Brief RPM dive when stopped.
 

I'm having a minor issue on my car where my RPM dips down a little too much after I bring my car to a stop. For example, say I'm driving my car normal and come to a stop sign my engine drops RPM's a little too much and I notice that my charging system takes a dip to about 12v too (because the engine rpm drops). This only happens for about 2 sec and then picks back up to normal idle and then the charging system goes back to 14.5 like normal.

I'm concerned about these drops on the system because I don't want to strain the charging system. My car has z1 urethane mounts so my car shudders when the RPM drops off. I understand that the mounts are stiffer so the car will shudder at lower RPM. I don't believe that my car did it last year after I had the mounts installed but I'm not sure. Maybe I'm just focusing on it more because of the engine mounts?

I've had my car back on the road for about 3 weeks now. Previously my car has been sitting outside for the winter. My first tank of gas I added STP Complete Fuel System cleaner. My car does not dog out at any RPM and feels right other than the quick dip in RPM at a stop.

I was thinking about cleaning the throttle bodies on my car (which has 55k miles) but figured I would stop here and ask what others thought first.

Thanks

That's interesting. Has the car been tuned ever? Has the car's battery been disconnected recently? Only 2 things that come to mind.

might be the ECU relearning stuff if it sat for awhile.

i know sometimes when i come to a stop and not in the right gear or brake hard in a high gear (like when dbags brake check me in traffic) i'll see a little dip in my RPMs but it jumps right back up for me. never noticed a charge system difference yet either.

i actually i'm a little worried that something in my tune changed over the winter from sitting so long.

At 55k miles, it would probably be a good idea to go ahead and clean the throttle bodies. It's not all that easy to remove the TBs but it's not all that difficult either. Check some of the DIYs on this site and see if you feel comfortable doing it. While you're in there, I'd go ahead and clean the MAFs (super-easy job; all you need is a big Phillips screwdriver and some MAF cleaner).

But that's just a shotgun "fix" - do some data monitoring/logging to help pinpoint the problem.

Nope, the car has not been tuned. The battery was dead as a door nail so I had take the battery out and charge it for 24hrs on a slow charge. The car starts fine but I'll go test the battery.

You think it would take 3 weeks? I thought it would only take a couple days at most.

depends on the milage. if you're only putting 5-10 miles a day on it then yea it could take longer.

take a normal car when they clear a check engine light or whatever. it takes about 75-100 miles to fully fix itself.

I'm going to be tearing into the car pretty soon. I'll hit the TB's and MAFs at the same time. I figured 55k on the ol' lady it's about time I clean them too but I wasn't sure if it was possible if the TB's (or MAF for that matter) could cause a dip in RPM.

I'll snag a video later so you guys can see what I'm talking about.

Eh, I already been through 3 tanks of gas.

well then lol should be reset or learned everything by now

throttle bodies.

that's it.

My guess would be battery.

My car does the same thing. Z1 mounts, but 20% UD pulley (so I don't look at the volts)

For me, it dips and comes back, back and forth. F-ing annoying... I have an auto as well, and it only does it in Drive. In park, I just feel mild vibrations

Just found this:

"Nissan throttle bodies are notorious for getting dirty pretty quickly. If your car has issues with the RPM dropping at idle, it is probably a dirty TB."

DIY Clean your Throttle Bodies - MyG37

The issue sounds like it fits the bill.

Good to know. I have a 7AT too and will be putting NST Pulleys on... if they ever ship.

I took my car up to Advanced Auto and had them test the battery. The battery has 313 cold cranking amps per what his machine said. I did a quick google search and the battery should be at 650.

Earlier today the temp was about 32 -35 degrees. It's now 65 degrees and the car is no longer dropping off time to time.

Maybe it's time for a new lightweight battery?

.... just remembered something.... I should check the water level in the battery.... bbiab

So.............. The water level was really low. I'll let that marinate for a day or so and check back.

I'll also clean the TB's next week.

My battery is brand new, and worse comes to worse I have an even newer OEM battery
No tune either.


I'll try the TB thing this weekend. Can I drench the TBs in cleaner?

I'll check with my brother to see how the TB's should be cleaned and let you know. My bro is a master mechanic with 20+ years exp.

I know the ideal "book" way, but if I cna disassemble them enough and just sonicate them in something. I'd rather do that.


Don't know how much I need to take off though.

i just used a micro fiber and some general cleaning solution. you'll see how dirty they are. just need to remove the intake tubing, or get it out of the way-pretty easy diy.



did this a ways back. it was always in drive at idle accompanied by a voltage drop. problem solved.

I just got back from my brother's place and talked to him about it. He said it's probably dirty TB's. He pretty much repeated verbatim what Critical said.

1.)Take the TB's off
2.) Spray a micro fiber rag with TB cleaner
3.) Wipe it clean.

It's still gonna vibrate like a mother with an auto though... ;-(

I don't give two sh!ts about the vibrations. :) I just don't want my charging system to be strained.

pull them off and clean them with maf cleaner/electrical cleaner

The spraying power from the can should be enough to blast any grim off with ease. no need to scrub or marinate anything:rofl2:

It's quite possibly the battery.

Failing that search for an intermittent vacuum leak -- the unmetered air will cause you to skew lean at idle; ECU will compensate with high positive fuel trim, run rich, and dip a bit as it attempts to work around the leak.

So much of the throttle on this car is done with VVEL, I doubt cleaning the TB's will fix. Probably not it.

I have that same problem then it started to turn on the oil light at the same time and then about 2 sec goes back to normal yesterday i bought a battery thinking it was it and nope it continue to do it. But the oil light did it when i changed my oil and theres no leaks i checked the dip and it was fine. I am loosing my mind help people!!!

Your ECU might be losing its mind... I'd check to see if have a bad oil pressure sensor if you aren't burning oil or actually losing pressure somewhere in the system... That might require a trip to the stealership and use of CONSULT III.

On that note: Do you have an oil cooler, either OEM or aftermarket?

While Jordo!'s bad sensor diagnosis has a good chance of being correct, I'd try powering down the ECM first. It's easy to do - remove battery cable and step on brake pedal for a few seconds, then reconnect battery. The car may run rough for a bit while the ECM relearns the engine and you'll have to reset the windows (procedure has been given many times; use search). You may also lose the radio presets.

The battery was the issue in my case... if anyone was wondering.

Thanks for the update.

Which battery did you end up purchasing?

Figured -- glad its fixed :tup:

Actually, my idle was intermittently doing this tonight out of the blue...

I'm probably about due for a battery too... I'm due for an oil change in another few weeks or so. I'll check the battery then.

I ended up putting a light weight battery in my car:
http://shop.advanceautoparts.com/p/a...000#fragment-3

I originally thought about putting in a lithium ion battery but it was cost prohibitive. Here is the thread for that:
http://www.the370z.com/engine-drivet...n-battery.html

Had the same RPM dip together with slight batter voltage drop when rolling to a stop at neutral (idle drops from 650ish to almost 500 RPM) (mine is 6MT). After a month's trail and error I finally fixed it. Since my cause is different than the original post's, so i though of sharing it.
The RPM drop happened about 3 out of 10 times back when my car was completely stock, but I didn't bother to fix it. Then the problem worsen after I replaced the stock intake to Stillen G3 CAI. It happens almost 90% of the time and I finally decided not to be lazy and treat her right. Thanks to Z family's posts I listed the potential causes and rearranged them in the order of least to most workload:
1) Gas: I used to getting gas from Costco. So I switched to Chevron. Nothing changed after 2 full tanks.
2) MAF sensor: cleaned it throughly and still nothing changed.
3) ECU reset: I thought the RPM drop might be caused by not resetting ECU properly during my install. But i don't want to go thought the hassle to reset power window, radio, service reminder... list goes on. To avoid all that, this is what I did. Most people have seen the crazy "cheat code" and here is one example: http://www.the370z.com/nissan-370z-o...ecu-reset.html
(post #6 by cdoxp800). Tried it but still nothing changed.
4) Throttle body: I was trying my best to avoid this since my CAI requires me to take the front bumper off (yea, once again being lazy). But with options running low I ended up cleaning the throttle body. I used this as my guide: DIY Clean your Throttle Bodies - MyG37
I was surprised how dirty mine are for the millage I have (45K miles). Got lucky and problem fixed!!! Now is day 3 since the fix and haven't had RPM dip with or w/o AC on.
5) O2 sensor or MAP sensor: luckily I got my problem fixed by #4. Otherwise I would of gone through the hassle to put the stock on and take it to the dealer hoping whatever they can find will be covered by warranty.

I've got the g37s, and I was having a similar problem, battery just changed. Try using seafoam for your fuel injectors and vacuum system, as well as cleaning out your upper intake manifold.

My car runs like new again.