NB8A limp mode

Sean · Postby **Sean** » Thu Jan 01, 2015 4:37 pm

Hey guys,

Been trying to help a non forumite sort an issue with his NB8A over the xmas break with very little luck.

The plan of attack was based on me taking my car which we know is functioning right and swapping out parts to solve the problem...

The dud car runs very poorly on start - it idles ok, but when you go to take off it's like it's running on two cyls or something until around 3500/4000rpm where it suddenly rockets to life.

Things we've checked/swapped out with known good parts from my car:
AFRs - all look fairly sensible.
New plugs
New leads
New fuel filter
Swapped coil (I fried a coil less than a month ago so the one we swapped in for testing is literally a few weeks old).
Swapped Fuel pump
Swapped MAF sensor....

Swapped the parts individually and together, and made no difference...

Here's the weird bit that leads to my question: If we unplug and run without the MAF sensor it runs pretty close to perfectly (slow to start). From cold right through and at low/high loads it runs flawlessly - the AFRs look pretty safe too.

So my question is what does the ECU bypass when we unplug the MAF? Is it switching to an RPM vs TPS based map and still using water temp etc? It seems odd that even my good MAF wont let his car run well, but in limp mode it seems fine. The goal is if I can figure out what is not used with the MAF unplugged we can swap that part out in the hope of fixing the issue.

No check engine light etc displays.

Got me stumped, and google doesn't seem to know what unplugging the MAF/limp mode uses.

Cheers
Sean

Magpie · Postby **Magpie** » Thu Jan 01, 2015 4:41 pm

hks_kansei · Postby **hks_kansei** » Thu Jan 01, 2015 5:01 pm

CHecked the o2 sensor?

my NB8A went through an issue somehwat similar, it would start and run on 3 cylinders, at high rev-lowload it would come back to 4.
Mine however would be fixed for a short time if I replaced the plugs.

My issue was a buggered o2 sensor telling the ECU that the car was lean all the time, hence the ECU just kept dumping in fuel, which led to it fouling the plugs.

Sean · Postby **Sean** » Thu Jan 01, 2015 5:37 pm

I figure it needs a good CAS / signal to run so well in limp? It'd have issues firing and be sluggish if the timing was out.

It's also running the tach at what appears appropriate.

O2 sensor was on my list, but it doesn't seem to show unusually high or low afrs (measured on a wideband).

I'm sure I have an old (but working) factory narrowband I can take and switch in for him on the weekend to be sure.

pezchops · Postby **pezchops** » Thu Jan 01, 2015 7:08 pm

maybe check plumbing between the MAF and the inlet manifold for holes/splits/restrictions .
It sounds like the volume of air entering the motor is different to what's been read by 2 different MAFs.
i dont know about the MX5 but an old BMW i had ran like your mates car.
it ran like a pig...i disconnected the 02 sensor and a default setting allowed it to run ok so i knew it needed replacing.

Sean · Postby **Sean** » Thu Jan 01, 2015 8:27 pm

Cheers Pez. Will get him to give that a go before I head back over.

speed · Postby **speed** » Fri Jan 02, 2015 8:31 am

You say it runs poor on start. What about once at operating temp? My thoughts are like the others. CAS, O2 or maybe coolant temp sensor.

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bootz · Postby **bootz** » Fri Jan 02, 2015 12:01 pm

Good luck, this is one of those interesting problems that a mechanic would charge a bomb to sort out.

Mr Morlock · Postby **Mr Morlock** » Fri Jan 02, 2015 3:54 pm

Have you actually had a OBD check? Just changing over parts is not necessarily going to fix it and it can take a very long time . A friend recently had a problem which ultimately resulted in the car ( MX5) not being drivable and all the guesses for the fault were wrong- the code from the OBD had it nailed to a component in virtually not time.

hks_kansei · Postby **hks_kansei** » Fri Jan 02, 2015 5:58 pm

Mr Morlock wrote:Have you actually had a OBD check? Just changing over parts is not necessarily going to fix it and it can take a very long time . A friend recently had a problem which ultimately resulted in the car ( MX5) not being drivable and all the guesses for the fault were wrong- the code from the OBD had it nailed to a component in virtually not time.

Unfortunately the '99/00 years didnt have ODB2.

They used a similar system to the NA (the short two pins and plug in an LED to count the flashes)

The problem is that the USA models had ODB, which means there is almost no information on what the early NB flash codes are, since most of the information comes from the USA (or Japan, where the obvious translation errors occur)

Sean · Postby **Sean** » Sat Jan 03, 2015 6:09 am

Spent some time on his car overnight.

Disconnecting the o2 sensor had it running what felt like faultlessly. Seemed odd given the wideband (read on a separate portable wbo2 sensor) readings didn't seem very different either with or without the o2 connected.

We decided to log the wideband to laptop - still didn't see what you'd think was a significant difference, so I suspect the dud sensor reading of the factory sensor makes something else happen in the factory ECU.

No OBD2 connector, but I can access a regular scantool with mazda adapter, just not til Monday, and by then I'm back at work and have little time to help out...

Looking like he'll be calling mania when they re-open to order a factory o2 sensor.

MLR · Postby **MLR** » Sat Jan 03, 2015 10:30 pm

I got a new one on ebay which was a very good price

Sean · Postby **Sean** » Sun Jan 04, 2015 1:50 pm

MLR wrote:I got a new one on ebay which was a very good price

Was it a generic one or with the mazda plug?

mania have them listed on ebay for under $100 shipped. It's not my money anyway, but that seemed fair to me - just they are away til next week - he's up and running unplugged in the mean time.

emily_mx5 · Postby **emily_mx5** » Sun Jan 04, 2015 4:18 pm

Repco/super cheap auto etc sell the O2 sensors, you can save money by buying the Bosche one that needs your original plug soldered onto its wires, only 4 joints.

Check the wiring in the engine bay, especially the chunk that runs across the front of the engine to the maf. I have seen many cars with cracked conjute/sheath and then the wires inside start splitting.

NB8As seem to have an issue with coilpack plugs too, and the wiring leading to them.

Sean · Postby **Sean** » Sun Jan 04, 2015 9:16 pm

emily_mx5 wrote:Repco/super cheap auto etc sell the O2 sensors, you can save money by buying the Bosche one that needs your original plug soldered onto its wires, only 4 joints.

Check the wiring in the engine bay, especially the chunk that runs across the front of the engine to the maf. I have seen many cars with cracked conjute/sheath and then the wires inside start splitting.

NB8As seem to have an issue with coilpack plugs too, and the wiring leading to them.

I've put a screw in bung where the sensor fits and pulled the sensor and wires (to the plug) completely out for him.

He's going to go and see what he can find (lucky bugger has this week off).