Hi:

I'm really stumped on this one, I changed my fuel filter yesterday and I'm getting a really high idle, when the car is in Neutral and at normal operating temperature the RPMs are at 1000, but when it's in Drive it's a 700 RPMs. My car didn't do this before, what can be the cause of this problem?

 

Any air which "sneaks" into the engine downstream of the Mass Air Flow Sensor is called False Air. The engine uses it but the Engine Control Module isn't aware of it. False Air causes a high idle.

Perhaps you disconnected a vacuum line in the process of replacing the fuel filter. Perhaps you dislodged the big black flexible air duct which conveys filtered air to the throttle body. These are examples of how you could have unintentionally created a path for False Air.

 

I'm going to check first thing in the morning before I go to work, today the idle was higher all the way up to 1600 RPMs in Neutral, so I am worried about this.

Thanks!!!

 

If the "False air" is not the problem, check the throttle linkage to see if their is constant tension being applied due to kinks or other reasons. The cruise control linkage would be the more likely culprit.

 

I'm going to check right now!!! Thanks guys!!!

 

I'm at work right now, I checked all of the vacuum lines and they are all connected, the throttle linkage is fine.

When I turn the AC on the RPMs do not go down, they just go up a little and then they come back down to the 1000 RPMs.

What I'm thinking is that maybe my fuel filter was very clogged, and that masked the idle problem, or maybe my ECU got used to the low fuel pressure????

If I can't get this fixed by my self before Saturday, I'm going to have to take the car to the shop.

Any more ideas will be greatly appreciated.

Thanks again!

José

 

If your fuel filter was that clogged fuel starvation would have limited your speed to 20 or 25 mph. At idle the engine's fuel needs are minimal compared to normal driving speeds.

There is no fuel pressure sensor, so the Engine Control Module doesn't know the pressure in the fuel rail. It just trusts that the pump can deliver the required 34-43 psi and that the fuel pressure regulator will bleed off any excess.

 

After changing my fuel filter I also experience increased idle RPM when the car is cold. When it warms up the idle falls back to the normal range...

 

I just took a look at my 95 and noticed there are a LOT of electrical connectors that could have been removed to facilitate the R&R of the fuel filter. Trace your steps back and look to see what you probably disconnected to remove the filter. If the idle problem happened AFTER the installation, you more than likely forgot something that is causing this. Allthough you should have a check engine light if a connector is not connected, you never know.

Double and triple check those vacuum lines! Does your cruise control work? Curious that,s all.

 

I'm pretty sure that everything is in it's place, I'm going to go check again, and I'm even going to disconnect and reconnect everything. I don't have a check engine light , I haven't checked the cruise control.

Thanks for your time and patience.

 

but did not have the problems you are having now. But I'm thinking my car always idled around 1000 w/ A/C and 500-700 w/out.

 

Good luck! Don,t live near San Antonio do you?

 

Daniel's got some pretty good answers to your problem. i as well though have an idle that high to begin with. if not that high, it's very close, sometimes a tad higher for a short period of time after starting, warm-up of course. you may not've noticed it before.

Adam

 

Do you mean San Antonio del Mar???

 

After I disconnected and reconnected everything, the idle got better, it went down to 800 RPMs, I'm going to test the car today, just to make sure.

Thanks guys!!! I feel better now!!

 

Cool deal!

San Antonio, Texas, BTW.

 

Nop, San Antonio del Mar, Mexico.

 

also, you can try to reset the ecu... everytime i do anything to the car related to the engine (intake, exhaust, y-pipe, etc... i normally warm up the car... then turn off the car... reset the ecu and let it idle for like 5 minutes at normal operating temperature with the radio, ac, light, etc... off.

that way the car can relearn the idle faster...

-V

 

That's a good idea!! I'm going to try it!

Thanks

 

I am unaware that the Maxima Engine Control Module can "relearn the idle". Can you substantiate this? Please provide pointers to any service manual, magazine article, Web site, etc. I'm always willing to learn.

 

I'll see what I can find. If I remember correctly, there were directions somewhere that also noted that after resetting the ECU, that certain driving characterisitcs had to be met before inspections for the ECU to learn (I think this was actually in the manual of my 1990 Maxima SE, and noted that if you did not want to perform the driving cycle, bring it to a Nissan dealership to do it for you...) I'll check up on that part of it more when I get home.

The Below is from the SE-R page... I'll keep looking for Maxima info... http://www.se-r.net/faq/ecu.html

***** I just installed my JWT ECU and the car is idling high. Is this normal or should I send it back to JWT?

Lee Alvarez (LEEA%EI-JFK@expd.com)
Do not worry, I had the same problem, so I called Jim Wolf & spoke with Clark, and he said that it is fairly common, and that after approx. 50 starts the short term memory in the ECU will purge the "bad" info that it has telling it to idle higher. One day you will start the car and without warning the idle will be back to normal, if my memory is correct, it took about 12 days, until mine kicked back down.*****



-V

 

http://www.nissanuniversity.ca/resou...g/199911-e.pdf

This is for a 1999/2000 Nissan, but it states on page 6/7...

"...This procedure must be performed so the ECM can learn the idle air volume that keeps the engine within a specific range during operation..."

I know this doesn't specifically apply to the 4th gen, but I'll continue looking for the 4th gen info...

-V

 

BTW DBM, these articles that I've posted do not point directly to the fact that the ECU learns anything from the fuel flow/fuel filter as this thread is about. But your question was whether the ECM/ECU/PCM/'BRAIN' on the Maxima was learning. It is also the reason why I seem to recommend resetting that module on many items and after most services or modifications have been added.

Also, keep in mind that there is a battery back up and that unplugging the battery on the Maxima doesn't reset the computer. You must rest it using the proper procedure. In some manuals, it states that you must turn the car to the ignition position 20 times, in some it states 50. I just turn the screw on the ECU.

Here's one more article I've found on how the ECU learns the voltage value of the TPS, pages 3-6 at:
http://www.nissanuniversity.ca/resou...g/199902-e.pdf

"...the minimum tps value is stored in the ecm... it can be adapted to changes in engine operating condition... it is a learned value in the ecm... if the tps postision is changed, idle control and timing can be affected util ecm identifies the tps minimum voltage... and has recalculated the soft idle switch values...ecm must recalculate soft idle whenever fail safe mode is entered... (component failure) such as MAFS or TPS becoming disconnected... equally important to erase the ECM TPS minimum value to allow the ECM to learn the new positision quickly..."

So it appears that the ECU does 'learn'.

I'm only posting these links because I think you'd be interested in reading these, not to question you Dan!!! I know I found much good information at these links!

So next time I need help, be sure to reply to my questions if you know the answer!!!

-V

 

The Engine Control Module certainly does learn and continually adjusts the learned parameters Long Term Fuel Trim and Short Term Fuel Trim. I was unaware of any learning which pertains to idle behavior. Thanks for the links!

 

I reseted the ECU yesterday, and the idle does go down now, but some times when I come to a stop I can feel the car idle high and then stop doing it. When the car feels like this and I pop the tranny into neutral the idle goes up to about 1200 RPMs and then after two seconds goes down to about 800 RPMs.

I checked the ECU codes before resetting it and it gave me 0505 code, the all clear code.

What do you guys think???

Thanks!!!!!