Topic: T.B. swap problem

[Sandman]

I did the 4.0 T.B. swap last weekend on my 91 YJ.  I did everything the post says to do.  My problem is when driving and I push in the clutch to upshift/downshift, the engine would only drop to 1500 rpm.  When I came to a stop with the clutch in, the engine would remain at 1500 rpm for about 5 - 10 seconds then finally drop to normal idle speed.

Well after driving it like this for maybe 50 miles, I changed it back to the 2.5L T.B. and everything is fine again.  With the 2.5 T.B. back on the engine drops to approximately 1000 rpm for 5 - 10 seconds when I push in the clutch, then it drops to normal idle speed.  The only thing that I can think of is that I didn't reset the computer and I made my own gasket (T.B. - to - intake manifold) out of "Asbestos Free - High Temp. Gasket Material (1/32" thick)."  Could using this type of gasket enable a vacuum leak?  I cut it out perfectly!  I also reused the AIS housing gaset and O-ring, but the TPS sensor does not have a gasket.

Any suggestions would be greatly appreciated because I am moving from Missouri to Mass.  I also am pulling a 4X6 cargo trailer behind this Jeep with a 14' canoe on top, so I need all the power I can get.  Especially when I hit the Appilation Mountains.  Thanks

[GreenBayjeeper]

Throttle return spring?  I just had to replace one of them, and it did very similar high idling.  Just a though.

[dunklervogel]

Reseting the computer is definitely something to try.  I did my swap and reset the computer and the first time I started it, it wouldn't even idle I had to give it gas or it would die.  But it quickly adapted and the next couple start-ups it idled normally and I could feel the improvement.  I'd imagine if you didn't reset the computer it would be used to taking in air from the smaller bore TB... and with the larger one on it would throw everything off.  Try reseting it.

[Jeffy]

If the sensors are all working properly, I'd check for a vacuum leak.  The high idle is a good sign for it, especually when you start the engine with the throttle plate closed.  If the idle is slow to drop you rev it, I'd look at the return spring like what mentioned before.  You didn't say whether this was on a YJ or a XJ or a TJ though.

[Sandman]

It is a 91 YJ. Hey GreenBay, my return spring seems nice and snappy.  Was your old one snappay or did it not even ruturn to the set screw?  That's another question.  Should I dare to tweek the set screw below the throttle linkage after verifying no leaks and resetting the computer?  I am under the impression that the set screw shouldn't be messed with since you have to take the T.B. off to adjust it.  Thanks for all the replies.

[Bounty Hunter]

Don't mess with the set screw, the idle is computer controlled, or in your case, controlled by a vacuum leak or faulty sensor or IAC.

[Sandman]

O.K. I won't mess with the set screw.  I'll reinstall the 4.0 T.B. and reset the computer first.  If that doesn't work any better, I'll check for vacuum leaks.  If no leaks are found I'll swap the 4.0 TB's return spring with the 2.5's.  That'll keep me busy for a while.  Thanks everyone for your input. 

[Jeffy]

Are you swapping the IAS Housing over or are you reusing the 4.0L housing?  If you mix those, you'll get a leak past teh plunger.

[Sandman]

No, I'm using the 2.5L AIS sensor and housing.

[Sandman]

Yep, I found out the problem.  Turns out to be a vacuum leak getting through both side of the trottle shaft   I found this out by spraying carb cleaner directly on both sides where the throttle shaft comes out of the TB.  Just to make sure, I put the 2.5 TB back on and did the same procedure.  No raise in RMPs when I sprayed the 2.5 TB's throttle shaft on both sides.

So my next question is does anyone know where I can get new throttle shaft bearings or am I out of luck on this one?  I looked at AutoZone, but they have nothing.  Just wondering if there is some type of rebuild kit for these things or not.  Anyways, thanks again for all of your support. 

[Jeffy]

Yep, I found out the problem.  Turns out to be a vacuum leak getting through both side of the trottle shaft   I found this out by spraying carb cleaner directly on both sides where the throttle shaft comes out of the TB.  Just to make sure, I put the 2.5 TB back on and did the same procedure.  No raise in RMPs when I sprayed the 2.5 TB's throttle shaft on both sides.

So my next question is does anyone know where I can get new throttle shaft bearings or am I out of luck on this one?  I looked at AutoZone, but they have nothing.  Just wondering if there is some type of rebuild kit for these things or not.  Anyways, thanks again for all of your support. 

I haven't heard of any rebuild kits since it's not really something that goes bad.  I wonder what happened to cause it.

[Sandman]

The only thing I can think of is that I took the butterfly valve off and then slid the throttle shaft out to clean everything up real good.  After that I lubed up the roller bearings and put it back together.  I guess I should've left it alone

[Bounty Hunter]

I doubt there is a rebuild kit.  I haven't looked at the bearings, is there a number on them for cross-reference to a timkin bearing?

Could be a worn out TB and you're SOL.

[Sandman]

I believe I repaired the vacuum leak at the throttle shaft!  One night I couldn't sleep and decided to inspect the throttle shaft bearings a little closer.  It turns out that each throttle shaft bearing has a square-cut seal on each side of them (4 total).  The seals on my 4.0 TB were in horrible condition so I picked them out with an awl and measured them with a caliper.  The dimentions I got from the measurements were:
- 1/2" O.D.
- .060" thickness (like measuring the thickness of a washer)
- 3/8" I.D. approximate (my seals were worn badly, that's why it's approximate)
** The throttle shaft diameter was measured at 0.372" which is very close to   
     3/8".

The next day, I went to Ace Hardware and got 4 o-rings of similar dimension.  I cleaned the bearings with carb cleaner, lubed up the o-rings and slipped them right in.  Then I lubed the bearings with high temp grease.  To be sure the throttle shaft wouldn't cut my new o-rings when sliding it in, I wire-wheeled the shaft and sanded any sharp points along it with my Dremel.

I put everything back together and installed the 4.0 TB, started up the Jeep and performed the leak check around the throttle shaft (sprayed carb cleaner).  No rise in rpms!  I'm not sure if this is a permanent fix due to using o-rings instead of square cut seals.  Also the material of the o-rings is not positively known, so they may break down sooner or later.  If that happens, I'll be sure to get Buta-N or nitrile o-rings which resist petroleum products very well.

The o-ring size I used was 1/2" O.D. X 3/8" I.D. X 1/16" thick.  The square cut seal's thickness is 0.060" which is 0.0025" thinner than the o-ring, but I don't really think that matters too much.  Sorry about the novel, I just wanted to explain in detail in case someone else runs into this problem in the future.  It may not be a permanant fix, but it should last for a while.

[Jeffy]

Nice, you fixed it.  Congrats!