Random misfire

Hi,

A misfire can be caused by several different things. Usually, however, it is something simple. Have you checked for engine vacuum leaks? Are there any codes stored other than the random misfire (P0300)?

If you haven't checked for leaks, take a look through this link. It explains how to do it.

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

Also, if it is a specific cylinder that is creating the misfire, it could be lower on compression. That can cause a random misfire and rough idle.

Let me know if there are other codes or if it is specifically a P0300.

Also, if there are no vacuum leaks and no other codes, I would suggest checking engine compression and fuel pressure. Here are two links that explain how it's done:

https://www.2carpros.com/articles/how-to-test-engine-compression

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

Let me know as much as you can. For example, is there an odd odor from the exhaust, do you ever notice smoke from the exhaust, is the engine using coolant, and so on.

Take care,

Joe

Hi,

So every cylinder is misfiring. It could be fuel pressure; however, the compression test is a good idea as well.

If you have a chance, let me know what you find or if you have other questions. I'll try my best to help.

Take care,

Joe

Hi,

It is centered on cylinder 1 now. The P0171 indicates a rich fuel mixture. Are you certain the fuel pressure isn't high? If I recall, you didn't test fuel pressure yet and a leak down. That is what I recommend at this point.

Here is a link that shows in general how it's done:

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

I attached the directions specific to this vehicle below. They include the manufacturer's fuel pressure specifications.

Do me a favor. If your scan tool can provide live data, let me know what the fuel trims are at operating temperature.

Let me know.

Joe

See pics below.

Hi,

I agree. Keep in mind, an O2 sensor can be an issue as well. I should be able to tell by the fuel trims what is happening. When checking, watch how the O2 sensor voltages are changing and let me know their range.

Take care,

Joe

Every pic shows -100 on the trims. The PCM is trying to eliminate the amount of fuel entering the engine. It's running very rich.

I have a question because I'm not familiar with your scanner. All the O2 voltages are in the 0 range. Normal voltages will be between.1v (lean) and.9v (rich). Do the voltages on the scanner vary or do they stay constant? Was the engine at operating temp?

An O2 sensor's voltage should cycle normally with constant frequency. An O2 sensor stuck at a fixed voltage (0 is what I see) is an indication of an open O2 circuit or lack of O2 sensor ground.

Let me know.

Joe

Hi,

If there is no variation in voltage at an idle, it sounds like we have bad O2 sensors. It should show some variation in voltage at idle.

Take a look through this link and let me know if you are comfortable performing it.

https://www.2carpros.com/articles/how-to-test-an-oxygen-sensor-02-sensor

Next, and the reason I asked about the engine temp when tested, is because the last pic above indicates the engine coolant temp at 78.8° F. If the engine is warmed up and that is what it is signaling, the computer is compensating a rich fuel mixture when the engine is much hotter than it thinks. Thus, the rich fuel mixture.

Let me know.

Joe

Okay, that makes sense. At this point, we need to check the sensors. If they are not changing voltage signals at all, I have a feeling they could be the problem.

Let me know if you are comfortable performing the test.

Take care,

Joe