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Rpms rev up and back down during steady speed

While driving at a steady speed of 35 mph or higher the rpms rev up and down after car is warmed up.They usually rev between 1500 and 2000 then back to 1500 then back to 2000 until I let off the gas.Sometimes it stops doing it and sometimes it keeps on.It does not do it all the time just sometimes.It will do it on the highway or just around town.Took it to the Jeff Gordon dealership and they said my evap hose was off and that was the problem.2 days later its doing it again.Im taking back to Jeff Gordon in the morning.Any suggestions would be great before I take it back and they charge me more money and not fix my problem.Thanks

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6 Suggested Answers

6ya6ya
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Posted on Jan 02, 2017

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SOURCE: Car "bucks" and sometimes engine dies-intermitently

I posted the original problem...after a few different tries, we discovered that it was the ECM (the main computer). Once replaced, the car has not had a problem since!

Posted on Sep 19, 2008

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stormbrewin
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SOURCE: LPG probs

How close to the exhaust is the supply line as the lpg supply line should be cold until it gets into the regulator not warm.Is the supply line heat shielded as it enters the engine bay or getting a full dose of exhaust heat?Is it the vapor injection gas system or the old style.What regulator and system are you using?OMVL ?sprint gas or other cheaper variants?It really sounds like a fuel delivery issue as you are not getting the volume of gas required at higher speeds.Commodore engines have issues with thier water system having air bubbles that cause air locks in the regulators by the time youve jumped out of the car to check for freezing it appears to be fine.You may have restrictions in the lpgas filters/locks.The water hoses,2,should be hot so hot you cannot leave your hand on them and if not freezing of the reg once again becomes the issue.Also the water hoses must be fitted in such a way that the top hose is suction and the bottom hose forces water into the reg,this is done to assist in eliminating potential air lock problems.To sum up your symptoms are consistant with poor water ciculation.

Posted on Nov 03, 2008

  • 5522 Answers

SOURCE: Car rev up and down at startup, also car jerks at release of gas

This sounds like a vacuum control problem. Since part of the problem is when the engine is cold; a thermal switch is venting vacuum until the engine warms up enough to shut down the thermal switch, thus restoring a steady idle. Vacuum can be affecting high rpms while running too. You could have a misrouted EGR vacuum line opening the valve at the wrong time. There are sometimes hose routing decals on the radiator shroud or under the hood. These can be visibly checked.

Posted on Apr 20, 2009

  • 2 Answers

SOURCE: car starts fine, but stalls at stops and takeoffs occasionlly

Most likely it is running too ritch. should be around 7%. Mine did ecactly the same thing and I had several things done . Had the throttle body cleaned and installed a O2 senser.runs great now.

Hope this helps

Posted on May 28, 2009

  • 1 Answer

SOURCE: 2004 jeep grand cherokee idling problem. Revs motor 200 to 500 rpms higher while driving.

I had the exact same problem on my 04 GC. I put some Techron Concentrate Plus Fuel System Cleaner in from Wal-Mart and it seemed to fix the problems right away. Let me know.

Posted on Jun 01, 2009

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