Engine misses while idling at a stop

PILOTBILL

MEMBER
2000 MERCURY VILLAGER
3.3L
V6
2WD
AUTOMATIC
80,432 MILES

Hello, I have question for you all. I have the van listed above with A/C. I have been trying for 2 years to find out why the engine misses while sitting at a stop sign or stop light. Other than the missing, the van run real fine going through towns (except for the missing sitting still) and on the highways, plus the van does have a lot of power. Over the last 2 years, I have checked, cleaned and/or replace the distributor cap, spark plug wires, spark plugs, throttle body. Even when the engine is missing, there is no check engine light on. The only time the light comes on, is only when I first start the engine, but then it goes right off. Do you all have any ideas what I should check or do? I am at a lost. Help please! Thank you all. Bill Hewitt, Comfort, Texas

1 Reply

JACOBANDNICKOLAS

MECHANIC
109,885 POSTS

Hi Bill,

If you feel a misfire at idle, it could be a few things. First, the components you mentioned above can cause it. Also, engine vacuum leaks, fuel pressure, a faulty oxygen sensor, and so on.

First, do you have a live data scan tool? If you do, I need you to check two things. First, let me know what the short-term fuel trims are, and second, let me know if the engine coolant temperature sensor is sending an accurate signal. For example, if the engine is at operating temperature, does the sensor indicate a temperature of approx 195°F?

If you don't have the scanner, we will need to check the fuel pressure to confirm it isn't too high or low, along with checking for vacuum leaks. Here are two links that explain how it's done:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Please keep in mind that the A/C places a heavy load on the engine, so pay attention to the engine's RPMs. Make sure they aren't dropping too low. That can cause a vibration that will resemble a miss. If you notice the RPMs are getting low, we will also have to check the idle air control valve. The powertrain control module should signal the IAC to increase idle speed to compensate for the added load.

Let me know what you find or if you have other questions.

Take care,

Joe