I have been troubleshooting an issue with my pickup that has been happening fairly consistently for the past year. After testing everything I can think of, I decided to turn to those with more experience for help. I found another thread that sounds like they are dealing with the same issue, but without resolution (https://www.mazdaforum.com/forum/maz...ly-gear-51107/).

The truck runs well while driving, but has a very rough idle and will die or stumble when the rpms are allowed to drop quickly. This happens only when the truck is in gear, and is repeatable when sitting still, or when approaching a stop with the clutch pushed in and the accelerator released.

CEL light stays off, but does illuminate briefly prior to starting. Codes were checked using the test connector method, but CEL light did not flash.

• On cold start: Elevated idle, drops slowly as the truck warms up, stabilizes at around 875 RPM.
• On hot start: Initial elevated idle, stabilizes after a few seconds at around 875 RPM.
• Behavior in neutral:
  • Idle is stabile at around 875 RPMs.
  • When RPMs are increased to around >2,000, and accelerator is released, RPMs will return to normal.
• Behavior in gear:
  • Idle is a touch higher than in neutral. Shifting is easy, so I don't believe the clutch is dragging.
  • When RPMs are increased to around >2,000, and accelerator is released, RPMs will drop to 0 (engine stalls), or sometimes the engine will catch itself at around 250 RPMs and return to normal.
• What has been tested:
  • Fuel pressure is within spec.
    • Engine off, Test jumper installed, key on:
      • No vacuum to regulator, natural bypass = 44 PSI.
      • Fuel line dead-headed = 58 PSI.
        • Pressure remains above 50 PSI after 5+ minutes of key off.
    • Engine on, test jumper removed:
      • 875 RPM = 35 PSI w/ vacuum line to regulator, 45 PSI w/o vacuum line to regulator
      • 2,500 RPM = 35 PSI w/ vacuum line to regulator, 45 PSI w/o vacuum line to regulator
      • Using a syringe in place of the vacuum line to the regulator, vacuum line plugged:
        • Full vacuum: around 30 PSI (did not record)
        • Full pressure: around 70 PSI and fluctuating slightly.
  • BAC Valve
    • Cleaned and new gasket installed. No change in performance.
    • Coil measures 10.3 Ohms at 55°F, which is slightly high.
    • Cold start RPMs are elevated when BAC is connected, not elevated when disconnected.
  • Coolant temp sensor
    • 0.9k Ohms @ approximately 110°F
    • 0.341k Ohms @ approximately 150°F
  • IAT sensor
    • 40k ohms @ approximately 38°F
    • 38.1k Ohms @ approximately 80°F
  • TPS sensor, values were not documented well, but I believe the measurement showed 750 Ohms at idle and increased smoothly as the throttle was opened.
  • MAF sensor
    • Cleaned, no difference.
    • Did not measure, but need to read up on this test.
  • Spark plugs:
    • Replaced plugs and cables 9,000 miles ago.
    • All plugs are normal light tan shade and clean.
    • Cables resistance is normal.
  • Distributor: Replaced cap and rotor recently as they were worn. No difference in performance.
  • Ignition timing: Checked and adjusted recently, noticeable change in overall performance, but not with the stalling.
  • Cleaned throttle body
  • Cleaned air filter (recently replaced)
  • Battery replaced recently
  • Inspected and tensioned balancer chain.
  • Vacuum level is 17mmHg @ 875 RPM, 19mmHg @ 2,500 RPM.
  • New O2 sensor, no change.
  • ECM was rebuilt in 2007 by ?AVM Pro?

I will post videos of the tach once I can convert them to an accepted format.

In Gear:

In Neutral:

 

You have a bad or broken CLUTCH SWITCH. These switches, when bad or broken, can definitely affect the idle speed with the clutch depressed. It is a simple on/off switch that tells the ECM when the clutch is depressed, and then the ECM adjusts the idle control accordingly. At least that's how I understand it, anyway!

Order a new Clutch Switch and install it. Looks like this.......




Mazda Part Number.....



As a test, you can unplug the electrical connector from the rear of the Clutch Switch and see if the idle issue improves, or continues, even without the clutch depressed. If I am not mistaken, when the clutch pedal is depressed, the switch is in it's OPEN state. Anyway, play around with it and see if it affects the problem that you are having.......if it does, then a new switch will restore the drivability issue.

Now......if you're interested in how things work......like I am......then you can pull your existing switch apart and see what is actually broken with it! If it is the spring inside of it, you may be able to find a small spring at your local hardware store and FIX your existing Clutch Switch.......like I have done, by swapping parts from several different Clutch Switches that I picked up from the Junkyards!

Here is some photos of them.......one had a broken plastic plunger.......the other a broken spring. Swapped the parts around and made a good clutch switch!









BTW......the Clutch Switch is located under the dash, where the brake & clutch pedals are, and it is a switch that is smaller than the Brake Light Switch and the Starter Inhibit Switch (for disabling the starter until the clutch is depressed).

Welcome to the forum, and let us know what you find under there!!

 

Awesome, thank you for the response MeB200! I was laying in bed last night thinking about your post in the thread that I referenced, and the clutch switch was on my list to check today. I was assuming that I would have issues starting the car without putting it in neutral if this switch was bad, but it sounds like you are saying that this might be separate from the starter inhibit switch. If that is the case, that makes perfect sense why it would be stalling like it is. I work as a controls engineer and troubleshoot this type of thing all day, but after work my brain just says no. I will definitely take it apart and see if I can fix it after running some tests.

I will check tonight and post back here on my findings.

 

Also, I believe that the plastic plunger "lands on" a rubber pad that can actually fall out/off and the plastic plunger would then go through the hole that retains the rubber pad.......and therefore NOT depress the plastic plunger! Look for that when you take a look.......if this is the case, your Clutch Switch may still work......it's just not getting depressed when it should be.

Honda cars brake lights are famous for the rubber pad falling off at the brake pedal and therefore affecting the brake lights.

 

Thanks again, I will definitely check that as well.

 

So here might be a better explanation of the Clutch Switch.......pulled from an older MX-5 forum thread:

"The clutch switch is an ON-OFF switch, which is wired to the ECU. When you depress the clutch, and your TPS sensor is in idle position (no throttle), the ECU will control the IDLE to the preset rpm."

Response:
"But when it's in neutral and the pedal is all the way up, the ECU is still controlling the idle. Even with the switch stuck all the way out, the car drove normally."

Response:
Because the clutch switch and the neutral switch (in the transmission) is wired in Parallel. The ECU using the signal to decide about the IDLE control. When you are stopped and in neutral, the neutral switch pulls the signal to zero, hence IDLE control active. If you are in gear, and you still stopped, you had to press the CLUTCH, so still there is IDLE control.

The clutch switch is good in situations,when you are moving, the TPS sensor is at idle position, and you remove the load (disengaging the drive train from the power train) from the engine by pressing the clutch pedal. The ECU will set the preset RPM, by activating the idle control."

 

That was it, thank you so much for the advice! The terminals were pretty dirty, but I think the culprit was a broken spring that prevented the plunger from reaching the contacts. I got it repaired and reinstalled and took the pickup for a test drive. The idle returned nicely to around 750 RPM regardless of the shifter or clutch position. I may look around for a replacement switch, but I think this one should last a while longer. Now to find the source of the rough idle and replace the 6 ball joints that are cracked!


Clutch switch for idle control has a removable plug on the hidden side. This must be removed first before unscrewing the switch from the bracket.


The old plunger was still good.


Contacts covered in some sort of 30 year-old grease.


Original spring was broken.


Cleaned contacts with rubbing alcohol.


Cleaned plunger with rubbing alcohol and installed a new spring from the spring drawer.


Packaged back up and ready to install.

 

Well (at least on the 2WD models) the ball joints are of the bolt-on type, so I didn't need any special tools.

 

Well (at least on the 2WD models) the ball joints are of the bolt-on type, so I didn't need any special tools.

And tie rod end replacement was straightforward.

 

Sweet! Isn't it amazing how something that minor, can cause a drivability issue that you could chase for months on end!

Glad you got that taken care of RJV!