Hello,
My turbo Na6 has been sitting for about a month while I did some repairs on it (new radiator and new turbo manifold). The car ran perfectly before doing so. During this I did not unplug anything electrical except for the radiator fan plugs. I did remove the AFR sensor but did not unplug it.

Upon finishing the repairs, I jump-started the car and just let it idle until it was warm to check for coolant leaks. The car was fine in this time, and I shut it off after the fans came on. It is winter here in Australia, and the car sometimes ran a bit rough at first on cold days (I expect due to bad warmup enrichment in the lower temps) but was perfect after about a minute.

When I first tried to drive it a few days later, the battery was dead, so I jump-started the car. The battery was fully charged after the following drive. After starting the car, it ran fine but I could hear an odd high pitched metallic noise when revving in the engine bay, so pulled over after about a minute driving up the road. This seemed to only happen when letting off the throttle, kind of sounded like dropping or hitting a thin piece of sheet metal. It sounded like this was coming from the turbo, so I checked that the turbine spun freely by hand still, which it did (nothing seemed to be obstructing it, and it must have been getting oil). I think now it was just some resonant thing from the exhaust vibrating against the gearbox. The car boosted fine.

I decided to keep driving, and got perhaps another 10 minutes away when the car suddenly started running very rough, misfiring and eventually stalling at a traffic light. I drove off the main road, and in doing so it came good after about another minute. Trying to drive home the car would constantly switch between being fine (like it was before sitting for a month) to misfiring badly all during a pretty steady throttle. When I was almost back it started to misfire very badly, to the point that the car was shaking. It did not come good again after this.

Today, I replaced spark plugs and plug leads (NGK leads and NGK BKR7E gapped 0.030"). The fuel filter was replaced about 20,000kms and 3 years ago, so I did not do this yet. The car was still misfiring very badly, basically after getting out of my driveway and 50m down the road. I have no idea where to go from here, or why this would happen from sitting, after being perfectly normal beforehand, and acting normal for the first 10 minutes of driving after sitting. It happens both when the car is cold and warmed up.

The car is running a megasquirt DIYPNP ecu and an innovate MTXL wideband sensor. It does not have a MAF anymore, there is a temperature sensor in the intake piping. The car has ran perfectly for 4 years turbocharged with the same setup. At the moment all I can think is a dead/dying sensor? Tomorrow I intend to log a short drive on the megasquirt and see if this shows anything obviously wrong. I will also check that the timing is still correct at idle, and recalibrate the wideband sensor.

Does anyone have any idea what could cause this and what my best course of action is now?

 

If you have a megasquirt the first thing you should do anytime it acts up is log. It's kinda calling up a doctor and saying I feel funny and idea what it could be? He needs to see you and do testing to see what's up. Megasquirt is the same way.

 

Isn't there a heat related issue with the CAS systems (and coils) in NAs?

 

Sure it could be the CAS, the coils, the wires, the temp sensor, the map sensor, the TPS, or just a huge list of things.......a log will tell you exactly what it is. Anything else is just guessing.

 

might want to clean and Re calibrate the O2 sensor

 

Hello all,
Sorry, I've been busy helping a relative that has ended up in hospital. I haven't had a chance to log a run in the car yet, but will do so tomorrow when I have some time!

What I did get a chance to do was to pull the new spark plugs (had the single start and run on them). What I noticed was that the two outer plugs, cylinders 1 and 4, were wet with petrol. The two middle plugs were completely dry and looked like they would after a normal drive. On the coilpack, 1 and 4 are shared by the same coil. I'm thinking that this could really point to that coil being dead then?

I pulled it out quickly and measured resistance of both separate coils, with both showing about 12Kohm (in spec and very similar), measuring between the red lead on the plug to the white and black leads separately gave me a very close 0.9 Ohms each too. Is it still likely to be the coil pack despite this? I don't have a great understanding of the spark system, would a bad CAS still cause one coil not to fire despite the coil being fine (making it look like the coil at fault)?

What I thought as a test, is starting the car tomorrow with the plugs reversed, so that this time the middle plugs go into the suspected bad coil, and the outer plugs go into the suspected good coil. If the wet and dry plugs swap positions would this prove that the coil pack is dead?

Thank you all for all your responses.

 

It's funny to me how you can have time to pull plugs and do all that diagnostics work on the coils and not plug a laptop into it.

Two plugs didn't die at the same time, the coil died or the ecm quit telling that coil to fire. Swap coil locations and see if the dead cylinders follow it. If they do it's the coils, if they don't then it's the ECM.

 

Update, still not diagnosed.
Today I switched the leads running into the coilpack around and cleaned and dried the plugs. The same cylinders again exhibited plugs wet with fuel while the other two were dry after running for a short amount of time. I guess this disproves the coilpack as the issue, as the one working fine before is now acting 'dead' (both coils must actually work). This would also mean that one of the lines feeding in must be the issue. I am at a loss as to what would cause only one of the wires to be able to fire a coil. I am getting 12v at both coilpack wires. Measuring resistance from the coilpack to the ignitor showed negligible resistance and no difference between both coils.

Next I measured voltage at the CAS plug with it unplugged. The ground wire is showing no resistance, I am getting 12v on the right plug, but only one of the signal wires is getting the 5v it should be (SGC) - the other is getting 11.5v for some reason (the white SGT signal wire). I am not sure if this points to an issue, but looking online seems to suggest both should get 5v? See pic below,



Next I put the car back together and recorded this short log of the car idling, then driving up one end of my street. It was still running normally at this point (the past few starts it has been instantly at the misfire - I guess whatever is dying must be hanging on) so I turned around and went back the other way when it started to misfire again. Not sure how to interpret it or if there are any issues shown in it?

 

Hello,
Sorry, I started writing my response early in the day so didn't see yours until I had posted. I do have a log now finally, and did decide to do what you said with the coils (see previous post). So you think it's most likely something in the megasquirt has died? This would make sense why it is giving odd voltages to the CAS I guess. Brings up a whole other issue trying to fix that! I guess replacing the transistor that controls the spark output?

 

I would start with verifying you don't have a shorted wire first and at that point yes I'd dive into the ECM. It's odd that it's intermittent when you're driving. That leads me to believe it's a rubbed wire.

 

Completely stumped on the issue now. Wiring has continuity from CAS to ecu. No shorts that I can find.

I bench tested the CAS by feeding it 12v and ground and measuring the signal output while spinning it - the SGC pin goes from 0-3.6v during revolution twice as expected. The SGT pin goes only from 0-0.21v but does do this four times per revolution. I guess the CAS is therefore putting out a normal signal apart from the lower voltages than expected (0-5v).

The 12v that the ecu is feeding the SGT wire with the CAS unplugged must be the issue I think (should be 5v). I have no idea however what part of the DIYPNP would be at fault to cause this. Have emailed diyautotune but no reply. At a loss where to go from here as the ecu looks fine internally. I can't rule out the CAS for sure, but from research it seems it's the ecu.

 

Update,
Realised I could better test the CAS wiring by plugging in the stock ecu but not starting the car. This time I got 0, 12v, 5v, 5v [ground, power, SGT, SGC] with the CAS unplugged as expected. Shook the wires and got no change.

I then plugged the CAS into the wiring while the stock ecu was still connected to better test it, and spun the CAS. I watched the voltage on the two signal outputs go perfectly from 5v to 0v during the rotation. So the CAS is fine presumably.

Tried this with the megasquirt and got the same result as before with the CAS unplugged, 0, 12v, 11.5v, 5v (again the odd almost 12v on one the SGT signal wire). Tested with the CAS plugged in and rotating it, the SGT output went from 11.5v down to 0.5v during the rotation. The SGC output went from 5v to about 0.3v (Did not actually hit 0v like it did when plugged into the stock ecu). I assume that even though the signal does seem to change from full voltage to ~0v, that the CAS cannot work with that input voltage (and that this was not always the case and an intentional design in the megasquirt?).

Something else I noticed was that the CAS clicked audibly when plugged into the stock ecu as it spun, but it never did when plugged into the megasquirt?

I guess this points to the wiring and CAS being fine and the megasquirt being at fault due to supplying the 11.5v instead of 5v? Unless the 12v is normal on a megasquirt and something I haven't looked at like the igniter has gone?

 

Apparently the 12v on the SGT pin is normal on a megasquirt (by design).

Changed the ignitor as it was the final thing that I know of in the ignition system that I hadn't tested yet other than the ecu. Didn't fix the issue.

The car ran fine for about a minute, and then I can almost feel when it's going to start misfiring. I can hear/feel some odd clunks from the front end while it's running fine then it's misfiring shortly after. Pulled over and wiggled all the ignition wiring and ecu wiring and none of it made any difference to the issue.

Completely at a loss what to do. Diyautotune haven't been able to provide anything meaningful on the issue yet.

 

Car is still broken. A local mx5 specialist has suggested that the car always has spark from their testing during the misfire, and that they think the car is overfueling badly. They suggested the ecu being the fault but said that the car was a bit beyond them as it is modified.

I know that the car has cylinders 1+4 and 2+3 shared for spark, but thought that injectors 2+4 and 1+3 were shared? Doesn't make sense to me that it could be fueling then if affecting only 1+4?