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Mazda 323F 1.8 1999 - Intermittent flat spot - Fixed #

Posted by Shane Gray on January 13, 2015, 4:49 pm

Year of Manufacture *: 1999
Engine Size and Code *: 1.8 FP
( Optional ) Scanner Used: Blink cods
( Optional) Fault Codes: 12 and 10

Hi,
I have this one in with a intermittent huge flat spot. Flash codes for IAT and TP sensor, I fitted both secondhand sensors and tested with scope. I have scoped MAF, Cam and crank and O2 sensor. O2 sensor was switching a little slow so fitted a new one I had in stock (just changed plug) and still the same. Vacuum gauge showing 25"hg and steady at idle. Smoked and showing no leaks, ran engine with O2 unscrewed(check blocked cat)and no difference.
When your in top gear it will go fine, so I have no petrol/pressure issues.
Any help please I,m scratching my head.
Shane

Re: Mazda / 323F - big flat spot

Posted by Pete Mutlow <iframe src='https://members5.boardhost.com/ukautotalk4/info/user=UKAT1569' width='330' height='300' border=0 style='border:none;'> on January 13, 2015, 5:50 pm, in reply to "Mazda / 323F - big flat spot"

Hi Shane,

Sounds like a fuelling issue to me. Vacuum seems a little high @ 25. I would be expecting about 21. I assume the vacuum holds steady throughout the whole rev range (after the initial dip). I would certainly have the fuel pressure checked, also the fuel flow. A vehicle would demand more fuel under acceleration rather than cruising in top gear. I believe the fuel filter is in the tank and I would change that as a matter of course. Does the pump sound noisy? I believe that is in the tank also.

I would also be interested in the gas readings both at idle and fast idle.

Best of luck

Regards

Pete M

Re: Mazda / 323F - big flat spot

Posted by Andrew Pethig on January 13, 2015, 7:45 pm, in reply to "Mazda / 323F - big flat spot"
Edited by board administrator January 14, 2015, 11:12 am

Can you graph TPS and front O2 at WOT on a road test to see if the o2 goes lean when the throttle is opened, this would confirm a lean condition, and give you some direction like fuel pressure or dirty MAF

Andrew

Re: Mazda / 323F - big flat spot

Posted by shane gray <iframe src='https://members5.boardhost.com/ukautotalk4/info/user=ukat3442' width='330' height='300' border=0 style='border:none;'> on January 14, 2015, 12:47 pm, in reply to "Re: Mazda / 323F - big flat spot"
Shane Gray

Hi,
Fuel pressure is 5 bar at idle and 2.5 bar when flat spot is present.
fast idle co 0.0% hc 23ppm lambda 1.03.
at idle co 0.02%
Cat glows red when flat spot present.
unable to scope coils as it has built in amplifier's. Have ordered new coils.
I have including scope graphs.
shane

Re: Mazda / 323F - big flat spot

Posted by Chris Angus <iframe src='https://members5.boardhost.com/ukautotalk4/info/user=UKAT5111' width='330' height='300' border=0 style='border:none;'> on January 14, 2015, 2:18 pm, in reply to "Re: Mazda / 323F - big flat spot"

I would go back and double check that vac reading, normally it should be about 20 on a good engine, that year i would expect closer to 18, but do a snap throttle test, it should drop to nearly zero then increase to about 25-26 then drop back to 20. if you have 25 at idle then you need to see 25 ish peak, but 25 at idle is too high anyway.

Double check fuel pressure too, this should be fuel return system and regulated at 2.5b, 5 bar is way too high.

Also what is lambda sensor voltage while it is flat?

Also to test the ignition, just set the voltage scale on your scope to low volts about 500mv and just touch the probe on top or side of coil, you will pick up the pattern this way, and then adjust scope and trigger.