gtsonoma

2002 Grand Cherokee Limited 4.7 HO Auto 166k miles
Engine will not rev over 2000 rpm while in park / neutral.
In Drive and driving has no power, will not rev over 2000 rpm.
Lots of hesitation, poor idle.
There are no codes in the ECM by either of 3 methods.
1. while holding down the odometer reset button and turn key to on, no codes, only the donE
2. ODB2 port with cheapie harbor freight scanner, no codes
3. elm327 ODB2 / blue tooth to torque tablet app, no codes

New plugs and a couple of injectors 12k miles / 1 year ago, new battery 9 months ago.
Plugs are black now but do cleanup easily.
Fuel pressure at the rail is 50 psi with minor fluctuation
A tight rag over the exhaust does not change the symptoms. I don't think there is a exhaust clog, but I'm not above sawzalling the thing apart

97grand4.0

Perform a fuel pressure leak down test to eliminate a fault in the pressure regulator or leak in the system aka injector.
You say plugs are black? How black
Other than that I would suspect high tension leads breaking down or other secondary ignition, like plug wires or coil pack

Jeepin'_Aint_EZ

How is your exhaust flow though? If a tight rag doesn't change symptoms then I would start there. You can simply unbolt the exhaust from the cat and see if there is a difference. This really does sound like classic symptoms of a clogged cat though.

gtsonoma

97grand40, blackest of black but they did clean up easily. No plug wires on these engines, 1 coil pack per plug and I would think that if a coil was bad it would throw a code and have a definite miss. If I pull any wire it does miss put I've not tried that recently, might be a good way get it to throw a code.

jeepin aint ez, exhaust flow seems as expected, I mean that with a tight rag I could not back up the flow enough to get the engine to stumble nor stall, I'll give taking the exhaust apart a try.

Jeepin'_Aint_EZ

Is the cat getting insanely hot?

gtsonoma

I have not noticed, I would think it would if it was plugged up, I'll get a temp on it next time I have it running

gtsonoma

Mystery solved, the cat in front of the muffler is plugged up. With the exhaust open at the cross member it runs great, but she won't like the sound.... :/

dave1123

BINGO! You could always stick a glasspak in there. On my 2000, there are no 02 sensors near it, but there are 4 upstream on either sides of the cats. The aftercat is just for added emission cleanup and doesn't effect mixtures. Does your state use a tailpipe sniffer?

Jeepin'_Aint_EZ

Nailed it. I would have hit the jackpot if I was a betting man lol.



http://www.rockauto.com/en/catalog/j...converter,5808

New direct bolt-on rear converter.

97grand4.0

Well played, old boy. That was my next guess. (not really). What tipped you off, the black fouled plugs?

Jeepin'_Aint_EZ

Not sure if you're talking to me but ima add my 2 cents anyway. The whole not revving over 2k rpm and lack of power is usually a quick indicator of an airflow restriction. I've seen it a few times working in a shop.

97grand4.0

Yep, I was. I discounted the not revving over 2k part. Was more focused on the black plugs...Come to think of it when I did mine it would rev but just seemed boggy, and my plugs would foul way more often than they do now. In fact I haven't even pulled the plugs in quite a while, just runs too good.

dave1123

Classic cat failure: My wife was driving home from work on the highway in her Dodge K-car when it suddenly lost all power. She was able to get it off the highway and call me. I started it up and it made a lot of exhaust noise from under the hood. The EGR valve had blown completely off the manifold! I nursed it home about 8 miles and the head gasket blew just before I pulled into the driveway! When I removed the cat NO AIR would pass thru it. Plugged solid! The entire matrix had collapsed.

That was the first time I ever had a "torque-to-yield" headbolt setup. You take all the new bolts up to torque, Then go 1/4 TURN MORE!

97grand4.0

*snicker* Dodge K car!