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Rough Idle and sucking noise

77K views 19 replies 9 participants last post by  foxmulder 
#1 ·
'98 Ram 4x4 360 w/ 144xxx miles.

I recently replaced the plenum gasket with the improved mopar gasket and Hughes plate. She also got new plugs, wires, cap, rotor, and some other stuff.

This was happening before the work, but soon after starting, regardless of engine temp, the idle will drop to about 500 RPM's and become very rough. Within a few seconds of this starting, there will be a loud sucking/hissing noise coming from the throttle body. It is air being sucked in the port that is controlled by the IAC motor. This will happen if I let it set when first started, or if I hop right in and drive. If I'm driving, there will be a sudden loss of power. Either way, turning the engine off and immediately restarting makes the problem go away and I can drive for hours after that with no problem.

The IAC appears to be functional, meaning I turned the key with the IAC out and the plunger moves in and out. It is not throwing any codes, and there are no codes pending. What could this be? I'd rather not start throwing money at replacing sensors, but does anybody know what could be causing this? Since it goes away with a restart, my thinking is a sensor somewhere, but I'm not an expert.
 
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#2 ·
Also, I know O2 sensors do not operate until the they hit 600 degrees. I don't know how long that takes, but this often occurs withing about 1 minute of starting the truck.
 
#3 ·
wow..so im not the only one with this..lol
ill definatly watch this thread.mine sounds like a dam evil ass race truck with the exhaust i have on mine when it starts acting up.
did yours ever start the hissing noise then all of the sudden the noise goes away,theres absolutly no sound from the motor,but its still running?iv had that happen twice.i could dam near whisper to the person next to me the motor got so quiet.
 
#4 ·
I have not had that. I've never let it go all that long because the motor will really start bogging down and shaking. I've been told it could be everything from MAP to O2 to IAC sensors. A friend is bringing over a high end scan tool, and we are going to pull all the refernce voltages and see what is acting up.
 
#5 ·
Mine did that a couple of times about a year ago when first started up. It would eventually idle normally and drive fine. Just before my annual road trip I cleaned the TB and the IAC and since then it hasn't done it. I suppose that was the problem.
 
#7 ·
This is the same problem I've been complaining about for over 3 years now with my '97 5.2L.

I don't have to turn-off and restart mine to push through the roughness... I simply let off the gas and get back on it (when it happens as I'm moving), and bump the throttle once when it happens as I'm idling.

The only thing I haven't changed out in attempt to solve this issue is the Throttle Position Sensor. I've cleaned the throttle body, replaced the IACV, both temp sensors, MAP sensor, cap, rotor, plugs, wires, coil, intake manifold gasket, cat, and both O2 sensors. It has thrown anything more than a 'misfire' code - and only on a couple of occasions (I have access to Snap-On and Genesis scanners).

Honestly, I think I remember the problem beginning after I replaced the O2 sensors. Since I'm pretty sure the sensors are different model numbers between the before- and after-cat units, I'm thinking maybe I might've reversed them. I also swapped in Bosch units... I wonder if that has anything to do with it.

Keep us posted.
 
#11 ·
Hooked up my scanner to it, and when the noise started, the bank 1 O2 sensor was reading .08 volts, and the bank 2 O2 sensor was reading .9 volts. I'm pretty sure the acceptable range is .1-.9. If I tapped the gas when it was occuring, then the voltages would max out at 1.00v on both banks.

Does this sound like bad O2 sensors? The truck has 146xxx miles, and these are more than likely original.
 
#12 ·
both my rams did this.....only on a cold start though.....in the morning after sitting over night.....start it up....45sec's later she run's rough TB starts sucking.......can pedal it threw this or shut her off as it starts running rough and restart and it runs great.......i do have an air/fuel ratio gauge and this issue starts as the gauge starts to swings to lean lower voltage .1v......if i shut it off and restart the gauge/02 takes much longer to become active another min or more........to me it seems that the pcm might be using the reading from the 02 to soon.....the o2 isnt hot enough 600c......need to scan to watch open/closed loop operation......does the pcm go in to closed loop using the 02's readings when the truck act's up?
 
#14 ·
I have the same problem with my 98. It sounds like it has a wild race cam in it when it idles every now and then and a loss op power is noticed. So is the conclusion possibly just changing out the 02 sensors?
 
#15 ·
What is the best way to clean the throttle body? I have a brand new engine in my truck as well, and it had this sucking problem with the old motor as well. So im thinking about just changing the 02 sensors and cleaning the throttle body. I already replaced the throttle position sensor already.
 
#16 ·
Best way to clean the TB is to remove it and use Carb cleaner on it with an old toothbrush. Just be sure to replace the gasket when re-installing it. Also be careful with the IAC as the plunger cannot be moved or it's ruined.
 
#17 ·
here's a vid of my cold start issue.....at 1min 20 it starts running rough.....at 2min 40 i shut her off and restart......if i drive b4 the rough idle it run's great.....ive replaced both o2's , map , iac , iat , all with in 6month's.....each part replaced changed nothing....

http://www.youtube.com/watch?v=U5zmIsob1qs
 
#18 ·
mine does similar, and even the stealership cant figure it out...when mine does it it boggs off the line, and if i let off the throttle or just leave the foot down it takes off like a rocket.
 
#19 ·
mine run's great if i dont let it warm up.....get in start it , do pre-flight check , n drive she run's great....cold start warm up and the idle goes to s**t tell i drive then it idle's great....what's puzzling is both my ram's had this....my 98 was brand new:dunno:
 
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