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Rough idle, RPMs jumping and seemingly low limiter

 
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  #1 (permalink)  
Old 27 Mar 2018, 04:53 am
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Default Rough idle, RPMs jumping and seemingly low limiter

Hey, y'all! I'm fairly new to the forums so I'll briefly introduce myself and my rig. My name is Zack, I'm a college kid from MN and I have owned my PT for about a year. She's a stock 2001 PTC Base. I've done basic regular maintenance on her but nothing big. I got this PT as a project. She was constantly overheating and the last owner just didn't have the time to get er ripping again. Long story short, the rad fan housing was loose and caused the fan fuse to blow. No air flow past the rad = bad day. I've since done plugs, wires, thermostat and some throttle body cleaning on top of regular maintenance. My main problems are that she's pretty dang loud, she idles really rough, gets terrible gas milage and wont rev past 4,500. I thought this all was normal but after driving a few other PTs, I'm starting to think I have a problem. Any insight? Any mods I should do to improve performance and increase longevity? I read a thread about cutting some wire coming from the ECU to help a bit with performance but I cannot for the life of me remember where I found it and what it was called. Hope to see some of the members I look up to and some new faces down below. Thanks again, friends.
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Old 27 Mar 2018, 07:57 am
Fk5 Fk5 is offline
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Default Re: Rough idle, RPMs jumping and seemingly low limiter

It's not normal.

Don't cut anything, or do any mods, until you have the car running right. Any codes? What is "terrible gas mileage"?
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Old 27 Mar 2018, 08:40 am
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Default Re: Rough idle, RPMs jumping and seemingly low limiter

Quote:
Originally Posted by ShakeMyShifter99 View Post
Hey, y'all! I'm fairly new to the forums so I'll briefly introduce myself and my rig. My name is Zack, I'm a college kid from MN and I have owned my PT for about a year. She's a stock 2001 PTC Base. I've done basic regular maintenance on her but nothing big. I got this PT as a project. She was constantly overheating and the last owner just didn't have the time to get er ripping again. Long story short, the rad fan housing was loose and caused the fan fuse to blow. No air flow past the rad = bad day. I've since done plugs, wires, thermostat and some throttle body cleaning on top of regular maintenance. My main problems are that she's pretty dang loud, she idles really rough, gets terrible gas milage and wont rev past 4,500. I thought this all was normal but after driving a few other PTs, I'm starting to think I have a problem. Any insight? Any mods I should do to improve performance and increase longevity? I read a thread about cutting some wire coming from the ECU to help a bit with performance but I cannot for the life of me remember where I found it and what it was called. Hope to see some of the members I look up to and some new faces down below. Thanks again, friends.
Quoted from above --- "I read a thread about cutting some wire coming from the ECU to help a bit with performance but I cannot for the life of me remember where I found it and what it was called"


MX-5 'Burp -Free' Mod. Procedure

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Last edited by wwpptc; 27 Mar 2018 at 08:46 am.
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Old 27 Mar 2018, 09:04 am
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Default Re: Rough idle, RPMs jumping and seemingly low limiter

I believe that mod is only for 2001/2002 PT's. By cutting one wire, it disables torque management and improves transmission shifting. I believe CREWZIN has done this one. It will not effect engine performance as long and the correct wire is cut. but that's not something you would want to do until the mechanical problems are fixed.

The OP needs to let us know if he has any troubles codes. That will likely tell the story. If there are no codes or if the codes are the P0300 series, then the most likely culprit is either trash in the fuel system or bad spark plugs wires. The noise may just be a leaking exhaust or EGR tube. A failing head gasket due to the previous heating might be in play. But if it's run for a long time since the heating problem, this is less likely.

Let us know about the trouble codes and we will go from there.
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Old 27 Mar 2018, 09:13 am
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Default Re: Rough idle, RPMs jumping and seemingly low limiter

You have a 2001!!!

The rough idle is probably broken motor mounts and a partially or mostly clogged cat.

The 4500 rpm thing is probably again, a partially or mostly clogged cat.

The bad gas mileage can also be the result of a mostly clogged cat.

These are just guesses on my part, but if it was my car, I would do these two things first.

I'm sure there are other possible causes of your problems and other members will jump in here shortly.


Jerry

Last edited by wwpptc; 27 Mar 2018 at 09:29 am.
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Old 27 Mar 2018, 09:43 am
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Default Re: Rough idle, RPMs jumping and seemingly low limiter

Quote:
Originally Posted by wwpptc View Post
You have a 2001!!!

The rough idle is probably broken motor mounts and a partially or mostly clogged cat.

The 4500 rpm thing is probably again, a partially or mostly clogged cat.

The bad gas mileage can also be the result of a mostly clogged cat.

These are just guesses on my part, but if it was my car, I would do these two things first.

I'm sure there are other possible causes of your problems and other members will jump in here shortly.


Jerry
Yep. This one is easy to check with a vacuum gauge. You should get 18 to 22 in-Hg at idle. Speed the engine up to 2500 or 3000 rpms and hold. If the vacuum falls off to almost nothing and doesn't come be up, it indicates a potential exhaust blockage such as the cat. You can also use a thermal gun but it's not as definitely as the vacuum gauge. For that, warm the engine up for about 20 minutes then check the temp at the front and rear of the cat. On a good cat on a lot of modern cars, the rear of the converter will be somewhere around 20 F degrees or more hotter than the front. If the front is hotter, then you may have a blockage.
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Old 27 Mar 2018, 10:53 am
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Default Re: Rough idle, RPMs jumping and seemingly low limiter

Quote:
Originally Posted by wwpptc View Post
You have a 2001!!!

The rough idle is probably broken motor mounts and a partially or mostly clogged cat.

The 4500 rpm thing is probably again, a partially or mostly clogged cat.

The bad gas mileage can also be the result of a mostly clogged cat.

These are just guesses on my part, but if it was my car, I would do these two things first.

I'm sure there are other possible causes of your problems and other members will jump in here shortly.


Jerry

Your noise could also be your exhaust gases that have found another way out at the joints on either side of exhaust manifold because of the extra pressure and heat that is building up because the cat isn't letting them through fast enough.

If you don't have a vacuun gauge, a simple way to check for a possibly clogged cat is, with the hood up, bring the rpms up to around 2500 rpms and then quickly release the throttle. If the rpms drop slower than normal back to idle, You have a suspicious cat. I realize that "slower than normal" is completely subjective in this case, but I think you can figure it out.

Clogged cats like this are usually caused by a few different reasons;

1. Your right rear taillight has burned out.
2. You need to pour a can of "RID-X down your upstairs toilet.
3. You have chinchbugs in your lawn. This can be fixed with liberal applications of "PREPARATION H" anywhere you feel is appropriate.

And last but not least --- You live in southern Indiana and unfortunatley there is no cure for that. It's the kiss of death!


Jerry
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Last edited by wwpptc; 27 Mar 2018 at 11:04 am.
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Old 27 Mar 2018, 11:04 am
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Default Re: Rough idle, RPMs jumping and seemingly low limiter

I believe THIS is the main reason for a clogged cat.

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Old 27 Mar 2018, 11:06 am
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Default Re: Rough idle, RPMs jumping and seemingly low limiter

I am suggesting you might have a defective EGR valve or the components going to the EGR valve might be leaking.
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Old 27 Mar 2018, 12:07 pm
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Default Re: Rough idle, RPMs jumping and seemingly low limiter

Steve has a good point.

All there is to the EGR system on a 2001 PT is a metal tube that runs from the back of the plastic intake manifold to the EGR valve mounted on the driver's side of the cylinder head. If the EGR valve is stuck open or if the tube is damaged (making noise and making the engine run rough), the PCM would likely throw a trouble code when there is no pressure change detected by the MAP sensor in the intake manifold.

By the way, this is why you cannot just block off the EGR valve on these cars while leaving the vacuum controller in place. It will throw a code just as I described if the EGR does not actually function. This one used to stump me until I did me some studying and "learnt" that the MAP sensor was the tattle-tail.

PS: I wonder if the OP is ever going to come back and tell us he's got any codes and/or if the suggestions we've given did him any good. A lot of the time I feel we do all this armchair mechanicing for nothing but our own edification.
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