2000 3.8 Rpm Drops low and Dies during slowing down, stopping, engaging clutch or braking

ColdStanger

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Jan 5, 2019
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2000 V6 3.8 Manual

My car runs strong, good power and good take off and throttle response. Recently bought this car knowing of this problem. The car when (slowing down, shifting manual slowing with clutch, letting off gas for a few seconds or stopping/braking to stop sign or in traffic, turning corners) my rpms drop down low quickly to around 500 I think and the car shuts off. It does this most of the time after the car has warmed up. usually if I start car first thing in day it will not cut out on me until the car warms up or after 5 miles or so of driving. If I either shift the manual to neutral and give it gas while stopped or turning corners it wont shut off.

Today it cut on me and the battery was dead when I tried to restart it . I tested it and replaced the battery because it was bad thinking that was the problem but the problem continued.

While changing spark plug wires one of the wires was not attached on a plug so I assumed that was why I was having problems but that didn't help although it did give me more power and less erratic rpm and stuttering while driving and gas smell gone.

I have replaced the TPC, PCV, I swapped IAC with another used one that came off a junk car, changed spark plug and wires, and replaced the battery. Cleaned and tested the MAF sensor by tapping on it while car is running (no engine change) and I also unplugged it and drove it and Im still having problems.Coil pack seems good as I unplugged each . Coil pack seems good as I unplugged each wire and got a constant bluish spark from each one. Cleaned the throttle body although it looked clean. New Air Filter (Cold Air Intake install)

I'm pulling my hair out trying to figure what the problem may be. I'm thinking maybe replace the EGR valve, Fuel Filter or crankshaft censor.

Car will just die down when slowing down or coming to stops or hard braking. RPM drops low first then it does out quickly

Any help would be greatly appreciated
 
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Onesick99GT

Active Member
Nov 20, 2018
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Dayton, Ohio
This sounds more tune related than a mechanical issue. Sounds like the car runs rich at start then leans out once it warms up. Just from this the maf tables aren't correct. Does this car have any aftermarket parts? Has a different tune been loaded to change it from stock? Without datalogging its hard to know what exactly is happening inside the motor. I would suggest downloading forscan software and buying the elm adapter on amazon. When I first started having issues with my mustang I replaced everything and still didn't find my issue until I bought the elm adapter for ford/mercury, downloaded the forscan software and watched my maf tables, fuel pulsewidth 1 and 2, Iac and rpms. Then I found out it was my tune giving me the issues and not mechanical. Datalogging is a surefire way of solving your issue cheaper than throwing parts at it.

View: https://www.amazon.com/OHP-ELMconfig-Adapter-Compatible-Diagnostics/dp/B01F0GVBWY/ref=sr_1_1_sspa?ie=UTF8&qid=1546743048&sr=8-1-spons&keywords=elm+adapter+ford+mercury&psc=1



The adapter above has been a lifesaver and has opened up another world for me when troubleshooting cars.
 

wmburns

SN Certified Technician
Aug 14, 2009
5,613
464
164
Houston Texas
Shutting off for no apparent reason is also a symptom of a bad battery, loose battery cable or bad alternator. Don't miss the chance for an "easy fix". Suggest stating with a through review of the charging system.

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-forums/resources/howto-perform-charging-system-voltage-drop-test.56/

Then when you know that the charging system is in "tip top shape", start trouble shooting the IAC/idle system itself. Does the IAC work? Does the PCM have control of the IAC valve?

Troubleshoot IAC idle problems 1996-2004
https://www.stangnet.com/mustang-forums/resources/troubleshoot-iac-idle-problems-1996-2004.13/
 
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ColdStanger

New Member
Jan 5, 2019
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This sounds more tune related than a mechanical issue. Sounds like the car runs rich at start then leans out once it warms up. Just from this the maf tables aren't correct. Does this car have any aftermarket parts? Has a different tune been loaded to change it from stock? Without datalogging its hard to know what exactly is happening inside the motor. I would suggest downloading forscan software and buying the elm adapter on amazon. When I first started having issues with my mustang I replaced everything and still didn't find my issue until I bought the elm adapter for ford/mercury, downloaded the forscan software and watched my maf tables, fuel pulsewidth 1 and 2, Iac and rpms. Then I found out it was my tune giving me the issues and not mechanical. Datalogging is a surefire way of solving your issue cheaper than throwing parts at it.

View: https://www.amazon.com/OHP-ELMconfig-Adapter-Compatible-Diagnostics/dp/B01F0GVBWY/ref=sr_1_1_sspa?ie=UTF8&qid=1546743048&sr=8-1-spons&keywords=elm+adapter+ford+mercury&psc=1



The adapter above has been a lifesaver and has opened up another world for me when troubleshooting cars.
Hey thanks! The adapter looks nice and I will be ordering it tonight to use with for scan! What do I look for in the app when using it?

The car has no tunes. Only a CAI that I installed after the problems not before. Actually I recently bought the car for two weeks ago with knowledge of the problem. The person I bought it from said he was told it might be the oil senor

I bought a fuel filter today I will be installing in the morning. I plan to replace that and the egr valve just because they look pretty old and I had plan on replacing some parts on the car for anyway so I might as well try and start with cheaper parts first to find the problem.
 

ColdStanger

New Member
Jan 5, 2019
7
0
1
42
USA
Shutting off for no apparent reason is also a symptom of a bad battery, loose battery cable or bad alternator. Don't miss the chance for an "easy fix". Suggest stating with a through review of the charging system.

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-forums/resources/howto-perform-charging-system-voltage-drop-test.56/

Troubleshoot IAC idle problems 1996-2004
https://www.stangnet.com/mustang-forums/resources/troubleshoot-iac-idle-problems-1996-2004.13/
Thanks. The Battery was replaced today with a new battery and I still have the problems.

But the die down problem does replicate as though the battery is dying because it just slowing shuts off like as though I turned the engine off via ignition
 
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ColdStanger

New Member
Jan 5, 2019
7
0
1
42
USA
Shutting off for no apparent reason is also a symptom of a bad battery, loose battery cable or bad alternator. Don't miss the chance for an "easy fix". Suggest stating with a through review of the charging system.

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-forums/resources/howto-perform-charging-system-voltage-drop-test.56/

Troubleshoot IAC idle problems 1996-2004
https://www.stangnet.com/mustang-forums/resources/troubleshoot-iac-idle-problems-1996-2004.13/

Alternator was tested and good. Battery was replaced

I did a quick voltage drop test but was kind of confused by the video. battery is well over 14v when started and even with load. Confusing part was when he was pacing the cord on the alternator. It looked like it was the negative cable and then he said he placed it on the Alternator case with positive end on the + battery

So I placed the Positive cord of my volt meter on the battery positive cable and place the negative end of my volt meter on the alternator case and also on parts of the car that was metal such as engine top manifold and screws on body (also i reverse the leads) and it read negative -12.50
 

wmburns

SN Certified Technician
Aug 14, 2009
5,613
464
164
Houston Texas
I did a quick voltage drop test but was kind of confused by the video. battery is well over 14v when started and even with load. Confusing part was when he was pacing the cord on the alternator. It looked like it was the negative cable and then he said he placed it on the Alternator case with positive end on the + battery
I'm not sure that you understand how the voltage drop testing works. One of the VOM leads does go to battery positive and the other goes to the end of the wire to be tested (for example the main alternator B+ terminal). It does not matter which lead is connected to battery positive (only unless you are trying to determine the direction of current flow). It seems confusing because this is not the normal way to test. One would think that voltage testing what amounts the same wire is a useless test. And if the current path is good, the voltage drop test will confirm this by being less than 0.250 volt.

So if the voltage drop test reading is below 0.250 volt THEN the test is good.

Test the alternator ground by putting one VOM probe on the alternator case and the other on battery negative. Test with the motor running and loads turned on. Again a good ground path should have a low voltage drop (below 0.25 volt).

And yes I know the battery was replaced. But do you know that all of the battery terminals are good? The voltage drop test will quickly let you know for certain.

So I placed the Positive cord of my volt meter on the battery positive cable and place the negative end of my volt meter on the alternator case and also on parts of the car that was metal such as engine top manifold and screws on body (also i reverse the leads) and it read negative -12.50
This is not a voltage drop test. But it does illustrate there is voltage drop some where in the system. Why? Because you stated the voltage was 14 volts at the battery. So where did the other 1.5 volts go? No telling because the current flow is going through soooooooo many different points as to render your test meaningless.

Bottom line. The test is supposed to be a quick diagnostic go/no go style test. But it is important to understand what is being tested, how to perform the test, and finally how to understand the results.

If you plan on getting the ODB2 scanner adapter I would like to encourage you to stop or slow down the "parts changing" method of trouble shooting. Try to test more and replace only the parts that are known to be bad.

Here's some more information on the ForScan windows based ODB2 scanner.

ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/

Did you go through the information on how to trouble shoot IAC/idle problems? Did you learn anything that you didn't know before?

Another quick test that you could do while waiting for a full function ODB2 scanner is to get yourself an "add a circuit" or "add a fuse". This will give an easy way to tap into a fuse. Use the "add a fuse" to tap fuse F2.2. Put a VOM on the pig tail and attach the VOM to a known good ground. This will give an easy way to monitor system voltage while driving.

What you are looking for is to see IF the voltage drops BEFORE or AFTER the stall starts to occur.

IF the voltage drops BEFORE the stall this means that the stall is being caused by the reduced voltage.

IF the voltage drops AFTER the the stall occurs, this means that the voltage is dropping because of low RPM's and is a symptom of the stall (not the cause).

To do this test with an ODB2 scanner one would graph the voltage, RPM's, MAF, IAC duty %, and recreate the problem. What you will be looking for is which PID is changing first. This can point you in the correct direction for the next set of tests.

1999-2004 MY fuse panel schedule:
https://www.allfordmustangs.com/forums/4-6l-tech/305933-1999-mustang-gt-2001-motor-swap-wont-fire-please-help.html#post2669271

https://www.oreillyauto.com/detail/b/littelfuse-4149/lighting---electrical-16777/fuses-16615/fuse-holder--universal--24423/8c82836bc809/littelfuse-holder-micro2-add-a-circuit/fhm20200zpa/4414324/2002/lincoln/town-car?q=add+a+fuse&pos=0

Note, IF the problem only happens when braking on a car with vacuum assisted power brakes, don't overlook the power brake booster using up too much engine vacuum as a possible cause. IE a vacuum leak in the power brake booster itself.
 
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ColdStanger

New Member
Jan 5, 2019
7
0
1
42
USA
I'm not sure that you understand how the voltage drop testing works. One of the VOM leads does go to battery positive and the other goes to the end of the wire to be tested (for example the main alternator B+ terminal). It does not matter which lead is connected to battery positive (only unless you are trying to determine the direction of current flow). It seems confusing because this is not the normal way to test. One would think that voltage testing what amounts the same wire is a useless test. And if the current path is good, the voltage drop test will confirm this by being less than 0.250 volt.

So if the voltage drop test reading is below 0.250 volt THEN the test is good.

Test the alternator ground by putting one VOM probe on the alternator case and the other on battery negative. Test with the motor running and loads turned on. Again a good ground path should have a low voltage drop (below 0.25 volt).

And yes I know the battery was replaced. But do you know that all of the battery terminals are good? The voltage drop test will quickly let you know for certain.


This is not a voltage drop test. But it does illustrate there is voltage drop some where in the system. Why? Because you stated the voltage was 14 volts at the battery. So where did the other 1.5 volts go? No telling because the current flow is going through soooooooo many different points as to render your test meaningless.

Bottom line. The test is supposed to be a quick diagnostic go/no go style test. But it is important to understand what is being tested, how to perform the test, and finally how to understand the results.

If you plan on getting the ODB2 scanner adapter I would like to encourage you to stop or slow down the "parts changing" method of trouble shooting. Try to test more and replace only the parts that are known to be bad.

Here's some more information on the ForScan windows based ODB2 scanner.

ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/

Did you go through the information on how to trouble shoot IAC/idle problems? Did you learn anything that you didn't know before?

Another quick test that you could do while waiting for a full function ODB2 scanner is to get yourself an "add a circuit" or "add a fuse". This will give an easy way to tap into a fuse. Use the "add a fuse" to tap fuse F2.2. Put a VOM on the pig tail and attach the VOM to a known good ground. This will give an easy way to monitor system voltage while driving.

What you are looking for is to see IF the voltage drops BEFORE or AFTER the stall starts to occur.

IF the voltage drops BEFORE the stall this means that the stall is being caused by the reduced voltage.

IF the voltage drops AFTER the the stall occurs, this means that the voltage is dropping because of low RPM's and is a symptom of the stall (not the cause).

To do this test with an ODB2 scanner one would graph the voltage, RPM's, MAF, IAC duty %, and recreate the problem. What you will be looking for is which PID is changing first. This can point you in the correct direction for the next set of tests.

1999-2004 MY fuse panel schedule:
https://www.allfordmustangs.com/forums/4-6l-tech/305933-1999-mustang-gt-2001-motor-swap-wont-fire-please-help.html#post2669271

https://www.oreillyauto.com/detail/b/littelfuse-4149/lighting---electrical-16777/fuses-16615/fuse-holder--universal--24423/8c82836bc809/littelfuse-holder-micro2-add-a-circuit/fhm20200zpa/4414324/2002/lincoln/town-car?q=add+a+fuse&pos=0

Note, IF the problem only happens when braking on a car with vacuum assisted power brakes, don't overlook the power brake booster using up too much engine vacuum as a possible cause. IE a vacuum leak in the power brake booster itself.
Hey thanks! The add a fuse tap can be bought at parts store? Is that the one you speak of? So basically I plug it I to the f2.2 fuse?

I'm going to get the Ford an adapter tonight? I'm just wondering what should I look for. Will you be able to help me read it when I get it?

I'm going to do the voltage test on the ALT in a few hours. I'll post my results

I more thing my oil level gauge most of time stays at 0 until I give it gas. And ski the electronic miles display on the dash doesn't lights up but some times it does it lights up. It usually lights up when the cat is warned up and problems occur I believe.

And sometimes in neutral while idling the rpm sits at 800 but goes up and down to around 1000 back and forth then sits back at 800
 
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wmburns

SN Certified Technician
Aug 14, 2009
5,613
464
164
Houston Texas
I'm going to get the Ford an adapter tonight? I'm just wondering what should I look for. Will you be able to help me read it when I get it?
All I can say is to encourage you to go back and review carefully the information provided. I very clearly attempted to point out that the ODB2 scanner will not just "tell" you what the problem is. It's a powerful tool that how useful it is depends mostly upon YOU. I'm here to help but I can't do it for you. Some assembly required.
The add a fuse tap can be bought at parts store? Is that the one you speak of? So basically I plug it I to the f2.2 fuse?
I gave a sample link to one possible source for the product. I was hoping that seeing a picture of the product would help you understand how to use it to tap into the car's electrical system and be able to test voltage while driving. Almost any/all Autoparts store sells them. I was attempting to give a "low tech" method to use when an ODB2 scanner is not available.

Myself personally I would perform the test by using the graphing capabilities of the ODB2 scanner.
I more thing my oil level gauge most of time stays at 0 until I give it gas. And ski the electronic miles display on the dash doesn't lights up but some times it does it lights up. And sometimes in neutral while idling the rpm sits at 800 but goes up and down to around 1000 back and forth then sits back at 800
I covered the "hanging" idle in the how to link provided. As well as the reason why under most conditions it is normal behavior when coming to a stop. On the other hand a bouncing idle at full stop is likely a symptom of the real problem.

The oil pressure sender sensor can and does go bad. I have replaced more than one over the years I have own cars (not just Mustangs). You should know if the engine oil pressure is good or not.

Here's some more information for your review on the dash issue.

Cluster PC foil trace repair.
https://www.allfordmustangs.com/forums/v6-tech/304608-my-odometer-goes-off.html#2657361
 
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Onesick99GT

Active Member
Nov 20, 2018
343
40
38
33
Dayton, Ohio
Once you get the adapter you can view almost anything on the car while its running and also save the files and upload on here for us to see. I will be more than happy to help walk you through using the software. Its super easy and will give you a lot of information. Lets just say once you learn how to add PID's and read them it will change everything you know about troubleshooting cars. My parents own all fords and I have found problems they didn't even know was there until I hooked it up ran a few test and found them. My dad was about to start throwing parts at it but the software pinpointed the exact issue. It gives you the codes like a obd1 and 2 show but also shows the history of cleared codes and gives you what each code is and what the problem could be. Its pretty much a ford manual built in with datalogging capabilities, which in my opinion is priceless and saves you the headache of throwing parts at it. The picture below is just a couple data logs I have from tuning my mustang.
Fuel Bank 1 and 2 Differences 2.PNG


Oxygen Sensors Voltage.PNG
 

ColdStanger

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Jan 5, 2019
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Thanks you all adapter should be here tommorow.

In the meanwhile I bought a $50 Walmart OBD and he capture live data. I’ll post that this evening

Good news is I bought a new sigh “autozone” IAC. That actually fixed my problem with the cutting off and stalling but here is the catch now at idle the rpm surges 300 or so Rpm and drops back down every ten secomds
Also while driving and shifting the rpm will get stuck then drop. And I can felling it while driving it a little.

So I figuring that the new IAC is bad? Although it did fix my problem.

And they sell a IAC with black vent. Is that one better?