QuestionTHE CAR HAS BEEN GARAGED FOR SEVERAL YEARS. DROVE WELL PRIOR TO STORAGE. SPARK PLUGS, AIR FILTER, OX SENSOR REPLACED, CATYLITIC CONVERTER REPLACED WAS GUTTED BY PREVIOUS OWNER, RECENTLY REPLACED WITH BY-PASS TUBE. CAR STARTS EASILY AND IDLES AROUND 500-600. IF YOU PRESS ON THE ACCELERATOR WHEN IN NEUTRAL THERE IS NO INCREASE IN RPM. MORE PRESSURE ON THE ACCELERATOR PEDDLE WILL CAUSE BACKFIRE. IF YOU LET THE CAR IDLE FOR 15 MINUTES THE SITUATION IS UNCHANGED, IDLE REMAINS 500-600 RPM. I HAVE REACHED INTO THE AIR FILTER BOX AND LIFTED THE THROTTLE PLATE?, SLIGHTLY AND THE RPM PICKS UP AND SMOOTHES OUT, SOUNDS BETTER. REMOVING THE RED VACUUM LINE ON THE DISTRIBUTOR HAD NO EFFECT ON IDLE. MEASURED 5 INCHES OF WATER ON A GAUGE I HAD. WATER NOT MERCURCY TO BE CLEAR ON THAT. THE BLACK HOSE TO THE VACUUM ADVANCE HAD MORE THAT 15" WG. TAKING THAT OFF AND PLUGGING IT HAD NO EFFECT. IM THINKING ITS EITHER THE VACUUM ADVANCE OR THE AIR VALVE/REGULATOR. WHAT AND WHEN DOES THE AIR VALVE/REGULATOR DO?
STEVE
AnswerHi Steve,
I'm glad you're trying to get that baby back out on the highway where she belongs!
I'd like to suggest another culprit besides vacuum advance. Given you can't get the car to go above idle, the advance mechanism is not even waking up, let alone acting poorly. The advance comes into play at much higher RPM's.
And as far as the air valve/regulator...I'm going to assume you are talking about the air flow sensor, although it strikes me there are several other components you may be referring to. I'll just cover the air flow sensor briefly and you tell me if I'm on the right track.
That "throttle plate" you lifted to get the idle to go up a bit is the most visible part of the air flow sensor, in fact the plate is called the sensor plate. It only reacts and raises/lowers as the volume of air being sucked into the engine changes. The actual throttle that you activate with your foot on the gas pedal is situated between the air flow sensor and the intake manifold.
But back to the air flow sensor, by virtue of its position in its bore, it knows how much air volume is streaming through... it is attached to an arm (picture a playground teeter-totter) and so as the sensor plate rises the other end of the teeter-tooter arm inside the fuel distributor moves in the opposite direction and mechanically meters out just the right amount of fuel to achieve the ideal mixture of air to fuel, close to 14:1. Assuming, as you say, the car was running well before, there is no reason to suspect this process is faulty.
However, what could be the cause, and is a major contributor to backfiring, is "false air". The meaning of this is that somehow air is entering the intake manifold that has not been measured by the air flow sensor...and as a result the 14:1 ratio is off. This false air is most often the result of vacuum leaks...or...holes in the ducting between the air flow senor and the intake manifold.
That's where I'd start looking. Let me know what you find.
Dave
