rpm's fluctuating when driving

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dennis55753

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1996
Engine
5.9
I've been noticing lately that when driving my truck but not under a load that my RPMs are fluctuating. Not very much probably less than 100 RPMs but you can hear the engine going up and down ever so slightly. It's annoying. It does not do this when accelerating but when going down the road between 45 and 60 miles an hour. RPMs do not fluctuate when idling.Also when I left my truck to idle in park it died on me several times but not very often. But it would start up right away and stay running.Check engine light or other warning lights have not come on. Put it on diagnostic tool and there was no trouble codes.I've changed the idle air control valve and the throttle position sensor a few days ago. The RPMs still fluctuate but I do not know yet if the motor will die again when idling because this did not do all the time. So I do not know if what I changed solved the stalling problem when idling. But in the past several months I've change the fuel pump, spark plugs, spark plug wires, coil, and a new battery. The motor runs great, starts good, accelerates good, shifts perfectly, and has good power. But it always got terrible gas mileage between eight and 11 miles gallon. But the fluctuating RPMs is annoying. Any ideals? The truck has 157,000 miles on it and a brand-new automatic transmission right from the dealer was put in at 143,000 miles. It's a ram 1500 4 x 4 with a 5.9 motor.
 
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bigdodge

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It's the throttle position sensor. I suggest getting one from the dealership.

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dennis55753

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You think the new one I put in is defective ?
 
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dennis55753

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What do you mean by QC? Have you personally seen people had problems with aftermarket IAC valves. I did buy a after-market one. Just curious! Your input is greatly appreciated.
 

bigdodge

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Yes, defective IAC and TPS. I have had both so many times I only buy parts from the dealer.

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dennis55753

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Sounds like you had the same experience? You know the truck runs great but going down the road when the motor is not under any load, cruising, at low miles per hour, you can hear the motor going up and down. This is very noticeable the other day deer hunting when we were cruising around.Going down the highway tonight on cruise control at 65 miles an hour the RPMs stayed faithful.
 

bigdodge

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It was very random.

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Yeret

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1999
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I've had a similar issue for years. Under light load between 1,500 and 2,000 RPMs, the engine has what can be best described as a faint surging. You can't feel it, but you can definitely hear it, courtesy of my loud exhaust. Increasing throttle and/or RPM steadies it.

Replaced TPS with a blow'Reilly's unit, didn't resolve it and the damned thing went bad in less than a year (occasionally stuck in a "loaded" position, creating a high idle). Replaced with genuine Mopar, but the faint surging is still there. The pesky issue even persisted when I loaded my tune.

Almost makes me wonder if the dizzy shaft has a slight wobble, causing the injector and/or ignition timing to fluctuate in that RPM range. :shrug:
 
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dennis55753

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Yeah it's very annoying. I'm not going to stick any more money into it. Everything else works great except that RPM fluctuation just like you have to. I talked to a few good mechanics and they had no ideal what would be causing it. I wish something would just quit and then I would know what it was. Thank you for your reply!
 

69er

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MAYBE just MAYBE try a new pvc valve?
 

MarineBSP

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Also check for small vacuum leaks - not as easy as it sounds. Tiny leaks are the ones that will generate random bouncing idle or faint surging at cruise - because the only time that small leaks matter is when the total intake flow is low and vacuum is high.

Likely you have addressed some of the possible culprits with the O-rings and seals for the parts that you replaced, but I would check all my vacuum hoses and their connections, even the ones that look solid. This kind of leak doesn't always show up in a smoke test.

Good luck with your quest !
 

Okiespaniel

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The reason a TPS was suggested is that over time a worn spot forms in the wire coil and provides a poor signal which results in the surging you get at highway speeds.

You see a TPS is like an old school volume control, also referred to as a Poteniometer. The shaft is the throttle shaft and the low voltage signal varies to the computer which helps it set timing, fuel curve, ext.

Once this worn spot occurs, the voltage is poor or intermittent and the computer can't decide where to set things, hence your surging.

Having made the mistake of using poorly built aftermarket sensors, which either fail within a couple of years or immediately, I try to procure OEM stuff. Remember that your Ram came with a 10 year 100k warranty, so OEM stuff is built to not fail within that period.
The brand Delphi is a good choice because they are an OEM supplier. Usually Delphi products are found at higher end stores like NAPA or Bumper to Bumper.
 
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KXRider1

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1995
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5.9 360
So I have a 1995 Dodge Ram 2500 that has the rpms fluctuating like this. We serviced the throttle body and put a new IAC in it and new gasket and some vacuum lines off the throttle body. My rpms do this about the same as yours. And sometimes acts up when firing. New fuel pump and lines. How did you find out it was the crank sensor?
 
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