At wits end with 86 Carrera troubleshooting

[AtomicDog]

My ’86 Carrera has not been running well at all. Mileage is 151,xxx. It failed emissions testing last week with the following results:

Idle HC – 913ppm. High idle HC – 511ppm (max is – 220ppm), CO% was 0.03 for both tests.

The car will barely idle, and has a misfire throughout most of the band. This is the obvious source of the high HC numbers, but not the only symptom. Low-load, steady state operation within a varying 200 rpm band (but always between 2k and 3k rpm) results in stuttering and bucking until throttle position is changed slightly. Idle does drop when the oil tank cap is removed.

So far, I have replaced; distributor cap and rotor, spark plugs, plug wires (new Magnacores), speed and reference senders, and multiple vacuum hoses.

I’ve also shifted AFM pickup to un-worn track, checked DME relay against new one, performed valve adjustment, tightened intake manifold bolts, checked operation of full-throttle and idle microswitches, checked temperature sensors for CHT (updated sensor in 2009) and AFM, run the car with idle microswitch disconnected, run car with O2 sensor disconnected, and cleaned and checked operation of ICV on the bench.

Next, I plan to isolate any remaining vacuum leaks, check fuel pressure, electrical ground connections and perform testing on ICV, CHT, and speed and reference senders while the engine is running.

I have searched the forum extensively, yet I am clearly missing something. Symptoms have been fairly consistent for over a year. Your help is appreciated.

[Joe Bob]

While you have run w/o the O2 sensor, have you considered replacing it?

[AtomicDog]

It's on the list, but further down... Should I move it up?

[Joe Bob]

My 3.6 exhibited those symptoms a few years back....ended up being the bad signals from the 02 sensor.

[AtomicDog]

Thanks, JB. Up the list it goes.

[sjf911]

Sounds like it could be lean misfire. Your fuel pressure may be low (? plugged fuel filter) or fuel injector malfunction (dirty, bad electrical, etc.).

[AtomicDog]

Steve,

Is that based on the low CO%?

Chris

[ejp]

Have you performed a compression test? This may help rule out anything catastrophic.

[john walker's workshop]

compression and leakdown tests next. the leakdown may show more than just a compression test. you need to know that you have a good base to work with. at 151K, it should have had a top end job done by now. was it? otherwise, i would suspect worn guides and the resulting very worn valve seats could contribute to your problem.

[Tippy]

Minus leak down test that is very wise, have you tweaked the AFM bypass screw at all in the past?

[AtomicDog]

No top end work, yet.

I asked for leakdown during PPI. Shop said that it would be pointless, as compression test was fantastic (~180#, IIRC) across all cylinders. BUT, that was at 87K miles... Was told at the time that not all 3.2s would exhibit the guide wear issue, and that if I might be one of the lucky ones.

Neither leakdown, nor compression have been checked since then.

I have never touched the afm bypass screw.

The confusing thing, is that the misfire is not steady on one or two cylinders. If I pull plug wires one at a time, there is always a slight degradation in idle. Plug inspection doesn't reveal any smoking guns, either. Shouldn't I have steady indication of a faulted cylinder?

BTW, thank you for the help. I'm tired of tearing my hair out.

Chris

[sjf911]

Originally Posted by AtomicDog

Steve,

Is that based on the low CO%?

Chris

That is a very low CO and certainly suggestive of lean condition (0.1% roughly correlates with an AFR of 14.7) but I don't know how your catalytic converter is affecting the level. Lean misfire is very easy to induce in these cars. I have a difficult time getting mine to idle well at anything more than an AFR of 14 (about 1% CO). I didn't see coil replacement on your list above.

[AtomicDog]

Coil is about 4 years old - which means nothing. Will add that to my list of checks.

Thanks

[john walker's workshop]

bosch silver coils are bad juju. very high failure rate.

[Tippy]

Notice any change in your brakes? The brake booster takes the largest amount of vacuum. If the diaphragm has a leak, it could result in a large vacuum leak and lean misfire.

[dhagood]

+1 on the 02 sensor. your car is exhibiting the exact same symptoms my 84 carrera did prior to changing out the 02 sensor; my car ran perfectly after the sensor switch.

ymmv of course.

[Jonny042]

Originally Posted by john walker's workshop

bosch silver coils are bad juju. very high failure rate.

Do they often fail partially, so that the car will still run, albeit poorly, or do they just plain stop working?

[GH85Carrera]

The original coil that was on my car for 15 years failed, and the car would miss and ran horrible. A new coil was a instant cure.

[GH85Carrera]

Originally Posted by john walker's workshop

bosch silver coils are bad juju. very high failure rate.

John, do you know of a source for a top quality drop in replacement coil?

[CCM911]

How about checking the fuel filter and injectors?