car dies when coming to stops at random?

[00_silvergt]

I'm going to clean the iac. my car shuts off sometimes coming to a stop. Hope I don't have to replace it

 

[CHUXSVT]

I'm going to clean the iac. my car shuts off sometimes coming to a stop. Hope I don't have to replace it

$45 bucks. its not too bad. very easy to replace.

 

[00_silvergt]

$45 bucks. its not too bad. very easy to replace.

I know. I had to replace it on a 2000 gt a while back. I got a lot going on right now. Trying not to spend any money on the car since its not even a daily driver. My wife isnt working and baby is due any day now. I guess eventually ill replace it when I get some extra money. Cleaning it is worth a try lol

 

[mwolson]

This is not a cheap hobby...

 

[00_silvergt]

It isn't a cheap hobby. Ive spent over $600 and I still consider it stock lol.

 

[9804SNAKES]

Sorry to disagree, but this will treat the symptom, but not the cause. You can accomplish the same thing as drilling a hole by adjusting the throttle stop screw to open the TB blade a hair. But that will then probably screw up the idle system.

SVT Pille, you will see if you datalog your idle that it is probably now out of kilter. You want to see your ISC duty cycle to be around 35% and the ISC integrator to be around 0. Your tuner should know this. You set your ISC duty cycle by slowly adjusting the TB stop screw while datalogging. Your tuner should then set the ISC integrator to 0 by adjusting the idle air tables in the tune.

The PCM uses air for coarse control of idle, and it uses spark advance for fine control of idle. You will also want to log your spark source to see if it is at a steady 9 when you idle. That means spark is in control of idle.

Idle problems can really be difficult to solve. If you try to solve them without datalogging, you are just shooting in the dark. And the first thing to do is ensure you don't have any leaks of metered air with a smoke test before you even start trying to fix idle issues.

However, the OP's problem is not an idle problem, it is a deceleration problem. I have seen this in cars that have larger than stock injectors, and that is solved by tweaking the dashpot function in the tune.

I have also seen the lack of a good VSS signal at the PCM cause this problem. The PCM uses a more aggressive dashpot function when the car is stopped than it does when the car is moving. If the PCM does not see a good VSS signal, it thinks the car is stopped and drops the RPMS very fast. But the load on the engine from the power brakes and steering is too much and causes the engine to stall. So this symptom can also be caused by the lack of a VSS signal to the PCM.

I know this is an old thread but your contribution whereby you address the deceleration problem was clutch for me. I am having the same problem as the OP but my IAC is fairly new. I am still going to clean it but my odometer has stopped working although my speedometer still works. What other symptoms would a bad VSS or bad VSS connection show? That is where I think I maybe should concentrate. Thanks in advance.

 

[mwolson]

I know this is an old thread but your contribution whereby you address the deceleration problem was clutch for me. I am having the same problem as the OP but my IAC is fairly new. I am still going to clean it but my odometer has stopped working although my speedometer still works. What other symptoms would a bad VSS or bad VSS connection show? That is where I think I maybe should concentrate. Thanks in advance.

If your speedometer is working, then your VSS is working. There is an outside chance that the VSS signal is making it to the speedometer, but not making it to the PCM. Unless your wiring is really screwed up, that is an unlikely scenario. You could run the car on a lift and look at the VSS signal at the PCM with a scope...

If your speedometer is working and your odometer is not working, then the most likely cause is that your odometer's gears have turned to dust. The second most likely cause is that the odometer motor has died. You can get replacement odometer gears on Ebay.

Regarding dying on deceleration, if you have have larger than stock injectors and a custom tune, it is not unusual for the tune's dashpot function to be screwed up. If that is your situation, you whould work with your tuner.

Other than that, the suggestions above are good. Clean your IAC and MAF, etc.

If you have the ability to datalog, it might tell more about what is going on in there. I assume you have no DTCs.

HTH

 

[9804SNAKES]

If your speedometer is working, then your VSS is working. There is an outside chance that the VSS signal is making it to the speedometer, but not making it to the PCM. Unless your wiring is really screwed up, that is an unlikely scenario. You could run the car on a lift and look at the VSS signal at the PCM with a scope...

If your speedometer is working and your odometer is not working, then the most likely cause is that your odometer's gears have turned to dust. The second most likely cause is that the odometer motor has died. You can get replacement odometer gears on Ebay.

Regarding dying on deceleration, if you have have larger than stock injectors and a custom tune, it is not unusual for the tune's dashpot function to be screwed up. If that is your situation, you whould work with your tuner.

Other than that, the suggestions above are good. Clean your IAC and MAF, etc.

If you have the ability to datalog, it might tell more about what is going on in there. I assume you have no DTCs.

HTH

Thanks for replying. I cleaned the IAC with carb cleaner and Seafoam. I actually soaked it for a couple hours and scraped any carbon off with a small wire brush. I reinstalled it and it still decelerated rapidly. Just so you know when it decelerates, it drops hard and dips below idle then comes back up. It only stalls occasionally. My injectors are stock.

With respect to the VSS, I do have a Diablosport handhled tuner and am running one of their most recent tunes. I drove the car with the tuner hooked up and the VSS reading on the tuner corresponded to the reading on the speedo. Is this a good way to test the VSS?

I also reinstalled the stock tune, thinking like you said that it might be the tune. It did the same rapid decelerating and dipping with the stock tune. I reinstalled the Diablo Tune and the problem was the same.

With reference to the gears in the odo, they might have turned to dust. I have read up in the HOW TO section on what to do reference those.

The only thing I haven't done so far is the MAF cleaning and will try that. My MAF is in the fenderwell due to a JLT CAI and I do have a K and N.

So my only other options appear to be to clean the MAF and if that doesn't solve, try a brand new IAC?

Funny I had never heard of DTCs, just always called them trouble codes. Had to google that one. No, no DTCs.

Thanks ever so much!:beer:

 

[mwolson]

With respect to the VSS, I do have a Diablosport handhled tuner and am running one of their most recent tunes. I drove the car with the tuner hooked up and the VSS reading on the tuner corresponded to the reading on the speedo. Is this a good way to test the VSS?

Perfect. Your PCM is seeing your VSS signal. We now know that it is not caused by a lack of VSS signal.

With reference to the gears in the odo, they might have turned to dust. I have read up in the HOW TO section on what to do reference those.

So my only other options appear to be to clean the MAF and if that doesn't solve, try a brand new IAC?

You haven't said if your injectors are larger than stock. Since you have a Diablosport, do you have a tune? In my SCT tunes, I can change the dashpot function to make this problem not happen, unless it is caused by a mechanical problem.

Lastly, what are DTCs?

Diagnostic Trouble Codes. Sometimes the PCM sets them without setting the CEL (Check Engine Light).

 

[9804SNAKES]

Perfect. Your PCM is seeing your VSS signal. We now know that it is not caused by a lack of VSS signal.

You haven't said if your injectors are larger than stock. Since you have a Diablosport, do you have a tune? In my SCT tunes, I can change the dashpot function to make this problem not happen, unless it is caused by a mechanical problem.



Diagnostic Trouble Codes. Sometimes the PCM sets them without setting the CEL (Check Engine Light).

Thanks again for the reply. You might have missed it, I tucked it up there with the paragraph on cleaning the IAC, but I have the stock injectors.

I have checked for trouble codes even in absence of the CEL and there are none. The tune I have from Diablosport is whatever their tuner comes with. I don't have a custom tune from a shop or a tuner (meaning person here). Diabloport every now and then updates their prepackaged tune and that's what I am running.

I am not familiar with the term "dashpot". I can modify the Diablo Tune so what parameter are you suggesting I modify that you modify in your SCT Tune? And let me ask you frankly, should I get rid of the Diabloport and just buy and SCT? I have had the Diablosport since way back when SCT didn't even have a handlheld for the '98 Cobra. That's the only reason I went with Diablosport.

 

[mwolson]

I did miss it, sorry.

I have only seen cars with tune-related issues die on deceleration when they have larger than stock injectors and an SCT tune. The Dashpot function is the part of the tune that controls how fast the RPMs drop on deceleration. It works exactly the way the old dashpots worked on carburetors back in the day. Here's a picture: Redirect Notice

I am not familiar with the Diablosport tuner, but I am pretty sure it is very limited compared to SCT's handhelds with their Pro Racer Package software. I don't know if the Diablosport lets you set the dashpot function. Since you are running a pretty stock setup, I'd try to stock tune to see if it still has the problem. If it does, then you must have a mechanical problem. If it doesn't, then it is probably tune related. What does Diablosport have to say? Might be worth a call to them.

With SCT, you will need to have a tuner (person, not handheld) help you unless you buy the PRP. SCT's PRP is pretty pricey, but you can do virtually anything to your tune, including screw it up good. You have to invest a lot of time and money if you go this route. That is the route I went, but I have spent a lot of time and money getting trained and practicing.

 

[9804SNAKES]

I did miss it, sorry.

I have only seen cars with tune-related issues die on deceleration when they have larger than stock injectors and an SCT tune. The Dashpot function is the part of the tune that controls how fast the RPMs drop on deceleration. It works exactly the way the old dashpots worked on carburetors back in the day. Here's a picture: Redirect Notice

I am not familiar with the Diablosport tuner, but I am pretty sure it is very limited compared to SCT's handhelds with their Pro Racer Package software. I don't know if the Diablosport lets you set the dashpot function. Since you are running a pretty stock setup, I'd try to stock tune to see if it still has the problem. If it does, then you must have a mechanical problem. If it doesn't, then it is probably tune related. What does Diablosport have to say? Might be worth a call to them.

With SCT, you will need to have a tuner (person, not handheld) help you unless you buy the PRP. SCT's PRP is pretty pricey, but you can do virtually anything to your tune, including screw it up good. You have to invest a lot of time and money if you go this route. That is the route I went, but I have spent a lot of time and money getting trained and practicing.

Thanks I definitely appreciate your time and wisdom. I will try the MAF Cleaning this week/weekend and will get back to you and everyone else for the good of the forums.

Good call on calling Diablosport I will check with them. I did try the factory tune and the same symptoms applied. So I am leaning toward something mechanical as you posted.

It's funny because I used to have the hanging idle issue which I usually combatted by shutting the car off and turing it back on at an intersection. That went away on its own and now I have rapid deceleration issue. So it may just be the IAC acting up with the weather. I read about guys having it stick in the cold and now it's hot in SO FL and I am getting rapid deceleration. Who knows. I am going to clean the MAF and if that doesn't help I will get a new IAC and cross my fingers.

 

[9804SNAKES]

$45 bucks. its not too bad. very easy to replace.

Where did you get the IAC for 45? From auto parts store to ebay, I keep seeing 75+. Thanks in advance.:beer:

 

[SilverNotch]

I had the same issue in my 01 cobra I replaced the IAC and it ended being an old tune from diablo predator. I downloaded the newest version and the car was back to normal. Now I'm stuck with the IAC that I bought from autozone and it has idle hang a bit sometimes when I push the clutch in. I should switch to the factory one.