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RPM drops when stopping

 
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Old 07 Mar 2018, 01:32 pm
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Join Date: Jan 2016
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Default RPM drops when stopping

I've got a similar problem to this thread but slightly different.

Rough Idle/RPM Drops

I have a 2007 PT Cruiser non turbo, i've always had issues with the ac at idle, works fine at highway speeds but when stopped for brief moment RPM's drop to about 500 and the car shakes before coming back to 800. I asked mechanic to change the fan but mechanic is adamant the fan works fine, living in a cold climate on a student budget, I've left it alone for know. AC isn't priority and car has never overheated. Is always at the middle position.

Recently, I started having a issue when braking suddenly; the idle would drop and the car would stall. It doesn't happen always but was occurring every so often. Also had weak acceleration with shaking one night. My mechanic wasn't able to figure out what was wrong.

Finally the issue got worse one night where stopping any where would cause the car to shudder and idle to get rough. I had a check engine light come up so went to mechanic and he read a P300 code. Had me replace the wires, plugs, and coil.

The stopping anywhere issue shudder was better. I still have the random stalling/shudder when braking suddenly (RPM's drop, in some cases they come back up and no stall. When it stalls, they drop and don't come back). I've also had another P300 code come up when starting and the RPM surged very high on a cold night. Mechanic said he cleared the code and sent me on my way.

Recently, the shudder when stopping has started appearing randomly; RPM's drop and then come back up, some days it occurs and some days it doesn't. Yesterday I tried to turn on the defrost and had the same shudder with the RPM dropping and the car shaking. I haven't had any issue with the defrost prior to this. My mechanic says he's not sure what else to look; he maintains the P300 and the idle issues should be wires and plugs and coil which he changed.

I'm wondering if you guys have any ideas?
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Old 07 Mar 2018, 01:42 pm
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Default Re: RPM drops when stopping

I would suggest you start the engine when it's cold, turn on the A/C, and see if the fan automatically comes on. If it doesn't come on one of the speeds on the radiator fan is not working properly. Post what you find.
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Old 07 Mar 2018, 01:54 pm
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Default Re: RPM drops when stopping

Quote:
Originally Posted by ptcruisersteve View Post
I would suggest you start the engine when it's cold, turn on the A/C, and see if the fan automatically comes on. If it doesn't come on one of the speeds on the radiator fan is not working properly. Post what you find.
Any tips on how to tell if the fan is on? When my engine is hot, i can hear it come on but with the noise of the ac it's hard to tell.
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Old 07 Mar 2018, 02:06 pm
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Join Date: Aug 2017
Location: Michigan, USA
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Default Re: RPM drops when stopping

After your car has sat all night and is cold, pop the hood, start the car, turn on A/C, step out of the car and see if the fan is on. Might work.
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Old 07 Mar 2018, 06:59 pm
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Join Date: Nov 2017
Location: Beaumont, TX
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Default Re: RPM drops when stopping

Thought I was on a different forum for a moment...

Have the exact same issue but with my 2015 Abarth... randomly stalls when stopping or clutching in while going slowly. It's like the fuel/spark cut when the RPM drops is just too long. No clue what's causing it, and on my car I've changed coils/plugs, reflashed the PCM to stock, bought a new battery (in case voltage drops were to blame)... It's so rare I can't properly diagnose it, and the only code I ever get is P0302 (misfire #2) but that code only appears right after I reset the PCM...

Looks like these random stalls are a hiccup with computer controlled cars that are almost impossible to diagnose.
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Old 07 Mar 2018, 07:11 pm
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Join Date: Dec 2017
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Default Re: RPM drops when stopping

The fan won't cause this issue, don't bother changing it.

A p0300 is random cylinder miss fire so look for a global problem.

I'd be removing the air filter housing and large hose that leads to the throttle body. Clean the throttle body bore and plate, when this gets coated with oil, it restricts air flow at idle leading to a rough idle / stall on a quick stop. .

The weak accel would not be caused by the throttle body gunk. I wonder if the transmission lock up torque converter is staying engaged too long ( sticking ). This would give a " I didn't push the clutch in when stopping " feeling and would give for sluggish acceleration.

This was a super common problem on 80's / 90's GM cars when the converter clutch solenoid valve stuck.
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Old 08 Mar 2018, 10:13 am
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Default Re: RPM drops when stopping

Other things to check for with the P0300 are the fuel pressure and the intake gaskets. Something causing air to get in after the throttle body can sometimes cause it. On mine, the gasket was blocking the injector port, but that caused a cylinder specific code. I think the composite gaskets have better retention. Any work done on it prior to the code popping up?
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Old 08 Mar 2018, 11:18 am
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Default Re: RPM drops when stopping

Quote:
Originally Posted by jasonboy View Post
Any tips on how to tell if the fan is on? When my engine is hot, i can hear it come on but with the noise of the ac it's hard to tell.
I would suggest when you check to see if the radiator fan is coming on with the engine cold just put a piece of thick paper between the radiator fan and the engine to see if the thick paper pushes away indicating the fan is working.
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