Idle issue with multiple codes FRUSTRATED please look

So my 98 xj ( auto 4.0) has started throwing a few codes and has a weird idle issue. The codes that have popped up here and there are, 306, 420, 171. the 306 and 420 i get pretty often, the 171 has only shown up once. It also gets a chugging idle once warm. It runs pretty much normal while driving but always comes back down to that chugging idle. and as a side note the exhuast does have a distinctive smell to it, not like fuel smell but almost a funky smell. not sure if that is lean or? I replaced all plugs, wires and the cap and rotor as well as o2's. it made no difference at all. This is my dd and the baby hauler since winter is here so i really need it to be dependable and im worried it is going to get worse. I can say it possibly has a cracked ex. manifold but i haven't found any cracks just hear a slight exhuast leak like a tick noise. any ideas???

How does the power feel when driving? Maybe a blocked cat?

changed the fuel filter lately?

306-Cylinder 6 misfire
420-Cat converter efficiency failure
171-Fuel system too lean

I would concentrate on the 171 first. Chasing the lower number codes first is always the best strategy as other codes can be byproducts of the lower number code.

The 171 could be because of a large vacuum leak somewhere (intake, etc) Vacuum inspection and testing would be recommended.

For engine management sensors, I would test the MAP and coolant temp sensor. You'll need a meter and a manual. Both can affect fuel/air ratio.

Verifying proper fuel pressure is easy to do and should be done early in the troubleshooting process. Must be done with a fuel pressure gauge. You're looking for 49.2 psi plus or minus 5 psi. The fuel filter in the 98 is part of the fuel pump assembly, located on top of the gas tank. Not generally serviced as a maintenance item unless testing or symptoms point to it.

thebrick said:

How does the power feel when driving? Maybe a blocked cat?

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power feels good, revs quickly and sounds good

DesertRunner said:

changed the fuel filter lately?

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not since i have owned the XJ but I was under the impression it wasn't servicable

birchlakeXJ said:

306-Cylinder 6 misfire
420-Cat converter efficiency failure
171-Fuel system too lean

I would concentrate on the 171 first. Chasing the lower number codes first is always the best strategy as other codes can be byproducts of the lower number code.

The 171 could be because of a large vacuum leak somewhere (intake, etc) Vacuum inspection and testing would be recommended.

For engine management sensors, I would test the MAP and coolant temp sensor. You'll need a meter and a manual. Both can affect fuel/air ratio.

Verifying proper fuel pressure is easy to do and should be done early in the troubleshooting process. Must be done with a fuel pressure gauge. You're looking for 49.2 psi plus or minus 5 psi. The fuel filter in the 98 is part of the fuel pump assembly, located on top of the gas tank. Not generally serviced as a maintenance item unless testing or symptoms point to it.

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thanks! I did check everything while i was doing the tune up but ill double check all vac lines etc. i have had coolant temp issue's before and i replaced it, wonder if possible it could have gone bad without throwing code? Only manual i have is a Haynes which probably doesn't help with testing these. Will check everything here in a few. thanks so much and ill get back my results asap

This sounds a lot like the issues I'm having with my Renix jeep.. I'm thinking its a bad injector..

My exhaust also smells awful. Did all the color of your plugs match that you took out?

ya the plugs looked all the same for the most part. the number six was good.

I checked the Fuel pressure and the ECT and all looks good. I did notice that today while driving to get my son i was at 50mph which is 1500 rpm's and left off the throttle and it dropped to 1200 and back to 1500 and kept doing this untill the jeep slowed down. I also noticed that i can feel the chugging when cruising. It just threw a code and i have not had a chance to check it tonight but ill check it tomorrow.

Maybe your #6 injector is sticking ... when its misfiring.. unplug the injector and see if the idle sounds the same... also.. taking the fuel rail out of the manifold and turn the key on, and see if any of the injectors drip..

You gotta look at the codes as a group of clues, don't just pick one and start running pinpoint tests.

So what do we have here: a misfire, cat efficiency and a lean code. The misfire is causing the lean code and your cat is now wasted from raw fuel overtemps. I would look at the ignition system first.

Misfires are detected via the crank position sensor. The pcm reads a slowdown in rpm every time, or frequently at least, when said cylinder takes its turn at bat.

Lean codes are tricky. A rich misfire will cause them every time because you have unburned, raw fuel that is high in oxygen content going past the sensor. Remember the oxygen sensor only reads oxygen, not true air/ fuel mixture.

The cat efficiency code: the front o2 should toggle up and down while the rear o2 voltage stays relatively steady. When the rear o2 voltage goes up and down same as the front the pcm infers that the cat is now chemically dead and no longer catalyzing.

So, take a closer look at the recently installed tuneup parts. Something is not doing its job.. and just a thought, the ticking could be a plug wire that is grounding out. If nothing obvious pops up see if you can sweet talk a tech into putting a kV meter on the ignition system.

jacobs, thanks! i do have access to a mechanic and any equip i need but i have been trying not to bother him. Maybe ill just have to. I wonder if a snap on scan tool will read what i need? it does real time readings of all sensors. Do you think the cat is toasted? should i remove it?

Find the cause of the #6 misfire, sometimes brand new plug wires are junk.. look for the simple stuff first. Yes, if you've been running on a dead miss and a P0420 came up, the cat is now trash.

swapped wires around and such and nothing has changed. i also pulled the wire on the #6 plug and it made a noticeable difference and about died. getting confused.

ok so i been tinkering around with it and trying to just check and clean all elec conections and try to test all sensors. so far i know the 02's are good and functioning properly, the map is good, ect is good, ignition system is in perfect order and im no longer getting a miss from #6 however i get a constant 171lean code now and it is spitting black out the tail pipe which i know is the ECU compensating for the lean code by dumping fuel. which gave me a whole 7mpg. Im going to re check Fuel pressure but im lost.

also is there anywhere that has the sensor readings for the 4.0ho motors? i found one for all renix sensors but not the HO. just want to double check what i got off the internet

Check all the fuses. The fuse box behind the battery. The idle problem might be coming from a blown 15amp mini fuse. Check them ALL.

which fuse are you talking about?? i know there is a fuse with the 02 heater circuit that is not labeled

didn't find any blown fuses and FP right at 49 ish. uggg i hate this

OK--you have a reported lean condition, but poor mileage and black sooty exhaust.

The PCM is getting input (or thinks it is getting input) that the fuel mixture is lean, so it is increasing the duration of the injector firing to enrich the mixture.

Now, you need to look for a possible exhaust leak upstream of the first o2 sensor. An upstream exhaust leak will cause the o2 sensor to read lean mixture, and the PCM will enrich the mixture to correct the issue. If the engine is in fact NOT running lean, then the rich mixture will get blown out of the tailpipe.

Get a 3' piece of vinyl tubing--some auto parts stores, most hardware stores--use it as a listening device, with one end stuck in your ear and then move the other end CAREFULLY around the exhaust listening for a leak.

Post back what you find.

I do have what i believe a leak at the header but i can't locate it visually. I have it at a buddies shop right now and he is looking it over. can even the slightest cracked header cause the problems i am having?? if so i will just get a APN because im sure it is leaking somewhere and not going through the trouble of pulling it without replacing it LOL

DementedXJ said:

I do have what i believe a leak at the header but i can't locate it visually. I have it at a buddies shop right now and he is looking it over. can even the slightest cracked header cause the problems i am having?? if so i will just get a APN because im sure it is leaking somewhere and not going through the trouble of pulling it without replacing it LOL

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Even a small crack can have an effect, and small cracks tend to become larger when the header gets hot.

You can use a 3'~4' piece of vinyl tubing, 1/2" up in diameter, and stick one end in your ear and move the other end around the engine bay--when you get close to the noise source you will know it.

thanks for the reply. we are working on it this weekend and hope i find something because i have to have it running for next week or i don't know what ill do. ill let you know what we find or don't find lol