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1990 cherokee smog problems

Old 07-02-2010, 03:34 AM
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Default 1990 cherokee smog problems

1990 cherokee not passing smog. replaced the nrg valve, put a stock airbox in, ran all new vacuum hoses to slots and evap canister, and used to emission test formula that cleans the pistons and valves. still not passing smog. any common problems like this for my year cherokee? 1990 jeep cherokee laredo 4.0 4x4 aw4 tranny. im thinking the cat is bad cuz it looks old mayb like its stock.. any other reasons before i have to spend $180 to get a cat put in? its a couple hundred numbers over the max smog rate... and wen i did all this stuff it went down around 200 points but still not good enough. need to get it smogged to be fully put in my name. thanks
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Old 07-02-2010, 04:31 AM
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Replacing Cat is always a good idea for passing emissions, especially on these older cars. Save yourself some money though, pick up a new cat and install it yourself. if you don't have a welder, clamp it on. You can find exhaust clamps at Autozone and the like. you can cut the old one out with a really good hacksaw or angle grinder if you can't get your hands on a Sawzall.
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Old 07-02-2010, 07:20 AM
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Do you know the actual numbers it blows?-that would help every one. On my 88 it first came back as a gross polluter,the o2 sensor was bad. Try to test yours its not to hard to ,your cat might be bad,or it might be doing overtime if your running to rich ,a new cat could give the same numbers
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Old 07-02-2010, 07:25 AM
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If your Jeep is capable of passing here is how to help it out... LEGALLY


Step #1 give it a tune up (change your oil, replace air filter and PCV valve, replace spark plugs, check wires and distributor cap and rotor button)
Step #2 Use a good fuel injector cleaner 1 tank before testing
Step #3 on testing day drive the car to operating temp preferably a good long freeway drive is best. You want to warm up the catalytic converter to make it perform best.
Step#4 air up the tires. make sure they are at the proper pressure.
Step#5 before testing make sure to turn off everything electrical and make sure the AC is off.


If after all that it still fails you can do a few.... well not illegal things but definitly not recommended things.

#1 remove air filter for testing (puts engine at risk)
#2 back off the adjuster on the rear drums (puts YOU at risk)
#3 air up the rear tires beyond recommended operating pressure (be very careful. Over inflating tires is dangerous!)
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Old 07-02-2010, 07:38 AM
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If you need a cat check with mudfrog (a Vendor here). https://www.cherokeeforum.com/f10/ou...nverter-45340/
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Old 07-02-2010, 08:18 AM
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and put some rubbing alcohol in your tank before you go in. it makes your exhaust run clean. alot of bmw guys in cali do that on their cars with cats that are hollowed out. its like a half gallon a tank or something. also, if your cat is good, make sure its all the way warm or it will give out bad numbers. you have to drive it for a while before going into inspection
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Old 07-02-2010, 09:06 AM
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be very careful with putting alcohol in your tank. Alcohol absorbs water, eats seals that aren't compatible and makes the car run lean. Too much can cause engine problems and fuel system damage.

Some people have used E85 instead of other alcohol. At least E85 is meant to be a fuel grade alcohol.
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Old 07-02-2010, 10:28 AM
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you need to post actual numbers on how it smog to be told how to fix it.example where the hc's high or the o2's they mean totally diferent things if your hc are high replacing you cat wont do a thing. Hc mean your engine is running lean wich could be multiple thing 02' running rich wich i would start with the tune up then cat.
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Old 07-02-2010, 04:29 PM
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ok the actual numbers are (first numbers are 15 mph second numbers are 25 mph) HC: max 123 and 103. measured 47 and 40. CO: max .73 and .93. measured .20 and .17. NO: max 711 and 613. measured 836 and 778. that last one was the only one it failed and that was of course the emissions test. thanks guys
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Old 07-02-2010, 07:42 PM
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Originally Posted by xXWhiteXjJeepXx View Post
ok the actual numbers are (first numbers are 15 mph second numbers are 25 mph) HC: max 123 and 103. measured 47 and 40. CO: max .73 and .93. measured .20 and .17. NO: max 711 and 613. measured 836 and 778. that last one was the only one it failed and that was of course the emissions test. thanks guys
That test shows your jeep is running lean. A cat will not fix the problem. The problem with jeeps as they get older is that when they were built they were set to get maximum fuel milage so there was not a hole lot of roon for carbon build up. and when the carbon builds up on the pistons and valves the compression raises slightly which makes them run lean. My first sugjestion is to go thru your intake ehaust bolts and makes sure they are tight. I just had this problem with mine and found that i had a major intake leak on my bottom side wich the o2 sensor could not make up for so once i tightend it all up and ran some lucas (not the cheap stuff the 12 dollar bottle of fuel injection cleaner and top end cleaner) thru it it it passed also you might change the 02 sensor to make sure its telling the computer the right stuff. Those might help enogh to get it to pass. Make sure you have no intake leaks. Also run a 92 octane thru it with the lucas the whole fuel tank. then fill it again only have way when you get it smogged. try that.
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Old 07-02-2010, 07:47 PM
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anddd where r the exhaust and intake bolts located? is it in the head? i havent taken apart a motor yet so i dont know to much about the inside of them
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Old 07-02-2010, 08:13 PM
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yes they are located on the right side (driver's side) of the motor.
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Old 07-02-2010, 09:46 PM
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Originally Posted by xXWhiteXjJeepXx View Post
anddd where r the exhaust and intake bolts located? is it in the head? i havent taken apart a motor yet so i dont know to much about the inside of them
The bolts that im talking about are the one on the right side of the head that hold the intake and exhaust manifolds onto the head if you have to many lose you might want to replace the gasket itself, it's like 17.99 at autozone or kragens. After you tighten them if they are lose when it cold start it and spray some starting fluid around where the manifolds meet and if the idle goes up you still have a leak.
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Old 07-02-2010, 09:58 PM
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don't use starting fluid... use something less flammable like WD-40 or something.
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Old 07-03-2010, 01:45 AM
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ok i checked all those earlier and the wer all tight. not even one of them was loose. sooo im thinking maybe a new o2 sensor cuz mine is really old.. probably 20 years old. and then replacing the cat and running some good fuel injector cleaner and see if that does it. and if not then i have no idea what to do
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