I’ve been battling a misfire at idle for a week or so, and last night, when starting the car to drive home from work, that glorious Transmission Fail light came on. Here we go... Stuck in 3rd gear, maximum line pressure.

There were no VCM codes for the Engine Control Module. I checked some known trouble spots including the kickdown switch (which years ago had an issue with worn-through insulation on the wiring, causing an intermittent transmission fault light). That checked out, no issues.

I started to look on the electrical side, as I noticed that pressing the Sport mode button did not cause it to illuminate the red LED nor the cluster indicator. I checked and swapped the transmission relay, it was fine. I even plugged in a known-good ECU I had laying around. No change.

My PDU has been having trouble connecting to the car. It typically takes an hour or so before it’ll establish connection. This seems to be temperature dependent. I laid the PDU on the roof, in the sun, while it attempted to connect and it finally did about an hour later. I was able to interrogate the Bosch TCU and received “Code 5: Throttle Signal Out of Range, Open Circuit or Short To Ground (1 Count)”

This seemed odd because I wasn’t getting a VCM code for the TPS, but because the transmission computer receives its signal from a separate track in the TPS, it’s possible. I unplugged the connector, flooded it with contact cleaner and plugged it back in (which I had done many times in the past week...), and sent the clear code command with the PDU. To my surprise, the code cleared and stayed off.

The PDU read the TCU memory again and came up with “Code 22: Pressure Regulator Current Flow Out of Range: Open Circuit or Short Circuit To Ground (1 Count).”

I cleared that code and it also remained off so I’m not sure if that was a glitch while disconnecting the TCU (mistakenly with the ignition on).

The transmission is fine now, no issues after starting the car a few times. Hopefully it remains off.

I had to leave for work, but I quickly performed a few Engine Setup Procedures in the PDU’s Powertrain Toolbox. One was setting of the Throttle Position Sensor. The PDU displays a bar graph which moves as the sensor is adjusted. In the middle of the bar graph is a box which represents the correct idle voltage with it’s appropriate upper and lower tolerances. Adjust until the bar graph is in the box, and you’re done. Mine was reading .42v so it had a ways to go to be within spec. All set now, but it didn’t help the idle misfire.

I performed the Idle Speed Adjustment which commands the stepper motor closed in one shot, and then shows a bar graph with the appropriate base idle RPM marked. Adjust the bypass screw until you reach the mark. I couldn’t adjust the idle speed because the engine was misfiring too badly at that low engine speed and the RPM was unstable.

Now that the transmission issue is sorted, I will perform a tune up to try to rectify the misfire. The cap and rotor are not too old, probably 10-15k miles old IIRC, and the plugs are only about 5k miles old. Wires and coil was done about 20k miles ago. Haven’t ever replaced the ignition amplifier.

The only thing I did recently that coincides with the idle issue is removing the CPS and cleaning the built up dried, hard crusty gunk on the sensor tip. The car didn’t start idling poorly until a week or so later. I did the usual ground cleaning, ECM reset, Intake plumbing cleaning, dead cylinder test, checked the plugs, etc.

I ran the car down to empty today and filled it up with my usual Mobil Premium. I haven’t gassed up at any other station in probably 6 months, so I doubt it’s a fuel issue. Pressure Regulator is only about 18 months old.

At 800-1000 RPM and above the engine is extremely smooth. At idle in Park and in gear, the stumble is quite noticeable.

Hopefully a new cap and rotor will help. I did see some pitting on the cap contacts. I’m really glad I have the PDU to sort out the transmission light! It’s very temperamental and can be frustrating but it’s helped me out many times.






Nick

 

Those rotors can be a PIA Nick, get a few.

I think it was just a week or two ago someone posted that there was a resistor in the rotor and it occasionally breaks down.
Maybe that isn't it, extreme low tech compared to your setup but would be my first check, followed by CPS connector.

Larry
now getting 1 month to the gallon

 

Thanks Larry,

I've poked around today and swapped in a spare cap and rotor I had in my parts bin. No change at all.

While I was under the hood, I had the car idling and lifted up the EGR valve by hand. The car stumbled only a hair more than it already did without operating the valve, which seemed odd. The diaphragm travel didn't feel right tho.

I grabbed a spare (thank goodness for spare parts!) EGR valve and compared. Sure enough, the one on the car was hung up and stuck OPEN a bit, making the car stumble at idle, yet at speed, it was perfect. It all makes sense now... Swapped the spare valve in, and Bingo! No more misfire at idle.

Hope this helps someone out there

Nick

 

Great news Nick! we all tend to forget about that EGR valve ...Great work!

Larry

 

Excellent, Nick!

Larry's right (as usual)! We all tend to forget the EGR valve!

Cheers,

Don