First a little bit about my car, its a 2001 A4 with vararam, texas speed longtubes (AIR del), Ls6 valley cover with a with catch can.
Long story short my 01 a4 went into limp mode this morning and when i went to pull the codes i got po300 ( missfire) and po0756 ( shift soleniod) car drove fine last night. You can feel the car misfiring, however oil pressure and level are good, no knocking (thank god), and Coolant temp & level are good.
When i get home from work i was thinking of checking the coil packs,(would i just use a volt meter to check that?) plugs, and wires. If those check out ok should i do a compression test? Any thing else i should check for? any help would be greatly appreciated.
Thanks
Charles

 

Check to make sure the Spark plug boots are tight. Last time I got a P0300 one boot was loose. Pulling codes from the DIC will NOT show the specific cylinder misfiring. A cheapy Actron can.

 

Take it down to your favorite autoparts store and have them scan for codes with an OBDII scanner (they'll probably do it for free) and that'll tell you which cylinder(s) is having the issue. P0301 = cylinder 1 P0302 = cylinder 2 etc.

Check your wires before anything else; especially since you have long tube headers installed. One of your plug wires may be loose or may be touching your headers which will damage it and cause this issue.

 

An old trick when you have a misfire. Pop the hood, at tight, start the engine and look for a light show (sparks). Give it a try.

 

sorry for the late update, I got a few mins to look at the car yesterday (i've been really busy with the family and renovating my house). First off thank you for all the advice you guys have given me, i started to look at the plug wires and noticed one of the boots was off the coil pack a little bit, so i made sure all the wires were nice and tight and i started her up and bam! sounded great. however this morning i went to take the car to work and i made it down the block and the flashing check engine light came on, started missfiring again. I have a sct handheld tuner if that doesnt show me where its missing at ill run it up to a local auto parts store and have them check it. Thanks again guys for the responses and adivce. Any more suggestions would be awesome. Thank again.

 

You'll have a better idea of where to look when you know what cylinder(s) is missing...

 

so i took my vette to autozone and oreillys and both there scan tools just said po300 random misfire. So i got the car on jackstacks and i will start going through my checklist friday night.Any suggestions ? the plugs and wires were installed last march when i put my tsp longtubes on and none of them are burnt or touching the headers. My only other guess would maybe a 02 sensor wire is touching the header and melted it. Would that cause random misfires? thanks again for your help guys.

charles

 

Here is my thread on the p0300 code. It was a bad valve spring.

Be careful about running a misfiring car in the garage, even with the doors open. I happened to have a CO sensor and it went off (it had a reading of 110. With a reading of 50, you need to get out of the room immediately).

Pull that valve cover - it will be a broken $3 valve spring.

 

The OBDII scanner should have shown which cylinders were missing. Was the motor running when you scanned for codes?

There are a TON of possible causes:
Bad ground connection on the back of the driver's side head
Vacuum Leak - Check for unmetered air entering the intake manifold. Look at all of your couplers and hoses
Plugs - If you pull them you'll probably be able to tell which cylinders are missing
Wires
Coil Packs
Injectors
Clogged Cats - You'll need to get a hold of an IR thermometer to diagnose this
Broken Valve Spring - Pull the covers and take a look
O2 Sensor Issue - Inspect the wiring to your sensors. You'd probably have other codes specific to 02 if this was your issue though.
Bad Head Gasket
Bent Valve
etc...

Start with the simple stuff and go from there...

Do you smell gas while your motor is running. Are you getting any black exhaust? How many miles on your Vette and how old are the plugs and wires?

You may find this informative:

DTC P0300
System Description
The powertrain control module (PCM) uses information from the crankshaft position (CKP) sensor and the camshaft position (CMP) sensor in order to determine when an engine misfire is occurring. By monitoring variations in the crankshaft rotation speed for each cylinder, the PCM is able to detect individual misfire events. A misfire rate that is high enough can cause the 3-way catalytic converter (TWC) to overheat under certain driving conditions. The malfunction indicator lamp (MIL) will flash ON and OFF when the conditions for TWC overheating are present. If the PCM detects a misfire rate sufficient to cause emission levels to exceed mandated standards, DTC P0300 will set.

Conditions for Running the DTC
DTC P0101, P0102, P0103, P0106, P0107, P0108, P0116, P0117, P0118, P0125, P0128, P0335, P0336, P0341, P0342, P0343, P0410, P0500, P0502, P0503, P1114, P1115, P1120, and P1220 are not set.
The engine speed is between 425-3,000 RPM.
The ignition voltage is between 10-18 volts.
The engine coolant temperature (ECT) is between -7 and +130°C (19-266°F).
The fuel level is more than 10 percent.
The throttle angle is steady within 1 percent.
The antilock brake system (ABS) and the traction control system are not active.
The transmission is not changing gears.
The A/C clutch is not changing states.
The PCM is not in fuel shut-off or decel fuel cut-off mode.
The PCM is not receiving a rough road signal.
Conditions for Setting the DTC
The PCM is detecting a crankshaft rotation speed variation indicating a misfire sufficient to cause emission levels to exceed mandated standards.

Action Taken When the DTC Sets
The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.
Conditions for Clearing the MIL/DTC
The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
Clear the MIL and the DTC with a scan tool.
Diagnostic Aids
Excessive vibration from sources other than the engine could cause DTC P0300 to set. The following are possible sources of vibration:
Variable thickness brake rotors--Refer to Symptoms - Hydraulic Brakes in Hydraulic Brakes.
Drive shaft not balanced--Refer to Vibration Analysis - Driveline in Vibration Diagnosis and Correction.
Worn or damaged accessory drive belt--Refer to Base Engine Misfire without Internal Engine Noises in Engine Mechanical.
There may be more or less cylinders actually misfiring than indicated by the scan tool.
Spray water on the secondary ignition components using a spray bottle. Look and listen for arcing or misfiring.
If there are multiple misfires on only one bank, inspect the fuel injector and ignition coil, power and ground circuits for that bank. Refer to Engine Controls Schematics .
Test Description
The numbers below refer to the step numbers on the diagnostic table.

If the actual CKP variation values are not within the learned values, the misfire counters may increment.

DTC P0135 or P0155 can be set because of a misfire.

Step
Action
Values
Yes
No

1
Did you perform the Diagnostic System Check-Engine Controls?
--
Go to Step 2
Go to Diagnostic System Check - Engine Controls

2

Important
You must perform the crankshaft position (CKP) system variation learn procedure before proceeding with this diagnostic table. Refer to CKP System Variation Learn Procedure .


Start the engine.
Allow the engine to idle or operate within the conditions listed in the Freeze Frame/Failure Records.
Monitor all of the misfire counters with the scan tool.
Are any of the current misfire counters incrementing?
--
Go to Step 3
Go to Diagnostic Aids

3
Are any DTCs other than P0300, P0135 or P0155 set?
--
Go to Diagnostic Trouble Code (DTC) List
Go to Step 4

4
Can any abnormal engine noise be heard?
--
Go to Base Engine Misfire without Internal Engine Noises
Go to Step 5

5
Does the scan tool indicate that the HO2S bank 1 sensor 1 or HO2S bank 2 sensor 1 voltage parameters are below the specified value?
200 mV
Go to DTC P0131 or P0151
Go to Step 6

6
Does the scan tool indicate that the HO2S bank 1 sensor 1 or HO2S bank 2 sensor 1 voltage parameters are fixed above the specified value?
900 mV
Go to DTC P0132 or P0152
Go to Step 7

7
Inspect the following components:

The vacuum hoses and seals for splits, restrictions, and improper connection--Refer to Emission Hose Routing Diagram .
The throttle body and intake manifold for vacuum leaks
The crankcase ventilation system for vacuum leaks--Refer to Crankcase Ventilation System Inspection/Diagnosis in Engine Mechanical.
The PCM grounds for corrosion and loose connections--Refer to Ground Distribution Schematics in Wiring Systems.
The exhaust system for restrictions--Refer to Restricted Exhaust in Engine Exhaust.
The fuel for contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
Did you find and correct the condition?
--
Go to Step 20
Go to Step 8

8
Turn OFF the ignition.
Disconnect the spark plug wire from the spark plug that corresponds to the Misfire Current counters that were incrementing. Refer to Spark Plug Wire Replacement .
Install the J 26792 Spark Tester to a good ground.
Start the engine.
Does the spark jump the tester gap, and is the spark consistent?
--
Go to Step 10
Go to Step 9

9
Remove the spark plug wire for the affected cylinder. Refer to Spark Plug Wire Replacement .
Inspect the spark plug wire. Refer to Spark Plug Wire Inspection .
Measure the resistance of the spark plug wire with a DMM.
Is the spark plug wire resistance less than the specified value?
700 ohms
Go to Electronic Ignition (EI) System Diagnosis
Go to Step 19

10
Remove the spark plug from the cylinders that indicated a misfire.
Inspect the spark plug. Refer to Spark Plug Inspection .
Does the spark plug appear to be OK?
--
Go to Step 11
Go to Step 12

11
Exchange the suspected spark plug with another cylinder that is operating properly. Refer to Spark Plug Replacement .
Operate the vehicle under the same conditions that the misfire occurred.
Did the misfire move with the spark plug?
--
Go to Step 18
Go to Step 15

12
Are the spark plugs oil or coolant fouled?
--
Go to Base Engine Misfire without Internal Engine Noises
Go to Step 13

13
Are the spark plugs gas fouled?
--
Go to Step 16
Go to Step 14

14
Do the spark plugs show any signs of being cracked, worn, or improperly gapped?
--
Go to Step 17
Go to Step 15

15
Perform the fuel injector coil test. Refer to Fuel Injector Coil Test .

Did you find and correct the condition?
--
Go to Step 20
Go to Base Engine Misfire without Internal Engine Noises

16
Perform the fuel system diagnosis. Refer to Fuel System Diagnosis .

Did you find and correct the condition?
--
Go to Step 20
Go to Base Engine Misfire without Internal Engine Noises

17
Replace or gap the spark plug. Refer to Spark Plug Replacement . If an improper gap is found, be sure to gap the spark plugs using a wire type gage.

Did you complete the replacement?
--
Go to Step 20
--

18
Replace the faulty spark plug. Refer to Spark Plug Replacement .

Did you complete the replacement?
--
Go to Step 20
--

19
Replace the faulty spark plug wire. Refer to Spark Plug Wire Replacement .

Did you complete the replacement?
--
Go to Step 20
--

20
Was the customer concern the MIL flashing?
--
Go to Step 21
Go to Step 22

21
Operate the vehicle at the specified value for 4 minutes.
Operate the vehicle within the Conditions for Running the DTC P0420 or P0430 as specified in the supporting text. Refer to DTC P0420 or P0430 .
Does the DTC run and pass?
2500 RPM
Go to Step 22
Go to DTC P0420 or P0430

22
Clear the DTCs with a scan tool.
Turn OFF the ignition for 30 seconds.
Start the engine.
Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.
Did the DTC fail this ignition?
--
Go to Step 2
Go to Step 23

23
Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?
--
Go to Diagnostic Trouble Code (DTC) List
System OK

 

Depending on how bad the misfire is, you can use an infrared thermometer to see what cylinder is colder than the others.

 

While a valve spring could be an issue, they do not come and go. Once they break, the engine will run rough...until it is fixed. Did you actually inspect the plug wire you found loose. Typically, if it has been loose for a while, the contact will be severely corroded. Go over everything, and take another look at the plug wire you found loose.

 

thanks lucky, I was planning on pulling them one by one and rechecking everyhthing and if that wasnt the case then open the valve covers and check for a broke valve spring like others have suggested.Or checking any wires anything around the headers. I hope the solution is simple.
Thanks again guys

charles

 

found this on the driver side... its where you plug in the racetronix wiring harness to move the back o2 sensors to the front...

I guess they were touching the headers...
and i found this on the passenger side ...

looks pretty melted imho so i'm going to order new harness's from racetronix.
Now im not saying thats it .... infact i wont know untill i get the new ones in from racetronix, how ever i'm still going to check all my plugs, wires, coils and valve springs... hopefuly i will get a chance to update later.
Please let me know if you guys think i'm on the right path with those pictures.

Charles

 

also sorry for the oversized pictures. But does any one know the part number for the square connector that goes to the 02 sensor.. Thanks again CF.

Charles