I've been chasing a problem with my '97 for months. Misfiring, high idle. CEL, throwing P300 code (multiple misfires), and a few other cylinder misfire codes.

I've looked for vacuum leaks (only found one, around the throttle shaft), replaced plugs, wires, cap, rotor, and coil.

No change.

Then it suddenly went to running stupidly rich (7 MPG!) for a couple of weeks, and then died on the road. A new O2 sensor and new CamPS and it was running again, but CEL stayed on and it was still rough & idled high, but not as bad. After a few days, the CEL went out on its own. (Maybe the carbon got cleaned out after the O2 was replaced?)

Since I had to be towed to a shop when it died on the road, I don't know what brand of O2 & CamPS were used as replacements.

Yesterday on the way home, CEL came on and it went to stupidly rich again. I can blow black smoke when I step on it.

This morning it's throwing these codes:

P0300 Multiple cylinder misfire
P0301 Cylinder 1 misfire detected
P0301 Cylinder 3 misfire detected
P0301 Cylinder 4 misfire detected
O0171 System too lean




Too lean? OR maybe that code was set before it started running rich?

Gotta run to work, but I plan to read the codes this evening. I cleared them this morning, so I'll have just my commute on it when I read them again this evening. I'll report back on what I get then.

Back to those vacuum leaks - I did find one around the throttle shaft. I'm not sure if that one is a problem or not. It's not huge, but I really don't know how big a leak has to be before it causes problems.

Any ideas?

 

P0171 Possible Causes:

INTERMITTENT CONDITION
RESTRICTED FUEL SUPPLY LINE
FUEL PUMP INLET STRAINER PLUGGED
FUEL PUMP MODULE
O2 SENSOR
O2 SENSOR SIGNAL CIRCUIT SHORTED TO GROUND
O2 SENSOR HEATER OPERATION
TPS VOLTAGE GREATER THAN 0.92 VOLTS WITH THROTTLE CLOSED
TP SENSOR SWEEP
MAP SENSOR OPERATION
ECT SENSOR OPERATION
ENGINE MECHANICAL PROBLEM
FUEL FILTER/PRESSURE REGULATOR (HIGH)
PCM

Too Lean is a response from the O2S/PCM indicating a high positive fuel correction (dumping fuel at the actual lean condition, resulting in running rich). Since you've changed the upstream(?) O2S, and addressed vacuum leaks, you should look at the fuel delivery systems, like clogged or dirty fuel injectors, fuel pressure; or even a clogged catalytic converter.

Check the O2S Heaters fuse in the PDC.

Check TPS voltage output at idle, and dead spot in the TPS sweep.

 

Thanks, Ken! Another list of things to look at.


BTW, same codes this evening, after clearing them this morning. The CEL didn't come on until a few minutes after it had reached operating temperature. Seemed to run rougher as I neared home this evening, so, possibly somewhat heat related, or at least, exacerbated by heat.

 

Review that list of possible causes.

The Engine Coolant Temperature sensor may be defective, or its wiring at the ECT sensor connector. Check the wiring and connector pins/cavities for corrosion.

ECT's are cheap. You may just want to shotgun a new ECT sensor and see if it helps. Testing of the ECT is too much of a PITA.

 

Check the map. It could be anything Ken listed but in almost every Chrysler when it's an issue that involves fuel it's either the map or the tps 90% of the time.

 

You can test the MAP sensor if you wish.

You'll need a digital voltmeter, with alligator clips for the meter probes.

You'll need two safety pins. Go to this tread: https://www.cherokeeforum.com/f2/gre...e-6558/index4/ and review post #53. Post #53 shows back probing the MAP sensor with #3 safety pins.

The MAP sensor input is the number one contributor to fuel injector pulse width (rich/lean).

The PCM uses the MAP sensor input to aid in calculating the following:

• Manifold pressure
• Barometric pressure
• Engine load
• Injector pulse-width
• Spark-advance programs
• Shift-point strategies (certain automatic transmissions only)
• Idle speed
• Decel fuel shutoff

Testing:

Using a digital voltmeter, back probe the MAP sensor connector while it’s attached to the MAP sensor and take the following readings.

Test the MAP sensor output voltage at MAP sensor connector between terminals 1 (Brown/Yellow tracer wire) and 2 (Dark Green/Red tracer wire).

With ignition switch ON and engine OFF, output voltage should be 4-to-5 volts.

With engine running, the voltage should drop to 1.5- to-2.1 volts with a hot, neutral idle speed condition.

 

Feh. I've been through FOUR meters trying to get a reading on this! Two of mine that are dead, one just... dead. Was fine last time I used it. ONe that gives some really interesting and totally bogus readings. Again, was find last time I used it. Then the last one, an El Cheapo analog meter that I destroyed my own self. Was getting ready to do the measurements, got called away for a few minutes, then asked to run a quick errand. Found it on the intake manifold when I got back.

Why, no, it didn't like that, why do you ask?

Then I dropped 60 bucks on what looked like a decent DMM from PepBoys.

Dead out of the box.

So, today, Amazon delivered my new meters. (I always like to have an analog around, along with my DMM.)

I've got a solid 5.2 volts with engine off and key on. Ground wire shows 1.3 ohms as measured to the battery terminal. That all looks good.

Here is a video of my measurements while running. Pretty much the same regardless of temperature, so I think I'm going to take Ken's advice and replace the ECT sensor.


So, what do you think of my readings?

 

.

 

Weird. That link worked perfectly when I tested it last night....

Anyway, I'm getting 2.3 -2.4 while running at operating temp. Goes higher (about 4v) if I blip the throttle, then drops down to 1.5 or so, then creeps back up and settles around 2.3 - 2.4.

When the engine was cold, it was about 1.7.

 

TPS shows .911v with throttle closed. Sweeps very cleanly up to 3.96v @ full throttle opening. (I used my analog meter for the sweep test. Looks really good.) Shows 5.2v on supply side. (all measurements with engine off)

Ground is good on both sensors: less than a half ohm, measured directly to battery terminal.

A fuel pressure test is next, along with checking the O2 heater function. (02 sensor is new.)

 

Problem identified. Finally. I hope.

After doing all the tests I could find on several forums, I was still getting nowhere. Finally took it into my local Jeep dealer.

They found a massive vacuum leak on the fuel rail, and the tech thinks the inside of the fuel rail is probably gunked up, too.

When I tested for vacuum leaks, I injected smoke into the intake system. I didn't see anything coming from the fuel rail, of course.

So, lesson learned - injecting smoke is a great technique, but it's not enough!

So, next stop, a new fuel rail and injectors.

 

Whoa!

Did the dealer identify where the vacuum leak in the fuel rail was?

Fuel rails don't have vacuum leaks but bad fuel injector o-rings at the intake manifold do leak (vacuum). This has been documented before.

You can check for vacuum leak(s) at the rail/intake by running the engine and dribbling water at the intersection of the fuel injectors and the intake. If the water sucks in then there's your leak.

If nothing else, pull the rail w/injectors and replace the o-rings, reinstall, and check again.

Felpro o-rings can be bought from the auto parts stores. You will need three packs of them. They come four to a pack.

 

X2^. The lower O rings on the injectors need to seal vacuum. (uppers would leak fuel). I found one of mine leaking a while back. If 94 still has the MAP up on the firewall, (I think it does), I'd make darn sure that brittle little tube to it is flawless! Any little crack or blockage will cause trouble.

 

I spoke to the service manager, not the tech, so something probably got lost in the translation. The exact words were "massive vacuum leak", so I suspect they actually found something. This is the first time I've taken my Jeep into this dealership, but so far, I like what I see. They told me symptoms that I hadn't mentioned to them.

My rail is a rusty mess, so much so that I am very skeptical of getting anything disassembled in one piece, so I'm just going to go with a new rail. It was on my "someday" list anyway, due to the rust.

I've PM'd Programbo about new injectors. At 228k, new injectors are probably due, and from the reading I've done here, the 4 hole injectors should be better.

DFlint, this is my '97, and I've smoke tested the rest of the system. Nothing except a small bit of leakage on the throttle body around the throttle shaft, on the front side.

 

I wonder if there's rust inside the fuel rail getting into the injectors.....