Jerking in 3rd Gear when cold 1999 MX5

[Kimberley]

My car is jerking a lot in third gear probably only the 1st and 2nd time I am in this gear when I start the car and has no power, people behind almost hit me. Only happens with a relatively cool engine, never cold in Cairns! First & second gear are fine & so is fourth. Had a new fuel filter installed (no difference) and had new spark plugs (no difference) - they were both my idea the mechanics came up with no other suggestions. Difficult to demonstrate as the engine has warmed up by the time I get to a garage. Have also just had a new clutch installed & new brakes all round before this problem started. Mechanics can't seem to find the problem so guess their scanner is not showing anything. Have read a huge thread on Whirlpool & now have a list of about 20 things it could be. If I take it to a mechanic that has an oscilloscope will that find the problem? Is it likely to be a Idle Air control valve issue or a timing belt problem. I do not want to replace everything under the bonnet just because the mechanic does not know. Have already spent $1820 (two different mechanics) & just want to find a decent mechanic who can give me the right answer. Have little faith due to my experience with the local Mazda dealership. I really don't know much about motors so would appreciate answers from members who have experienced the same problem & fixed it. Had a new coil pack a number of years ago but when that went the car only ran on 2 cylinders. Also had a new fuel pump nearly two years ago.

[bear2230]

Where are you located Kimberley?

[taminga16]

Cairns!
Hi Kimberly,
Plug leads or a vacuum leak could be things to look at.
Greg.

[93_Clubman]

Yeah, replace spark plug leads if not already done.

[Kimberley]

Where are you located Kimberley?

I live in Cairns. Leads were checked when the spark plugs were replaced last week.

[Mr Morlock]

what is required is a good i.e. helpful mechanic. We should not confuse the issue with what work has been done on the car already, I don't quite understand why the car is only exhibiting the problem - hesitating or misfiring - in 3rd gear - the components are the same so why 3rd gear-I don't know. I think the car needs to be dríven by a mechanic to at least experience the problem and not just replace parts. If the plugs and replaced last week I would be going back to the mechanic and ask them to re look at it.

[93_Clubman]

How were the spark plug leads checked? If by replacing the old leads with new or known good leads, & the problem persisted, then ok. But if the leads were just 'checked' or 'tested', which doesn't always pick-up the problem, then you still need to try the car with a new or known good set of leads.

[taminga16]

Mr M,
Load can easily highlight a lead or plug breakdown.
Greg.

[Kimberley]

[quote="Mr Morlock"] - hesitating or misfiring - in 3rd gear - the components are the same so why 3rd gear-I don't know.

I can also get it to hesitate in 2nd but mainly in 3rd & after a few minutes driving no more hesitation in any gears. So only a cool motor. I went to the mechanic & he said he would keep the car for a few hours & then scan it as he drove along. The fault is obviously not showing in the ECU. I wonder about the scanners with some mechanics as often they tell me they are not working properly. Hence why I asked if I need a mechanic with an oscilloscope & maybe really good scanning tools not just run of the mill ones.

[Okibi]

A 1999 doesn't have an ODBII port that mechanics might hope to use to diagnose the problem.

Your mazda dealer might still have the computer and plug to connect to the ECU diagnosis point.

It would be good if there was another MX-5 enthusiasts in Cairns you could swap a few components with an narrow down the issue, tricky since it only happens in the cold.

[hks_kansei]

Fault codes only really work when everything the computer expects to happen does.

If a part still works, but sends out teh wrong signal, the ECU won't realise and probably won't throw a code (my car had that issue)


As for the jerking, try and recreate it, but in another gear.
It seems incredibly odd for the issue to only happen in 3rd (although, 3rd tends to be the gear where the car is under load often)

Put the car in second, then try to accelerate from a low speed with more throttle than you normally would (about the same speed as a leisurely jog)
That should load the car up enough to recreate the issue. If it does, we've narrowed down that it's not 3rd gear itself, but that it's the engine load causing it (and that by pure luck, 3rd gear is where you normally load the engine enough to cause the issue)


If it is misfiring under load, which it sounds like, the first checks are:
Spark plugs (the mechanic, or yourself if you feel like it, can look at them and see what colour the end is. Very black, and maybe wet, is a sign that a plug isn't sparking and it's wet with fuel)

Spark plug leads - try a known good set, the manual says to test with a multimetre, but I've had leads that tested well cause misfiring, and leads that tested bad run perfect (and every combination in between)

Ignition coils (sounds like you've checked these)

Oxygen sensor.
if the sensor is reading incorrectly then there is a possibility the car is putting the wrong fuel mixture into the engine. Too much fuel and the spark plugs can get fouled and won't spark, causing jerkiness.
This turned out to be the issue on my car a few years back when I had very similar problems.

[JBT]

What HKS_K said ^^^

A set of plug leads, and even another set of spare plugs, will be money well spent on a 15 year old car.

[speed]

Fault codes only really work when everything the computer expects to happen does.

If a part still works, but sends out teh wrong signal, the ECU won't realise and probably won't throw a code (my car had that issue)


As for the jerking, try and recreate it, but in another gear.
It seems incredibly odd for the issue to only happen in 3rd (although, 3rd tends to be the gear where the car is under load often)

Put the car in second, then try to accelerate from a low speed with more throttle than you normally would (about the same speed as a leisurely jog)
That should load the car up enough to recreate the issue. If it does, we've narrowed down that it's not 3rd gear itself, but that it's the engine load causing it (and that by pure luck, 3rd gear is where you normally load the engine enough to cause the issue)


If it is misfiring under load, which it sounds like, the first checks are:
Spark plugs (the mechanic, or yourself if you feel like it, can look at them and see what colour the end is. Very black, and maybe wet, is a sign that a plug isn't sparking and it's wet with fuel)

Spark plug leads - try a known good set, the manual says to test with a multimetre, but I've had leads that tested well cause misfiring, and leads that tested bad run perfect (and every combination in between)

Ignition coils (sounds like you've checked these)

Oxygen sensor.
if the sensor is reading incorrectly then there is a possibility the car is putting the wrong fuel mixture into the engine. Too much fuel and the spark plugs can get fouled and won't spark, causing jerkiness.
This turned out to be the issue on my car a few years back when I had very similar problems.

Am I right in thinking that the O2 sensor does less fuel trim then say the coolant temp sensor? If so, couldn't it be that? Just makes more sense to me especially since it only happens when cold. Maybe check that it is connected first, just in case it was disconnected to help find faults.
Am totally guessing here!


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[NitroDann]

o2 does almost noithing.

[hks_kansei]

No idea, but I know that my sensor was telling the ECU it was lean, constantly.

So it would just pump in fuel until it fouled the plugs.

New o2 sensor, no problem.