P0108, P0132, rough idle and surging at 2500+ rpm

Hello, my 2000 XJ (4.0L, AW4, NP231, 230,000+ miles) is running poorly with codes P0108 (Map Sensor High Voltage)and P0132 (Left-Upstream O2 Sensor Shorted to Voltage).
I first noticed the jeep running poorly when driving in a hilly area in the rain. The next day on a 30 minute drive I couldn't keep up speed over 45, and acceleration was sluggish. At some point in this drive, the CEL lit up.

It stalls from either a warm or cold start, unless I give it gas for 10-20 seconds, then it idles. The idle is rough and surges slightly. If I give it gas slowly (in Park), it will rev up to 2500+ but then it surges violently with an occasional backfire. The high rpm surge sounds really sad.
When driving, light acceleration up to 2500rpm is fine, including normal shift points. Hard acceleration bogs the engine until you back off, but it has not stalled. It makes for slow and frustrating driving.

What I've done/history:
-The idle doesn't change if the MAP is disconnected. Possible wiring issue?
-I checked the MAP sensor voltage (brake bleeder pressure test), and it appeared fine.
-Swapping a cheap parts store MAP did not change anything, the original is back on.
-Swapping a cheapo TPS just gave me a TPS low voltage code, so the original is back on there.
-The spark plugs are black and sooty, and the fuel pump sounds fine.
-A pressure test on the intake indicates I may have clogged catalytic converters (2 pre cats + 1 regular cat).
-Engine/chassis grounds appear fine, recently upgraded 6mo prior.
-Engine has a history of P0303 (Cyl 3 misfire) associated with heat soak. One day it was flashing with this code (hot start up. 2 hours down the road after wheeling), but ran normally after cooling down for 10 minutes.
-Mileage has been getting worse this year, down from 19hwy/15city to 17/12, with 14-15mpg average.

I have not yet checked the upstream 02 sensor.
The jeep is parked for now, and plan to test the wiring on the weekend.

My plan of action includes:
Test continuity of wiring harness between sensors & pcm, confirm voltage from harness to sensors, check all grounds.
Test 02 sensor if possible, maybe replace 02 anyway (Bosch?).
Replace MAP and/or TPS with MOPAR parts
Remove/replace main cat and check for restrictions, then maybe remove exhaust manifold to replace pre-cats.

Does anyone have new ideas or suggestions? If its not a failed sensor or broken wire, what else? Could a clogged cat cause the codes and surging?

Thanks!

Follow the trouble codes. I think you are on the right track. The PCM cannot tell the difference between faulty wiring, and a faulty sensor, continuity testing is always a good idea. Engine sensor testing is simple and should always be done is you suspect a weak or faulty sensor.

Always buy genuine Jeep sensors and NTK O2 sensors. The Jeep 4.0L and Bosch O2 sensors do not play well together.

The stalling at start up and having to feather the gas pedal suggests low voltage.


For 1996+ Jeeps, the OBD-II engine computer (ECU/PCM) re-boots, and the idle settings are deleted, when you: (1) let the battery run down by leaving the lights or radio on, (2) disconnect the battery for an extended period of time, (3) when the battery goes bad and needs replacing, (4) when the alternator is not properly charging the battery, (5) or when poor battery connections result in a voltage drop at the PCM or poor battery charging. Loose, corroded, or damaged battery cables or ground wires may also cause or contribute to the problem.

A faulty Coolant Temperature Sensor may also cause similar symptoms. Testing of the CTS is recommended as part of the diagnostics. A poor ground at the ignition coil can also cause similar symptoms.

The reboot symptoms are: (1) having difficulties starting the engine without depressing the gas pedal, and (2) the engine will have a low idle and probably stall unless you keep your foot on the gas pedal. (3) The ECU will usually relearn the idle settings after a short period of driving and the engine will start and idle normally.

Begin with basic trouble shooting of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables or connectors and replace as needed. Copper wires should be copper color, not black or green. Do the same for the grounding wires from the starter to engine block, the ground wires at the coil, and the ground wires from the battery and engine to the Jeep's frame/body. You must remove, scrape, and clean until shiny, the cable/wire ends, and whatever they bolt to. Jeeps do not tolerate low voltage, bad connections, or poor grounds.

If the symptoms reoccur, you need to have your battery and/or the alternator Load Tested at a repair shop or auto parts store. This is usually a free service they offer to bring in potential customers. Handheld testers are very inaccurate and will usually pass marginal alternators. Even with low battery voltage, the engine will start, but the ECU can re-boot multiple times and a new battery may be required.

Thanks Tim.
I checked the grounds and cleaned them, but no changes.
As for the coolant temp sensor, it bench tested fine, and computer seems to be reading them properly, and match the dash temp.

Continuity testing of the MAP connector to the pins in the PCM connector all read 1-3 Ohms, which is great. I performed the resistance pressure test on the MAP sensor again, and the back-probed sensor wire gave the proper voltage based on pressure (2.9C @ 10in/hg, 1.9v @ 15in/hg). With ignition on, the MAP connector's power and sensor wire are both reading 5.1V (harness wire to battery or harness ground wire).

I could not find anywhere, Should the sensor wire from the harness should read 5V or not?

However, the computer/PCM is not registering anything from the MAP at all based on my OBD2 scanner with Torque. I reconnected everything, and drove it to a shop for better diagnosis. Their scanner verified that the computer is not seeing any MAP voltage at all. When I got home we rechecked the MAP harness continuity to the PCM, with the same low resistance.
I am going to order am oem MAP this week and try it, just to see if the computer can sense it, but I'm doubtful.

At this point, I think the computer may be bad. The pins/sockets themselves are clean and blemish-free in the computer-harness connectors. I'm not sure what else to test for continuity, or where to look.

Thanks

The OBD-II sensors are all supplied 5 volts.

With the map disconnected there should be no voltage on the sense wire, only the power feed. 5v on the sense wire is a problem, and is either shorted to the 5v ref or the pcm is internally faulted

Thanks Tim and Tripled. I suspected 5V on the sensor wire was a problem.

I ruled out the computer by swapping in an identical model, with no changes.

I have and OEM MAP sensor on order, and I am looking into the wiring diagram to check routes for a short. I am hoping for an obvious short somewhere I can patch.

The dig continues.

Update:
I found that my wiring harness is melted to itself across most of the intake manifold.
The melty mess starts near the splice off the firewall, and continues at points across the intake manifold. This branch of the harness controls the injectors, all throttle body sensors, 02s, and some temp sensors. In teasing the wires apart, I found a definitive short of the MAP sensor wire to its 5V source, hence the P0108 MAP high voltage.

I'm sourcing a local wiring harness now. If that doesn't work out, I found one on ebay for $100+ shipping.
If I can't find an intact full engine harness, I may resort to carefully spicing in new wires for most of it, or using part of another harness to splice in.

The theory is that a previous owner seriously overheated this jeep and had at least the head replaced. Evidence includes these melted wires, some markings on the engine, new motor mounts, and a dent in the fender form an engine hoist. I've only had this jeep for 20,000 out of its 230,000+ miles, and I'm not going to give up now.

trippled said:

With the map disconnected there should be no voltage on the sense wire, only the power feed. 5v on the sense wire is a problem, and is either shorted to the 5v ref or the pcm is internally faulted

Click to expand...

Wrong

With the key on engine off, and a DVOM you should see 0v, 5v and 5v. The signal wire has a 5v diagnostic carrier voltage. 5v sensor feed to power the sensor and 0v on the sensor ground