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1997 P0300 weirdness



 
 
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  #1  
Old March 12th 11, 05:41 PM posted to rec.autos.makers.mazda.miata
Hal
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Posts: 233
Default 1997 P0300 weirdness

All - my 1997 with 109k on the clock has developed an annoying habit
of flashing the CEL at highway speeds recently. I never lose any
power, and the car runs fine throughout the RPM band.

I found a TSB regarding this issue and checked the gap between the CKP
and the crank sensor plate. It was out of spec, so I adjusted it. I
also cleaned all three grounds in the engine bay and reset the light.

The problem came back. The only other thing I can mention is that my
speedometer cable broke a few months back because the speedometer
itself locked up, so I took the speedo apart, oiled the bushing, and I
repaired the cable with a small piece of model airplane all-thread.
The speedo works now but the pointer is a bit jumpy because the end of
the cable is not perfectly straight. And while I was in there fixing
the speedometer I noted that there is a reed switch on the magnetic
drag ring that is probably the feed to the computer for the vehicle
speed.

So..going out on a limb here.....does anyone know...do the VSS and CKP
sensors work in tandem to detect misfires? If so, I may need to
replace the speedo cable to fix this problem....

Thanks,
Chris
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  #2  
Old March 13th 11, 03:17 AM posted to rec.autos.makers.mazda.miata
KWS[_2_]
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Posts: 16
Default 1997 P0300 weirdness

On 3/12/2011 9:41 AM, Hal wrote:
> All - my 1997 with 109k on the clock has developed an annoying habit
> of flashing the CEL at highway speeds recently. I never lose any
> power, and the car runs fine throughout the RPM band.
>
> I found a TSB regarding this issue and checked the gap between the CKP
> and the crank sensor plate. It was out of spec, so I adjusted it. I
> also cleaned all three grounds in the engine bay and reset the light.
>
> The problem came back. The only other thing I can mention is that my
> speedometer cable broke a few months back because the speedometer
> itself locked up, so I took the speedo apart, oiled the bushing, and I
> repaired the cable with a small piece of model airplane all-thread.
> The speedo works now but the pointer is a bit jumpy because the end of
> the cable is not perfectly straight. And while I was in there fixing
> the speedometer I noted that there is a reed switch on the magnetic
> drag ring that is probably the feed to the computer for the vehicle
> speed.
>
> So..going out on a limb here.....does anyone know...do the VSS and CKP
> sensors work in tandem to detect misfires? If so, I may need to
> replace the speedo cable to fix this problem....
>
> Thanks,
> Chris


The first thing that comes to mind is the tendency of Miatas to eat
spark plug wires. If yours haven't been replaced in the last 30K miles
or so, you might want to throw a set at it and see if that makes the
problem go away.

On line, I found this. The assumption is that a P030X code might be a
number where X is 1 to 4 if a specific cylinder is affected:

"Random misfires that jump around from one cylinder to another (read:
P030x codes) also will set a P0300 code. The underlying cause is often a
lean fuel condition, which may be due to a vacuum leak in the intake
manifold or unmetered air getting past the airflow sensor, or an EGR
valve that is stuck open."

If it was me, I'd do the plug wires first if they are getting due for
replacement.

Good luck; let us know what you find.

Ken
  #3  
Old March 14th 11, 04:52 AM posted to rec.autos.makers.mazda.miata
Hal
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Posts: 233
Default 1997 P0300 weirdness

>
> The first thing that comes to mind is the tendency of Miatas to eat
> spark plug wires. If yours haven't been replaced in the last 30K miles
> or so, you might want to throw a set at it and see if that makes the
> problem go away.
>


It has, as best I can tell, the original 1997 plug wires. But a bad
plug wire should cause a loss in power, right? I never feel even a
slight hiccup ......

> On line, I found this. The assumption is that a P030X code might be a
> number where X is 1 to 4 if a specific cylinder is affected:
>


According to my scan tool, it's a P0300 code. "Random/multiple"

> "Random misfires that jump around from one cylinder to another (read:
> P030x codes) also will set a P0300 code. The underlying cause is often a
> lean fuel condition, which may be due to a vacuum leak in the intake
> manifold or unmetered air getting past the airflow sensor, or an EGR
> valve that is stuck open."
>


I tested EGR last night, the valve holds vacuum and nearly kills the
engine when I start to open it manually at idle, indicating that the
passages are clear..and that the valve shuts correctly.
I will check for intake leaks tomorrow with some carb cleaner but I
would think that a leak there would cause a misfire..which I just
never seem to feel. And the fact that it only happens at freeway
speeds where manifold vacuum is going to be lower than at a low-speed
cruise/idle condition seems to rule the intake manifold out...at least
a little bit. A vacuum leak, if I have one, at cruise is going to have
less effect on the overall mixture than that same leak at idle, at
least that's how it seems to make sense to me, but I'm not an
expert....


Chris
  #4  
Old March 14th 11, 11:10 AM posted to rec.autos.makers.mazda.miata
charlie
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Posts: 66
Default 1997 P0300 weirdness

On 3/14/2011 12:52 AM, Hal wrote:
>>
>> The first thing that comes to mind is the tendency of Miatas to eat
>> spark plug wires. If yours haven't been replaced in the last 30K miles
>> or so, you might want to throw a set at it and see if that makes the
>> problem go away.
>>

>
> It has, as best I can tell, the original 1997 plug wires. But a bad
> plug wire should cause a loss in power, right? I never feel even a
> slight hiccup ......
>
>> On line, I found this. The assumption is that a P030X code might be a
>> number where X is 1 to 4 if a specific cylinder is affected:
>>

>
> According to my scan tool, it's a P0300 code. "Random/multiple"
>
>> "Random misfires that jump around from one cylinder to another (read:
>> P030x codes) also will set a P0300 code. The underlying cause is often a
>> lean fuel condition, which may be due to a vacuum leak in the intake
>> manifold or unmetered air getting past the airflow sensor, or an EGR
>> valve that is stuck open."
>>

>
> I tested EGR last night, the valve holds vacuum and nearly kills the
> engine when I start to open it manually at idle, indicating that the
> passages are clear..and that the valve shuts correctly.
> I will check for intake leaks tomorrow with some carb cleaner but I
> would think that a leak there would cause a misfire..which I just
> never seem to feel. And the fact that it only happens at freeway
> speeds where manifold vacuum is going to be lower than at a low-speed
> cruise/idle condition seems to rule the intake manifold out...at least
> a little bit. A vacuum leak, if I have one, at cruise is going to have
> less effect on the overall mixture than that same leak at idle, at
> least that's how it seems to make sense to me, but I'm not an
> expert....
>
>
> Chris


1997
When was the last time the fuel filter was replaced?
Original Plug wires should be replaced, usually with Mazda "Blues"
(If nothing else, new plug wires help seal the plug wells - - IF you use
dielectric grease at the the top seal.)

Although not as fragile as the 99 coils,bad plug wires can cause some
really puzzling problems. (As can such things as a flaky igniter.)

A friends Honda started acting up, with various intermittent problems.
Minor power loss at highway speeds.
Tach readings occasionally dropping out.
Occasional ECU light set, clears without any action.

The car started to intermittently crank without start, and if it did
start, the tach did not read.

To make a long story short - -
No start
Eventually, the fuel pump relay was replaced. This worked for a few
days, then the no start returned. Relay drive circuit from the ECU
failed, and was repaired. Original relay failure or troubleshooting by
an inexperienced mechanic may have damaged the drive circuit. Many
relays have a diode across the relay coil. If this diode fails, the
inductive "kick" from the relay coil can damage the drive circuit.

The car ran, but the random no tach reading still persisted.

A check during one of the no start events showed erratic ignition timing.

Final Solution
Repaired ECU fuel pump relay output circuit.
Replaced coil and igniter(electronic ignition module)
Plugs and wires were replaced, since the wires were quite old, and
brittle. (Not to mention about 90k miles and over ten years on them.) We
might have bothered to test them, but did not, due to the visual condition.

Old wires with a carbon/graphite core are generally very bad when flexed
while testing.
  #5  
Old November 14th 16, 06:19 AM posted to rec.autos.makers.mazda.miata
[email protected]
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Posts: 1
Default 1997 P0300 weirdness

I know this was 5 years ago, but did you ever figure out your problem? I have the exact same issue with my '97.

I think I figured it out, I found a Technical Service Bulletin for the P0300 code and it says that if your vin is under a certain number (JM1NA35**V0727842) then your problem is caused by a mis-logic of the ECM/PCM. My vin is lower than this number so I guess I'm going to search for an ECM out of a later '97...
 




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