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Rough idle

Tiny
JONATHAN WHITE
  • MEMBER
  • 2004 NISSAN ALTIMA
  • 2.5L
  • 4 CYL
  • AUTOMATIC
  • 200,000 MILES
When it Parked it has a rough idle and if you run the heater or A/C it runs rough but if you out it in neutral it will clear up.
Saturday, January 22nd, 2022 AT 2:15 PM

7 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,747 POSTS
Hi,

I'm not 100% sure what you indicated. Does it have the rough idle in park with nothing else turned on, or do you have to turn the heater or A/C on?

If it is only with the AC/Defroster on, it is likely related to the idle speed control valve which should increase engine RPMs when the engine is under a load.

If it happens even when the things are off, then oftentimes it is the result of an engine vacuum leak.

Tell me, is the check engine light staying on when the engine is running?

Here is a link that explains how to test for an engine vacuum leak. It is a good idea to start with the basics. However, let me know as much as you can. Additionally, you mentioned the heater being on causes it. Is it the heater or when the defroster is on? I ask because the AC compressor is actuated with the defroster. Also, let me know if you feel it when the heater/ac is on high and low both.

Here is the link I mentioned.

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

Let me know.

Take care,

Joe
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Saturday, January 22nd, 2022 AT 4:58 PM
Tiny
JONATHAN WHITE
  • MEMBER
  • 5 POSTS
Yes, the check engine light is on. The rough idle does it in park while sitting still even with no load but gets worse if you run anything; radio A/C Etc. When you start moving it'll clear up but while you're idling you can put it in neutral and it seems to do better, I'm sorry I guess I should have mentioned it had a complete motor replacement the top end of the motor was rebuilt. Thank you for any help you can give me. I have recently found three bad motor mounts. And MPG is not too good.
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Saturday, January 22nd, 2022 AT 9:05 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,747 POSTS
Hi,

Okay, the first thing to do is check for vacuum leaks. Since there was work done, it could simply be a broken or unplugged vacuum hose.

If that doesn't help, I need you to scan the computer for diagnostic trouble codes. I realize most people don't own a scanner, but oftentimes, a parts store will do it free of charge. This is important since the check engine light is on.

At this point, here is my theory. First, if there is a vacuum leak, it will cause the oxygen sensors to think the fuel mixture is too lean (not enough fuel). They signal the powertrain control module (PCM) indicating a lean mixture and the PCM tries to adjust for the lean mixture by adding fuel to the engine. So, we end up with lower MPG and a rough idle. Oftentimes, there are even more issues. Pay attention to smells from the exhaust. A rich fuel mixture produces a very strong exhaust smell.

On the other hand, if there is no vacuum leak, the fuel pressure could simply be too high causing the rich mixture. Again, that would cause the MPG issues and poor idle. However, I have a feeling my first theory is correct. If you have the computer scanned, that will help me make that determination.

If by chance you do have a scan tool that can read live data information, let me know what the short-term fuel trims (STFT) are at operating temperature.

Let me know what you find. I will watch for your reply.

Joe
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Saturday, January 22nd, 2022 AT 10:12 PM
Tiny
JONATHAN WHITE
  • MEMBER
  • 5 POSTS
I have just noticed oil being blown out the oil cap, so I took it off while it was running, and it has a lot of air blowing out. Is there a cause for that or is that post to be normal? I've never run into that before. I found no vacuum leaks. I'm going to have code scanned and will update but will be tomorrow before I can. Thank you for help guiding me through this. It has been colder out than normal and it's not doing it so bad until the car has been driven for a while but in warmer weather it does it as soon as you start the car.
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Sunday, January 23rd, 2022 AT 9:20 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,747 POSTS
Hi,

That shouldn't be happening. It is normal to have some air come out with the cap off, but not blow oil out of the cap.

When an engine is running, it produces crankcase pressure. That pressure is released via what is called a PCV valve (positive crankcase ventilation). Basically, engine vacuum draws the air back through the engine for re-burning.

The PCV valve is located on the valve cover. I attached a pic below of it. I need you to first confirm the hose to it isn't cracked or plugged at either end. Next, remove the PCV valve and check to see if it is plugged.

Let me know what you find. Also, you mentioned the top end of the engine was rebuilt. Was this your engine or a different one? Do you know how many miles were on it?

Let me know.

Joe

See pics below.
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Sunday, January 23rd, 2022 AT 1:50 PM
Tiny
JONATHAN WHITE
  • MEMBER
  • 5 POSTS
I checked the PCV hose and valve the hose still look good PCV valve looks to be clean-ish very oily. The installation under hood is soaked in oil of course it all over valve cover plus alternator and fuse box under hood it blows quite bit more while accelerating. It has approximately 175,000 to 200,000 miles I got the car from a good friend of mine he had a second Altima that he switched the whole front clip only thing you didn't change was the motor mounts and all of them are bad I've worked on another of motors but never had one blow the cap off just by loosening it I loosened it to see what it would do it blow the cap off. I'm use to older engines, not really familiar with imports though. Thanks for all the help. I know it's all a process of eliminating I've never run into a problem like this one as I was running it today it it didn't have as rough of an idle it was still doing it just not as bad when I went to park it for the evening it was getting worse with running a load the fan and radio is what was on it seemed to have more power than it has had. I have found no vacuum leaks. I know it has a new idle control sensor and a couple other sensors are nearby. I'm going to ask him tomorrow which one supposed to be, but I don't think anything was ever done with the PCV valve all hoses under the hood was put back new when had motor swap it was done about a year ago last time codes where read it said misfire on number 1 or 2 I think it's said oxygen I'm going firt thing in morning to have them re-read them I will update as soon as it's done again thank you caes it's really got me stumped.
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Sunday, January 23rd, 2022 AT 5:17 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,747 POSTS
Hi,

Make sure the PCV hose isn't plugged. Most times when hoses are replaced, they are the smaller ones and not the PCV. Remove it and try blowing through it. Also, the same with PCV.

What is happening sounds like blow-by. When the compression rings or cylinders wear beyond a certain point, compressed air escapes past them causing excessive crankcase pressure. If the hoses are all clear, we will need to do a compression test.

Let me know what you find.

Joe
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Sunday, January 23rd, 2022 AT 5:51 PM

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