Old wive's tale.

Go hold your hand over your tailpipe and tell me how a vacuum could be caused at the manifold.

 

It's a crack in the exhaust manifold not a restriction? High speed gases passing by draw air in through it? It's the same principle as back pressure helping evacuate exhaust gases from the combustion chamber. That's assuming that the exhaust gases are passing by quick enough near the crack that the pressure inside the manifold there is lower than the pressure in the engine bay? IDK if it would occur in between exhaust pulses or not but I think it's possible? You would definitely know more than me though. You were a Jeep tech though right? Does a cracked exhaust manifold make the downstream O2 sensor show more oxygen in the exhaust enough to throw the fuel trim way off?

 

Downstream sensor has nothing to do with fuel trim. It monitors catalytic converter efficiency.

You're digging a hole............

 

Seriously just an honest question. Bad wording on my part I was referring to the 1rst O2 sensor downstream of the crack which in the non California Jeep's for sure does fuel trim.

 

Actually the downstream O2 does measure Cat efficiency. The ECU can then make finer adjustments to the A/F ratio on top of the upstream O2 reading.
Now, the ECU is set to monitor the NOx content at the cat. Excess O2 in the system causes "changes" to these NOx readings and can cause a "tweak" to the final A/F ratio or other idle/power problems.

 

the venturi effect is real. however, no measurable results will occur because of a minute crack in the exhaust manifold of a stock jeep. i've seen very slight changes caused my exhaust leaks in a/f occur on 600+ hp small cube, boosted engines with egt on all cylinders and a/f on both banks. it does and will occur, but not on this stuff.

also, on obd2 - the downstream o2 does nothing but trip a CEL. it offers 0 to change how the vehicle runs. it's a tattletale, nothing more.

 

Yeah. Constant 4.0 liters of pressure versus a tiny crack......

 

I just put in a new water pump on my 4.0. It also has a new thermostat, tubes, and a new belt. I torqued it properly to my knowledge. I idle out of the parking lot and everything sounds normal, normal start up and everything. But when I get to the road and accelerate (going up to 2000rpm at least) it starts to click and the clicks get farther apart when the rpm's go down and the click are really short when the rpm's go up. It's very alarming, but after 45 sec to a minute it goes away runs normal......Any suggestions?

 

Does anyone know where to find stock length front bumpstops for a 1999 Cherokee? It appears the only option is extended or doesn't specify whether they're extended or not. I need 3/4 more bumpstop or so and figured I would put some stock ones in since it's so harsh hitting my diy ones now.

 

Thinking this is what you are looking for. 52004295 is the part # out of my '00 parts catalog.

http://www.morris4x4center.com/coil-...FY9MDQodnO8ETg

 

We have no clue on how big of a crack it is. Thats why I said it is possible. And this is a different system than a race motor. On a Ford EEC and the GM ECU, the downstream is just a CEL tripper, not so much on the ChryCo. On CA style ECUs, it helps finer adjust the upstream O2 voltage.

 

obd2 is obd2. the given sensors on different manufacturers can't (and don't) legally do different things. there is a standard.

even if the downstream o2 does anything, the effect is so small it's not worth mentioning. mine has been cut and ziptied or removed and ziptied for a duration of at least 60k miles with no change in engine operation. a good way to prove or disprove this theory is use the software like hptuners to monitor the engine. they've just recently cracked dodge ecus after years of attempting. after using my laptop and hptuners software for the better part of 10 years i can say with 100% certainty that with regard to gm the sensor voltage is just that. something to trip a cel.

 

I'm just saying that this whole theory of a crack in an exhaust manifold causing drastic changes in O2 sensor readings, and causing major driveability problems is unfounded.

Show me some proof. There's a gajillion 4.0s out there running around with exhaust cracks.....not experiencing driveability issues.

 

Then you just admit to deleting federal emissions equipment. Don't do that here.
Also a standard is a MINIMUM to be met. You can exceed a standard.

I myself am guilty of some things too, I'm sorry. So let's just try to get along here.

I will allow links to outside sources IF that link is to explain or teach to someone to help answer their question.

 

Thanks! Ordered some and hopefully it'll work out great!