3rd Generation Specific (1993-2002) 1993-2002 Discussion including performance modifications and Technical Support Sections.

IACV cause of starting issue

Old 07-08-19, 11:51 PM
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IACV cause of starting issue

93 touring on jack stands (ha)

95-100 psi comp each face, both rotors
New pump, FPR, stock rails, injectors (cleaned and tested)
Newer NGK wires, 30ish min runtime on new NGK plugs
PFC with base map, O2 feedback disabled (more on that later)
Optima Redtop 12.5 standing voltage, 11.5ish when cranking (shown on controller)
190ish rpm cranking (shown on controller)

Went to start the car to check the gauges I installed: WB, water and boost. It would crank fine but It would only loudly pop out of the exhaust with a few poofs. A few times it would almost catch, but wouldnt come up to idle. Key on, before cranking it nothing would seem amiss. After cranking the IACV would be buzzing and wouldnt stop until I shut the ignition off. Rinse and repeat. I pulled the plugs, they did not look wet but had a layer of carbon on them.

Previously, it would start no problem after a loud pop from the exhaust, it would crank for another second and run fine. The last time I started it was about a month ago to burp the cooling system after installing a banzai billet neck. It fired, idled and revved no problem, coming up to temp without issue. The first time I ever had the car running it idled poorly if I had the O2 feedback on - very rich and would not hold a steady idle. Disabling O2 feedback solved this.

What would cause it to go from running to not with seemingly no change - the only thing new that I am noticing is the buzzing IACV.
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Old 07-09-19, 08:09 AM
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The idle air control (Mazda calls it ISC, idle speed control) only sets the air bypassing the throttle body to set the amount of air to maintain idle. The car should start fine even with it unplugged.

I have seen the buzzing ISC before, I can't remember the root cause of it.

Most likely the car has fouled plugs and/or is flooded from short term starts. I'd put a new set of leading plugs in and see if it will start. Fouled plugs can *look* great, I've seen some that were so fouled they only gave off a weak yellow spark but to the eye they were shiny and new.

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Old 07-09-19, 08:19 AM
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Dale's probably right about the plugs/flooded engine.

It's possible that it's the base map on the PFC is in need of tweaking/tuning or the leading coil is weak but I'd start with the easiest to fix, new plugs and a deflood.
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Old 07-11-19, 05:18 PM
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New plugs and deflooding didn't change the behavior of it popping / not starting at all. Confirmed fuel pressure is Good and that I have spark on all the leads. Not sure where to look next.
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Old 07-11-19, 08:57 PM
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Mine did this after it had been sitting for a year+ while I worked on it. After I made it start once or twice, its never given me starting/running problems since then (even after sitting for another month or two without running it).

I got some extra vacuum line and hooked it to the extra nipple on the back of my UIM, and used the starter (with fuel + ignition disabled) to suck in some premix oil, then I sprayed a very small amount of starter fluid in the hose and cranked the car.

If it starts, I'd definitely let it run for a little bit.

I wouldn't try this more than once or twice honestly, and its not a long term solution if you continue to have starting problems just as bad. YMMV this is definitely the hammer approach.
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Old 07-11-19, 11:50 PM
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I'll give starting fluid a try. I have the intake elbow off at the moment so it may be easier than rigging up vaccuum lines. A friend who runs a local shop known for rotaries suggested that the primary injectors could be the culprit - possibly clogged or completely failed, so fuel pressure would be present but no fuel would be injected when cranking. Is there a way to confirm this in situ? Is there a way to remotely trigger the injectors with a PFC for diagnostic of the injectors themselves or the circuit? EDIT- another thing I recall, looking at the PCF sensor readout, it highlights the water temp - WTRT at 4.99V, and only shows dashes for water temp while in monitor mode.

Last edited by DPeanut; 07-12-19 at 12:23 AM.
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Old 07-12-19, 10:13 AM
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If you're water or air temp sensor is bad, it will likely over/under fuel the engine. I'd check that you are getting an appropriate temperature readout for your sensors if you're getting 5V for one.
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Old 07-12-19, 03:27 PM
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Air temp was spot on with outside temp. I'll look closer into the temp sensor and if the sensor has failed or the wiring has. Is there a sensor resistance vs temp chart for the oem sensor?
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Old 07-13-19, 10:49 AM
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I think there is in the shop manual.

Also check the TPS, seem to remember if you hear the ISC buzzing/cycling it could be a misadjusted TPS.

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Old 07-14-19, 11:31 PM
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Took a look at my TPS settings and they fell dead center in the adjustment range and rose as normal as far as I could tell with a digital multi-meter and not an oscilloscope. The coolant sensor test gives a resistance reading at 20 and at 80 I love the examples they list in car and motorcycle manuals to test various components. For the water or oil sensors, thermostats ect, boil them, for the IAT; shine a lamp at it until it warms up enough haha.

Turns out my water thermosensor failed open, showing the PFC that the water temp was somewhere in sub zero temperatures and fueled accordingly. Jumping the two pins showed my 0.00V on the controller. New sensor and it fired within 2 seconds.
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Old 07-18-19, 11:08 AM
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Originally Posted by DPeanut View Post
Took a look at my TPS settings and they fell dead center in the adjustment range and rose as normal as far as I could tell with a digital multi-meter and not an oscilloscope. The coolant sensor test gives a resistance reading at 20 and at 80 I love the examples they list in car and motorcycle manuals to test various components. For the water or oil sensors, thermostats ect, boil them, for the IAT; shine a lamp at it until it warms up enough haha.

Turns out my water thermosensor failed open, showing the PFC that the water temp was somewhere in sub zero temperatures and fueled accordingly. Jumping the two pins showed my 0.00V on the controller. New sensor and it fired within 2 seconds.
I was suspecting a bad sensor, glad you found it. I had the same thing happen before. A good fix is to calibrate the extreme opposites of the sensor in the ECU to where you can still limp the car if the sensor goes bad.
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Old 07-19-19, 12:08 AM
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Might not be a bad idea to throw a 300 ohm resistor dummy plug in one of the bins as a limp home safety.
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