I am now noticing when I start the car the RPMs bounce up and down for like 10 seconds. The engine races, idles, races, idles, races, etc. This happens pretty quick so it's not like it races for a few seconds then idles, it literally goes up and down fairly quick.

If I put the car in gear to take off, it continues to bounce. However once I get on the road it runs just fine.

One thing I also experienced is when I punch the gas it sometimes will hesitate or feel like its cutting out and then accellerates. This doesn't happen all the time.

But it's getting to be pretty consistent on a cold engine the RPM's bounce. Someone said it could be the IAC.

 

Sounds like a sticky or plugged IAC to me too ... do you still get the bounce if you step up on the gas? The rationale behind this is that once you opened the throttle a bit, IAC operation becomes irrelevant. Let us know ...

Cheers,
cougar_fan

 

I should have asked this first because not all cars are made equal ... do you have the old-style mechanically-actuated throttle (by cable) or is it electronic (drive-by-wire)? My post is only valid for classic throttle.

Also, you said the engine races then idles ... well, even a fully open IAC may not leave enough air in to make the engine race, I think ... how many RPMs mean "race"?

 

When I say RPM's it is like bouncing from 1000 to 1500 and back n fourth. When I give it gas it does stop bouncing however when I take the foot off gas the RPM bounces again.
I did throw the IAC in some carb throttle cleaner but thinking I might need a new one.
The IAC plugs in.

 

I'd say, it's a sticky IAC ... the PCM tries to correct too low rpms by slightly opening the valve ... which doesn't respond to small actuation commands ... so the PCM drives it stronger ... and stronger ... until the PCM responds, except it now opens too much, so the engine speeds up above the prescribed idle ... so the PCM now tries to close the IAC ... which is sticking again ... so the PCM drives it stronger in the opposite direction... until the IAC closes again, except this time it goes too far towards the closed end ... I bet that's what happens.

Unfortunately, the carb cleaner may not work on every IAC. On my 94 Cougar, for instance, Ford clearly states that IAC should not be cleaned - if if it malfunctions, replacement is the ONLY option.

HTH ...

Cheers,
cougar_fan

 

GRRRR - Well installed the IAC and nothing!
RPM's still bounce from 1000 to 1500 on a cold start. So I have thus far replaced the CamSensor and IAC. Almost ready to take it to the auto mechanic, have them sit in it and see what's going on.

 

I don't recall responding to this problem at an earlier time,but could you provide some info like year, model, engine and any other info you feel might assist in suggesting a possible solution. There are loads of problems that just cannot be corrected on the internet , so don't feel discouraged. Every problem is and has it's own solution and we can only suggest some possible remedies. There are some problems that are characteristic to certain models and just maybe we can come up with a possible solution before you go broke or bald from pulling your hair out. I'm sure there are other folks reading these posts and possibly have an answer. So, how about some more info and just maybe someone will hit on the cause and solve the problem.

 

Honestly I was thinking of including that when I originally wrote the question - my apologies!

2000 Mercury Sable Sedan 6cyl (Is Not a DOH)
94K miles

 

I would think you have a vacuum leak at one of these three places, PCV hose to the PCV valve, the upper intake plenum to lower intake manifold gasket or possible , but not very common the lower intake manifold gaskets. There are no special ways to verify the last two except for fuel trim values using a scan tool. When disassembled the gaskets look fine ,but they still are leaking. Let us know if you decide to tackle the job yourself.