ElMartillo

91 XJ Sport 4.0 Automatic 4x4

My ‘89 consistently gets 15-17mpg

On it’s fourth tank of gas now since I bought it a few weeks ago, this new to me ‘91 has been closer to 12-15 mpg.

Upon startup it sometimes revs a bit high initially, sometimes starts normally, sometimes when warm misfires/stumbles a bit but then smoothes out. Otherwise runs great once started no matter what, though it feels a little less peppy than my ‘89 and ‘96.

I have replaced plugs, entire distributor assembly (the one that came in it had side play in the rotor shaft), cleaned IAC. 4-hole refurbished 703 injectors on the way.

Does it make sense to clean O2 sensors in an attempt to improve mileage?

Thanks.

4.3L XJ

No, it does not. How long since it was changed?

ElMartillo

No idea. Like I said in the original post, I bought this one a few weeks ago. It has 107K miles. I can safely assume the sensors are original to the vehicle.

tjwalker

Replace it. Oxygen sensors have a finite lifespan and they aren't that expensive; cleaning not recommended. Replace it with an NTK oxygen sensor, which is original equipment for your vintage XJ. Don't use Bosch and don't use any of those total crap no name sensors floating around on ebay.

However, that new oxygen sensor is not going to help you with symptoms on initial startup as the oxygen sensor isn't even in play yet. Your engine stays in "open loop" for a period of time before it starts taking input from the oxygen sensors in closed loop mode.

I'd also recommend finishing up that tuneup with new quality plug wires and cleaning the idle air control and throttle body. More on this below.
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The Idle Air Control (IAC) is mounted on the back of the throttle body. (front for 87-90) The valve controls the idle speed of the engine by controlling the amount of air flowing through the air control passage. It consists of a stepper motor that moves a pintle shaped plunger in and out of the air control passage. When the valve plunger is moved in, the air control passage flows more air which raises the idle speed. When the valve plunger is moved out, the air control passage flows less air which lowers the idle speed. Over time and miles, the IAC can get carboned up which can have an adverse affect on idle quality. Cleaning the IAC may restore proper function and is an easy procedure to perform and good preventive maintenance so it is never a bad idea.

CLEANING THE JEEP 4.0 IDLE AIR CONTROL

Remove the air filter cover, associated hoses and the rubber boot that goes from the air filter cover to the throttle body. Remove the IAC with a torx driver (2 bolts; one can be kind of hard to get to)

“Gently” wiggle out the IAC from the throttle body. Gasket on the IAC can be re-used if it is not damaged

Clean the IAC with a spray can of throttle body cleaner; inexpensive and available at any place that sells auto parts. Throttle body cleaner is recommended rather than carburetor cleaner as it is less harsh, safe for throttle body coatings and is best for this task. Use cleaner, a rag and a toothbrush and or Q-Tips. Be gentle; don’t twist or pull on the pintle that protrudes from the IAC as it is fragile and you could damage it.

Thoroughly spray clean and flush where the IAC seats in the throttle body with the same spray cleaner

It is also a good idea to clean the entire throttle body itself, the butterfly valve inside of the throttle body and all associated linkage as long as you have things disassembled

Reinstall IAC and check idle quality

ElMartillo

Thanks for the reply and info.

Though it generally runs well, I’m in the process of getting this new to me vehicle in top running condition. I like to perform one repair at a time to see what the results are. Throw a bunch of new parts on at the same time and you don’t know what fixed the issue.

I suspect the injectors may be the source of my startup issues, and will be a good upgrade anyway.

I’ve already cleaned the IAC as mentioned, which helped with a previous issue.

After the injector upgrade, I’ll monitor mileage and the startup issue.

To clarify then, can the oxygen sensors, if old, affect mileage?

Thanks again.

BlueRidgeMark

Oh, shut up. You make way too much sense!


Yessir!

Could also be a vacuum leak, since it's there right at start up.

ElMartillo

I know, I’m a freak like that.

Tell me why you think a vacuum leak could be the culprit. Consider that the startup issues seem to be inconsistent. Dead cold, warm, doesn’t seem to be an issue.

BlueRidgeMark

Vacuum leak = too much O2 in exhaust. Computer dumps in more fuel to compensate for the lean condition. And trust me, vacuum leaks can be intermittent!

4.3L XJ

I would like to respectively disagree. It is a cracked header that has too much O2 in the exhaust. The ECM will compensate for a vacuum leak, which can cause other issues. You don't have a cracked header do you?

BlueRidgeMark

So you're saying that vacuum leak will not lean out the mix, causing the ECM to dump in more fuel?

4.3L XJ

That is correct. However a worn out O2 sensor will deliver a weak signal and cause a rich mixture because the ECM thinks it is running lean and give it a little extra go juice

cruiser54

I always make sure the intake manifold fasteners are snug. They always had a tendency to loosen up.

BlueRidgeMark

I don't believe it is. A vacuum leak will add air to the air-fuel mix, making it more lean, which will cause the ECM to add fuel. The ECM is monitoring the TPS and the O2 sensor, adjusting the fuel accordingly. The vacuum leak messes up the relationship between the TPS position and the amount of air in the mix. It's like having a mis-calibrated TPS.

cruiser54

I've never been able to wrap my head around an exhaust system under pressure pulling outside air in......

Not to mention how many Jeeps I've had with cracked exhaust manifolds, and when observing the fuel trims, oxygen sensor movement, and MAP readings with the factory tester, were all fine......

97grand4.0

How many cross counts do you see on a healthy 02 sensor? (front)
Mine looks like about 1 per second. Thinking that might be slow.