Misfire on cylinder number two, need a vacuum diagram

Hi,

Usually, a vacuum leak won't cause one cylinder to misfire, but rather a random misfire. However, anything is possible.

As far as the vacuum schematics, below are two pics. The first is for emissions. The second is related to the HVAC system.

Here is a link you may find helpful when trying to locate a leak:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Let me know if this helps or if you have other questions.

Take care,
Joe

Hi,

What code has turned up related to the TPS? Also, when you heard the noise that sounded like gravel in the hubcap, was it from the engine or under the vehicle?

Let me know all the present codes. It will help point me in one direction or another.

Also, make sure nothing made its way into the throttle body.

I know you have a P0302, but what other codes are present?

Let me know,
Joe

Mark,

The two codes, 121 and 401, as you know are related to the MAP and the EGR. For the 121 to set, It indicates there is a skewed Manifold Absolute Pressure (MAP) signal or faulty MAP sensor. This can be tied into the MAP, throttle position sensor (TPS), a wiring problem, or faulty PCM.

I'm going to provide the diagnostics. However, before testing, let me know if there could possibly be an issue with the catalytic being plugged. That will create a difference in the manifold pressure. I remember you mentioning the rattling sound like stones in hubcaps. I'm wondering if the internal portion of the converter is loose. It's just a theory at this time but hang in there. We'll figure it out. Also, there could be an issue with the MAP and not the MAF which you cleaned.

Here are the diagnostics. The attached pics are the actual flow charts. Let me know if this is something you are comfortable doing.

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1998 Oldsmobile Achieva V6-3100 3.1L MFI VIN M
P0121
Vehicle ALL Diagnostic Trouble Codes ( DTC ) Testing and Inspection P Code Charts P0121
P0121
Diagnostic Chart

pic 1

Diagnostic Chart

pic 2

Wiring Diagram

pic 3

CIRCUIT DESCRIPTION

The Throttle Position (TP) sensor circuit provides a voltage signal that changes relative to throttle blade angle. The signal voltage will vary from less than 1 volt at closed throttle to about 4 volts at Wide Open Throttle (WOT).

The TP signal is one of the most important inputs used by the PCM for fuel control and most of the PCM-controlled outputs. The PCM monitors throttle position and compares actual throttle position from the TP sensor to a predicted TP value calculated from engine speed. If the PCM detects an out of range condition, DTC P0121 will be set.

CONDITIONS FOR SETTING THE DTC

No active TP sensor or Manifold Absolute Pressure (MAP) sensor DTC.

The engine is running.

MAP reading is below 55 kPa or above 70 kPa.

Predicted throttle angle is not close to actual throttle angle.

ACTION TAKEN WHEN THE DTC SETS

The PCM will illuminate the Malfunction Indicator Lamp (MIL) during the first trip in which the diagnostic runs and fails.

If equipped with traction control, the PCM will command the Electronic Brake and Traction Control Module (EBTCM) via serial data (CKT 800) to turn OFF traction control and illuminate the TRACTION OFF lamp.

The PCM will store conditions which were present when the DTC set as Freeze Frame and Fail Records data.

CONDITIONS FOR CLEARING THE MIL/DTC

The PCM will turn the MIL OFF during the third consecutive trip in which the diagnostic has been run and passed.

The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.

The DTC can be cleared by using the scan tool Clear Info function or by disconnecting the PCM battery feed.

DIAGNOSTIC AIDS

Check for the following conditions:

Skewed Manifold Absolute Pressure (MAP) signal or faulty MAP sensor. An incorrect MAP signal may cause the PCM to incorrectly calculate the predicted TP sensor value during high engine load situations. Check for an unusually low MAP reading. This condition can cause DTC P0121 to be set.

Poor connection at PCM. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.

Damaged harness. Inspect the wiring harness for damage. If the harness appears to be OK, observe the Engine Coolant Temperature (ECT) display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the display will indicate the location of the fault.

If DTC P0121 cannot be duplicated, the information included in the Fail Records data can be useful in determining vehicle mileage since the DTC was last set. If it is determined that the DTC occurs intermittently, performing the DTC P1122 and DTC P1121 Diagnostic Charts may isolate the cause of the fault.
See: A L L Diagnostic Trouble Codes ( DTC ) > P Code Charts > P1121
See: A L L Diagnostic Trouble Codes ( DTC ) > P Code Charts > P1122

TEST DESCRIPTION

The numbers below refer to the step numbers on the Diagnostic Table.

2. An incorrect MAP signal may cause the PCM to incorrectly calculate the predicted TP sensor value during high engine load situations. Check for an unusually low MAP reading. This condition can cause DTC P0121 to be set.

12. This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is being replaced, the new PCM must be programmed. Refer to PCM Replacement/Programming.

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The last pic below shows the MAP sensor. Make sure there is vacuum to it and no leaks.

Let me know what you find.

Joe