Codes P0300 and B1S2, O2 sensor fluctuating

Tiny
TOTALMC
  • MEMBER
  • 2006 FORD FREESTYLE
  • 6 CYL
  • 150,000 MILES
Working on the vehicle listed above with a V6.

There is a misfire on #2 and #3 (both are bank 1).

Spark plugs were replaced.

Live data numbers look fine to me, except for downstream 02 on bank 1 fluctuating (almost mirroring the upstream sensor).

Both fuel trims look good, from -5 to +8 and the three other O2 sensor numbers are normal.


I can't figure out why I'm getting a misfire code, when those numbers look right, but I am not getting a p0420 for a bad catalytic converter even though that downstream sensor's numbers would suggest it.

Any ideas are greatly appreciated, and thanks in advance.
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Thursday, April 28th, 2022 AT 8:26 AM

12 Replies

Tiny
KEN L
  • ADMIN
When you pull the spark plugs on number 2 and 3 what do they look like? It sounds like you have two injectors that are having a spray pattern problem. Here is how to remove the injectors and how to check the spray pattern to see if this is the issue. I have included diagrams below to show you how to use your car as well. Please upload what the plugs look like so I can see what's going on.

https://www.2carpros.com/articles/how-to-test-a-fuel-injector

Also let's make sure the gaskets for the intake are not leaking, this guide can help:

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

Check out the diagrams (below). Let us know what happens and please upload pictures or videos of the problem.

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Friday, April 29th, 2022 AT 9:36 AM
Tiny
KENNY K
  • EXPERT
A multiple cylinder misfire is nothing more than multiple cylinders that are misfiring but not consistently.

So, you may be getting 2 cylinders that are counting but they are not consistent enough to set a code for those cylinders.

So, we need to focus on the two that are misfiring. The reason you are not seeing the o2 sensor data change is because it is registering misfires due to not seeing the proper crankshaft acceleration each time the spark plug fires.

So, it may be burning the air/fuel, but it is not accelerating properly. This means we could have a compression issue causing the power to escape and not accelerate the crankshaft like it should.

This means we need to do two things. First, we need to check the compression on all the cylinders and find out if we are losing compression on these two.

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

If we are not, then we need to monitor the intake vacuum using a vacuum gauge and find out if we have intake valves hanging open. I attached a chart below that will help with this.

Next, we need to check for an exhaust valve hanging open and we can use an old school trick by holding a dollar bill over the exhaust pipe and it should feel like it is being pushed out. However, if it feels like it is getting pulled back in at all then that is an exhaust valve closing creating suction.

Lastly since these two cylinders are next to each other, we need to ensure we don't have a head gasket failure between them.

https://www.2carpros.com/articles/head-gasket-blown-test

You can also test this by doing a cylinder leak test.

Please run through all this info and let me know what questions you have, and we can go from there.

Thanks
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Friday, April 29th, 2022 AT 9:42 AM
Tiny
TOTALMC
  • MEMBER
Thanks, guys, for all the input

Okay, so, I performed a vacuum test, the numbers were perfect - steady 19 at idle, when you hit the gas fluctuated down and up then back again.

I'm pretty sure this passed test eliminates a lot of the other tests to perform (I was looking through the list of things that the vacuum gauge would show if they were faulty).

Hopefully there's no need for a compression or leak down test now, seeing I passed the vacuum test.

I still have yet to remove the manifold to check the injectors and plugs, I was hoping there was maybe something else I could do first before digging into the engine.

Thanks again for any insight.
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Friday, May 6th, 2022 AT 11:19 AM
Tiny
KEN L
  • ADMIN
Did you ever plug the spark plugs to see what they look like?
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Friday, May 6th, 2022 AT 2:35 PM
Tiny
TOTALMC
  • MEMBER
I did not. The plugs were replaced recently, normally I would just pull one to read the condition. I was just too lazy to remove the intake manifold to get to the rear bank of plugs.

But I guess that's going to have to be my next move.
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Friday, May 6th, 2022 AT 2:52 PM
Tiny
TOTALMC
  • MEMBER
So today I pulled the intake, and the spark plugs look fine.

Performed a compression test and had 175 - 180 across all cylinders but 2 and 3, which still fell within spec at 165 and 175

And as stated before it passed the vacuum test with flying colors.

Cleaned injectors and inside the intake, then reassembled.

The customer drove it around this afternoon and the check engine light has yet to return (when he said it normally would have come back already) - so I'm hoping this solves it.

If not, and the misfires come back, what would be my next move?
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Tuesday, May 10th, 2022 AT 9:55 PM
Tiny
KENNY K
  • EXPERT
So, if you have misfires and the check engine light comes on then we need to rely on o2 sensor data. However, you mentioned that the o2 sensors were reading correctly.

This is what is odd. If you have an actual misfire the o2 sensors should be reflecting this. The short-term fuel trims would be rich or lean depending on what is happening. If they are not, then I suspect you have an o2 sensor issue.

What happens is when the engine is running rich or lean, the PCM adjusts for it, and it bases this on the o2 sensor data. If the o2s are not telling the PCM that it is rich or lean, then it will never adjust, and the engine will just misfire. This is the most common cause of only the P0300 misfire with no other specific cylinder's misfire.

So, I would look at the fuel trims and see if they are correct while it is misfiring and if they are then I would suspect the o2 sensors are the issue. If you are getting misfires on only one bank, swap the upstream side to side and see if the misfire moves.
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Wednesday, May 11th, 2022 AT 6:48 AM
Tiny
TOTALMC
  • MEMBER
Both upstream O2 sensors have recently been replaced and numbers read perfectly, but I did notice some intermittent fluctuations downstream on Bank 1, where my cylinder 2 and 3 misfires reside.

It would basically hold between.7 -.8. But then occasionally mirror the upstream numbers for a moment, then back to holding around.75.

This seems to be the only reading that's out of place.

Now there's an unrelated issue; the oil light illuminates on and off, even though oil is fresh and full.
Is there a way I can check oil pressure?

Thanks again.
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Wednesday, May 11th, 2022 AT 8:28 AM
Tiny
KENNY K
  • EXPERT
This is what makes me think we have a possible sensor issue. The reason is, why does the downstream o2 sensor see a rich condition and hold it at a rich condition voltage but the upstream is not showing a rich condition.

So, if the downstream is showing rich then the PCM could be pulling fuel and it is affecting 2 and 3 to the point of them starting to actually misfire.

If this is the case, then the upstream should be showing lean if they are reading accurately.

If it were me, I would swap the downstream if the upstream are new and have been replaced since this issue started. However, if they are new and this started after then I would not trust the upstream are accurate.

As for the oil pressure issue, we can help with that, but we need to get a new post started for that. People who have that issue will not find the solution under this post, so we need to start a new one on that issue.
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Wednesday, May 11th, 2022 AT 5:37 PM
Tiny
TOTALMC
  • MEMBER
I was not aware the downstream O2 sensor would affect fuel trim, thought it was basically a cat performance monitor. I was under the assumption that the fuel trims are controlled by the data from the upstream O2 sensors.

I'll replace that downstream sensor, and hopefully the issue clears up.

Thanks again for all your input.
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Wednesday, May 11th, 2022 AT 6:02 PM
Tiny
KEN L
  • ADMIN
Please let us know what you find. We are interested to see what it is.
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Thursday, May 12th, 2022 AT 9:33 AM
Tiny
TOTALMC
  • MEMBER
I have not had the chance to get back to that vehicle, but I will post what happened once I do.

Thanks again
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Saturday, May 28th, 2022 AT 9:01 AM

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