Revs up when braking at sixty mph and higher and sometimes engine surges at idle

Tiny
JAKESAU93
  • MEMBER
  • 2005 DODGE RAM
  • 4.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 89,000 MILES
My truck is a 1500 SLT. The truck revs up from 1000 rpm to maybe 1200 when I am braking but only if I am braking from sixty mph and higher and drops below 1000 rpm to normal idle. Sometimes the engine starts to surge at idle and in park the rpm will surge up and down, but in drive I can tell the engine surging but the rpm do not fluctuate. I can stop the engine surging by pumping the brakes. No engine codes and I have checked the brake booster and it seems to be operating normally and holds vacuum. Check valve and line coming from the intake manifold looks to be good also. This is driving me crazy. It is not a huge issue, but I do not want it to become a bigger issue long term.
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Sunday, January 22nd, 2017 AT 9:58 AM

6 Replies

Tiny
JOHNNY G.JR
  • MEMBER
Try cleaning both sides of throttle body assembly
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Sunday, January 22nd, 2017 AT 5:54 PM
Tiny
JAKESAU93
  • MEMBER
Thank you for the advice. I cleaned the IAC even though it is brand new, but I will try cleaning the whole throttle body.

Thanks again!

Jake
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Sunday, January 22nd, 2017 AT 8:08 PM
Tiny
CARADIODOC
  • EXPERT
If you get a nice idle flare-up to 1500 rpm at engine start-up, the idle speed motor is working, and carbon isn't the issue. Consider watching the idle "steps" on a scanner when the problem is occurring. The Engine Computer places the AIS motor to one of 256 steps. As the number increases, the armature pulses to a new position, and the threaded shaft retracts the pintle valve to expose more of the air passage around the throttle blade. Step 32 is typical for a properly running engine. You'll find it near step 50 with a single cylinder misfiring on a V-8 engine.

What you're looking for is whether the step number goes higher or lower when engine speed increases. If the step number goes up when speed increases, the computer is requesting that higher speed in response to some other sensor reading, most commonly an incorrect reading from the coolant temperature sensor. If the step number goes down, even as far as "0", when speed increases, the computer is trying to reduce engine speed, but without success. If that is the case, the entire idle speed motor system is working correctly and you need to figure out why engine speed is going up. That is almost always due to a vacuum leak. Given your description of how the symptom acts, I'd be looking for a vacuum leak that opens up when the engine rocks and tugs on hoses.

Also don't overlook the need to relearn "minimum throttle" after the battery has been disconnected or run dead. That usually results in hard starting and stalling from low idle speed, but it can cause erratic idle speed too when the computer tries to overcome the low idle speed.
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Monday, January 23rd, 2017 AT 7:31 PM
Tiny
JAKESAU93
  • MEMBER
Thank you for the detailed explanation and diagnosis it was very helpful. I found the engine RPM increasing a bit during deceleration when braking to be normal and I was over reacting. The random engine surge is still a mystery and I have been looking for a vacuum leak but no luck so far, I figure could be the brake booster or possibly the cruise control because it's not working but I ruled them out during tests. Coolant temperature sensor is something I should check but it looks to be displaying proper values with my scanner during the engine surges. Also would you be able to tell me why it only surges in park?
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Tuesday, January 24th, 2017 AT 8:35 PM
Tiny
JAKESAU93
  • MEMBER
Also my throttle is straight cable controlled from the accelerator pedal not fly by wire. Is there still a re learn procedure with cable driven?

Thank you for your help!

Jake
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Tuesday, January 24th, 2017 AT 8:38 PM
Tiny
CARADIODOC
  • EXPERT
Yup. The Engine Computer controls idle speed on both throttle systems; the common sense, inexpensive, reliable cable and the ridiculously-over-complicated and frustrating electronic systems that put Toyota in the news a few years ago. It has to know when to do that, and it knows that by seeing the TPS voltage that corresponds to closed-throttle. That voltage must be learned because it will be different for every sensor that could be installed. Learning minimum throttle eliminates the need to adjust the TPS mechanically when it is replaced, as was the case many years ago when some aftermarket fuel injection conversion kits first showed up on the market.
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Wednesday, January 25th, 2017 AT 12:43 PM

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