Quick RPM Drops -> Power Loss & Gas Mileage

So I've had my 99 4.0 XJ AW4 for a year and a half and gas mileage has always been terrible (10 city, maybe 14-15 highway bone stock). Needless to say I've read every gas mileage thread on here. I'm investigating everything I possibly can to bump my mileage and lately I've been focusing on what I think is odd engine behavior.

When I lift off the throttle completely, I feel like my RPM's drop much faster than they should. For example, if I push it a little to about 3,000 RPM's and then let off the gas completely before the engine can upshift, RPM's drop to around 800-900 in at most 1 second, followed by a blip up to about 1,000-1,100 where it stays steady until I slow to a complete idle. She idles around 750 pretty smoothly but with what I think is probably the characteristic 4.0 occasional stutter. With the Jeep parked and the hood up, I can blip the throttle by hand and notice this same type of behavior (after letting off the throttle, the engine slows and then "rebounds" a bit, noticeable both by sound and a by a bit of a shudder as the RPMs rise again).

I've been meaning to take a video of the tach while driving to demonstrate just how quickly the RPM's drop. I'm thinking something like a vacuum issue/exhaust leak/backpressure buildup might be to blame, but I can't find any vacuum leaks though I do hear a sort of low "airy" howl when letting off the throttle (sounds like it's coming from just in front of the passenger side firewall, but strangely more noticeable from the cabin than from under the hood.)

I have no engine codes. Here's a list of all gas mileage related things I've done since I've owned it (within the last 10,000 miles). Many of these things were probably ok but with 106,000 miles on her, I changed them anyway:

new Champion Truck plugs, Mopar wires, cap, & rotor
new O2 sensors
new Coolant Temp Sensor
new Intake Air Temp Sensor
new stock Thermostat
new Magnaflow Direct-Fit Cat
new OEM exhaust manifold (w/ the accordion-style joints)
new CCV orifices and grommets
cleaned Throttle Body
multiple Sea Foam treatments
new Revo's at 36 PSI
Mobil 1 in the diffs
Mobil 1 High Mileage in the engine

The only things I haven't done are cleaned the IAC, and haven't checked the MAP. In general, it runs pretty smoothly but seems down on power (my old girlfriend had a 5-speed 4.0 wrangler, which is my only base for comparison.) Any ideas on what might be causing RPM's to drop so quickly? Call it naivete or blind optimism, but I'm hoping that solving this would get me closer to the 13/14 city and 18/19 highway that I'm expecting out of the Jeep. I'll try to take a video today to demonstrate.

I dont see a new TPS in that list!!!

Is the Torque Converter TC locking up?

Check your TPS and as you said the IAC

Don't seem to be any problems with the TC. Transmission shifts fine and no slipping. But since you mentioned it the RPMs don't drop nearly as fast when the Jeep's in park and I blip the throttle. Only happens when driving, so I wonder if it's something TCU or otherwise computer related. I plan on running a data log with ODB software of MAP, RPMs, throttle position, etc. Maybe I can post a graph of that stuff later. Thanks for your reply.

Just checked the MAP and TPS. MAP is spot on. Service manual specs for the TPS are between .26 and .95 volts at closed throttle and "less than 4.49 volts" at WOT. Mine tested at .91 at closed and 3.9 at WOT. Voltage changes smoothly throughout the throttle range. I'm wondering if anyone knows or thinks that the 3.9 is excessively low. The throttle plate opens fully so it's not due to anything mechanical. I'd think that the readings from my TPS would give me a limited range of the full available throttle response. Could it be possible that a new TPS would be in the same range or would I be guaranteed to get a greater range? Anyone? Still need to clean the IAC too...

1. IAC (clean or replace)
2. Injectors (replace)

I would clean the IAC. Also check and see if it works with the engine running see if you can see the plunger moving when you turn on the A/C or go from park to drive. I have an 88 XJ and you can see it work with the intake boot off not sure about yours since it is newer. Also I would check the injectors or upgrade to some mustang injectors. My 88 XJ has 205,000 mile on it I thought it ran good until I put replaced my injectors with a set a friend gave me. WOW what a difference it made the Jeep revs faster, the idle is smoother, gas mileage went way up, it will burn off the 31x10.50 mud tires on concrete when before it wouldn't hardly peel out going around a corner.

Cleaned the IAC. It was very sludgy. Noticed a little less of the RPM "blip" that was happening after I left off the gas and a slight (very slight maybe 50 RPM) change in idle speed when shifting from Park to Drive. Never did that before, it just always idled at the same speed. Still have suspicions about whether the pindle is working properly. You can't see the pindle through the slot in the throttle body and I didn't have an extra pair of hands so I think I'll pull it and check it with the engine running when I can round up a helper.

As for the injectors, no leaks as far as I can tell and no codes. I've been tempted to go the Mustang route but right now I'm trying to fix what may be broken before I start upgrading stuff. Thanks for all of your replies.

I had a similar problem...athough I don't think mine was quite the same. When I would let off my rpm would drop instantly below 700...sometimes even stalling the engine when it did. It was worse when I was running the AC...which was making a noise on the passenger side though. I replaced battery cables and battery because it seemed like no matter what I did, I could not get a good connection. The problem went away completely after that.

swampass2 said:

Just checked the MAP and TPS. MAP is spot on. Service manual specs for the TPS are between .26 and .95 volts at closed throttle and "less than 4.49 volts" at WOT. Mine tested at .91 at closed and 3.9 at WOT. Voltage changes smoothly throughout the throttle range. I'm wondering if anyone knows or thinks that the 3.9 is excessively low. The throttle plate opens fully so it's not due to anything mechanical. I'd think that the readings from my TPS would give me a limited range of the full available throttle response. Could it be possible that a new TPS would be in the same range or would I be guaranteed to get a greater range? Anyone? Still need to clean the IAC too...

Click to expand...

Check the input voltage to the TPS first. It should be close to 5 volts. If it is low, like less than 4.7 volts, the problem may be voltage loss in the wire from the ECU (PCM). If the input is 5 volts, or close to 5 volts, you may have lost enough span, or range in the TPS min to max voltage span in the output to cause your problem. A span of 0.85 to about 4.25, with a 5 volt input would be optimal.

Ecomike said:

Check the input voltage to the TPS first. It should be close to 5 volts. If it is low, like less than 4.7 volts, the problem may be voltage loss in the wire from the ECU (PCM). If the input is 5 volts, or close to 5 volts, you may have lost enough span, or range in the TPS min to max voltage span in the output to cause your problem. A span of 0.85 to about 4.25, with a 5 volt input would be optimal.

Click to expand...

Should have mentioned that input voltage to the TPS checked out fine. Also, I just pulled the IAC again. Not sure if it's working right. Here's what happens:

1) Turn ignition on, engine off, IAC pintle retracts.
2) Start engine, pintle pushes out slightly.
3) Put it in drive, pintle pushes out more.
4) After that, no matter what I do, the pintle doesn't move again until I turn the engine off and then turn ignition on again.

Also at one point, I think with the engine in drive, the pintle moved out until both the pintle and the spring popped off. After reseating the pintle (properly I think), I noticed that the motor was turning but the pintle was going nowhere. So at this point, I'm thinking replace both the TPS and the IAC.

swampass2 said:

Should have mentioned that input voltage to the TPS checked out fine. Also, I just pulled the IAC again. Not sure if it's working right. Here's what happens:

1) Turn ignition on, engine off, IAC pintle retracts.
2) Start engine, pintle pushes out slightly.
3) Put it in drive, pintle pushes out more.
4) After that, no matter what I do, the pintle doesn't move again until I turn the engine off and then turn ignition on again.

Also at one point, I think with the engine in drive, the pintle moved out until both the pintle and the spring popped off. After reseating the pintle (properly I think), I noticed that the motor was turning but the pintle was going nowhere. So at this point, I'm thinking replace both the TPS and the IAC.

Click to expand...

Sounds like physical damage to the IAC.

How did you get the engine to run with IAC removed from the throttle body?

engine runs with IAC rmoved but still connected to the wiring harness. just revs high from the extra air coming through the IAC hole to the throttle body.

swampass2 said:

engine runs with IAC rmoved but still connected to the wiring harness. just revs high from the extra air coming through the IAC hole to the throttle body.

Click to expand...

OK, but I would think a clean throttle body with no IAC would red line the engine at a very high idle, but I don't know for sure. Could there be some loose trash hung up in the IAC throttle port?

I pluged an old , bad, IAC into my throttle body, and wired up the loose IAC for testing to avoid too high of an idle during the testing.

I'm still sort of stumped. Put in a new TPS and it showed the same voltage range, so I put the old one back on. Replaced the IAC and it idles higher when first started, put it in park and the RPM's go down, but then after that no change in idle speed (i.e. if I shift back into Park the revs don't go up at all) RPM's are still dropping fast when the Jeep's in Drive, but not when I rev it in Park. I'm starting to fear that maybe there's a problem at the ECU end or maybe with the TCU not sending the proper signal to the ECU about whether the tranny's in drive or not. Any tips on how to diagnose those two issues or any other thoughts? Thanks.

You need to periodically disconnect the battery for a while, then reconnect and drive it so the computer can relearn the sensors and control devices. All that disconnecting and reconnecting is confusing the computer and letting it store bad data. May not be your problem, but it sure adds to confusion and diagnostic troubles. Do the battery disconnect, relearn every time you swap a part, before deciding if it had any useful affect.