7 mpg, stinky exhaust, carbon coated spark plugs, and check engine light

[Bobkat]

Here we go! Please don't thrash me for "throwing parts at it". Ok here is what I have done so far to my 1998 TJ with a 2.5, which I purchased 2 months ago. It had an intermittent miss at 2000rpm. 1. Replaced the Idle control valve which was actually stuck out and frozen. Then the check engine light came on. The code was bank one too lean. I was told to replace the oxygen sensor, so I did the upstream one. Then I totally cleaned tge throttle body. I replaced the spark plugs which were totally carboned fouled. I runs much better but the gas mileage is horrible! 7mpg!!! Someone online suggested I replace the throttle control sensor, so I ordered one of those. Any ideas guy? Halp!

 

[TheBoogieman]

Welcome to WTjF. Nice looking Jeep. Does it run fine now, but gets sucky gas mileage? Any codes now? What brand O2 sensor and plugs?

 

[AndyG]

Have the cats been checked?

Be thorough. 7 mpg is bad bad .

 

[Bobkat]

Welcome to WTjF. Nice looking Jeep. Does it run fine now, but gets sucky gas mileage? Any codes now? What brand O2 sensor and plugs?

Champion plugs. The oxygen sensor was just what Advance Auto had. I don't know the brand. Nope I have one of those blueteeth code reader doohickeys and it still keeps putting out the same code and check engine light.

 

[Bobkat]

Have the cats been checked?

Be thorough. 7 mpg is bad bad .

It does not throw a code for cats. It has 126,000 miles. It doesn't seem like a bad cat would make it have bad gas mileage. The cat is probably bad now though because the plugs were so badly carbon fouled. Every time I start it, it pukes out carbon onto the floor behind the exhaust pipe.

 

[AndyG]

Sometimes low exhaust output by restricted cats can be felt by hand ...

A handheld thermometer can show no heat passage at the cats -

 

[Rickyd]

An exhaust leak before the o2 sensor will trick the motor into running rich.that will give poor mileage and carbon on the plugs.

I have seen cracks hidden on 2.5l exhaust manifolds imbetween the block and manifold. A small mirror on a stick might help find a crack. The manifold gaskets at the head and pipe are other places to look.

PS get ntk/ngk o2 sensors for our jeeps. Cheap ones don't usually work

 

[Bobkat]

An exhaust leak before the o2 sensor will trick the motor into running rich.that will give poor mileage and carbon on the plugs.

I have seen cracks hidden on 2.5l exhaust manifolds imbetween the block and manifold. A small mirror on a stick might help find a crack. The manifold gaskets at the head and pipe are other places to look.

PS get ntk/ngk o2 sensors for our jeeps. Cheap ones don't usually work

I know the owner before me put a new exhaust manifold on and a new gasket. I will give the fasteners a torque check.

 

[williambmac]

Reset the computer, disconnect both battery terminals and hold them together for 30 seconds.

 

[Bobkat]

I know the owner before me put a new exhaust manifold on and a new gasket. I will give the fasteners a torque check.

Aha!!!!! Three bolts on the exhaust manifold where loose!! I tightened them and the lean code did not show up again!! Yay!! Thanks so much!!

 

[MikeE024]

Have the cats been checked?

Be thorough. 7 mpg is bad bad .

Agreed and I've been there.

It does not throw a code for cats. It has 126,000 miles. It doesn't seem like a bad cat would make it have bad gas mileage.

I would not assume the last part.

If the catalyst is broken or melted (plugged up), it won't do its job properly...and if that happens your downstream sensor will pop a code. The engine is an air pump, and it can't move air if the exhaust is plugged up.

A bad Cat will result in a sulfur/rotten egg smell because the atoms aren't splitting the hydrogen sulfide coming out of the combustion process that would result in oderless sulfur dioxide. If you don't smell that then there's a good chance the catalyst is still doing its job.

The cat is probably bad now though because the plugs were so badly carbon fouled. Every time I start it, it pukes out carbon onto the floor behind the exhaust pipe.

The cat might be ok (see above), but I'd still remove it to see if the carbon needs to be back-flushed off. And check to make sure the catalyst isn't plugged/broken/loose.

- -

That said, I had a similar situation and that was with new cats installed. I was getting exactly 7mpg on the highway because my intake manifold had a vaccuum leak due to a hose coming loose. I had lean codes and the system was pouring in fuel to keep me from running lean. My suggestion is to do a good inspection of this. Ask for help if you don't know all of the locations to check.

I smelled a heavy smell of gasoline because the system was dumping too much fuel into the system to account for the vacuum leak.

BTW, I fixed the vacuum leak by connecting the loose hose, but I also had an issue with the B2S2 wiring that was shorted between the transmission. That's why you see the O2 code in the following screenshot along with the codes from the vacuum leak.

Additionally, a pressurized smoke device (using shop air) will make quick work of knowing if you have vacuum leaks, but it's not necessary for finding something obvious like a disconnected hose.

 

[MikeE024]

Also, if things are really mucked up with carbon deposits, you could have a valve not properly seating.

You could try running some seafoam spray through the throttle body area while revving around 2,000 RPMs (follow their instructions). Or you could do an Italian tuneup.

If it were me, I’d first check for a manifold leak, then do the seam foam, and then do an Italian tuneup…all before checking on the cats bc it sounds like there’s a good chance you be sending more carbon bits to the cats from the combustion chamber.

 

[Bobkat]

I know the owner before me put a new exhaust manifold on and a new gasket. I will give the fasteners a torque check.

I have updates! I tightened up the exhaust manifold which had 4 loose bolts. Then it threw a code for the ECT sensor! I did some vestigatin and found that a bad ECT sensor will cause 7mpg, carbon fouling and put the motor into limp mode, which by the way it did! I put a new one in and cleared the code. Let us hope this works!

 

[AdventurePig]

I have updates! I tightened up the exhaust manifold which had 4 loose bolts. Then it threw a code for the ECT sensor! I did some vestigatin and found that a bad ECT sensor will cause 7mpg, carbon fouling and put the motor into limp mode, which by the way it did! I put a new one in and cleared the code. Let us hope this works!

Welcome and Nice work!

I look forward to hearing how it goes now.

 

[MikeE024]

I have updates! I tightened up the exhaust manifold which had 4 loose bolts. Then it threw a code for the ECT sensor! I did some vestigatin and found that a bad ECT sensor will cause 7mpg, carbon fouling and put the motor into limp mode, which by the way it did! I put a new one in and cleared the code. Let us hope this works!

I must have missed you mentioning going into limp mode.

Would be interesting if that fixed your lean issue as it’s certainly possible for fresh O2 to cause a false lean condition. For some reason my exhaust manifold leaks don’t pop a code. I need to swap the JBA manifold in later this summer.