1991 3.0 195K

Did the California smog test in order to sell the truck:

.. ............................. %CO2 .....%O2.......... HC ........... CO%

IDLE 1018RPM............ 15.3....... 0.2 .......... 178 .......... 0.11 ......... Fail HC

2500RPM.................... 15.3....... 0.2 ............ 20........... 0.01.......... Pass


I'm failing on HC at idle (but 1018RPM seems high) can't find any vacuum leaks, was high on the HC AT IDLE (109) 3 yers ago but passed, it idled at 861 then vrs 1018 now.

Will start with a minor tune up but any BTDT will be helpfull.

Thanks, Jim

 

Primary systems that affect HC (which is unburned fuel) include mixture control, ignition and exhaust after-treatment.

Primary air/fuel feedback sensors (such as O2 sensors, MAF) can be common causes of high HC.

Ignition system must be working properly. If a cyclinder is misfiring that can also cause high HC.

Your cat converter could also be failing.

Have you tried pulling codes?

 

OK, no stored codes but when I did a KOER I got a code 13 (Cannot control rpm
low during self test or idle speed control inoperative)

I also get a 0.0% reading on the Idle air control when doing a DCL real time scan.

That sounds like a good place to start but where is the Idle air control on a '91 3.0? Got my Ford Shop Manual out all I can find is a Air Bypass Valve. Looks like it might control idle air but I'm a little confused on the terminology

Thanks, Jim

 

Same thing.

Air Bypass Valve = Idle Air Control Valve (a/k/a the "IAC").

When's the last time the motor was tuned up? New air filter? You might try cleaning the MAF and even pulling the intake tube off and cleaning it out, as well as cleaning the throttle body. There are links in the Technical Info Sticky Thread on how to do that.

You want a clean burning system with proper a/f mixture.

 

I agree that cleaning the throttle body is a good idea. I'd also consider spritzing the heck out of the intake manifold. Change the oil if it has any great mileage on it, as it will release extra HCs thru the PCV that are beyond ECM control. A clean air filter would help if you can't see the sun thru the one you have. Listen at idle for the phtt pht of an intermittent miss, 'cuz it will cause HC (unburnt, you know) to be exhausted.
My 2.3 failed when it had less than 6k miles. Costly experience as I ended up paying for a second test, along with a 'clean intake system' at $100 and it STILL failed. Referee checked that the ECM was reporting code failures, and gave me a sticker. Less than 2 years old. Since then, I've had good luck using an O2 safe 'gumout' to clean the intake before getting tested.
If you run a clean engine, doing reasonable care and maintenance, it should pass. The HC #s (imho) dirty at idle and clean at higher rpms indicate the catalytic is functional.
The code that you are reporting is caused by the idle setting being too high. I'd bet that you have the idle screw on the throttle body screwed in far enough that the idle cannot be controlled 'low' by the IAC, and the ECM is letting you know that it cannot make the idle slow down when running the test. When I had the 2.3 fail, they found a TSB that allowed them to bump the hard (screw controlled) idle up to over 900 rpm. Still failed. Since then, I've dropped it back to what was originally spec'd, and it has passed. 1013 is a couple hundred rpm too high, IMHO, and if you disconnect the IAC, and the idle still is high, your 'hard' idle is too high. (It is 'hard' in that it is done by hardware -- throttle stop screw -- rather than the software in the ECM controlling the idle)
tom

 

Put in new plugs, cap and rotor, Cleaned the IAC and found that someone had been playing with the hard idle screw. Got that adjusted correctly and went down for a retest, my HC at idle dropped from 178 to 120 and I passed. 121 would have failed so you can't get any closer. But that was good enough to sell the truck. Picked up a 96 4.0 5-speed, a real sweet truck.

Thanks for your comments,

Jim