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Chris (aka-Moose)
Chris (aka-Moose), Technician
Category: Car
Satisfied Customers: 43967
Experience:  16 years of experience
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1998 Mazda B2500 manual trans with erratic idle. Initial

Customer Question

1998 Mazda B2500 manual trans with erratic idle. Initial codes P0340 P1506 P0455 P1401 As I was under the truck, I noticed the hose from the canister had popped out. Put it back in.
Replaced Cam Sensor, EGR Vac Sol, IAC, DPFE. Cleaned throttle body and MAF sensor. Every time I start it, even now, it barely runs at all and I have to hold the throttle open a bit to keep it running. After about 30 seconds or so, something warms up enough to stop the stalling and change to just a high idle of about 1500, and it runs amazingly quite smoothly at that level. I hit the throttle a bit and it revs up to about 3000 and then slowly but surely comes back to 1500. Every time I start it cold (and have erased the codes), no CEL, but after it dies or I kill it and restart it, I have a CEL with always the same codes remaining: P0340 and P1506. The others have not returned.....yet.
Now it gets interesting; if I take it for a drive, accelerate and then let off the throttle, the RPM stays at 3000 UNTIL I bring the vehicle to a stop, at which point it will very slowly idle back down to 1500. How weird is that? I'm at a stand still, told customer she will probably have to take it to a shop with a scanner, unless someone has a really good answer.
Submitted: 5 months ago.
Category: Car
Expert:  Chris (aka-Moose) replied 5 months ago.

Welcome, I'm Chris (aka Moose). I normally log in after 3 pm central. Please make sure to let me know exactly what is needed from me so I can provide EXCELLENT 5 STAR service.

  1. Unplug the IAC valve and verify that engine RPM drops to a low value of about 500 RPM or less.
  2. If the RPM drops, monitor the White/Light Blue wire at the IAC solenoid with a lab scope to verify it shows a typical saw tooth wave form signal.
  3. If the IAC control signal does not look normal, check the WT/LB wire for a potential short to ground in the harness. If the circuit is OK, replace the PCM.
  4. If the idle is high after the IAC is unplugged, pinch the air line to the IAC closed with a pliers to verify that the engine RPM drops. Check for sticking throttle or cable. See if the base idle screw has been adjusted.
  5. If the RPM drops very low or stalls, then the IAC installed was bad.
  6. If the RPM doesn't drop low, inspect the PCV valve.

Monitor the CMP on a labscope. The signal should be clean and uniform, if not then unplug the alternator and see if its better. If so then replace the alternator.


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Thanks Chris

Expert:  Chris (aka-Moose) replied 5 months ago.

Hello again Chris here,

I am contacting you because the post has not been closed or rated. We have 7 days to get a post rated before it times out. When a post times out, that means its no longer on my active list (if you reply I never know it.) You have already paid the fees to get this question answered, but it appears your not pleased, or need more help.

Let me help some more. Is there anything else you feel I could offer so you would be pleased with my help. I have the option to call you if that would be better.

Thanks Chris

Customer: replied 4 months ago.
They picked up the car Thursday night. Don't know yet if they will be bringing it back or not to have me give it a second look.
Expert:  Chris (aka-Moose) replied 4 months ago.

Is there anything else you need from me or are you please with my help. This post is due to time out and close tomorrow.

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