Topic: Weekend upgrades for my YJ & TB question

[Bounty Hunter]

I have the stock 4 liter TB (60mm bore), TB spacer

I believe the stock 4.0L TB is a 55mm bore.  One of the greatest improvements to boring out a TB is removing the taper below the throttle plate, making throttle response instant.  In stock trim, the throttle plate has to move 1/4" or more before any air gets between the throttle plate and the tapered bore beneath the throttle plate.  Look down the neck of the TB and move the throttle, see for yourself.

I've got a comparison pic over in the classifieds section comparing the 4.0L TB to the 62mm TB's I'm now selling (cheap). I've also got 62mm spacers.

jfrabat, I would remove the IAC housing and clean it thoroughly, along with the IAC plunger.  If that doesn't work, try installing a different IAC plunger assembly.  Mine idles at around 6-700 with a 62mm TB.

[jfrabat]

I believe the stock 4.0L TB is a 55mm bore. 

I always thought it was 56mm, but I guess both are too close to call!

Ijfrabat, I would remove the IAC housing and clean it thoroughly, along with the IAC plunger.  If that doesn't work, try installing a different IAC plunger assembly.  Mine idles at around 6-700 with a 62mm TB.

Yeah, I did that...  And adjusted the hex screw that adjusts the throttle plate (I forgot to do that the first time I installed the 4.0L TB, which is why it was idling at 1,500RPMs then!).  And I installed a new plunger.  But I'll clean it again just in case.  I do think it is possible that I have a vacuum leak somewhere, though, and that could be the culprid. 

But the idle is now at the same level it was with the 2.5L TB, so I am pretty sure it is not the TB that is causing the higher idle.  It does take it a couple of seconds to bring the idle back down when getting off the freeway (as soon as you let go of the throttle, it is around 1,300RPMs, but then goes back down to 900).

Felipe

[Bounty Hunter]

Could be bad seals on the throttle shaft bearings, if not leaking anywhere else.

[aw12345]

spray around the throttle shaft and throttle body with propane or some brake cleaner even a spray bottle with water works to find vacuum leaks

[Bounty Hunter]

I gave this some more thought, are you sure you have the correct IAS housing for the 2.5L?  That higher rpm sounds like it might be a larger diameter 4.0L IAS housing with the smaller diameter 2.5L IAS sensor.  This would let the extra air by resulting in the higher idle.  Just a stab at it.

Your IAS housing should have 2.5L cast into it in large numbers, I've got a bunch if you need one.

[jfrabat]

I am pretty sure it is the 2.5L one...  As well as the plunger.  I think it may have to do with a couple of things; one is that the throttle plate of the bored out 4.0L TB was a very tiny (visibly if you take the TB and look through it into a light) gap on the sides (gap is less than 1mm, but I think it is still letting a bit of air through).  What I think this is causing is the initial high idle when I get out of the highway.  Eventually, the IAS brings it back down, though.

The other thing I am suspecting is that I have a vaccum leak somewhere, because I had the same idle speed with the entire 2.5L TB when I bought the rig.  By the way, the idle I am measuring by reading the RPM's from the dash, so it may be a little lower, but I doubt it, because it "feels" like what it is reading.

In any case, how much are you selling the 2.5L IAS housing for?  And do you have the gasket?  I may get one from you to have both TB built (that way I can temporarily replace the 4.0L to work on it and get it working right, while not being stranded without my daily driver).

Felipe

[Bounty Hunter]

I would ship you a IAS housing for free.  It could be caused by poor machining if that much air is getting around the throttle plate.  Maybe you need one of my 62mm TB's, only $85

Pictured here on the bottom:

[jfrabat]

I would ship you a IAS housing for free.  It could be caused by poor machining if that much air is getting around the throttle plate. 

In that case, I'll take you up on the offer (I'll send you the address via PM).  I checked yesterday, and the IAS housing IS the 2.5L.  As for the machining, I am not sure, because at the end of the day, I am ending up with the same idle I had with the stock 2.5L TB, so that is why I am thinking more it is something else...

[Bounty Hunter]

It's in the mail, IAS housing and sensor.  They're a little dirty on the outside, but you can deduct cleaning from the price 

[jfrabat]

Thanks a lot, bounty...  Now I am debating weather to do the mechanical work I was planning for this weekend or not because there is a Jeep activity about an hour from me that I want to go to!  But I do have to get moving on checking for leaks at some point!