Emissions Test and Check Engine Light

Greetings fellas,

I have a '96 Classic w. 4.0l, AW-4 w/ 120-plus miles and just moved to Frederick, MD, where I need a emissions test this month. On que, my check engine light keeps appearing. It seems to go off on a full tank of gas. Here's a few symptoms maybe you can help me with. I just changed my oil a few weeks ago. Put Valvoline max life in (1st time thought I'd try), a can of 6 cyl. Restore, and a bottle of STP injector cleaner. I noticed I smell gas more now. Could it be sludge in the CCV/air hose? No airfilter gunk yet though. Suppose I could open her up and take a look. I also have not changed my OX sensor yet. I changed my plugs, wires, and rotor, etc. at around 100k, so I'm not too concerned there. I'm the only owner and have been ritual w/ most all fixes. I've heared that my gas cap could need replacing maybe too.

Does anyone live outside DC or Balt. that has a OBD II tester who will help me diagnose it? How much is one anyway, maybe I'll buy one. Like most Jeepers, I have little to spend and wrench my own, but want it to be done right w/ as little cash and frustration as my clock is ticking on this emissions test.

Thanks in advance for the help!

PLATZIE

try a new gas cap

I actually sniffed the cap last night. No fumes from the cap. I agree though, start with the least expensive part.

Also, I forgot to mention. Last night on my way back from work, I stopped at a light and had a wierd dip in RPM kinda like a sputter. I was worried, but once I accelerated it was fine. Then at another light it did it again. No signs of that problem today though.

I may make a list and just start replacing parts at this milege. I did searches and am getting worried about CPS's, map sensors, etc. I just had my NSS cleaned and put back in and a new fuel filter.

Anyother ideas?

Just out of curiousity why did you put all that garbage in the jeep for ??
120K is barely broken in, really. Get that 'Max life' out of there, go to a good quality oil like Mobi-1, Castrol Syntec or Valvoline synthetic. Put on a good oil filter like a Mobil-1, K&N [no frams] or Purolator, depend on how much cleaning you want it to do.
Fill the tank up, add a large bottle of Chevron Techron.
Pickup a set of Champion plugs, cap, rotor, wires FROM THE DEALER, change the air filter to brand of your flavor though the OEM from the dealer seems to cost less.
That max life is a really low detergent oil and is made for tired engines more than anything else. Properly maintained your 4.0 is good for well over 300,000mi easy.

The oxygen sensor should be changed every 80,000 miles, max. When it gets "tired," the usual result is an over-rich mix, which could account for the gas smell. It also dumps more unburned hydrocarbons down the exhaust, which tends to burn out catalytic converters rather quickly.

First place look in your owner's manual and make sure your are current on all the published maintenance, especially the "emissions" items. On my 88 XJ they have you change the following every 50,000 miles: cap, rotor, wires, gas cap, CCV harnesses on the engine. There is also a 30,000 mile spark plug change interval.

You are at the mileage where the stuff you mentioned could be in need of replacement. You said your engine idle drops when waiting at a stop light. That is the indication of a dirty idle air controller (IAC). If the passage gets dirty then the IAC can not get to the right position when the engine computer requests it.

Go pay a mechanic you trust to remove the throttle body from the manifold, clean it and then reinstall with a new gasket. Once my dealer mechanic cleaned my throttle body he hooked up the test set and checked over the operation of the engine. he found my idle too high and did the adjustment to bring it into spec. On my 88 XJ you have to calibrate the TPS, he verified it was set correctly. For me it was the best 1.0 hours of dealer labor I had paid for, I had a clean trottle body and knew the XJ was running right.

I recommend you get your throttle body cleaned by a mechanic who understands XJ and the 4.0 engine. My guy could tell from the vacuum readings how dirty or clean my intake manifold was. There nothing wrong with paying a professional who knows his stuff. If you don't have a mechanic you trust then call a dealer, talk to the service dept and tell them your problem. Ask who would work on your XJ when you bring it in, once you have a name then ask to talk to that tech. Phone interview him to get a sense of his knowledge or experience. Then when you show up go out and talk to the mechanic, let him know you are a real person. The ask if you can watch him work, my guy doesn't mind and has asked me "does it do this...." while working so it a win-win for me.

Also after you have it cleaned and checked over and the problem continues consider replacing the IAC. It is just a stepper motor and they do wear out. I just changed mine at 150,000 miles because the XJ would die when I slowed down and the engine would flood. I had to floor the accelerator to open the butterfly to get air in to flush out the gas, then it would start. Once the new IAC was installed, no problems. Parts wear out, might be smart money to replace TPS, MAP, Inlet air temp sensor, CPS and any other sensor providing the engine computer with data to run the motor. I started aroudn 120K miles to replace sensors and it did make a difference in how the XJ ran. The sensors you have work but are degraded, they may be giving readings with larger errors which cause the engine not to run as well. I recommend a sensor replacement program for any XJ with over 120K miles on it. Buy one sensor every time you get paid, that makes it less painless and allows you to mail order if a place like rockauto.com has the best price.