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Running Problem, Misfire Help!!!

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misfire
3K views 7 replies 6 participants last post by  Calmdown 
#1 ·
Hey all over the last few weeks I have been a very unhappy MX owner, basically heres a run down of the problem...

I have a 1.6 uk mk1, iv owned it for 8 months, done about 1,200 miles in that time and it been fine but over the last few weeks she has been misfiring, the revs have been falling off to the point where she almost stalls. when you put the car in first gear the revs will drop off and some times even cut out on me! the car will cut out some times going along as well. The car normally ideals fine after she is warmed up. but lately she got really bad to the point where its undrivable dew to the misfire being aggressive.

Now this is what I have done so far,

-The rocker cover was leaking so I replaced the gasket.

-I have read in numerous places that the coil pack plays up so I got a replacement, no change

-The spark plugs where thick black and I have only done 800 miles on them so, I replaced them took it for a long drive (50 miles) and they are jet back.. over fuelling?

-I have read mixed things about HT leads, get after market, don't get after market. I have aftermarket lead on (8mm) and I put the standard ones back on my car and it wouldn't even start, so my HT leads are fine?

-The coolant /engine temp sensor has been replaced to as when I got the car the wire was cut.

-I have adjusted the idle speed (possibly to high) but that's stopped the dropping off when I put it in first and pull off at lights.

-Redx

AND the cars still running ruff!! can anyone please help me!!! as I bet its something really simple! but its stressing me out the point where i'm just going to scrap it and go back to a German car!
 
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#2 ·
When you say the coolant temp sender was replaced was it the one at the back of the engine as that's the one that needs replacing. If you replaced the sensor on the thermostat housing at the front this would not solve the issue as this is just the sensor that tells the rad fan to kick in when things are getting hot. If the one on the back of the engine was not replaced I would start there.
 
#3 ·
+1 on the ecu temp sender on back of the engine, if its faulty and not sending a signal back to the ecu its runs in safe mode which is a richer map

Failing that id say 02 lambda sensor
 
#4 ·
Thanks guys, yeah I did the one at the front, where abouts its the second one? can could some one link me the part? because i had real issues with mazda getting the coolant temp sensor? they showed me a drawing that looked like it was done with a super nintendo! and one was £43 and the other £50, and had to be ordered with no refund, in the end i ebayed it £15 and go the right part lol

On another note, i just drove the car first time in a week and a half, and nothing, no idle, no misfire, no cutting out, I dont believe the issues gone but, i'm abit pissed i'v been walking to work! I will get a o2 and the temp sensor and a fuel filter just for good measure?
 
#5 ·
The coolant temp sensor is under the coil pack, it has a blue top on it just remove the coil pack that's held in place by 2 12mm bolts at either side and there's a pain in the arse 12mm bolt at the bottom of the coilpack that holds it to the engine, you will need a ring spanner to undo it as it's too tight for the ratchet, and you will see the sensor. You can buy them on mx5parts.co.uk.
 
#6 · (Edited by Moderator)
It's unlikely to be the oxygen sensor if the vehicle runs "rough" from the outset, this parameter won't be used by the ECU until the engine has reached a sufficient temperature as the type of oxygen sensor used here is ineffective until it is heated, and also it's not possible to reach a stoichiometric running condition on a cold engine without some severe sacrifices.

Do you have a multimeter? There are a host of checks you can do before you start wasting money on parts. The ECU temperature sender is a potential culprit here as it sounds like the vehicle is in a lean-running condition but other sensors or fueling issues could be causing the problem.

Also, I can show you a fleet of modern German vehicles with a whole host of erratic misfires and idling faults..... Certainly do not purchase an oxygen sensor yet!
 
#7 ·
When I replaced my ht leads - I mixed two of the leads up - making it run real interestingly.

I also found that occasionally the plug leads sometimes popped up and occasionally filled up with water - not sure from where?

So, I guess what I'm saying is exhaust all the stupid and obvious things before you start spending again ;-)
 
#8 ·
You mentioned that you messed with the idle air screw. Pretty sure you need to jumper something, then set it (once warmed up) for a specific RPM.

Black plugs are indicative of the car not shifting into "use the sensors for fuel mapping" mode.

There's another check using an LED+Resistor (or test light) between the B+ and GND IIRC that you can decipher the flashes on, and it'll tell you what the ECU is taking a shit over. If it's the O2, you can wire in a generic Lucas one for pretty cheap.
 
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