Hi all,

I just joined the forums today and am hopeful some of you may be able to help me with an issue I'm having with my 1997 Jeep Cherokee Sport. My XJ has about 210,000 miles and has been pretty rock solid reliable for me.

However.

Since about April of last year, I've noticed an issue that rears it's head when I either
1. Let the car idle for a prolonged period of time
2. Do alot of in town driving with short drives and multiple stops where I'll shut the car off and only let it rest for 15 minutes or so

This is generally what I observe when it either the idle gets rough and dies or when I attempt to start it after frequent stop/start cycles:
Car will stumble at idle, I'll hold the throttle open to build the revs, while it tries to build revs the engine stumbles and I hear popping (presumed backfires through the exhaust). Sometimes if I get above 2K, the engine smooths out and it returns to a smooth idle and everything is fine and I take off down the road. Other times I can't get it to build revs, it stumbles and is basically completely unresponsive to throttle input. I usually get it going again by doing a series of things - cranking the motor with the throttle full open (sometimes it will catch briefly but never start) and letting the engine cool with the hood open for ~15 mins. So far, it's never failed, I've always gotten it to start and smooth out and let me on my way. When it's doing this backfiring, the exhaust smells raw, like a diesel, as you might expect.

Other things noted with this issue:
1. Poor fuel economy. I'm generally 16-18 MPG combined with my 2" lift and 30" K02s. Ever since this issue got worse, I'm more like 12 MPG
2. Decrease in power. Engine feels smooth but it does feel like a dog.

With all this said, it appears to me I'm dumping too much fuel into the engine. Definitely for hot starts and maybe in general while driving. This thing definitely behaves like an old carburetor car with a flooding carb.

In the past 3-5 months I have replaced/done:
1. Wrapped fuel rail and injectors in insulation
2. Thermostat
3. Radiator
4. Fan shroud
5. Cap, rotor, wires, plugs
6. Intake/exhaust manifold gasket
7. Exhaust manifold
8. O2 sensors (both)
9. Water pump

I'm almost thinking whatever controls the air/fuel ratio is not working correctly or maybe whatever controls the cold start circuit (i.e. car thinks it's cold so it enriches the air/fuel mixture)

Thoughts? Thanks!

 

This almost sounds like a bad MAP sensor but wouldn't I have a CEL on? I don't. Also wouldn't this be a problem at all times not just hot and while starting? What else controls cold starts? Coolant temperature sensor?

 

Nothing?

 

Well, I will post the results of my fix so that it may help someone else with a similar problem. It seems like every other thread on here has to do with a drivability or starting issue.

Here is a list of items I think could be the culprit
- Coolant temperature sensor
- MAP sensor
- IAC valve (attempted to clean this once before)
- Coil
- Fuel pump (just because this seems to a common issue and it's failing can present a wide variety of symptoms)

I'm doubtful it's the TPS or CPS - not sure I've ever seen an issue like this resolved by their replacement.
It would be helpful if anyone can contribute some additional ideas about what might be causing what appears to be a rich fuel mixture and apparent flooding when the car is hot.
Can anyone provide an explanation of what the upstream and downstream O2 sensors control exactly?

 

the upstream o2 sensor controls your air fuel ratio, the downstream basically verifies the cat is working properly, what brand o2 sensors did you install when you changed them out, alot of guys on here have has issues with everything but the factory o2 sensors, also the iac valve may be a decent place to start, however make sure you clean the whole throttle body not just the iac valve itself if the port gets plugged up with carbon it will not be able to adjust to smooth your idle regardless of the iac being clean itself, also check for vacuum leaks, you can either spray starting fluid(small amounts on all vacuum ports and lines) and wait a few seconds if theres a leak your idle will bump up, map sensor could be the culprit but I would assume you would have a check engine light, I had a similar issue when my upstream o2 sensor was stuck in high range it did not throw a code but after replacement it cleared everything up. Hope this helps you out

 

Cranking with the pedal all the way down initiates "clear flood" mode which cranks the engine but disables the fuel injector pulse. If you detect that engine stumbles and tries to start when doing this, it is indication that it is indeed flooding and has some unspent fuel in there.

Your next step is to get an OBD2 scanner, dirt cheap, and can typically be bluetooth connected to an android or iphone (depending on which scanner you get). I got mine (a BAFX OBD2 bluetooth scanner) from amazon for about $22. It will show you what your fuel trims and O2 voltage readings are doing.

I would be looking for these possibilities:
1. Upstream O2 sensor is not in the correct/reasonable voltage range and is causing the computer to trim the fuel to the rich side.
2. MAP sensor is misreading air flow and adjusting mixture too rich
3. Ignition coil is failing when hot and causing a couple of cylinders to stop firing, producing what appears to be a rich situation.

1 and 2, will express themselves as higher than zero fuel trim values.

3 could present as a negative fuel trim because the O2 sensor will detect the unspent fuel (or the overabundance of free oxygen in the exhaust) and could actually be trimming lean, while the system is remaining rich.

Typically if the fuel trims remain too high or low for a period of time, it will eventually throw a check engine light for fuel system too rich/lean (respectively). But sometimes the trims can be bad, but not -so- bad that they throw a code.

Anything that remains at higher than +10% or less than -10% is cause for concern. Typically, they want to hover around 0 (zero).

 

Ok, so I meant to post several days ago after both of you chimed in. Thank you by the way.

First, to address Nathan. I believe I used Bosch replacement O2 sensors for both. I have a suspicion there might be something going on with the O2 sensor because either I routed the upstream sensor incorrectly or it was like that and I hooked it up anyway, but the wire ran around the exhaust manifold and was in contact with the exhaust manifold, melting the wire casing. I've re-routed it but haven't yet attempted to investigate the condition of the wires.

Second, Jordan. Awesome advice with the scan tool. How have I not bought one? Well I did, I just used it.

For the 1st test, I started the car up and let it idle for 15-20 mins and watched the data when it was running "correctly"

Short term fuel trim % was hovering basically around 0 the whole time. Coolant temp looked good. Map looked good. .41 ATM at idle. TPS responded to throttle input correctly. Seemingly all good.

I kept flipping through the data as the car continued to run, looking for changes. Well, I started to notice the engine start to stumble. Then flipping through my diagnostics I noticed the short term fuel trim %had increased to over +10% (I'm assuming + indicates adding fuel). Idle continued to get worse (but never actually died). By the time I finished and came inside, it felt like the car was missing on a couple of cylinders (revving the engine never seemed to clear it up this time) and the short term fuel trim was now at +28%! CEL also came on (noticed it after the scan tool indicated there were 1 confirmed issue and 3 pending).

The one confirmed was a "too lean condition"
The 3 pending were "random misfire" "cylinder 3 misfire" and "cylinder 4 misfire"

So the ECU thinks it's running lean and is dumping fuel indicated by the +28% trim I'm seeing.

Am I interpreting that right?
Also, I'm note sure if I'm reading the tool right but it seemed to indicate that it only "saw" one O2 sensor?

 

Never mind I think it's reading both O2 sensors. 0211 bank 1 I assume is the upstream and 0212 bank 1 is upstream.

I noticed 0211 was near .8 volts and a bit erratic jumping between .3 and .8 volts constantly.

 

I did a fuel pressure test. Sitting at idle, the pressure is 22 PSI. What is normal?

 

49.2 +- 5 - So you definitely have a fuel pressure issue.

 

And Bosch O2 sensors have been known to be junk in a Jeep. Try unplugging the O2 sensors and see what happens.