2000 XJ running rich I think..

I got a new muffler put on with an exhaust tip yesterday and this morning the inside of the tip is caked with black soot. So I think its running very rich...

Rebuilt engine, less than 2000 miles. 10w30 valvoline conventional oil.

There are no other issues that I'm aware of that would cause this. I run a Scangauge II an the idle is good. Temp runs about 190º. Runs and drives very nice. No CEL's ever. I haven't had a chance to check but it appears I'm only getting about 10mpg city driving according to the SG II. I just filled up the tank and reset the odometer so I'll confirm this when I refill it again.

Any other issues that could cause all that instant black soot? That doesnt seem normal at all.

It's staying in closed loop... SCII is reading the temp only as 190ish. I have a 195 stat..

My guess is the coolant temp sensor??

anyone feedback?

No Check Engine Light ? Black residue in the tailpipe is normal, what besides the black stuff make you think it's running rich ? The thermostat is not on/off at the exact temp rating, as long as it's not too cold and not too hot, the 190* temp is close enough to normal.

More importantly, what are the O2 sensors doing, and showing for data ?

No CEL's at all. Just poor gas mileage and its closed loop as soon as I start it??

I don't know what the O2 sensors are doing? I'm going to check my scangaugeII and see if that tells me anything about them. What should I look for?

I'm also going to replace to replace my coolant temp sensor and check my fuses to see if any are blown.

I feel like the mileage should be a lot better than what it is... something isn't kosher.

What makes you think it is rich? Do you smell " rotten eggs"?
Soot can be cause by abnormal oil consumption.
You have 2k on the engine? How did you break it in?

Was it a straight rebuild, or a stroker? Any other modifications?

190 is abnormal in and of itself. Should be running 210.
PCM runs Open loop until all sensors come on-line.
If temp is low, it won't throw a code, just won't go closed loop.

My guess for it being rich was the instant black soot and the low gas mileage. There is some odd smell after shut off.. not really rotten eggs but burnt plastic? I don't know if that's normal or not, and its not too strong.

I broke it in as directed, 2000 rpm for 20 mins. First 500 miles were easy driving under 3000 rpm.

It is a factory rebuilt engine, no mods.

I notice from my SC that the temp sort jumps around.. one second it can be 185 the next its 192. I think thats way low, not sure why? The cooling system is stock.

It starts and goes into closed loop immediately. Maybe I have a bad O2 sensor(s) in addition to something else?

The heated O2 circuit should put it into closed loop very fast. I would imagine you'd be in closed loop, which is what you want, within about 10 seconds of running. Once in closed loop, the PCM begins utilizing data from the upstream O2(s) and begins adjusting short term and long term fuel trim.

I'm getting between 10-12 this winter in my XJ and I've pulled live data from a DRB III and everything is fine. I think winter gas sucks and our XJ's are thirsty beasts. Are you idling for extended times, say for warm-up? Sitting in drive-thru's? I have a short commute and often stop for coffee and it kills the mpgs. When I get out on the freeway the overhead console says 21 mpg so I'm not really concerned.

As Tim says, black soot on the tailpipe is normal. We do not drive low emission or partial low emission vehicles.

I'm not convinced you have a problem at all.

On the highway the SC reads about 14-15mpg.. I know something is off there. Plus that temp is way too low?? My dash reads up near 205, but the SC it telling me a lot less. I might have a couple things going on. I hope I don't but I have to look a little deeper just to make sure.

Stop focusing so much on the SC. The temp is not too low. A thermostat is rated to be fully open within +/- 15 degrees of its rating. You're right there. If you're concerned, get an IR gun out and take a reading on your t-stat housing. With the IR gun, you should be able to mark when the cooling fan cycles on and off. I believe it comes on at around 215. How is your coolant level? If it's low you could be getting a false reading contributing to the discrepency.

Your Jeep is running smoothly with no warning lights or signs of distress other than poor fuel mileage. I hate to break it to you......bricks with iron heads and blocks suck fuel. What gear ratio and tire size are you running? Have a rack on top or travel with lots of gear?

Fill your tank a few times and see how many miles you're getting out of the tank. Report that back here. On the highway, how fast are you driving when you're seeing 14-15 mpg? I get about 21 mph on level ground going exactly 55 mph. If I bump it up to 70, that number takes a drastic drop.

I still think you don't have a problem. You could always sell it to us cheap?

190-215* is normal operating temperatures. Live data often shows lower temps than the dashboard gauge.

Keep records of mpg's for several weeks. Each fill-up note percentage of miles that were city or hwy. Keep hwy speeds under 65mph.

My 1999 gets 18-20 mpg hwy and winter formula gas drops the mpg's about 10-15 %

No roof racks or heavy bumpers. 30.3" Cooper AT3 tires and 2.5" lift.

I changed the coolant temp sensor this morning and its reading 195-197 now. So I think that is more accurate.

On the black soot.. I went from a factory muffler to a Magnaflow, so maybe the open design is just pushing it through vs catching it like before.

I'm going to monitor the gas mileage and see my readings. I also need to change the speedo gears so my odometer is accurate.

I also ordered new NTK O2 sensors. These are factory, so its possible they might be giving me a poor A/F mixture. The cat is looking pretty ratty too. I'm going to order a replacement there. Any recommendations?

It might be possible I'm chasing a ghost, but if nothing else I'll make things more efficient.

Temps sound more like what I would expect. As as stated above a IR reading will tell you how close the dash is.

I guess that's not how I would have broke it in, but it's not my engine
Soot will probably go away as rings seat better.
As engines break in, the gas milage generally improves. Had a few new fleet vehicles and they get better as time goes on.

'00 and '01s suck on gas milage as it is.

You have a lift, and largely than stock tires. Are/ have you corrected for the tire change?

Good point, hopefully mileage will get better as the engine breaks in.

I ordered the yellow 33 speedo gear and should get that this week along with the new O2 sensors.

I was going to order this Thunderbolt Spun cat with the O2.. any feedback on that? I've only read good things and think it'll go good with the Magnaflow muffler and 2.25".

A couple of quick notes:
O2 Sensor. From closely monitoring my Sensors, I know that they are up to temperature and the engine goes into closed loop operation after approximately 30 seconds. I have to put the "approximately" disclaimer in as it obviously takes longer when starting out from freezing or subfreezing temperatures.

If you know how far off your odometer is, recalculate your mileage. To be fair, once you fit larger tires and lift a Jeep, the mileage drops. It is a matter of increased drag from the lift and the extra power required to turn the larger tires. No getting around it. Particularly if stock gears are still in the axles. I run P285/75 (32.8") tires with 4.56:1 gears and the AW4 transmission. This results in a ~10% torque increase over stock. This is to say the engine turns faster in any given gear over when it was stock.

You are reporting ~15MPG on the highway. This is, actually, not all that bad.

ECT Sensor. From what I have found on my '98, the Factory ECT gauge is pretty accurate. I run a Davies-Craig EWP115 electric water pump operating with a set point of 90(C) degrees. The gauge on the dash and the PCM agree with the pump controller to within a very few degrees.

Soot. In the "old days" a black sooty exhaust was a sure sign that the engine was running an over rich mixture. This is no linger true The days of a nice, light, grey tail pipe are long gone. Unleaded fuel burns differently and leaves the soot behind. It like to drove me mad when I got my first unleaded vehicle. Admittedly, it would be a short drive...

A side note on electric water pumps. I like them. Couple a PWM controlled pump along with an aluminium radiator and controlled fans it is a good, if expensive, solution. I am running the Flex-a-lite aluminium radiator with the three fans 10" they provide. But I do not use the fan controller that came with. The DC controller has the ability to turn on a fan when the coolant temperature is 3(C) above set point.

I have the fans controlled as follows:
The passenger side fan runs continuously with the ignition. This is to provide full time cooling air for the alternator. The other two fans cycle when either the PCM or the DC controller calls for fan operation. I have them wired via an "OR" logic circuit comprised of three relays.

The Heep no longer sees operating temperature greater than 100(C). The only down side is that the heater is not as warm in the Winter. If we had typical "it gets and stays" cold Winters, I would reprogram the pump controller to add 5(C) during the Winter months. But, alas, we can see 70(F) in January around here. Usually, right after a subzero day...

The black soot is normal. Just ignore it. I would focus on your fuel consumption issue, if actually present. Get correct mileage and then calculate the mileage by hand. Don't rely on the scan gauge thing.

98NWCherokee said:

I'm getting between 10-12 this winter in my XJ and I've pulled live data from a DRB III and everything is fine. I think winter gas sucks and our XJ's are thirsty beasts.

Click to expand...

It seems like this year's winter gas is extremely crappy. I've heard from a few people that their engines are pinging a bit this winter. I know my 5.2L in the ZJ isn't too happy with the gas this winter. MPG down, slight pinging.

Yep, once I get the right speedo gear in there I'll know the odometer will log correct miles and go from there.

New engine + winter gas is most likely the issue.