My jeep is running pretty good, but once an hour or so while wheeling it will stall.

If I have the trans in D or R with my foot on the brake hard enough that the vehicle is stopped, idle will drop real low and basically stall. If it gets low and I drop the trans into N, the RPM will climb back to normal almost instantly.

The CEL will come one when the idle goes low, as code 33, high MAP voltage. When idle comes back up, the light turns off.

Replaced the MAP sensor, does the same thing no matter what. If I disconnect the map vac line and plug it, and plug the map sensor port, the same code 33 comes up, which makes since. Basically when the Jeep goes too low of an idle, it doesn't create any vaccuum and sets the code. Same thing if I plug the hose so there isn't any vaccuum at all.

Any guesses? I'm not sure if my gearing and or hyd assist has anything to do with it or not….

What ecm and chip are you running?
What sensors did you retain.
Lack of a VSS can cause idle proplems when coming to a stop.

Are you running an auto or manual. does the chip match the setup, did you hook up the pk/n wire?

The ecm will also command a keep alive signal if the idle speed drops below the preset limit.
Then it will shoot the IAC open to try and get the rpm's up.

Is your IAC working properly?

Have you performed a minimum idle speed and IAC reset procedure?

Is your TPS set to ~.54v at idle?

thanks for the fast reply

1 no chip, 89 ECM from a truck with a 350/man trans and a Painless harness
2 Using all sensors except no VSS is wired, but I have it in a box lol...
3 I am running a TH400, so no elec needed. I have wired the N switch both ways and it does the same thing both ways
4 I have cylced the IAC in the past to test it, so I assume it is working
5 I have done the IAC reset described on the Painless website as follows
5.5.2 Base Engine Idle Adjustment (all TBI & TPI)
A. Turn on ignition but do not start engine.
B. Jumper A & B of the ALDL as if you were checking codes.
C. Wait 30 seconds and then remove the plug from the IAC motor.
D. Remove jumper from ALDL.
E. Start engine and adjust idle speed with adjusting screw at throttle lever
(there may be a cap covering the adjusting screw that will need to be
removed and discarded).
F. Shut off engine and disconnect battery for one minute.
G. Plug the IAC connector back in and then reconnect the battery.
H. Start engine and check for proper idle speed.

haven't checked TPS voltage at idle. Do I just unplug the TPS harness and check across 2 terminals? Any idea which 2?

The actual numbers and BCC off the ECM and chip would be better, like 1227747 and ASDW....
But assuming,

You are running an automatic with the manual bin. just disconnect the pk/n wire completely. Manuals have it open.

Here's the steps I normally recommend on initial system set up.

1. If you don't have the free winaldl program and a cable to monitor your system...get it!!! Google!!

1a. Set your timing properly. Most are 0° with the bypass disconnected.

2. IAC reset/min idle speed- This sets the throttle plate on the throttle body to a specific rpm. I recommend about 500 rpm for most engines. Procedures- Go to sheet 7 of the pinout diagrams. At the top is the ALDL connector. You need to put a jumper between pins A and B of the connector. These are pins A9(wht/blk) and A12 (blk/wht) on the ECM. This puts the ECM into aldl mode. Turn the key on (not start) and the IAC should drive completely closed. You will normally hear a buzzing from the IAC. If you have the winaldl up and running on your laptop, you should see the IAC went from 145 to 0. Now unplug the IAC connector. turn the key off. Remove jumper. Start motor. If the idle is too low you may have to keep it running. On the front drivers side of the throttle body is the adjustment screw. It may be behind a silver plug. If so just use a nail or punch to poke it and remove it. Then use a #20 torx bit to adjust the idle speed to at least 100 rpm less then your warm idle speed. if 600 then set to 500 rpm. You are looking for the lowest consistant idle your motor will do. When set, turn off motor and reconnect the IAC. I try to keep the IAC count to at least 40 with the motor warm and idling.

3. TPS-throttle position sensor. With the idle set, you need to now set the TPS. This need to be set to about .54V. I normally shoot for about .6V.
The TPS is on the passengers side and will have 2 screws holding it on. The factory TPS will not have much room in the holes for adjustments so I use a drill bit or file to elongate the holes as needed. To adjust, just loosen the screws and trist the TPS. You can monitor the voltage on Pin B of the connector with the key on or at pin C13 of the ECM. You can also see it on the sensors tab of winaldl!! Anything away from .54V will also show as a percent of throttle. Such as .2 percent or 1.7 percent...depending on how far from .54 you are. I try to keep the idle setting to within 1 percent.
This is also a good time to check your TPS. Once it is set, just watch the voltage as you open the throttle. you should see a steady rise in voltage up to about 4.5-5V at WOT-wide open throttle. If it jumps up and down or skips some then you may want to replace it.

4. Fuel pressure- The system calculates the fuel needed based on the initial settings for injector size at a specific pressure. If the pressure changes then the fuel calculations will be off. I recommend checking the feed and return line pressures at the very beginning. This can save a lot of time later. The pressure is less then 15PSI so a simple cheap gauge is all that is needed. I picked up a HF fuel inj tester for $7 on sale. Comes with a T and a short rubber hose to install it.
The feed side pressure should be about 13psi and should not change with rpm or load. The return should be near 0psi.

Thanks, sounds like I did my IAC adjustment right, but didn't do anything with the TPS. The park wires aren't connected. Timing is factory set. Fuel pressure is 15psi

Maybe just wiring the VSS in will do the trick, as I don't have a laptop to monitor anything.

Thanks, sounds like I did my IAC adjustment right, but didn't do anything with the TPS. The park wires aren't connected. Timing is factory set. Fuel pressure is 15psi

Maybe just wiring the VSS in will do the trick, as I don't have a laptop to monitor anything.


What do you mean by timing is factory set? have you checked it?

15psi is a little on the high side for a factory setting. have you checked it while reving the eng to see if it stays constant?

If your return has any restriction in it, it will also raise your inlet fuel pressure.
You should check it also.

the fuel pressure stays constant 14-15psi (holley blue, no regulator). The return line doesn't have any sharp bends or kinks.

Timing is set to whatever my GM manual said, I don't recall off hand. I'm not a GM guy by any means