Just a few 4.0 questions

[comanche tom]

Ok, so why does the 4.0 lean out at wide open throttle? and also why, when its warmed up does it not pickup as well as it does when its cold. I have a 91 and it is the high output, and also does not have an electric fan. I don't know if those things have anything to do with that. I'm somewhat aware that these are known of problems but I would like to fix them, so any help would be good. thanks...

[Eagle]

(a) What makes you think it leans out at wide open throttle?

 

(B) It may have more acceleration when cold because before the engine reaches operating temperature the ECU runs in "open loop mode," meaning it doesn't read the oxygen sensor to meter the fuel, it uses a pre-programmed fuel map that's coded into the ECU and basically runs extra rich for warm-up. (Much as cars with carburetors used to have to run with a choke partially on until they warmed up.) The performance should not degrade when it warms up and switches into "closed loop mode," so perhaps your oxygen sensor is in need of replacement.

 

Which could also affect (a).

[comanche tom]

Hey thats some good info, thanks. I think that it leans because whenever the throttle is wide it almost refuses to accelerate(opposite of what I expect to happen). There is a hesitation then it just feels as if there isnt enough fuel being supplied to match the ammount of air. Also partially because Ive read that about the 4.0 in an article comparing it to the v6 that comes in the new JK. The writeup mentioned that that was one of the downfalls of the mighty 4.0. don't get me wrong, I wouldnt have any other engine over the 4.0. And I think I will replace my O2 sensor.

[Eagle]

Hey thats some good info, thanks. I think that it leans because whenever the throttle is wide it almost refuses to accelerate(opposite of what I expect to happen). There is a hesitation then it just feels as if there isnt enough fuel being supplied to match the ammount of air. Also partially because Ive read that about the 4.0 in an article comparing it to the v6 that comes in the new JK. The writeup mentioned that that was one of the downfalls of the mighty 4.0. don't get me wrong, I wouldnt have any other engine over the 4.0. And I think I will replace my O2 sensor.

Lack of acceleration can also be caused by a clogged catalytic converter.

 

The V6 that comes in the new Jeeps is a passenger car engine that doesn't develope torque, it developes horsepower -- at high RPMs. On the street it's a nice engine, but it is not an engine that was designed to do what an off-road vehicle is intended to do. IMHO you should ignore anything you see in an article that suggests the new Jeep engine is an any way better than the 4.0L engine, because that tells me either (a) the writer doesn't know anything about off-road vehicles, (B) the writer is a shill for DaimlerChrylser ... or both.

 

I've only been driving a 4.0L Cherokee for 19 years and 260,000 miles (that's just on the original XJ ... I also own several other XJs and enough MJs that I've lost count for the moment). Your post is the FIRST time I have ever heard anyone claim that the 4.0L has a "problem" with not getting enough fuel at high RPM.

[aemsee]

Could also be as simple as a restricted fuel filter. Have you ever changed yours?

[512tr]

I don’t think it will help to replace the O2 sensor.

 

The O2 sensor have one mission: Send signals to the PCM to maintain the AFR to 14,7:1, but under certain conditions this AFR is too lean.

 

When the engine is cold, when acceleration/deceleration and at WOT (ca. +60-70% throttle opening) the engine is in open loop….it needs a mixture richer than 14.7 AFR (leaner during deceleration).

 

So when the engine accelerate, it switch into open loop and the PCM mainly use signals from the MAP, TPS and CPS sensors to determine the correct fuel table to use.

 

The o2 sensor signals is only used by the PCM/ECU in closed loop operations, in open loop conditions the signals are ignored.

 

The problem" with not getting enough fuel at high RPM", is also new to me.

 

I have two 4.0L HO's and one Renix and have never experienced or read about the”downfalls of the mighty 4.0”.

[comanche tom]

Hey, My cat and my fuel filter are brand new. so they arent the cause of this. This kinda stinks, I thought at least sombody would share this problem. Also, the problem is not occuring at high rpm's its goin on whenever i step on the pedal regardless of my rpm. Another thing to consider is that its a manual, I don't think i have that many sensors.

[Eagle]

Fuel injected vehicles don't have an accelerator pump like old carburetors did. The purpose of the accelerator pump was to avoid that lag when you mash the throttle.

 

When you're driving at a constant (more or less) speed, the throttle plate is partially open and the air column flowing through your throttle body is moving at a basically constant velocity. When you mash the throttle, the plate opens up wide, but the engine RPMs haven't increased yet. So the velocity of that air stream drops significantly. Same volume of air, but flowing through the throttle body slower. In theory the velocity should be the same at the point where the intake runners enter the head (which is where the injectors are located), but maybe not. In a carburetor, the slower air flow doesn't such as much fuel out of the venturis as a faster column of air, so they used the accelerator pump to give it a shot of gas to enrich the mixture until the engine RPMs could catch up and get the airstream moving again.

 

With an electronic injection system, there's the throttle position sensor that tells the ECU (or PCM, depending on year) what position the throttle plate is in. Maybe your TPS is out of adjustment and the ECU isn't "getting the message" that the throttle is wide open.

[Oizarod115]

i never have lag at WOT unless I'm under 2k rpm.

 

but if i was going to WOT-it at 2k RPM i would downshift so i never have this problem...

 

something is defffinately out of whack.

[512tr]

Regarding the engine management you have the same sensors as the automatic.

 

When you step the pedal, two sensors are instantly activated, TPS (Throttle Position Sensor) and the MAP (Manifold Absolute Pressure).

 

The TPS is connected to the throttle blades on the TB (Throttle Body), and send a increasing volt signal to the PCM (Power Control Module) as the throttle blades opens (you floor the pedal).

 

The TPS hooks directly to the throttle blades to tell the PCM via a voltage signal not only how far the throttle is open or closed, but also the rate at which the throttle opening is changing.

At idle (throttle plate closed) the output signal to the PCM is +0.2 volt and the volts increase as the plate opens, at WOT the volt should be just below 4,8 volt.

 

The MAP sensor read the absolute pressure (vacuum) in the inlet manifold, and send an increasing signal to the PCM as the throttle plate opens and the absolute pressure increases (vacuum decreases) in the manifold.

Output voltage with ignition on (no start) 4-5 volts, and at idle 1,5-2,1 volt.

 

Both these sensors send increasing volt signals to the PCM, in order to supply more fuel, by increasing the time the injectors are open.

 

You also better check your fuel pressure, should be 31 PSI at idle and 39 PSI at WOT, (31PSI with vacuum connected to the fuel pressure regulator, and 39 psi without the vacuum hose connected)