves are in good health.

[jadziasman]

James,

The symptoms you describe suggest one of the following as the culprit of your engine miss at steady rpm under load.

1) The fuel injector timing is controlled by the ECM based on the 20 inputs it receives from the various sensors. The ECU contains memories for the basic discharge durations at various engine speeds and manifold pressures. The basic discharge duration, after being read out from the memory, is further modified by signals sent from various sensors to obtain the final discharge duration. lgnition timing is also controlled the same way but with fewer inputs. So it is possible that the ECM is trying to control fuel injection and ignition with bogus inputs. If so you will have to troubleshoot them one by one.

2) One of the cylinders has a deteriorating condition within the power cell. It could be something simple like carbon build up in the VX piston combustion bowl - which is pretty deep compared to other civic models. Or something more serious that is causing pre-ignition and additional carbon build up and wear. You said your compression test came out good in all cylinders which probably rules this scenario out.

In the case of my first VX with 155K miles, it was #2. I had a damaged cylinder that caused loss of power and poor mpg that deteriorated over time. An engine rebuild was the only solution I could come up with so I decided to sell it instead of repair it. Someone eventually did fix it but I'll never know the story - Carfax records indicated the car was still on the road three years after I sold it.

[jadziasman]

Oops - I meant our friend in Ohio not James specifically.

[slurp812]

Does that thing have an EGR valve? My 94 accord had clogged egr ports. ran like crap anywhere between 2 and 3000 rpms under light throttle. Also my plugs were showing a bit lean. But thats a lean burn engine, so I am not sure what the plugs should look like in an engine like that...