Backfire under the hood

Hi,

Often times a backfire is caused by either a lean fuel mixture or a rich mixture.

First, take a look through this link and let me know if it helps in any way:

https://www.2carpros.com/articles/engine-backfires-while-running

Also, if it is loud enough to record and upload for me to hear, that may help me. Keep in mind, I'm not sure what procedure was used to service it. However, many of these shops feed a cleaner through a vacuum hose. So, you may want to check for any hoses that were left disconnected. Here is a link you may find helpful:

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

One more thing. If you have a scan tool that has live data, let me know what the short-term fuel trims are. That will let me know if it is a rich or lean mixture.

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

Take care,

Joe

Hi,

I do hear it. Were you able to test for a vacuum leak? Also, does it only do it that one time and not again after starting?

Let me know.

Joe

Hi,

In a perfect world, you want the trims to be 0, but that rarely happens. When it is running, are they changing between negative and positive numbers?

Also, is pic 1 indicating the oxygen sensors' voltages?

Let me know.

Joe

Hi,

When first started, the engine runs in a closed loop. Basically, it runs at a preset from the manufacturer. Once temperature is reached, then it goes into an open loop. At that point, the computer determines fuel trims based on readings from the oxygen sensors.

Normal conditions for trims are between -10% and +10%. The negative number indicates the computer is trying to lean the mixture and the + indicates the opposite. Being between 1 and 3 is actually good, but on most vehicles, it will go into the negatives and then back.

Since you never see negatives, the comp is trying to add fuel. Are you certain there are no vacuum leaks? Is there an exhaust leak before the catalytic converter?

I have a feeling something was left disconnected causing a vacuum leak.

Let me know.

Joe