Help! Idle drops low when stopped, occasionally dies.

cornfish

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Hey Guys,

I have an issue that I have been having with my car for years now and I need to get it fixed. I have taken it to many shops here in town as well as taken it to Ken (viper tech in Portland, Oregon) and no luck. I have spent thousands of dollars on this issue and NO ONE can fix it, and as I am sure you guys can understand, I am getting really angry.

Anyway, here is the issue:

* Quite often, when stopping the car, the RPM comes down too low, occasionally killing the car.
* When this starts to occur, this happens a lot, but there are days that I drive to work without this issue.

Not much more I can say other than that. It ONLY happens when I have been driving and if it idles without load, it seems to be fine. I have replaced numerous parts on the car trying to fix this issue and no luck. I have cleaned sensors, replaced O2 sensors, changed plugs and plug wires more than anyone should ever do on their cars.

The following works was just done by a Viper tech in Portland:

* Valve adjustment
* Idle speed motor replaced
* Replaced cold thermostat with stock one (coolant flush obviously)
* Plugs and Plug wires changed
* Some gaskets replaced
* Side sill panted (don't ask, long story)
* Complete checkup

I drove the car back from Portland with the idle only dropping below normal once, and only for a quick 1/2 second but yesterday it was back to misbehaving.

It is a 1998 GTS with 21k miles on it now. Here are the mods that have been on the car from pretty much day one, which SVS installed.

* 72mm Throttle bodies
* Larger bore airbox
* Roller Rockers
* Cams
* SVS Coated headers
* SVS single tip rear exhaust
* 3.55 gears
* Calibration box that doesn't seem to be working anymore because the car is 5 mph off (***??)

Any help would be appreciated as I have no clue what to do now. I love my car and just want to drive it normally again :(
 

-FROG-

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Had the same basic problem with my 2005 Mustang GT. I took off the throttle body. Noticed it had slight dirt / gunk in there, so I cleaned it out with a rag to wipe away the dirt. Installed the throttle body and the car has ran PERFECT for the last 6 months since I did that.

My assumption is... there was just enough build up of dirt, that it preventer the throttle body flap from sealing properly when it closed. It was a small amount of dirt, but nonetheless, it shouldn't have been there, so I removed it.

Good luck debugging your problem.
 

speedracervr4

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Have you looked at the TPS? You can check it with a multimeter before replacing it (part is cheap anyway). I would think IAC motor, but you've already replaced it.
 

uvbnbit

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Tried re-setting PCM?


re: speedo recalibration, If it's a Roe, call Sean with your info. gears, tire size/diameter. Inside the box are settings that can be changed. Check with him about the settings
 
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speedracervr4

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Greg might be on to something...Do you have engine management system installed other than the stock pcm? The cam might be to radical for the stock pcm to control effectively. Might want to look in to an SCT if you don't already have one.
 

tlcobra

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My 98 also does the similar thing. I idles so slow the car sounds like it has a big cam. Lope lope lope. I had a broken wire on tps it helped some. Still idles low. Car bucks sometimes nasty. I have changed parts also.
 

KenH

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Weak battery might cause this.

I would reset your PCM and let it relearn the TPS range. You can do this by removing the 3 large connectors on the PCM and letting it sit for a few minutes. Reconnect them and turn the key to ON (engine off) and depress the throttle a couple of times slowly to allow the PCM to learn the TPS range.

If you have a voltmeter, you can measure the TPS to make sure it is working correctly. With the key ON (engine off), you should measure about .5V with the throttle closed. With the throttle wide open you should measure something above 3.5V. If you open the throttle slowly, the voltage should increase smoothly and not jump around.

Something else that you can try is to adjust the throttle stop. This is not normally an adjustment, but you can use it to try cracking your TB open just a bit when closed and see if that helps the situation.

I had this exact same problem years ago and I think it might have been the throttle stop adjustment that fixed it for me, but I don't remember for sure.
 

KenH

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I should probably mention that this throttle stop is to synchronize the opening of the 2 TBs. One (drivers side I think) should open just before the other. If you mess with it, mark it so that you can get it back to where it was if you should need to.
 

dave6666

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According to Tator, the only way to truly reset the PCM is to ground the + battery cable to the chassis. Then after reconnecting the cable do a TPS reset by turning the key to run but not start, casually depress the gas pedal to max and release, repeat 4 more times, turn key to off, wait 30 seconds and motor on.
 
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cornfish

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Hi Guys,

Sorry it has taken so long for me to reply. Let me see if I can shed some more light on the situation.

Had the same basic problem with my 2005 Mustang GT. I took off the throttle body. Noticed it had slight dirt / gunk in there, so I cleaned it out with a rag to wipe away the dirt. Installed the throttle body and the car has ran PERFECT for the last 6 months since I did that.

My assumption is... there was just enough build up of dirt, that it preventer the throttle body flap from sealing properly when it closed. It was a small amount of dirt, but nonetheless, it shouldn't have been there, so I removed it.

Good luck debugging your problem.

I have cleaned the throttle bodies a few times since the issue started. I cleaned them last night as well as adjusted the bolt to make sure that the drivers side throttle body opens up just a hair before the passenger side, no luck.

Check the post I have running on the IAC motor.

I replaced the IAC motor twice within a month (The first place I took it to replaced it and then it was replaced again by the Viper tech 3 weeks ago.) I do not have the SCR Tuning software though..

Have you looked at the TPS? You can check it with a multimeter before replacing it (part is cheap anyway). I would think IAC motor, but you've already replaced it.

Yep! The TPS was replaced about a year ago. This was done when I just gave up and tried to replace everything

He said it is new. I would also agree about checking (or replacing) the TPS. Doesn't sound like dirty TB's but why not clean them. I'd remove 'em and hose them with throttle body cleaner.

Yep, did that last night and no luck. I am dying here.. I think I posted about this issue a few years ago too

Tried re-setting PCM?


re: speedo recalibration, If it's a Roe, call Sean with your info. gears, tire size/diameter. Inside the box are settings that can be changed. Check with him about the settings

Yep, the viper tech reset the PCM when he was working on it. With all the work he did, it was working pretty well until the day after I got it home. Apparently my ECU re-learned bad habits.


For the speedo calibration, I will call Sean.

I think the car needs to be tuned by a pro.

I hear you.. My issue is that the car used to run fine, with all the mods and everything with no problems. A few years ago, this issue cropped up and no I can not fix this to save my life.

Greg might be on to something...Do you have engine management system installed other than the stock pcm? The cam might be to radical for the stock pcm to control effectively. Might want to look in to an SCT if you don't already have one.

I don't have anything for the Viper. Funny, I do for my STi but not the Viper. This just ***** for me because I have spent SO much money on this and since it used to run perfectly for a few years, I was hoping that this would be something that one of the countless places I have taken it to could fix it, without making me broke. Apparently this isn't the case.

My 98 also does the similar thing. I idles so slow the car sounds like it has a big cam. Lope lope lope. I had a broken wire on tps it helped some. Still idles low. Car bucks sometimes nasty. I have changed parts also.

Really? You mean there was a broken wire on the TPS sensor connector? I am going to look at that right now..

Weak battery might cause this.

I would reset your PCM and let it relearn the TPS range. You can do this by removing the 3 large connectors on the PCM and letting it sit for a few minutes. Reconnect them and turn the key to ON (engine off) and depress the throttle a couple of times slowly to allow the PCM to learn the TPS range.

If you have a voltmeter, you can measure the TPS to make sure it is working correctly. With the key ON (engine off), you should measure about .5V with the throttle closed. With the throttle wide open you should measure something above 3.5V. If you open the throttle slowly, the voltage should increase smoothly and not jump around.

Something else that you can try is to adjust the throttle stop. This is not normally an adjustment, but you can use it to try cracking your TB open just a bit when closed and see if that helps the situation.

I had this exact same problem years ago and I think it might have been the throttle stop adjustment that fixed it for me, but I don't remember for sure.

The battery was the first thing that I replaced, since my first viper about 12 years ago had some crazy issues that the battery fixed.


The viper tech also reset my ECU when he was working on it.


I don't have a voltage meter but I should. I will go get one and check it out.


I did screw with the Throttle Stop a bit with no change in anything. Not sure what is going on, other than I want to beat the crap out of something

I should probably mention that this throttle stop is to synchronize the opening of the 2 TBs. One (drivers side I think) should open just before the other. If you mess with it, mark it so that you can get it back to where it was if you should need to.

Of course, I forgot to mark the last position.. Well, at least it didn't help nor hurt anything so far. I think I have it back to roughly where it was (I know, that is bad.)

According to Tator, the only way to truly reset the PCM is to ground the + battery cable to the chassis. Then after reconnecting the cable do a TPS reset by turning the key to run but not start, casually depress the gas pedal to max and release, repeat 4 more times, turn key to off, wait 30 seconds and motor on.

Wow, that is crazy. I am going to do this since it can't hurt.


All and all, thanks guys for the responses. Nothing is working currently and I am really depressed because of it. Any more help would be appreciated!
 

Jack B

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You have done a lot of work and the symptoms may have changed, please give us an detailed overview of how it now reacts - all the details
 

EllowViper

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I ended up trying several TPS's. One would work great with the stock TBs and another one wouldn't. I put it in the ACCUFAB TBs and idle like crap. Put a different TPS on the ACUFAB TBs and it idled perfect. I didn't believe it so I went back and forth several times and sure enough, the TPSs were the things causing MY issues.
 

DrumrBoy

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He said it is new. I would also agree about checking (or replacing) the TPS. Doesn't sound like dirty TB's but why not clean them. I'd remove 'em and hose them with throttle body cleaner.



SVS undoubtedly told the previous owner that they replaced it with a new one......but in my experience with those hosers gives it only a 50% chance of being done. 100% chance of being billed, but only a 50% chance of having actually been installed. Cleaning it is easy to do so at least the OP can eliminate one possibility quickly.
 

DrumrBoy

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Whoops, never mind....I missed the post about you having changed it yourself recently as well.
 

NI-KA

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I saw that you replaced the battery. I had same symptoms but battery fixed. I replaced the battery twice.

I agree with Greg Good. I vote for getting the car to a dyno and tuned by pro. Try to work with a vendor here that does tuning. If location is an issue I would still try and work long distance with someone who knows vipers real well. With all the frustration already invested; maybe shipping it to a vendor/tuner here and let them live with the car by driving it daily for awhile?

I really can appreciate your frustration. I have had my share of challenges with my viper. I did some of the work myself and had a number of different vendors help.

Like many here, when I am in there I might as well make it better.

Just keep in mind it is fixable. :2tu: Hang in there.
 

Ratical2

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According to Tator, the only way to truly reset the PCM is to ground the + battery cable to the chassis. Then after reconnecting the cable do a TPS reset by turning the key to run but not start, casually depress the gas pedal to max and release, repeat 4 more times, turn key to off, wait 30 seconds and motor on.

I know this is obvious, but sometimes it needs to be said. Remove the Positive Battery Cable from the Battery Post BEFORE grounding it. I know, 99% of you knew that already.
 

bluesrt

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all you need to do is find a tech that knows how to actually work on a car,and understands a working engine and electronics, anybody- its hard to find a real tech these days, 95% are just parts changers and bull****ters,and butchers... kill the dyno, they blow up engines
 

Camfab

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It's really sounds like a tuning issue. Mine as well as others have had the same issue. Call Dan at the former DC Performance, I'm sure he can hook you up. I can tell you that my car recently has started to misbehave as well. It starts to buck like a **** within a specific RPM range. It used to do it very slightly and could be controlled by your driving style, however it has become much more pronounced as of late.
 

Solid Red 98

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This is going to sound odd and possibly unrelated, but how does your temperature gauge behave? I had various idle issues, most notably, idle hang. Tried a lot of the things mentioned above. It wasn't until I replaced the temperature sensor that the Idle hang vanished and hasn't returned. Also check the condition of the smog sensors. Replacing them lowered my idle about fifty rpm. Suffice to say my car has been a pleasure since these two items have been addressed. Many cars seem to develop ECU ghosts as they attempt to learn their driving environment, and invariably cop a presentient attitude. Good luck!
 

Viper Specialty

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Very typical issue to tune out with a more aggressive camshaft. Sometimes it can be quite a challenge to get rid of it.
 

Jerry Scott[CO]

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If your car has a reduction pulley on the crank damper, you will get this problem when the battery is low in charge. The reduction pulley slows down the serpentine belt to take some load off the engine when reved to red line. The downside is that at long idle times, the battery is not charging and will run down if you have a heavy electrical load on the alternator. This causes low voltage to the coils and the car will keep dying. If the car does have the reduction pulley, you may want to go back to the stock pulley and belt. It will then be charging when the engine is idling.

Jerry
 
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cornfish

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Hey Guys, I am back with an update :)

Ok, so after playing around quite a bit, I might have taken care of this issue (for the time being.) So I opened up the battery compartment and replaced the battery. After putting in the new battery, one of the ties was a little loose but I left it in there anyway. That night the car wouldn't start at a park in town where I met up with one of my buddies to take pictures. After towing the car home and replacing both battery ties, the car was running well until I fixed my AC. With my headlights on and the AC at full blast, I could get the car to dip at idle again (not like it was before, but anything is bad at this point.) So I ripped out the HID headlights that I had in the car and have not had an issue since.

This tells me that the HID's were pulling power even though they were off, and pulling too much power when they were on. If this is/was the issue I am going to be **********, as I am sure you guys can imagine.

I will keep you all updated on the current status but as of now, I have COLD AC (with the $4 AC fix posted here that worked like a charm) and the car isn't stalling at stoplights anymore. I am loving life right now.. I just hope I didn't jinx it right now with that comment :)

Thanks for all the help!

- Ron (cornfish)
 

-FROG-

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Ahhh... the sweet world of debugging issues. Good find. What's the link to that A/C fix thread? I can't find it.
 

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