Re: OT Bronco II Stumbles But Passed Emission Test
Research showed that Oxides of Nitrogen NOx forms at about 3000 degrees. Slightly below that temp very little are formed, slightly above that "magic" temp you get alot - 12 different isotopes of it. A normal combustion engine's combustion temperature is slightly more than that - 3100-3200 degrees.
By expermentation, they found that adding a small amount of inert gas into the chamber significantly lowers that temp - below that "magic" temp. First experiments used Helium and Argon. Too expensive, too rare, not enough on the planet. It spaced the molecules apart slightly, lowering the temp slightly -- akin to spacing the BBQ charcoal slightly apart.
Evan Arnold - of LA Trade Tech fame was experimenting with it too. He found that exhaust gas works too. The exhaust has very little oxygen in it - it's all been burned already. The result is a relativly inert gas - essentially a "dead" gas.
By injecting a small amount of that gas into the chambers, it did the same as Helium - and it's free!
In 65? Chrysler failed to meet emissions standards in California, they could not sell one of their engines in Calif! Crisis time.
Evan had already been playing with the solution. He told Chrysler how to fix it - he was their hero! They paid him a bunch and gave him a new Station wagon. I rode in it!
His solution was to drill small holes in the intake's floor into the exhaust crossover, introducing a small amount of exhaust gas into the intake.
They drilled the holes and installed a screw-in orfice - called them Floor Jets. It was a small constant leak, but it worked. They were back in business again!
Find an old Chrysler - open the throttle wide and look down - you can see them right under the carb. They wear bigger, runs terrible like that. Unscrew the jet, put in a pipe plug -- works great!
Later on it was decided it was better to be able to control the amount recycled, so on came the EGR valve. The inert gas is only needed at higher loads, warm. They controlled it with ported vacuum, with a curve similar, yet different, than the spark advance. -- He He -- Another vacuum port on the carb to confuse us! They inhibited it to operate only when warm with a temp valve, sometimes even a transmission switch. More hoses!!!
That caught on.
The famous earlier computerized Fords use a position sensor right on top of the EGR valve - to sense the position of the valve. Those valves were still vacuum operated, but the MCU needs to know if it's open, closed, or how much between.
That input signal has a profound effect on mixture and timing. Unfortunately the way they made it, it's too sensitive. As the EGR spring wears, and the valve seat wears, the EGR opens slightly sooner than expected. That in itself isn't so bad, but the sensor's plunger gets out of position - the plunger moves up slightly too far, causing the MCU to start compensating for the EGR when there isn't any. That = Stumble!
Dealers simply replace both the EGR Valve and the position sensor for lots of $$$ - but it happens again soon.
The "fix" is as I posted. I'd like a nickel for every one I've done.
Newer Fords the EGR is operated directly by the computer - no more vacuum. He He - again they copied GM, but had to get around the patents.
Some engines nowadays leave a small amount of the inert exhaust in the cylinder by modifying the cam's grind for temp lowering. They don't even have ERG Valves. When you replace the cam on those with one that doesn't do that - they really WAKE UP! But may not pass the dyno type testing. The reason dynos are used for testing is to check for Oxides of Nitrogen - NOx - they put it under load.