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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

Reply to
Hal
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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

Reply to
KWS

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 ......

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

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

Reply to
Hal

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.

Reply to
charlie

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

I think I figured it out, I found a Technical Service Bulletin for the P030

0 code and it says that if your vin is under a certain number (JM1NA35**V07 27842) 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...
Reply to
konradgeorgeson

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