Good afternoon - I'm having an ongoing issue with driveability in the morning when the car is cold. I do not get the same problem when i leave work after the car has been sitting for 7-8 hours oddly.

This morning I started the car and it settled down to a very rough idle for 10-15 seconds before dying. Second start the idle shot up to about 1,000 rpm before settling down to 500 and then stalling. Third startup was rough but was OK to drive. By the time I got to work, which is only about 4 miles, the car was idling perfectly.

Things I have done so far to fix this problem:
new battery
new battery terminals
new alternator
checked all grounds. Interior and engine bay and used dialectric great on all (except engine block and starter which were tight).

As I said the problem is intermittent. 3 days it will be fine, today it was bad again.

I have NO issue with starter turning the motor or with the car starting. It dies at idle.

Any ideas what to check next? Thanks!

 

All the things you did with the battery and grounds have nothing to do with your concern…do you have a scan tool ??…I’d be looking at maybe a faulty ECT sensor or possibly a vacuum leak or dirty throttle body can cause stalling.…we can only guess here…if you don’t have a scan tool seek out an auto diagnostic shop in your area…they deal with driveability issues.

 

Do you have a fuel pressure gauge that you can stick on the fuel rail so you can monitor fuel pressure during a cold startup? Normal fuel pressure is about 58 psi. If fuel pressure drops off while idling from a cold start, that would be a huge clue.

 

Thanks for the replies. I don’t know why I never remember to pull codes unless there is a check engine light. Here’s what I got:

P0121_00 throttle position sensor

it was quite dirty and oily used brake cleaner and a toothbrush

P0300_00 engine misfire
plug and wire looked fine. switched coil for cylinder 3 and 5 and will see tomorrow

P0621_00 generator L terminal circuit
new alternator (non factory) started throwing “service charging system” warning

I started with charging system because I get a bunch of variation in voltage while driving

 

You can’t go by “plug and wire looked fine”…coil packs rarely if ever go bad on these cars and what I tell all members of the Forum with misfires that other than changing plugs and wires you will have little success without some advanced diagnostic equipment…so you have a single cylinder misfire and is it current or history DTC ??…the 0621 is L Terminal or “turn on” signal which is the orange wire at the alternator connector…I don’t know if you have or know how to use a DMM so I won’t go any further…I saw in a previous post you have some grounding issues…you get that resolved ??…I also see this is a salvage car…best to let a professional handle this and if you do have other issues I’d preface the post by “this is a salvage car”…you’re opening a can of worms on this one.

 

Thank you for all the replies. Here is an update. Last night we pulled the codes I posted, and then we went ahead with this work:

* changed the spark plugs and wires. I have read conflicting things about guys who say to only use AC delco plugs, and guys who swear by the NGK, but we went with NGK
* removed and cleaned the throttle body - it was VERY oily and dirty.
* removed and checked the MAF sensor - looked fairly new and it was clean and installed in the correct direction.

This morning I drove the car to work. It did not stall at all and had only mild driveablity issues. Only code present when I got to work was the P0621_00 that we never had before installing the Autozone alternator last weekend. I think we will switch it back and see what codes we get next.

I should add some context. I bought this car at a salvage auction as a project to work on with my 17 year old son. It was involved in a front end collision. It did not have water or flood damage. To say the car is no good electrically because it has a branded title I think is unfair. This is the 5th project car we have worked on together, and part of the "project" is troubleshooting/learning and talking to people who know more about a specific model than we do. The car has 113,000 miles, and besides a couple of small issues, runs fantastic.

I'll check back after we change the alternator back to the stock one.

 

As far as your P0621 which involves the alternator "turn on" signal you would normally see around 4.8-4.9 volts at the orange wire (Terminal B) if you back probe it with the engine running…voltage will drop slightly if an electrical load is put on the car…it could be a wiring issue, bad ECM or alternator....if this is a long project you should get yourself a 1 year subscription ($60.00) to Alldata DIY...it will have all the wiring diagrams and service information you'll need....you don't need to remove the whole TB to clean it...the only thing that needs cleaning is the bore area where the throttle plate closes on...per service info the TB gasket needs to be replaced whenever it is removed...another good reason to have service information....a few weeks ago a member removed the TB to clean it and had some idle problems...he did not change the gasket....you don't want to be building in more problems than you already have.

 

That looks like a great resource. Thanks for posting. And pretty cheap as well!

 

This is info for your P0621.,,looks like an error in circuit testing for your engine step 3...should be less than 1 volt ignition on.

Diagnostic Instructions

• Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
• Review Strategy Based Diagnosis for an overview of the diagnostic approach.
• Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptor

DTC P0621Generator L-Terminal Circuit

Diagnostic Fault Information

Circuit Short to Ground Open/High Resistance Short to Voltage Signal Performance B+ P0621 P0621 — — Control P0621 P0621 P0621 P0621 Signal P0622 P0622 P0622 P0622

Circuit/System Description

The engine control module (ECM) uses the generator turn on signal circuit to control the load of the generator on the engine. A high side driver in the ECM applies a voltage to the voltage regulator. This signals the voltage regulator to turn the field circuit ON and OFF. The ECM monitors the state of the generator turn on signal circuit. The ECM should detect low voltage on generator turn on signal circuit when the ignition is ON and the engine is OFF, or when the charging system malfunctions. With the engine running, the ECM should detect high voltage on the generator turn on signal circuit. The ECM performs Ignition ON and RUN tests to determine the status of the generator turn on signal circuit.

Conditions for Running the DTC

Ignition ON Test

• No generator, crankshaft position (CKP) sensors, or camshaft position (CMP) sensor DTCs are set.
• The ignition is in RUN position.
• The engine is not running.

Run Test

• No generator, CKP sensors, CMP sensor DTCs are set.
• The engine is running.

Conditions for Setting the DTC

• During the Ignition ON test, the ECM detects high voltage on the generator turn on signal circuit for 5 seconds.
• During the RUN test, the ECM detects low voltage on the generator turn on signal circuit for 15 seconds.

Action Taken When the DTC Sets

DTC P0621 is a C Type DTC.

Conditions for Clearing the DTC

DTC P0621 is a C Type DTC.

Diagnostic Aids

On vehicles equipped with a sense circuit connected to the voltage regulator. This circuit must have voltage for the charging system to operate properly. This circuit is used as a reference to control system voltage. All circuits must be tested at the generator before replacing any components. Refer to system schematic to confirm if the vehicle being serviced uses the sense circuit as part of the charging system.

Reference Information

Schematic Reference

Starting and Charging Schematics

Connector End View Reference
Component Connector End Views

Electrical Information Reference

• Circuit Testing
• Connector Repairs
• Testing for Intermittent Conditions and Poor Connections
• Wiring Repairs

DTC Type Reference
Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference
Control Module References for scan tool information

Circuit/System Testing

LS7 Engine

1. Ignition OFF, disconnect the X2 harness at the generator.
2. Ignition OFF, test for B+ between the B+ terminal D X2 and ground.
   • If less than the specified range, test the B+ circuit for a short to ground, an open/high resistance.
3. Ignition ON, test for less than 1 V between the control circuit terminal B and ground.
   • If greater than the specified range, test the control circuit for a short to voltage. If the circuit tests normal, replace the ECM.
4. Engine running, test for greater than 3.5 V between the control circuit terminal B and ground.
   • If less than the specified range, test the control circuit for a short to ground, an open/high resistance. If the circuit tests normal, replace the ECM.
5. If the circuit tests normal, replace the generator.

LS3 Engine

1. Ignition OFF, disconnect the X2 harness connector at the generator.
2. Verify that a test lamp illuminates between the generator B+ terminal D X2 and ground.
   • If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance.
3. Ignition ON, test for 4.5–5.5 V between the control circuit terminal B and ground.
   • If greater than the specified range, test the control circuit for a short to voltage. If the circuit tests normal, replace the ECM.
   • If less than the specified range, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal replace the ECM.
4. If the circuits test normal, replace the generator.

LS9 Engine

1. Verify that a test lamp illuminates between the generator B+ terminal and ground.
   • If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance.
2. Ignition OFF, disconnect the harness connector at the generator.
3. Ignition ON, test for 4.5–5.5 V between the control circuit terminal 1 and ground.
   • If greater than the specified range, test the control circuit for a short to voltage. If the circuit tests normal, replace the ECM.
   • If less than the specified range, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal replace the ECM.
4. If the circuits test normal, replace the generator.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• Generator Replacement
• Control Module References for ECM replacement, programming and setup

 

Just to let you know NGK makes the the AC Delco spark plugs for GM. They are an OEM supplier for a lot of car companies.