low idle, wants to die at idle only

1989 aw4 4.0
so i have had a low idle on the jeep for as long as i can remember,like around 250rpms on the tach. it might be higher than the tach reads though, ...
it has never seemed to affect anything, but recently the idle started dropping really low to nothing like its surging and doesnt want to idle good. it drives fine down the road, but the idle surging low is bothering the hell out of me.

the IAC fell apart in the throttle body last time it did something like this, and i replaced it, and it fixed it. just checked, the IAC is good this time.


the o2 sensor and tps are new this year, new coolant temp senor, cap, rotor, plugs, and wires, new used ford 19lb injectors in jan , exhaust manifold gasket, throttle body gaskets,
grounds are all 0 or 2 guage wire, and big alternator wiring too.
but my first guess is that its maybe the IAC or TPS?
i have just replaced the MAP sensor, ive been wanting to replace it for a long time anyway.
but it did nothing really, i checked all vacuum lines, no issues there.

maybe the tps, iac, ???

im thinking it could be the ballast resistor for the fuel pump is going out?
clogged injector?
maybe bad spark plug?

1. Did you clean the throttle body yet? A thorough cleaning of that is never a bad idea if you haven't done it.

2. If you replaced the IAC recently, it probably isn't that

3. Have you tested AND adjusted your throttle position sensor (yours is adjustable) You said you put a new TPS on, did you adjust it after you installed it?

4. Fuel pressure testing with a fuel pressure gauge would not be a bad idea. You are looking for 31 psi at idle, and then when you disconnect the line to the fuel pressure regulator, the pressure should jump to approx. 39 psi

This is not a ballast resistor or a spark plug issue.

yah ive adjusted the TPS to spec, a few times
but not recently...lat time was like in the spring.
i cleaned the heck out of the throttle body when i had it off earlier this year
the iac was an old used one, so i wonder if it could be bad.
ill throw my fuel pressure tester on the thing in a few minutes.
my gut instinct says TPS and IAC.
i totally spaced on fuel pressure tester.

ill also test/adjust the TPS as well.

birchlakeXJ said:

1. Did you clean the throttle body yet? A thorough cleaning of that is never a bad idea if you haven't done it.

2. If you replaced the IAC recently, it probably isn't that.

(Unless it was a cheapo or used IAC.)

3. Have you tested AND adjusted your throttle position sensor (yours is adjustable) You said you put a new TPS on, did you adjust it after you installed it?

4. Fuel pressure testing with a fuel pressure gauge would not be a bad idea. You are looking for 31 psi at idle, and then when you disconnect the line to the fuel pressure regulator, the pressure should jump to approx. 39 psi

This is not a ballast resistor or a spark plug issue.

Click to expand...

Agree with birchlakeXJ. The ECU sends extend/retract commands to the IAC to control idle speed. One CRITICAL element of data input the ECU relies upon to determine the IAC's correct position--the TPS.

How's your battery? With a bad battery(that will still hold enough of a charge to start the engine) the engine will idle slow, and eventually slow enough to stall, but otherwise run fine.

new battery in January 2010.
i also upgraded the positive and negative cables to 0/2 gauge, same goes for all engine grounds.
ive adjusted my TPS multiple times, just not recently. ill adjust it after the rain stops .
ill also look into a new IAC, although this one seems to work, ill wait until i check fuel pressure and adjust the TPS.

Have you checked the EGR valve?

egr is something to check, i havent. ill look into the manual to see how to test it

Sorry, don't mean to thread jack but i have an 89 with the same exact problem. I've got a feeling this thread will answer all my questions. But i don't know what the IAC is... Can some one help me out?

nochance9 said:

Sorry, don't mean to thread jack but i have an 89 with the same exact problem. I've got a feeling this thread will answer all my questions. But i don't know what the IAC is... Can some one help me out?

Click to expand...

The Idle Air Control (IAC) is mounted on the back of the throttle body. The valve controls the idle speed of the engine by controlling the amount of air flowing through the air control passage. It consists of a stepper motor that moves a pintle shaped plunger in and out of the air control passage. When the valve plunger is moved in, the air control passage flows more air which raises the idle speed. When the valve plunger is moved out, the air control passage flows less air which lowers the idle speed. Over time and miles, the IAC can get carboned up which can have an adverse affect on idle quality. Cleaning the IAC may restore proper function and is an easy procedure to perform and good preventive maintenance so it is never a bad idea.

CLEANING THE JEEP 4.0 IDLE AIR CONTROL

Remove the air filter cover, associated hoses and the rubber boot that goes from the air filter cover to the throttle body. Remove the IAC with a torx driver (2 bolts; one can be kind of hard to get to)

“Gently” wiggle out the IAC from the throttle body. Gasket on the IAC can be re-used if it is not damaged

Clean the IAC with a spray can of throttle body cleaner; inexpensive and available at any place that sells auto parts. Throttle body cleaner is recommended rather than carburetor cleaner as it is less harsh, safe for throttle body coatings and is best for this task. Use cleaner, a rag and a toothbrush and or Q-Tips. Be gentle; don’t twist or pull on the pintle that protrudes from the IAC as it is fragile and you could damage it.

Thoroughly spray clean and flush where the IAC seats in the throttle body with the same spray cleaner

It is also a good idea to clean the entire throttle body itself, the butterfly valve inside of the throttle body and all associated linkage as long as you have things disassembled

Reinstall IAC and check idle quality

ive also been having poor fuel economy, like worse than the normal bad MPG.
i have a warranty on the TPS, so ill replace it tonight and adjust the new one for free.

ill pull the throttle body and clean it, even though its very clean as of now, it wont hurt anything.
then i will try to clean the EGR. im not sure what the best way to clean it is, but ill search for that.

ill also check my fuel pressure and double check for vacuum leaks again.

i plan on replacing my IAC since its my old stock one as a spare.

did all the egr tests tonight
its good, the egr solenoid is good, and im pretty sure the vacuum transducer for the egr is good as well.

checked for vacuum leaks with no luck. and when i induce one, the idle increases not decreases.

tested fuel pressure 31 psi with vacuum, 39psi without.

tested TPS, got 9.7 input voltage
8.1 output voltage
the ratio is .83 like its supposed to be,
but i thought the input voltage was supposed to be like 5 and the output voltage around 4.15??? thats what i got when i installed it i think... is that out of spec?
i am thinking about tapping into the harness and splicing out new grounds???
i will double clean all of my grounds, but they look fine..

i cleaned all grounds thoroughly, still same voltage readings of input 9.7 and output 8.1.
i also have huge ground wires, and they are fresh, and i already cleaned my grounds three times this year...

It sounds sad. You should get out and wheel Jon!

I can stop by and drink your beer while you try to figure it out. I don't think I would be much help but then again you wouldn't have to drink alone.

i think the jeep is sad,
it hasnt been wheeled since the rubicon in september
if you were to stop by, that would be awesome josh, i will happily supply as much beer as you can drink simply for the company in the gargage, between my three rigs i am always in the garage alone wrenching, company is much appreciated out there...

ill be working on it again on tues night(tomorrow)

so ive been reading my massive chiltons for the 84-91 Xjs, its like 4 times as thick as a normal chiltons and has tons of diagnostic stuff and diagrams and other stuff.


it has a good trouble shooting section for the TPS:

first check for voltage with the key on ,
should be around 5 volts.


disconnect the connector at the TPS

it says if voltage is not close to 5 volts, then first check continuity between TPS pin C and ecu pin C-7

if no open circuit there, then test for continuity between ecu pin D-3 and TPS pin B

if no open circuit there, then test for continuity between ecu pin C-15 and TPS pin A.

if still no open circuit then replace the ECU.

interesting.
well just for the hell of it , since the TPS is under warranty still from june 09, im gonna replace it first.

aaa

blondejoncherokee said:

aaa

Click to expand...

???

I would try another voltmeter, yours may have packed it in.

You shouldn't be getting more than 5.0 volts input at the TPS, measured from "A" positive to "D" ground, with the connector off the TPS and the key ON. IF you are getting higher voltage than that you need to solve that issue before going any further.

Possible causes--there is a short in the harness to another positive power supply other than the ECU; the ECU is FUBAR.

IF using a new/different voltage meter you find you are getting something closer to 5.0 volts (usually slightly less), then that is your "reference" voltage. IF you have correct "reference" voltage put the connector back on the TPS and then measure "B" positive to "D" ground by back probing the connector--using a couple of small paper clips with straightened legs will make that easier--now you are looking for the "output" voltage and it must be adjusted to 83 percent of "reference" voltage for an AW4 vehicle or 17 percent of "reference" voltage for a manual equipped vehicle.

iver replaced a good amount of TPS's and adjusted them , for my jeeps and ferinds Xjs, as well as other cars.
ive never seen the voltage that high on a XJ TPS
yah i figured i have to start searching for shorts. im hoping its a short and not a bad ecu but im sure ill figure it out tonight.

Thanks birchlakexj, lots of good information.