Idling too high

nissan
sentra

#1

I have a 2008 Nissan Sentra SE-R. I recently had the battery and PCM replaced, reason being whoever had the car before me put the wrong battery in and it eventually fried the PCM over time. After I got the car back from the mechanic I noticed it was idiling too high.

I plan on calling them back tomorrow, but has anyone had experience with this?


#2

Hard to believe a battery could be installed that was so wrong it fried the PCM. Details?

Car batteries differ in physical size and internal design, but they are almost all 12 volts. The other differences are not sufficient to harm a PCM. I think someone is making excuses.


#3

I wanna know who told the OP that a 12 volt starting battery would fry the computer?

Tester


#4

You have to perform a relearn procedure for the throttle on Nissan’s of this era when the PCM is replaced.


#5

This is what’s written on the receipt, “battery was the wrong one and had an internal short that took out the PCM”. He also told me when he checked the battery the volts kept dropping and rising.

Let me add that I know next to nothing about cars. I originally took it in because it wouldn’t start, the engine wasn’t turning over at all. I jumped it and still nothing, the lights came on but that was it.


#6

Find a new mechanic!

Tester


#7

Do you think it’s worth it to take it back in and see if there’s anything they can do? The PCM they put in was used if that makes a difference. I’ve already spent a lot of money on the repairs/towing.


#8

Well, the battery could be the wrong one. That wouldn’t fry a PCM.

It could have an internal short. A fuse or fusible link should keep that from harming anything, no???


#9

I guess it’s possible a faulty battery might not provide enough capacitance and allow voltage spikes from the alternator to reach the PCM. The alternator isn’t able to keep the voltage regulated properly without the capacitance provided by the battery. But even a failing battery usually provides plenty of capacitance so that doesn’t happen.

High idle rpms can be a tough problem to diagnose. Idle air control, intake manifold air leaks, brake vacuum booster air leaks, etc. I had that problem on my Corolla, turned out to be the idle air control gadget had malfunctioned. Before venturing too far down that path however, make sure the re-learn procedure mentioned above has been done and enough time & miles has passed that it has a chance to take effect. A new computer mated with a new car works fine b/c everything is new; but as the car ages stuff inside the engine starts to clog up a little, and the computer has to adjust the engine parameters to keep it running & idling well. So it still works fine with an old computer matched to an old car. When the computer is replaced then you got an old engine/new computer situation, and it takes some major adjusting for them to get used to each other.


#10

I figured the repairs were somehow related to the high idling because I didn’t have a problem with it until I picked it up from the shop. Hopefully this is an easy fix.

I’ll definitely follow the instructions in the video before I do anything else. Thank you for your input!


#11

That is total bull.


#12

I don’t know about auto electronics, but all the electronics I use (and build) have voltage regulators that keep the voltage applied to electronic parts correct, which means not-too-high and not-too-low.

One of those 216-volt batteries the electric cars use?

My pickup idled high no matter how I set the idle control screws until I put in a new accelerator pump. I suspect OP’s car doesn’t have a carburetor though. Are there no mechanical adjustments to set the idle of fuel injectors?


#13

It is not unheard-of for a PCM to become damaged while operating the vehicle with a failed battery or from an improper jump start.


#14

On my early 90’s electronic fuel injected Corolla that’s still possible for idle rpm, but unlikely on a 2008 model. By 2008 the computers took complete control of idle rpm on most vehicles. Even if there were an air bypass screw to adjust the idle rpm, it probably wouldn’t be effective in OP’s case. It only works correctly if the engine is otherwise properly operating. There appears to be something amiss in the operation of the OP’s engine management system.


#15

As usual a lot depends on the system design but imagine what would happen if the battery had an intermittent short in one or two cells. What would you think the alternator would be doing in that case? And the voltage at the battery and control electronics? They can handle an occasional transient but repeated exposure could kill it.


#16

This is just bad design. I’d never design a circuit with a semiconductor without voltage regulation and over-voltage protection. It’s easy and cheap. It’s in every device I’ve taken apart except those that run on little batteries.


#17

There is protection built in but there will be a failure on rare occasion.
When jump starting a vehicle with a failed battery if the charging system voltage the PCM will shut down, that is why we sometimes have to keep the jumper box connected while moving the vehicle from the parking lot to the shop.

People warn against disconnecting the battery with the engine running, operating with a battery that has no internal resistance is the same thing except now it is “bull” and he needs a new mechanic.