Power loss & sounds like a WRX

[Purecaboos]

It has been a few years since i played around with petrol engines, having recently sold my diesel Jeeps and purchasing my new to me NA MX5.

Yesterday while travelling home from a 'short drive' I took a roundabout at my usual speed and when planting my foot on the throttle it did not launch like usual but rather made a ff.fff.ff noise and went out slowly. I presumed it was low on fuel as it was just under 1/4 tank and it was pulling a lot of sideways force.

travelling further on it occurred again in a straight and I continued to resume low fuel, ended up making it to a fuel station where I filled up with fuel.

Make it a few more km's down the road before it started doing it again....

got home (eventually) and checked under the bonnet, the bracket supporting the AFM that was installed for the cowl intake kit had snapped clean off on one of the welds and the AFM was just bouncing around on the inside of the guard. Took it out, checked it over and tested it as best I could and it appears to be OK. Put it back together and a short test drive provide it OK.

Cut to this afternoon, did a short run to the vet (2.5km) and back with no issue. Then a run into Moss Vale (20km) and about 5 k's out of moss vale it starts again, sounding very much like a crappy WRX exhaust now.

After a 15 minute wait and chat it starts normal, and I drive out of Moss Vale a few km's and it starts again.

I am thinking either fuel, or electrical as it works OK cold. Although there is a very loud high pitched tappet like noise from the rear of the engine and it does not sound like a tapped noise...to loud and can be heard about the other tapped noises.

I pulled out plug 1 and 2 separately while idling and it got worse, so I know those 2 cylinders and firing.

Going to pull plugs tomorrow and check them out, also going to test the plug leads. Anything else I should check while I am there? Can I test the coils? My working on engines experience does not include these new fancy computer parts.....carbies, oil filled coils and points with vacuum advance....

Thanks in advance...sorry for the long post :-)

Greg

[bruce]

Spark plug leads.

[hks_kansei]

yeah, as above, plugs and leads


Mine did similar, kept fouling plugs.
My o2 sensor had crapped out and was sending a constant lean signal to the ECU, so it kept adding more fuel until it fouled a plug and dropped a cylinder.

SO yeah, check plugs/leads, replace them, and see if that helps.
if it comes back, look into why it's possily overfuelling.

[Okibi]

I had an 02 sensor come loose years ago, it had a similar effect.

[Purecaboos]

yeah, as above, plugs and leads


Mine did similar, kept fouling plugs.
My o2 sensor had crapped out and was sending a constant lean signal to the ECU, so it kept adding more fuel until it fouled a plug and dropped a cylinder.

SO yeah, check plugs/leads, replace them, and see if that helps.
if it comes back, look into why it's possily overfuelling.

Interesting, it has fouled the exhaust with black crap and its pops on upshift and it didn't when I picked it up in November last year.

Anyone know how to test an owner sensor?

Can I use a wide band with the stock ecu?

[hks_kansei]

Widebands won't work with a factory ecu.

A narrowband puts out a voltage between 0 and 1 volt, essentially just enough to see if the mix is at 14.7afr, above, or below.
It can't see how much above or below.

A wideband usually outputs a broader voltage range, and needs to be calibrated etc. Most show from 7 to 22 afr i think. (Mine does anyway)

Some widebands do however have a narrowband output, where essentially the controller converts the signal to a simulated narrowband. That should work with a standard ecu if you wanted.

But, it's worth noting that it's not as simple as shoving a wideband sensor into the oem exhaust bung.
My Bosch sensor needs a thicker bung than oem, and also needed to be at least 22inches from the exhaust ports (oem is about 8inches)

So yeah, it can be done but it really depends how much effort.you want to go into for a gauge.

[93_Clubman]

Yesterday my NA6...made a ff.fff.ff noise and went out slowly.
...AFM was just bouncing around on the inside of the guard.
Today...sounding very much like a crappy WRX exhaust now.
...there is a very loud high pitched tappet like noise from the rear of the engine but it does not sound like a tappet noise...too loud and can be heard above the other tappet noises.
...going to test the plug leads. Can I test the coils?

...it has fouled the exhaust with black crap and its pops on upshift and it didn't when I picked it up in November last year. Anyone know how to test an o2 sensor?

Testing plug leads doesn't always reveal US leads as they tend to breakdown under load.
Coils & O2 sensor can be tested & how to & specs should be in the workshop manual.

have ORC'd the NA6 service manual so you can search words.
https://www.dropbox.com/s/o7f9ukuup4dacg6/1990_Manual_MX5.pdf

That said, NA6 coils were a weak point & O2 sensor is fairly much a consumable. Many NA6s also ran a bit rich in part of the range, so blackening of rear bumper wasn't uncommon.
All of that said, think there may be at least a couple of things going on given the short space of time between all of your above. So, if you haven't already, also check the AFM plug is properly & securely plugged in.

[Mr Morlock]

these are just guess's. The WRX noise is likely a cylinder is dropping out. All the usual suspects - check the plugs - I doubt it a bad plug is usually bad all the time . My own vehicle had a long term intermittent drop out and ultimately it turned out to be coil packs. You cannot really get them checked with any degree of certainty. I recently took it to a repairer to check the OBD ( which you dont have) and got no read out- he guessed coil packs. Some years ago I stupidly just replaced one pack instead of 2 ( NA is different) and that left another pack which was old and maybe was failing. If you do buy coil packs go for OEM I reckon. MX5 in UK ( Portsmouth) is really competitive and seems very professional for parts./ service . The noise seems a bit of concern - you might run that past a mechanic who can hear it. Manga on the forum is also a hands on wiz and very much hands on.

[bruce]

Plug leads. All NA6s do it.

[Purecaboos]

Plus are all checked, all 4 of them are a nice grey/brown colour and the gap is 1.1mm. They are NGK R v groove plugs.

Checked the wire to the oxygen sensor is still connected.

Going to see if I can find some time for a short test drive this afternoon.

Looking MX5mania, they have a nice new COPS system. Is it worth installing the COPS to replace the 2 coils and leads as I think it works out cheaper?

then again, they are out of stock and my frown gets bigger every day I can't drive my mx5 :-(

Cheers

Greg

[93_Clubman]

Is it worth installing the COPS to replace the 2 coils and leads as I think it works out cheaper?

Yep - btw, did you check the loom plug is plugged into the AFM plug properly & securely, given it was hanging down bouncing around on the inside of the guard?

[Purecaboos]

Is it worth installing the COPS to replace the 2 coils and leads as I think it works out cheaper?

Yep - btw, did you check the loom plug is plugged into the AFM plug properly & securely, given it was hanging down bouncing around on the inside of the guard?

I did, it has been rerouted from the original way as there is less strain in it now.

I will need to rebuild the bracket as it is a dodgy fix, but I ran out of alloy welding wire and haven't had time to pick up some more.

massive storm is heading our way, so out to administer some 'calming' goodies so they don't freak out....probably wont get a drive in now :-)

[hks_kansei]

Re: the o2 sensor, the ecu will only read from it when in.closed loop mode, which is at idle and cruise.

High load, high throttle position, and high revs the ecu enters open loop mode where the o2 sensor is ignored.

If you unplug the o2 sensor the ecu will be forced into open loop constantly, which may help to see if the sensor is giving a false lean making the edu add too much fuel at closed loop.

[project.r.racing]

sounds like spark plug leads. The description of the symptoms sounds like spark plug leads. The issue is fairly common on a BP engine.

I'd start with the easy and common stuff first, then work you way onto the more difficult stuff first.

To check the lead is easy and a 30 second job. Whle the engine is running, pull on lead out at a time and hold parallel to the engine had cover and about 5mm away from it. Rotate the solid section while holding it parallel and 5mm away from the cover.

If you find spark come out from anywhere other than the end of the solid section, then you have a hole in the plug lead and the spark is earthing before getting to the plug.

It is a 30second test worth doing before moving into fuel system and wide bands etc...

[Purecaboos]

Re: the o2 sensor, the ecu will only read from it when in.closed loop mode, which is at idle and cruise.

High load, high throttle position, and high revs the ecu enters open loop mode where the o2 sensor is ignored.

If you unplug the o2 sensor the ecu will be forced into open loop constantly, which may help to see if the sensor is giving a false lean making the edu add too much fuel at closed loop.

So if I unplug the sensor, then it will always run in open loop and use only the fixed tables? Can I surmise from this that if the phenomena goes away then it is the sensor?

I have unplugged cylinders 1 and 2 (separately) and each time the miss gets worse, I can't get to 3 and 4 easily as they are under the other leads. I will play later when I get a chance.