cletus141

Hey guys,

Did some searching, but it produced lots of reading about NOx failures, and limited results searching for high HC.
Gist:
Idle:
HC: 1600ppm measured, limit 220ppm
CO: pass (they don't give numbers here unless it fails)
CO2: pass
NOx: pass

2500rpm:
all pass.

1991 XJ 4.0L

Smog guy said it is running rich, but wouldn't that also have high CO levels? I know I have an exhaust leak, just haven't located it yet. I have read that a leak after cylinder head before o2 sensor could lean out the exhaust gasses and make the engine think it's running lean and dump more fuel. But wouldn't that give me high CO? Most things I've read suggest that misfire is the most common cause of high HC, My plugs, wires, etc aren't very old. I've passed my previous 2 smog checks without major tunes, but checking the plugs, wires, cap, and rotor is easy enough.

Thanks,
Chris

cletus141

Anyone have any ideas here?

derf

How well does it idle?

Could be that all you really need to do is clean the throttle body. The Idle Air Control circuit gets gummed up with gunk over the years and it can affect how well it runs at idle.

I just cleaned the whole throttle body on a 4.0 out of a TJ and it made a huge difference.

cletus141

a little rough @ idle

derf

There's lots of videos on youtube on cleaning the throttle body. It's cheap (a couple of paper towels and some carb cleaner) and takes half an hour or so. You'll be surprised at how much better it runs when it's clean. It will also likely help with your high HC numbers at idle. Allowing in the right amount of air at idle makes a big difference.

cletus141

Thanks!

97grand4.0

Of course what any tech would need to answer your question would be to look at your scan tool data.
What are your short and long term fuel trims?

wpatters1229

I have had the same problem.
1993 Jeep Grand Cherokee 4.0l engine in CA.
Fail first round of smog test the HC at idle failed by 8 pts (Max is 100) Measured 108.
Did a full tune-up. New plugs, cap and rotor...tested all wires as good resistance. Changed the oil and filter. Ran fuel injector cleaner. Installed a new Air Sensor in the exhaust pipe.
Did a re-test and it failed again but by only 1 pt (101).
I had also checked all vacuum lines and found no leaks.
Any ideas on what to do next?
It has 198,000 miles. Maybe a carbon cylinder cleaner??
It runs great but this is making me crazy.
I have heard of moving the idle a little higher will get it to pass...that true?
Here is a picture of the results.

egleaves

When my Jeep failed smog for HC I gave it a Seafoam cleansing and then it passed with flying colors.

One full can in the gas tank, one full can down the intake. They make a spray can with a little hose for the intake treatment.

cletus141

I haven't solved this yet. But I'm fairly confident that it is my exhaust manifold, which I think is cracked and is a fairly common thing for the 4.0L motors. I'll post once I replace it.

Xavierag

anybody know what could be the problem, i have a 95 jeep grand cherokee and changed all spark plugs, with boots and it keeps miss firing and check engine light keeps blinking.. Please help.. thanks

cletus141

Have you checked for exhaust leaks? It sounds like that is my problem. I definitely have one, just haven't remedied it yet.

wpatters1229

I read a bit more and one thing I did not check was the two PVC or whatever they are called that are on the top of the valve cover. I took off the hoses, which are clean and nice and sprayed a bunch of carb cleaner into the elbows and into the valves or whatever they are. I think they are just small holes. I then let them soak and using my air compressor shot air into them with the oil filler cap off. You could feel the air rushing out of the oil fill opening. I would think they are clear and free to suck fumes as they are supposed to. Not sure if they had a problem at all.
I then did one other thing and adjusted the idle to about 850 from 725...using the screw on the throttle body. I have read that higher idles gets better results on the idle test.
While it was idling I took carb cleaner and shot it around all the injectors, vacuum lines and around the base of the throttle body. I also shot around the intake manifold and exhaust manifold connection to the head. No change in idle. I think all is tight and no leaks.
The only thing left is to run some more cleaner through the gas and then drive the crap out of it before taking it back to the smog guy...UGH!!

wpatters1229

I have done all the things I said including Seafoaming using the spray and adding some to the oil and gas. Now just to drive until I can add a new load of gas.
Being it only missed by one point in the HC makes me think that if I get it hot enough and drive it hard before going in it could pass. If it doesn't I guess I would have to go to a referee to see if I can get clear for two years. It is such a good running car to just dump.
I think CA is trying to get rid of old cars. I would buy a newer one if I had the money.

lawsoncl

Don't use the screw to monkey with the idle. The IAC controls the airflow to control the idle. By mucking with the screw to bump up the idle you're effectively creating a vacuum leak. On the restest, make sure the car is good and warm for the test. Sometimes you sit and the engine and cat aren't fully up to temp when they do the idle test.