Dreaded P0505 will not go away. 99 XJ police package. Here’s what I’ve done so far...

Replaced:
-IAC (twice, two different brands and looks like prior owner did the same)
-TPS
-MAP
-Distributor cap/rotor/pickup coil
-plugs & wires
-ignition coil
-clock spring
-Battery & terminals
-downstream o2

-Cleaned the throttle body and checked PCM pins to make sure none were bent.

-Disconnected battery and did reset procedure for the PCM multiple times.

Code will not go away. Horn doesn’t work still. This may have been a coincidence at the time - prior to clock spring replacement - but I removed horn relay from passenger kick panel and it barely ran. Unless I removed the wrong relay, I thought that was odd which led me even further to thinking it was the clock spring because airbag light was constantly on.

Well, airbag light is off, TPS code is off but p0505 won’t go the heck away. I haven’t checked the wiring harness near the firewall THOROUGHLY yet. I did a quick visual and it seemed fine.

I have never used a multimeter to read voltage or ohms so if you go into that talk, I’ll be lost.

Do you think the next culprit is a new PCM (if wires check out fine in the harness)? I found one locally, same year and trans for cheap. Or could it be another faulty tps/idle?

 

But how is it running at the moment?

 

It seems to be running fine with possibly a very very slight miss occasionally. I think the idle is a tad high too, but I’ll have to check again today due to frustration last night that the code wouldn’t go off. Drove it around to try and let the computer learn again.

 

I haven't seen where you or the previous owner addressed the possibility of a vacuum leak.
https://www.700r4transmissionhq.com/...jeep-cherokee/

 

I’m unsure of what previous owner tried or didn’t try aside from IAC. They weren’t mechanically inclined.

today, I re-routed the plug wires, took out kick panel horn relay (horn wasn’t working but relay was fine) and forgot to put the relay back in. Started it up, no more CEL. Not sure what made it go away, but I’ll assume plug routing(?).

Edit; cel is back upon startup

 

 

The following is from the 2000 service manual. You can back probe the ECU or the IAC pins to check for the 12v and ground signals, but I'd just do a simple continuity check to see if there's a broken wire somewhere. Generally, broken wires on the xj harness are right at the junction under the fuel rail towards the back of the engine.

(From the FSM)
The PCM uses the IAC motor to control idle speed
(along with timing) and to reach a desired MAP dur-
ing decel (keep engine from stalling).
The IAC motor has 4 wires with 4 circuits. Two of
the wires are for 12 volts and ground to supply elec-
trical current to the motor windings to operate the
stepper motor in one direction. The other 2 wires are
also for 12 volts and ground to supply electrical cur-
rent to operate the stepper motor in the opposite
direction.
To make the IAC go in the opposite direction, the
PCM just reverses polarity on both windings. If only
1 wire is open, the IAC can only be moved 1 step
(increment) in either direction. To keep the IAC
motor in position when no movement is needed, the
PCM will energize both windings at the same time.
This locks the IAC motor in place.
In the IAC motor system, the PCM will count
every step that the motor is moved. This allows the
PCM to determine the motor pintle position. If the
memory is cleared, the PCM no longer knows the
position of the pintle. So at the first key ON, the
PCM drives the IAC motor closed, regardless of
where it was before. This zeros the counter. From
this point the PCM will back out the IAC motor and
keep track of its position again.
When engine rpm is above idle speed, the IAC is
used for the following:
• Off-idle dashpot (throttle blade will close quickly
but idle speed will not stop quickly)
• Deceleration air flow control
• A/C compressor load control (also opens the pas-
sage slightly before the compressor is engaged so
that the engine rpm does not dip down when the
compressor engages)
• Power steering load control
The PCM can control polarity of the circuit to con-
trol direction of the stepper motor.
IAC Stepper Motor Program: The PCM is also
equipped with a memory program that records the
number of steps the IAC stepper motor most recently
advanced to during a certain set of parameters. For
example: The PCM was attempting to maintain a
1000 rpm target during a cold start-up cycle. The last
recorded number of steps for that may have been
125. That value would be recorded in the memory
cell so that the next time the PCM recognizes the
identical conditions, the PCM recalls that 125 steps
were required to maintain the target. This program
allows for greater customer satisfaction due to
greater control of engine idle.
Another function of the memory program, which
occurs when the power steering switch (if equipped),
or the A/C request circuit, requires that the IAC step-
per motor control engine rpm, is the recording of the
last targeted steps into the memory cell. The PCM
can anticipate A/C compressor loads. This is accom-
plished by delaying compressor operation for approx-
imately 0.5 seconds until the PCM moves the IAC
stepper motor to the recorded steps that were loaded
into the memory cell. Using this program helps elim-
inate idle-quality changes as loads change. Finally,
the PCM incorporates a “No-Load9 engine speed lim-
iter of approximately 1800 - 2000 rpm, when it rec-
ognizes that the TPS is indicating an idle signal and
IAC motor cannot maintain engine idle.
A (factory adjusted) set screw is used to mechani-
cally limit the position of the throttle body throttle
plate. Never attempt to adjust the engine idle
speed using this screw. All idle speed functions are
controlled by the IAC motor through the PCM.

 

Just discovered I have the police package. When idle I’d lower, the ext switch works to bring it to 1000rpms. But last night it hasn’t left 9-1000rpms consistently when in park, or drive. Damn thing wants to go. So the switch obviously didn’t change a thing.

Also discovered that when turning ignition on, p0505 code shows up. I clear the code and drive as long as I want and it doesn’t come back until I shut the xj off. Prior, it was coming on right away when driving.

Still at a loss here. I still notice a very tiny miss at a stop light. Small, but enough for me to be bothered by it. I’m about to take it to a shop.

 

After a number of heat cold cycles the IAC body could warp, change shape. I have seen the special screws actually bottom out, and the body then cannot squeeze the gasket,,,, and you have vacuum leak ...
I removed mine, carefully filed the body FLAT again, then added a small flat washers to give the screws a chance to flatten the gasket.... my leak went away.