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View Full Version : High RPM Problem - Possible Torque Converter or Clogged Cat?


woodkrawler
July 19th, 2010, 17:12
Vehicle Info:

'99 MJ/XJ
4.7L Stroker w/850 miles
AW4 Tranny

Symptoms:

While driving home from the beach this weekend with the cruise on at 65mph the Jeep downshifts from OD to 3rd going up a long hill. Once it shifts to 3rd it begins to bogg down. RPMs wouldn't hardly go over 3K and the Jeep begins to lose speed. I kill the cruise control and recover speed around 50mph.

With the cruise off I continue to push the Jeep to rev higher than 3K with hardly any luck. :confused1 With the Jeep in Park or Neutral the engine will easily rev to red line. While moving in 4LO it will rev to well above 3K. After further testing tonight the problem only occurs at WOT. Under normal driving conditions it's everything is fine, but push the Jeep any and it struggles to get over 3K.

Tranny fluid and filter were just changed when engine was installed in June and I have "properly" checked it per the FSM and the level is fine.
Unplugged the TCU and all gears are fine, but it still won't rev above 3K.

I can only come up with 2 possible problems? Torque Converter is unlocking above 3K.

OR

Cat is clogged. New MagnaFlow Carsound cat was installed with engine install in June, however I've been running Hi-Zinc Comp Cams oil since break-in. Could it have clogged the cat within 850 miles? I have NOT had a CEL since day one.

What do you guys think?

JJacobs
July 19th, 2010, 17:22
A torque converter can't cause that, that's not the way they work. A torque converter is a fluid coupling mechanism that imparts most of the crankshaft rotation into transmission rotation. The torque converter clutch causes all of the crank rotation to turn into transmission revolutions. The TCC isn't usually locked up under higher throttle conditions anyway.

I would put a fuel pressure gauge on it before condemning the cat..

joe_peters
July 19th, 2010, 18:23
If your CAT is plugged you wouldn't get much in the way of RPMs loaded or unloaded.

Hook up a vacuum gauge. From idle slowly raise the RPMs--if the CAT is plugged the vacuum will drop off as the back pressure builds.

winterbeater
July 19th, 2010, 18:31
Could also be ignition failure under load. A bad spark plug or wire or possibly cap. I've had it happen to me, where it worked fine until it was place under load, and then the spark can't jump the gap, but finds a path of lower resistance.

woodkrawler
July 19th, 2010, 18:33
If your CAT is plugged you wouldn't get much in the way of RPMs loaded or unloaded.

Hook up a vacuum gauge. From idle slowly raise the RPMs--if the CAT is plugged the vacuum will drop off as the back pressure builds.

I don't own a vacuum gauge. I guess I can pick up one at any parts store or rent one? Is there a paticular place I should hook it up to on the intake?

joe_peters
July 19th, 2010, 20:20
I don't own a vacuum gauge. I guess I can pick up one at any parts store or rent one? Is there a paticular place I should hook it up to on the intake?

The port where the CCV vacuum line attaches from the rear of the valve cover will do. You should be able to rent one, and they don't cost an awful lot considering all the information they can provide.

How to read a vac gauge: http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm