Car won't start after new injectors

Tiny
THETFORD96
  • MEMBER
  • 1991 DODGE DAYTONA
  • 2.5L
  • 4 CYL
  • TURBO
  • FWD
  • MANUAL
Hanged my stock injectors to 1600cc injectors the car started fine but after I shut it off I went back 5 min later and it will start asking as I have the gas pedal to the floor and it will smoke horrible but as soon as I take my foot off gas it shuts off. What could be the problem?
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Saturday, June 6th, 2015 AT 9:20 PM

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Tiny
CARADIODOC
  • EXPERT
What are you trying to accomplish? The Engine Computer knows exactly how long to hold the injectors open to get the precise amount of fuel based on fuel pressure on one side, intake manifold vacuum on the other side, barometric pressure, and the volume of the cylinder. It is going to pulse the injector for the same length of time during each intake stroke. A larger injector will flow more fuel at those same set of conditions and will flood the cylinders.

What you're doing is the same as saying you want an air compressor that reaches 150 psi to replace the one you have now that can only reach 120 psi. You're just pumping your tires up to 40 psi, so having the higher maximum pressure doesn't mean you can pump your tires to 60 psi. It might mean you can pump the air in faster with possibly less-than-desirable results, like exploding the tire if you know it normally takes 60 seconds to fill it. With the new compressor, it might only take 35 seconds to fill that tire to 40 psi.

Your Engine Computer does something similar. They all start out with a set of "look-up tables" that are pre-programed in at the factory specific to your engine. Every engine is different though. One may have a few more thousandths of intake vale lift or a few less degrees of duration. One car might have a slightly-restricted fuel filter, stronger fuel pump, it could have high mileage and low compression, etc. There's a zillion things that can result in that pre-programed fuel metering calculation being just a hair too lean or too rich. That is picked up by the oxygen sensor.

Suppose the computer sees the exhaust is slightly too lean. If you were watching live data on a scanner, you might see the injector pulse width change from 34.4 milliseconds to 34.5 milliseconds. That gives the injector a little more time to spray a little more fuel. That adjustment is made continuously as you drive. The adjustments occur right now, and are referred to as "short-term fuel trims, STFT).

At some point the computer may recognize that it is always having to add 1.2 percent more fuel above what the engineers designed in every time that same set of conditions appears. At that point it moves that correction to the "long-term fuel trims. (LTFT). From there on it uses that corrected value as the starting point, not what the engineers programmed in. You would see that listed on the scanner as "LTFT: +1.2". Those numbers will rarely change while you're watching the scanner. The short-term numbers will be changing constantly.

What this has to do with your new injectors is the computer is still calculating injector pulse-width times based on what it was designed for. The volume of the cylinder hasn't changed. The fuel pressure is not monitored. The computer simply knows what it's supposed to be. You haven't changed intake manifold vacuum. The computer looks at the readings from the MAP sensor to know vacuum. (In fact, Chrysler is the only manufacturer that has been able to consistently make all of their engines run right without needing a mass air flow sensor). That MAP sensor also reports barometric pressure before the engine starts running and vacuum is developed. So as far as the computer is concerned, nothing has changed, and he is going to hold the injectors open the amount of time he was programmed to.

What he doesn't know is instead of a little elf tipping a thimble to pour in fuel for 34 milliseconds, there's some Goliath in there tipping a five-gallon pail for 34 milliseconds.

The next step in this process is when the computer modifies short-term fuel trim numbers. He doesn't know why there's WAY too much unburned fuel in the exhaust, He just knows there's too much. (More on that later). To correct that he shortens the pulse-width time more and more in an attempt to bring the mixture back where it should be. In a perfect world he might have to modify the injector pulse width to, ... Lets say 10 milliseconds to achieve the right mixture. There's two problems with this wishful thinking. The first is most computers can only modify the fuel metering by around plus or minus ten percent. That's all the capability they design in because that's way more than enough range to take care of any normal conditions or variables the computer might encounter.

The second problem is to open an injector, an electromagnetic coil has to be pulsed with enough current to make a strong enough magnetic field to tug a heavy pintle valve off its seat to open it. There's either around 50 psi fuel pressure, or closer to 12 psi if you have throttle body injection, pushing that valve closed. There's a whole pile of intake manifold vacuum pulling that valve closed. It takes a really strong magnetic field to overcome those forces and get hat valve opened up, and then, it takes some time to get that valve moved to the fully-open position. There's a lot of mass, relatively speaking, to that valve. The current waveform can be graphed with a diagnostic scope, and you can actually see from that exactly where the valve starts to open and where it is fully-open. About the time it gets fully-open, that 34 milliseconds is up and it's time to close again. There is no physical way the computer can open that injector for just ten milliseconds to reach the perfect mixture. This is like adjusting a kitchen sink faucet to adjust the flow of water through the Hoover Dam! There just isn't enough control.

So you have a need to shorten the injector pulse-width, (time) to a value that's too short for the injectors' capabilities, and you need a percentage of correction that is way beyond what the computer can do. To add even more to this sad story, if the computer WAS able to achieve its goal of shortening the injectors' on-time to get the right mixture, it would load up and run miserably until that happened, THEN, every time you disconnect the battery or let it run dead, all those modified look-up tables get erased and you have to start the relearning process all over. This happens on every car after every time the battery is disconnected, but because the variables have such a small range, the engine runs well enough that you never even notice there is the need for that relearning process or that it's taking place while you're driving.

As for the stalling, there's two things to be aware of. Add to the list of clues and observations the horrendous smoke, it is surely the result of drowning the engine with way too much fuel. The unneeded extra fuel doesn't have any oxygen to go with it so it just sails out the tail pipe, wasted. (As I said previously, more on that later).

The second thing applies to your car if you had to disconnect the battery, and it applies to all Chrysler products. You normally won't notice those fuel trim look-up tables being rebuilt as you drive, but the computer also lost the "minimum throttle" value it had stored. When that is lost from memory, the computer will not know when it has to be in control of idle speed. The typical symptoms are the engine may not start unless you hold the accelerator pedal down 1/4", you won't get the nice "idle flare-up" to 1500 rpm at start-up, and it will tend to stall stall stop signs. To meet the conditions to trigger the relearn, drive at highway speed with the engine warmed up, then cost for at least seven seconds without touching the pedals. The computer needs to see very high manifold vacuum for more than just a fraction of a second to know you're coasting, and when it sees a voltage from the throttle position sensor that is staying nice and steady during that time, it knows your foot is not holding the accelerator pedal down. Most mechanics perform a short test-drive to do this relearn if they had the battery disconnected rather than try to explain that to their customers and make them try to do it.

Okay, as promised, I have one more comment so I don't add to the confusion. The oxygen sensors in the exhaust system can not measure unburned fuel. That's why they're not called "fuel" sensors. They can only measure unburned oxygen. During normal operation, the computer switches the mixture between too rich and too lean about two times per second with the average being perfect. When it's lean, unburned oxygen gets stored in the catalytic converter. When it goes too rich, the unburned fuel mixes with that oxygen and is burned. The computer can't look at the absence of oxygen to detect a rich condition, but it can look at how long it takes to switch back to too lean once it commands the injectors to do that. That's how it detects a rich condition. I simply wanted to add that to avoid any confusion in that part of my wondrous story.

Bottom line, ... Put the right injectors back in. All larger ones do is give you the capacity to inject more fuel, but you can't make use of it. If you're looking for more power or higher top speed, you're still in the wrong area. First of all, you need a specific volume of fuel for each intake stroke. That doesn't change when the engine goes faster. If the engine runs at a higher rpm, the injectors just fire more times per minute, but for the same length of time each firing. If you need more power, the computer has plenty of room to increase the amount of time it holds the injectors open during each intake stroke. You may see 38 milliseconds on the scanner instead of 34 milliseconds.

The injectors are not the weak link in the chain. You're limited by the volume of the cylinders, the diameter and lift, (opening height) of the valves, and how long the lobes on the camshaft hold the valves open.
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Saturday, June 6th, 2015 AT 11:36 PM

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