Very subtle vibration/ miss (?) At low idle

99 XJ stock daily driver, 230k, well maintained. One owner. I love this car. Had it all my life and it's been incredible.

Only problem... its had the exhaust manifold crack/ leak forever. Always ran fine so never bothered fixings but it's got pretty bad so ordered a replacement.

Recently I've been noticing a very subtle intermittent vibration, almost like a slight missfire after it warms up and the idle drops to 500rpm.

Starting, driving, everything else is normal. If I hadn't had this car so long I prob wouldn't notice.

Cleaned the IAC
Changed the plugs
Cleaned the rotor/ cap contacts

No changes.

What should I be checking?

My intuition says this is an ignition issue. I plan to replace all the ignition components. I saw him someone on YouTube post a video of very similar symptoms and he replaced the ignition coil and not solve the problem so that might be top of my list.

Could my neglected exhaust manifold have damaged my O2 sensors? Can I check them? AVG MPG is about 17. Passes emissions.

Injectors?

I don't think it's a vacuum issue because idle is not high.

Do I just start replacing things one by one?


She is my baby and I feel like she is sick any help diagnosing her would be greatly appreciated.

Also no codes. Engine buns no oil.

Both my xj's have the idle stumble ... Very faint and I just don't bother with it. I've always attested it to old injectors with improper spray shape since they are so old .

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Never shotgun random parts at an undiagnosed problem/symptom.


Try some fuel injector cleaner in the gas tank. Make sure ALL the tune-up parts are fresh. Test the ignition coil. Test the O2 sensor function.

The Jeep in-line 6 cylinder engines are not known for their smooth idle, if it doesn't stall, it is probably just old age. My CJ-7 idles 2x worse than my XJ's.

Tim what are your thoughts on seafoam down the throttle body of these old motors for buildup carbon?

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SlowXj said:

Tim what are your thoughts on seafoam down the throttle body of these old motors for buildup carbon?

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Can potentially cause more harm than good.

If the valves are having issues functioning because of carbon build up, follow the JEEP TSB 09-003-03 for this issue. www.wjjeeps.com/tsb/tsb_wj_0900303.pdf . Otherwise, keep all your engine additives in the gas tank or the oil pan.

Seems like the idle miss could be due to the cracked exhaust manifold messing up O2 sensor readings. Have you looked at the fuel trims? Assuming that the fuel trims that are available for monitoring are all the fuel trims the engine uses, then a cracked exhaust manifold may show up as a consistent rich long term trim.

I once read that the ignition system worked the hardest
when the engine was at idle. Something about the slower
combustion event in the cylinder.

I'd check the old and worn out ignition wires. They should
have about 8-12K ohms per foot when checked with an
ohmmeter.

Tim_MN said:

Never shotgun random parts at an undiagnosed problem/symptom.


Try some fuel injector cleaner in the gas tank. Make sure ALL the tune-up parts are fresh. Test the ignition coil. Test the O2 sensor function.

The Jeep in-line 6 cylinder engines are not known for their smooth idle, if it doesn't stall, it is probably just old age. My CJ-7 idles 2x worse than my XJ's.

Click to expand...

Since the fuel rail is coming off and they are the original injectors I was thinking that might be a good time to replace them? Do they go bad or are the stock one likely to be better than anything I'm going to buy anyway?

How does one check the O2 sensor and do I only need to worry about the one on the intake or all of them?

I also read fuel pressure delivery problems can cause this kind of behavior and so I will try and get the pressure gauge on the fuel rail as well.

I don't know how reliable doing a resistance test on things like the coil will be? It seems that there are a number of people reporting the components look ok on the meter but ended up being faulty anyway which is why if I can't come up with anything I was considering just replacing stuff that prob is due to fail anyway?

Just about everything can be tested with a basic volts/ohms multi-meter, search on Google, or check out YouTube. BleepinJeep and ScannerDanner are probably the best on YouTube.

I believe in preventative maintenance by replacing parts that have not yet failed, but both my 98 and my 99 have the original injectors with almost 192,000 miles and with 284,000 miles. I did notice two injectors with cracked plastic and replaced them with junkyard injectors. Start with the cheap and easy stuff, add some injector cleaner to the next two gas tank fill-ups. The O2's closer to the engine are more important, but the testing is simple.


When you buy parts, keep mind that most auto parts stores sell cheap crappy Chinese made parts, some that even come with a " Lifetime Warranty ". These parts are poorly manufactured and/or made from inferior materials. They are often out of specification, or even failed, right out of the box. The ones that are not faulty many times will have a short service life before they fail. Always buy top quality replacement parts and genuine Jeep engine sensors. Numerous threads detail long and frustrating searches for a "problem" that ended up being cured simply with genuine Jeep repair parts.

Cheap parts are cheap for a reason.

I get that. In many cases now though some oem is no longer available.

I replaced cap, rotor, plugs, iac, coil, wires... no change.

I'll do the upstream o2 sensor and exhaust manifold next.

Does the 02 sensor in the intake contribute to idle? I see many threads about the Upstream O2 sensor and idle but not that one.

The O2 sensors are in the exhaust. The upstream one is before the cat, and downstream is after. Before replacing the O2 sensors, you might want to first get your hands on a OBD2 scanner, and check the fuel trims. There are now a bunch of loggers/scanners that plug into the OBD2 port and then can communicate with your phone over bluetooth. You'll need an app such as Torque installed on your phone. I find these very handy for diagnosing some issues.

I will order a better code reader.

My current one says:

Oxygen Sens Mon: Incomplete
OXYGEN sens Htr: Incomplete
Egr system: N/A
HtD catayst: N/A
Evap system mon: incomplete
SEC air system: N/A

Is this normal or does it indicate I've got a bunch of sensor problems?

Yeah, get a better one. I believe the incomplete is referring to some ODB2 tests the computer runs that take a while to complete before the results are available to the scanner.

Get one that can display the various sensor readings (not just status codes and error codes). The VeePeak BLE+ combined with Torque on my phone works great. $50 total investment but well worth it for a OBD2 vehicle that often has something going wrong with it.

A lot of good tips posted. Sometimes slight vibrations can be caused by driveshaft universal joints starting to go. With engine Off, crawl under the XJ and rotate the driveshaft back & forth with one hand while holding the universal joint with the other hand. If there's any play in the universal joint they need to be replaced.

For the slight miss, I'd run some injector cleaner through the tank. Likewise, I'd pull/check the plugs and spray a can of GUMOUT carb cleaner into all cylinders, re-install the plugs, let set overnight and fire it up in the next morning. After a little smoke, any stuck piston rings should free up.

Best regards,

CJR

Benthos said:

My intuition says this is an ignition issue. I plan to replace all the ignition components. I saw him someone on YouTube post a video of very similar symptoms and he replaced the ignition coil and not solve the problem so that might be top of my list.



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Injectors?

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I've got this exact miss as well. Super light and only possible to feel at idle. Does yours feel like it's specifically happening once per engine cycle (one cylinder only?).


I can report from my own experience that brand new injectors and ignition coil did not change anything at all



Compression on my engine is all within spec as well. The only lead I have right now is TSB 09-003-03. I pulled all my rockers and found that the exhaust valve on cylinder 6 was not rotating. I'm wondering if the valve stem is totally gunked up with carbon.