Deep Dive into Drivability, or Lack Thereof

[Deputy Dodge]

My 99 4.0 5spd started failing me last spring and I can't go much deeper to figure out what's wrong. It stated with a CEL for O2 codes. I replaced both with NGK/NTK, only to have codes come back. Currently P0132, 135, and 138. It also blows fuse 27 for the HO2S heater circuit. My thought was that there is a short somewhere, potentially shorting power to the sensor wires of the O2. But even with the fuse blown I'm still having engine trouble. Most notably if I shut it off when hot and on a trail ride, it won't idle right and surges for a while before mostly clearing up, and at steady throttle it will power on/off at steady intervals, as if you were giving it about half throttle for 2 seconds then off the throttle for 2 seconds. That part comes and goes no matter how long it's been running. The other issue is if I have an extended idle time such as when airing up tires it will bog and sound like it is running on 3 cylinders. If I catch it and rev it up, it might go back to a normal idle. If you've made it this far, stick around for my diagnosis train of thought. BTW, from a cold start it will idle good for about 3 seconds before running rough. It's still in open loop so my thought is not so much on the O2 side of things.
New cap, rotor, plugs, wires, and coil, mostly because it's new to me and I don't know the history of it.
New air filter in stock air box.
Using a cheap scan tool, I showed 14.5% TPS and 10" vac at idle. An actual vacuum gauge showed 18" vac at idle. A voltmeter connected to pins 1 and 3 gave a proper 5V reference at the MAP sensor. Pin 2 gave 1.3-1.4V at hot idle, which is below spec of 1.5-2.1V, but I'm not quite ready to condemn it yet. Revving the engine to 2500rpm gave mixed results...sometimes it had the ON/OFF throttle symptom and sometimes it didn't. The cheap scanner may not be interpreting the voltage correctly and giving a false reading of 10"....or the MAP really thinks it's that low.
Using an expensive Autell scanner, my upstream O2 stayed pretty low at .2 to .38V, and the downstream stayed hi at .8 to 1.0V. If the MAP was giving a false reading this could cause similar conditions but the intermittent factor has me questioning that determination.

At this point I'm leaning to leaky fuel injectors or a bad PCM. I'd rather not load the parts cannon, so I'm open to ideas. What do you think???

 

[RMETeeJay]

I would address the O2 heater wiring issue first just to rule that out and get it working right. I have never traced the heater wire all the way, but you should be able to follow it back and inspect. Clear all codes and see if you can trigger any more.

From there, go back to ICE basics. It's good that you did a full tuneup. I'd get a visual indicator of healthy spark at each cylinder. I believe that your year has the Schrader valve on the fuel line so go ahead and verify fuel pressure, leaving it hooked up so you can monitor during poor running conditions. Since that MAP voltage reading is lower than spec, I'd also inspect the computer and sensor's ground points and clean them up if they look crusty.

 

[Deputy Dodge]

Static fuel pressure is in spec and the rough running, rythmic cutting out and bog symptoms point more to ignition. Crank and cam sensors typically work or they don't. Not much in between. Can leaking fuel injectors or an exhaust leak cause this?

 

[JKP]

On my bluetooth scanner, the MAP reading is in pressure not vacuum. Are you sure yours was displaying in vacuum?

 

[Deputy Dodge]

A Manifold Absolutely Pressure sensor measure intake manifold pressure, which is always negative (vacuum) unless it is at wide open throttle. Then it is at atmospheric pressure, which is zero psi. My cheap scanner showed 10 inches of mercury, InHg, and my expensive one showed 423 mmHG. 423 is pretty close to 16.5 inches, which is closer to the actual 18in on the mechanical gauge...but still reading low. The good news is that a steady and high vacuum signal indicates a good base engine, no leaking or sticking valves and reasonably good timing.

 

[Deputy Dodge]

@Jerry Bransford, any thoughts???

 

[IOwnCalculus]

I would address the O2 heater wiring issue first just to rule that out and get it working right.

Agreed here. You have known faults on at least one upstream sensor, which means the computer has no idea what the actual AFR is, and are potentially trying to send 12V into a sensor input.

 

[Jerry Bransford]

@Jerry Bransford, any thoughts???

I wish I did, it's a puzzler to me too.

 

[JKP]

A Manifold Absolutely Pressure sensor measure intake manifold pressure, which is always negative (vacuum) unless it is at wide open throttle. Then it is at atmospheric pressure, which is zero psi. My cheap scanner showed 10 inches of mercury, InHg, and my expensive one showed 423 mmHG. 423 is pretty close to 16.5 inches, which is closer to the actual 18in on the mechanical gauge...but still reading low. The good news is that a steady and high vacuum signal indicates a good base engine, no leaking or sticking valves and reasonably good timing.

On the Jeep MAP sensor atmospheric pressure is 29.9 in Hg or 14.7 PSI if you are near sea level as can be seen with the engine off and reading the MAP data.

If your scanner is reading the sensor in pressure, not vacuum, you subtract it from 29.9 to get the vacuum reading. So, 29.9-10=19.9 in Hg vacuum.

The price of the scanner should make no difference as it is only displaying the data supplied by the PCM.

Intake manifold pressure is only neg relative to atmospheric pressure. The MAP sensor reads positive pressure, not vacuum.

At idle, my 06 reads 9 to 11 in Hg pressure depending on if the A/C is on.

 

[Deputy Dodge]

On the Jeep MAP sensor atmospheric pressure is 29.9 in Hg or 14.7 PSI if you are near sea level as can be seen with the engine off and reading the MAP data.

If your scanner is reading the sensor in pressure, not vacuum, you subtract it from 29.9 to get the vacuum reading. So, 29.9-10=19.9 in Hg vacuum.

The price of the scanner should make no difference as it is only displaying the data supplied by the PCM.

Intake manifold pressure is only neg relative to atmospheric pressure. The MAP sensor reads positive pressure, not vacuum.

At idle, my 06 reads 9 to 11 in Hg pressure depending on if the A/C is on.

Where are you getting your information? 29.9 inHg is a perfect vacuum or absolute zero. 14.7 psi is 0 psig, or atmospheric pressure (1 atmosphere). The 2 are not the same and with the engine off it can't be both, and will never be 29.9 under any condition. The map sensor only sends a voltage signal which is interpreted by the PCM. My cheap scanner may not be converting the voltage to the proper vacuum reading. My good one is. I think it was actually showing 450 mmHg, which is 17.7 inHg, which is really close to the 18 shown on my mechanical gauge. And one more reason to not condemn it as a bad sensor...yet.

 

[Deputy Dodge]

Agreed here. You have known faults on at least one upstream sensor, which means the computer has no idea what the actual AFR is, and are potentially trying to send 12V into a sensor input.

With the fuse blown there is no voltage to the heater circuit, and I can watch the O2 cycle on the scanner. In open loop it shouldn't matter what the O2 says because pcm isn't watching it. The fault should be fixed but I'm not seeing it as the root cause of the intermittent rough idle or surging/falling rpm. I keep coming back to leaking injectors causing the O2 to be all over the place and the pcm trying to compensate. Probably a good time to check my new spark plugs.

 

[JKP]

Where are you getting your information? 29.9 inHg is a perfect vacuum or absolute zero. 14.7 psi is 0 psig, or atmospheric pressure (1 atmosphere). The 2 are not the same and with the engine off it can't be both, and will never be 29.9 under any condition. The map sensor only sends a voltage signal which is interpreted by the PCM. My cheap scanner may not be converting the voltage to the proper vacuum reading. My good one is. I think it was actually showing 450 mmHg, which is 17.7 inHg, which is really close to the 18 shown on my mechanical gauge. And one more reason to not condemn it as a bad sensor...yet.

Yes, yes they are the same. You are confusing vacuum and pressure as I stated earlier. The sensor reads pressure, not vacuum. With the engine off, key on, the PCM sees atmospheric pressure 29.9.

 

[NskLJ]

My 99 4.0 5spd started failing me last spring and I can't go much deeper to figure out what's wrong. It stated with a CEL for O2 codes. I replaced both with NGK/NTK, only to have codes come back. Currently P0132, 135, and 138. It also blows fuse 27 for the HO2S heater circuit. My thought was that there is a short somewhere, potentially shorting power to the sensor wires of the O2. But even with the fuse blown I'm still having engine trouble. Most notably if I shut it off when hot and on a trail ride, it won't idle right and surges for a while before mostly clearing up, and at steady throttle it will power on/off at steady intervals, as if you were giving it about half throttle for 2 seconds then off the throttle for 2 seconds. That part comes and goes no matter how long it's been running. The other issue is if I have an extended idle time such as when airing up tires it will bog and sound like it is running on 3 cylinders. If I catch it and rev it up, it might go back to a normal idle. If you've made it this far, stick around for my diagnosis train of thought. BTW, from a cold start it will idle good for about 3 seconds before running rough. It's still in open loop so my thought is not so much on the O2 side of things.
New cap, rotor, plugs, wires, and coil, mostly because it's new to me and I don't know the history of it.
New air filter in stock air box.
Using a cheap scan tool, I showed 14.5% TPS and 10" vac at idle. An actual vacuum gauge showed 18" vac at idle. A voltmeter connected to pins 1 and 3 gave a proper 5V reference at the MAP sensor. Pin 2 gave 1.3-1.4V at hot idle, which is below spec of 1.5-2.1V, but I'm not quite ready to condemn it yet. Revving the engine to 2500rpm gave mixed results...sometimes it had the ON/OFF throttle symptom and sometimes it didn't. The cheap scanner may not be interpreting the voltage correctly and giving a false reading of 10"....or the MAP really thinks it's that low.
Using an expensive Autell scanner, my upstream O2 stayed pretty low at .2 to .38V, and the downstream stayed hi at .8 to 1.0V. If the MAP was giving a false reading this could cause similar conditions but the intermittent factor has me questioning that determination.

At this point I'm leaning to leaky fuel injectors or a bad PCM. I'd rather not load the parts cannon, so I'm open to ideas. What do you think???

I would find the short in the O2 heater circuit , you may find the harness has rubbed other wires through at the same spot

 

[Deputy Dodge]

Yes, yes they are the same. You are confusing vacuum and pressure as I stated earlier. The sensor reads pressure, not vacuum. With the engine off, key on, the PCM sees atmospheric pressure 29.9.

View attachment 358709

Ford calls their map sensors MAP/BARO sensors because they read both. As soon as the key is turned on and before the engine cranks it takes a reading. Also any time the TPS shows WOT it takes another reading. Those are the barometric pressures. With the engine running it measures vacuum. Whatever your definition of "pressure" is, unless you have forced air induction your intake will always be in a vacuum, or negative pressure, except at WOT. The main purpose of the map sensor is to determine engine load so the pcm can more accurately determine fuel rate. If it's providing false info then the pcm could be under or over fueling resulting in my rough running. My inHg seem accurate but the output of 1.3V when the pcm us expecting 1.5-2.1V could be causing issues. I don't want to start throwing parts at it butttt...
I would still like to know where you got your information on what the Jeep MAP sensor reads.

 

[JKP]

OK, the barometric pressure here today was 29.23 per the MAP sensor. The MAP sensor is reading 10.93 at hot idle, no A/C. This is all manifold absolute pressure. So the engine vacuum is 29.23-10.93=18.3 in Hg. See attached pics from scanner.

 

[JKP]

You can read the description of MAP sensor operation in the FSM, which is located in the Resources Section on the forum.