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High,,To anybody who can help...I have a 92 Bronco 351 and am trying to pass a ca smog test ive ben dreding for some time.

My question being is that i failed my most recent test
(15 mph--Max HC 78 and i got 140)
But last year the max was 112 and i got 112 passing.

So does the max hc emissons change from station to station or is this As*hole just trying to make a quick doller?
Even with a max of 112 i no ill still fail but its a way shorter margin that i have to correct to pass again.

Any help would be appreaciated..Thx in advace,Chris

Super Moderator
25,807 Posts
yo Chris;
This is from various sources;

Hydrocarbon failures mean unburned gasoline is passing through the engine and entering the exhaust. The three most common causes include ignition misfire, lean misfire and low compression (typically a burned exhaust valve). Ignition misfire can be caused by worn or fouled spark plugs, bad plug wires or a weak coil. Lean misfire results where there is too much air and not enough fuel, so check for vacuum leaks, dirty injectors or a fuel delivery problem. In addition to these, hydrocarbon failures can also be caused by oil burning due to worn valve guides, valve guide seals and/or rings


A high HC reading is the result of poor combustion. The most typical cause is a poor ignition system. HC is nothing more than fuel that makes it out the tailpipe without being burned.

So, your first step on the way to compliance will be a complete ignition tuneup: plugs, wires, caps and rotors. These parts are quite reasonable at any of the Big Three suppliers. The only exception to that might be the wires-- still "reasonable" but maybe just a little price shock. Resist the urge to spend on exotic multi-electrode plugs too; their only purpose is to provide extended life on new cars. The Bosch platinums will work just fine. Don't succumb to the temptation to get "bargain" wires either. The care and time required for wire replacement make installing good wires the best investment. Some folks have replaced aging ignition coils as part of the process too, while everything is out and accessible.

The CO reading tells you about the fuel mixture. For the most part, HC and CO are unrelated, unless the mixture on a cylinder is lean enough to cause a misfire. Lean conditions result from vacuum leaks at hard/cracked/broken hoses, leaking gaskets and boots around MAF/MAS sensors, and partially clogged fuel injectors. Old vacuum and emissions hoses undoubtedly need replacing just due to age on many cars, so getting all new ones and installing them in one session is not a bad way to go.

A poor oxygen sensor might cause a lean condition severe enough to create a misfire, but an engine with reasonable ignition will easily fire with that slightly lean mixture.

So, your marching orders would read ignition tuneup as described above, a visual inspection of hoses for vacuum leaks, followed by a listen and spray, and finally a test with a good high-impedance DVM for output from the oxygen sensor.
The chart below lists some of the kinds of problems that could result in abnormal gas readings

I read awhile ago that the PCV may have a hand in high HC combined w/other thAngs shown above.
A neglected PCV system will soon fail to function and the result can be
expensive as well as troublesome for the car owner. If the crankcase is not
adequately ventilated, the motor oil will quickly become contaminated and heavy
sludge accumulations will begin to form. Internal parts, not protected by the
motor oil, will begin to rust and/or corrode due to the water and acids that will
become trapped within the crankcase. If the PCV system is not functioning
properly, the flow of crankcase vapor into the intake manifold will not be properly
This, in turn, will upset the fuel/air mixture for combustion and cause rough idling
or even stalling of the engine. Furthermore, intake and exhaust valves, in
addition to spark plugs, may well be burned and rendered useless, prematurely
affecting performance and requiring expensive repairs. To assure trouble-free
performance of the PCV system and, in turn, the engine and vehicle, routine
maintenance of the PCV system is absolutely recommended and required.
A PCV valve should never be cleaned and placed back into service. Cleaning a
PCV valve will result in a clean PCV valve; not a new PCV valve. There are
contaminants that will remain in the PCV valve that can never be flushed out.
Additionally, there is an amount of wear that will be experienced by the spring
that cleaning cannot replace. The recommended replacement intervals are a
maximum of 12 months or 10,000 miles (16,000 km). Since vehicles and
operating conditions vary, the valve may have to be serviced more frequently. If it
is suspected that the valve is sticking or if there is evidence of sludge, the valve
should be replaced.
All hoses or tubes used in the PCV system should be cleaned and inspected. If
any cracks or breaks are noticed in the hose, it should also be replaced. All hose
connections should be inspected to assure an air-tight seal.
Proper servicing of the PCV valve system will help reduce overall vehicle
For additional information, contact:
Filter Manufacturers Council
P.O. Box 13966
Research Triangle Park, NC 27709-3966
lastly; can you do a Self Test for Codes?
a Self Test by Steve83 do the KOEO = Key On Engine Off portion first & Remember to have engine @ Normal Operating Temperature before doing the KEY ON ENGINE Running portion

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