Q: It sputters bad when i hit the gas pedal to hard and want to die

asked by on June 05, 2016

When the engine is cold it doesnt run half bad but once it warms up i got to baby the gas pedal or it wants to die

My car has 200000 miles.
My car has an automatic transmission.

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When an engine is cold, it needs to have rich fuel condition to run. Only when the engine has warmed up can it run at full power and efficiency. The thermostat not only keeps your motor from overheating, but it also keeps it running at an optimum temperature. From an engineering perspective, gas motors actually run a few degrees to cold to maximize there fuel mileage. If motors ran at higher temperatures, the metal and gaskets begin to fail much faster. So there is a balance that has to be maintained. Hot enough for greater efficiency and cool enough not to damage gaskets and other materials under the hood.

For a cold motor to run, it must have a rich fuel mixture, or it just won’t run. The choke was the first solution to this problem. If your not aware, the choke was a second throttle plate that restricted the amount of air entering the motor, but the amount of fuel did not change. The ratio between air and fuel has effectively changed. On modern fuel injected engines, the computer manages the motor in what is called open loop. It is using a set program for cold running instead of actively making fuel and air changes to optimize the motors running.

Your car runs well when cold. We can conclude that this is due to the rich fuel condition present with a cold motor. So the question you need to answer is, is there to much fuel when it warms up, or is there to much air?

A couple of possibilities come to mind. First would be the coolant temperature sensor. If it is giving the computer the wrong reading, the computer will think your truck is cold when it is not. This can cause numerous issues. But one thing that makes me doubt this, is the need to baby the gas pedal. Usually when there is plenty of fuel, giving it more throttle will result in strong acceleration.

The next likely possibility is the MAP sensor. This is one of the primary sensors the computer uses to calculate fuel mixture. The PCM (Powertrain Control Module) will not use it when it is cold. It will have a vacuum hose attached to it and an electrical connector with several wires. Make sure the vacuum hose doesn’t have a hole in it, or isn’t plugged. When you disconnect it fro the MAP sensor when it is running, you should feel a vacuum. If not, you know where to look. Check for damaged wiring or connector.

Next, you could have a vacuum leak. This can be in the intake manifold, a vacuum hose, or a the throttle body. There are several ways to check for this. You can use a smoke machine, which is expensive, or you can use some starter fluid, brake cleaner, or carburetor cleaner. Spray it around the anywhere there might be a vacuum leak. If the motor changes RPM when you spray in a specific spot, you are on the right track. Your looking for a vacuum leak. Spray sparingly. If you fog the entire engine compartment, you won’t know if it is just being sucked into the throttle body, or an actually vacuum leak.

A good trick to be sure your not chasing a phantom vacuum leak, is to add propane into the intake while driving. If the motor responds well to the addition of propane while driving, you can be sure you have a lean running condition. Which I believe is the most likely problem here. You will need to use a long vacuum hose with a valve on the propane bottle to add propane while driving. This takes some creativity, but is a very valuable test when diagnosing such a problem.

Good luck. I hope I have given you a diagnostic track to follow that helps you figure it out. However, feel free to contact a certified mechanic if you need help with these checks. They will be able to diagnose why your car is dying firsthand and fix it accordingly.

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