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Power increase with RPM....

T

Truckedup

NAXJA Forum User
Location
Western NY state
All my XJ's do this,the 96 I used to have and 97 and two 99's I own now.All are 4.0 5 speeds.
Let's say you are in second gear at 2500 rpm then floor the gas pedal.The engine accelerates good then around 3500 rpm there's an increase in power that's noticeable.It's less noticeable in higher gears but still there.
I've felt this effect on some other vehicles like 4.3 Chevys.

Is this engine management programming to deal with emissions?
 
Sounds like you are going from closed loop to open loop, which is normal under acceleration.

The PCM sees the TPS position and MAP sensor signal and will change the injector pulse width (enrichen mixture) in response to the increased throttle opening.

Sounds like a normal response to me.
 
I guess I don't beat on my 4.0 enough, I typically shift right around 3000-3200 :roll:
 
When N/A, at WOT and 3500 rpm is when it does go into open loop.
 
If you add in a switch and give the MAP sensor signal wire about ?4.65-4.8v, which is like 0 vacuum, would need an adjustable voltage output to see what is the minimum to do it b/c if too much voltage it will throw a CEL.

Best is to turn the key on, engine off and backstab the MAP plug and see what is your 0 vacuum signal voltage is and then thru your switch feed it about .1-.2v more than your measured voltage and that should do it. Any more and it will throw a CEL.

Use a ELM327 bluetooth OBD device,$12-15, and Torque PRO,$5, android to see closed/open loop status.

You can always feed it boost.:laugh3:
 
Boost :firedevil

The instant you open the throttle fully the intake vacuum goes to near zero.So since the increase in power happens after the rpm gets over 3500 then the vehicle computer is delaying open loop.So I believe a switch might not help ??
 
The later models hit peak torque and horsepower later in the RPM band. Some of it is tuning or motor management or even improved fuel and air delivery, most of it is simple physics, the higher the RPM the more horsepower. Most if the horsepower gains in the HO are because they moved peak torque higher in the RPM band.
 
'Near zero' but not 0. That is where the problem is. By feeding the ecu a voltage signal from the MAP sensor, like equivalent to 0.5-0.0"vac-1/4pound boost, will switch the ecu to open loop instantly no matter what rpm, the ecu doesn't wait until 3500rpm.

At 'near zero' which is really like 1-1.5+"vacuum still(thank you too small TB) the ecu waits until 3500rpm before it goes into open loop at WOT.

Being able to monitor open/closed loop status is great and driving around and seeing when it actually goes into open loop. Very rarely actually.
0.5-0.0"vacuum goes into open loop, the problem is a N/A 4.0 never sees 0.5-0.0"vacuum.

Monitor your MAP sensor voltage if you can. See it's range under varied driving conditions.
 
Marty,Ok, I understand how the engine management works but I'm little stupid on the latest stuff. I looked up the two devices you mentioned but the Android? I'm thinking cell phone but my cell is a phone only :D I know people with I Phones but maybe not an Android.. I assume the Apple stuff won't work?
 
Sorry, non apple smartphone is 'android'.
Torque doesn't do app for Iphone.
 
IDK about PC.
I've also used ultra-gauge.com .
 
Just check first, I found that not all OBD2 scan tools show 'Open/Closed' status. To me that is one of the most important items to see.
 
I know nothing about the Jeep/Chrysler ECUs, but who care's if it's in open/closed loop?

As soon as you push the throttle wide open, vacuum (ideally) goes to nothing (atmospheric pressure) and the commanded fuel at that load/rpm is delivered to the engine...is it not?

How much authority is the O2 given in closed loop to alter the fuel injector pulse from the standard? What triggers the ECU to switch to it's open loop static fuel table? I can't imagine the ECU is triggered to open loop at a certain RPM (as stated above), rather it's at a certain load (kpa (or whatever units they use) ~ 90 - 100?).

Like I said, I don't know anything about the ECU, since I come from the world of Megasquirt, but I have a feeling it is just the nature of the engine's operation rather than fuel delivery (i.e. the grind of the cam, intake, etc.)
 
A simple way of seeing if the PCM dropped out of Closed Loop is to hook up a scanner that reads live data and watch the fuel system activity when you punch it. A scanner worth its salt should show Open/Closed Loop.

10 to 1 says it will drop out of Closed Loop as soon as the gas pedal hits the firewall. Of course, what you will see is the TCM kick the tranny down so the RPM will increase until it shifts.

Not rocket science.
 
The science is that even at WOT, pedal to the metal, it will not drop out of closed loop to open loop until around 3500rpm. I've tested it many times.

Yes in closed loop, the ecu with feedback from the O2 sensor will try and keep the AFR at or near 14.7 or what it's target is, within it's range of adjustments.
Only in open loop does the ecu ignore the O2 sensor and only follows the fuel and timing tables regardless of the final AFR. Mind you the programming of the fuel tables is set as to know what target AFR is desired.

Only in open loop can I add fuel, as much as I want, to achieve nearly any AFR, like from 11.5 to 14.5 and anywhere in between without the ecu trying to pull fuel out to get back to it's target closed loop AFR.
 
This is a very interesting topic.I still think the delay in fuel curve enrichment is for emission standards. I'm pretty sure emissions are not tested at wide open throttle.
As I mentioned earlier I noticed this same effect in 4.3 Chevy stick shift trucks so I don't think it's the engine coming up on the cam so to speak.On a 88 Chevy with TBI I disconnected the o2 sensor and the engine seemed more responsive.
Do the auto tranny 4.0's have the same "surge" as the 5 speeds?
 
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