running a little rough

[RF600]

I'm hoping someone can help me out. I went to get a smog check and the HC's were good then they went high. Good then high. The guy doing the smog said it would likely fail, so he didn't do the test. There was a light popping out of the exhaust as well. I believe it's unburnt fuel that is causing the issue? I have done recently, plugs, wires, cap, rotor, fuel filter, oil change. 

 

I pulled 1 plug and it looked lean. The electrode was pretty white in color. I have a different MAF and it didn't change how it ran. Yes it's used, but I figured I would try it. It starts up great, no issues there. I have not tried to pull any codes. No check engine light, so I'm expecting no codes to be present.

[Slartibartfast]

So it's going back and forth between good and high? The mixture's actually supposed to bounce back and forth a little when it's in closed loop, that's how it controls the mixture with a narrowband oxygen sensor. Maybe the sensor's on its way out and it's responding very slowly? There is a test mode for the oxygen sensor, might be worth running that to see if the light's flashing like it should. 

 

Some codes don't trip the light, so I'd run those too on the off chance.

[adamzan]

The o2 sensors are so cheap, I'd just replace it if you're unsure of it's age/status.

 

Make sure the timing is correct, as well as egr, etc is functioning correctly. I hated doing this test when they had it here, would always fail and make me waste hundreds of dollars.

[RF600]

When it was warm at the smog place, the hc readings would be really good. Then it would jump way up, then back down. Too much fluctuation for the test. 
 

The egr test, push up on the valve, does what it should, run rough. I have put a new o2 sensor in. When I unplug the maf, it dies. Compression check is good, around 175 for all cylinders. 
 

Everything that I’m checking says things are good. 
 

The spark plugs show lean. The electrodes are grayish white. That would be a lean misfire, right? 
 

I will see if any codes are present. 

[RF600]

No codes are present

[adamzan]

It is possible that the cat is bad. If everything on the fuel management side is working as intended. Or maybe their sniffer is broken.

[Slartibartfast]

Bouncing back and forth sounds more like an EFI issue than a cat issue to me.

 

Lean condition--check the vacuum lines, the intake tube between the MAF and the throttle body, and the MAF itself. If the MAF is covered in schmutz, it'll read less air than is actually going past it.

[RF600]

On 3/16/2026 at 5:15 AM, adamzan said:

It is possible that the cat is bad. If everything on the fuel management side is working as intended. Or maybe their sniffer is broken.

New cats within 5 years, if I remember correctly. 

 

20 hours ago, Slartibartfast said:

Bouncing back and forth sounds more like an EFI issue than a cat issue to me.

 

Lean condition--check the vacuum lines, the intake tube between the MAF and the throttle body, and the MAF itself. If the MAF is covered in schmutz, it'll read less air than is actually going past it.

I have put on a used intake tube. There doesn't seem to be any cracks. MAF has been cleaned. Even tried a different MAF that I have and no change. I have tried the brakeclean method with no results. Possibly the lines under the intake? Kind of hard to spray under the plenum. I haven't pulled the plenum off yet. It's not a problem to do, I just don't want to do it if I don't have to. The fuel pressure is where it's supposed to be with the regulator connected and disconnected. 

 

I haven't pulled the plenum yet. Let's say I pull the plenum and find no bad lines. What would be my next thing to do?

[adamzan]

Have you tried going to a different shop, maybe a 2nd opinion would be good. These trucks are pretty simple as you know. The only other thing I could think of is the BPT valve (part of the egr) or maybe your iacv or aacv is broken some how allowing extra air in. Mine had this problem and it caused a lean condition, but oddly it always passed the emissions test.

[RF600]

32 minutes ago, adamzan said:

Have you tried going to a different shop, maybe a 2nd opinion would be good. These trucks are pretty simple as you know. The only other thing I could think of is the BPT valve (part of the egr) or maybe your iacv or aacv is broken some how allowing extra air in. Mine had this problem and it caused a lean condition, but oddly it always passed the emissions test.

There are 2 people in the county that I reside that do the older vehicles. I suppose at some point I may have to travel to a different county in order to get a smog. The issue of finding a different shop is finding one that will actually help. I am going to pull the plenum off and look at the vacuum lines under it. It will give me a chance to ohm the injectors as well. I can look at the iac valve while I have the plenum off. The iac has been changed before after it was giving me an idle issue. 

Edited March 21 by RF600

[Strato_54]

I still see no mention of checking the timing. If the dizzy is out, it's not gonna run right. Adam was on the right path (ha) when he mentioned it but OP I don't think you've checked that yet.

[Slartibartfast]

Maybe dripping fuel from an injector, or leakage from the pressure regulator into the intake? I'm still trying to think of what would make it oscillate. 

 

Also thinking about it again, I would expect a vac leak to cause NOx issues, not HC.

[RF600]

19 hours ago, Strato_54 said:

I still see no mention of checking the timing. If the dizzy is out, it's not gonna run right. Adam was on the right path (ha) when he mentioned it but OP I don't think you've checked that yet.

If the timing was that far out, I should have starting issues or backfiring. It starts right away, no problem. 

[RF600]

13 hours ago, Slartibartfast said:

Maybe dripping fuel from an injector, or leakage from the pressure regulator into the intake? I'm still trying to think of what would make it oscillate. 

 

Also thinking about it again, I would expect a vac leak to cause NOx issues, not HC.

Because the plugs are pretty white, I’m assuming a lean misfire. Especially with the little pops out the exhaust. 

[RF600]

I pulled the intake, nothing wrong as I suspected. A couple of the vacuum lines were showing signs of getting old, but nothing that was definite. I did replace the lines in question. There was no change to how it runs.

 

The dizzy is tight. I don't have a timing light to verify what it's set at. I wasn't able to turn it by hand. I can buy a cheap one or rent one from the parts store. 

[adamzan]

I don't know if you've already tried this, but there is another diagnostic mode in the ECM that will blink the lights in a certain pattern and will let you know if the mixture is within specification. If you don't have access to the factory service manual, I can probably find it for you.

 

Actually it is contained in this post. https://www.nissanpathfinders.net/forum/topic/21-ecu-self-diagnostics-for-87-95-pathfinder/

[RF600]

14 hours ago, adamzan said:

I don't know if you've already tried this, but there is another diagnostic mode in the ECM that will blink the lights in a certain pattern and will let you know if the mixture is within specification. If you don't have access to the factory service manual, I can probably find it for you.

 

Actually it is contained in this post. https://www.nissanpathfinders.net/forum/topic/21-ecu-self-diagnostics-for-87-95-pathfinder/

I have the manual saved. I'm pretty sure I checked that mode on the ECU. I can always try it again and see what happens.

[RF600]

I may have caught a break. I drove the Pathfinder and was hearing a hissing noise. Pretty high pitched. It was coming from the intake tube. I could squeeze it and it would stop. Squeeze it a different way and it would start again. 

 

I have ordered a new intake tube. Hopefully it is decent quality. It's new but made in China. I am gathering things to make my own smoke machine just in case.

[adamzan]

That could definitely throw things off. These rigs are getting long in the tooth.

[RF600]

The diy smoke machine didn't result in the intake tube showing a leak. The EGR BPV valve did leak. From what I have been able to find, that's normal with the engine off. 

[gamellott]

Stupid question. Did you check to make sure that stupid vacuum line on the back of the intake is still attached, or blocked of accordingly? I can't tell you how many times I've done that and it caused it to run rough.

[RF600]

15 hours ago, gamellott said:

Stupid question. Did you check to make sure that stupid vacuum line on the back of the intake is still attached, or blocked of accordingly? I can't tell you how many times I've done that and it caused it to run rough.

When I removed the intake to look at the injectors and the line underneath, everything appeared to be good. I don't recall specifically looking at that line. The intake tube is whistling off and on. I'm going to address that first, if that doesn't fix it, on to other things.

[adamzan]

The one they are talking about is the one that connects the intake manifold vacuum to the fuel pressure regulator, not very long and easy to miss.

[gamellott]

6 hours ago, adamzan said:

The one they are talking about is the one that connects the intake manifold vacuum to the fuel pressure regulator, not very long and easy to miss.

Yes, that one!!!

[Slartibartfast]

I had a whistle from the intake tube on mine a while back. Drove me nuts. Near as I could tell, it was the sharp edge on the bung that the pipe feeding the idle air control goes into. I "reshaped" it a little with a big drill bit and it's been quiet since. Engine ran the same whether it was whistling or not, though.