Genuine Urgent Help Please

[CrispyMK]

Hi All,

I'm after some urgent help with my MX5. The car has been running a bit rough since I brought it, running rich and generally not on top form. It will idle absolutely fine and once above 3000rpm feels like a turbo has kicked in (it doesn't have a turbo). As of a couple of weeks ago if you rev the car from idle it tries to stall before coming back to life. When slowing down, as you come to a stop it will do the same thing.

This morning the car was absolutely gutless pulling away in first gear until it got to around 3000 rpm it just didn't want to move. Anything with a slight incline was almost impossible. I've had to drive it to work like this but need to know if there is anything I can do to stop this for the drive home. I'm planning on leaving late as the car isn't really safe to be driven around like this.

To help with your suggestions:

I know that the car is over fuelling and I think that the culprit is the AFM.
I do have pretty much a full toolkit in my boot.
I can possibly get access to other tools and maybe sprays if needed.
The car has had a new oxygen sensor in the last month
New plugs and leads in the last 4 months

Any help or suggestions are welcomed.

[MX-5 Lazza]

It does sound like a sensor is the issue. Over 3k it will run from maps in the ecu, below that it runs using the sensors.
The main culprits would be the O2 sensor (which you say has already been changed), AFM, Engine temp sensor and CAS.

I think I'd try to find someone else with a Mk1 1.6 locally who will let you try their AFM on your car to see if that fixes it. If it does, get a known-good one from the likes of Autolink.

[CrispyMK]

MX-5 Lazza said:

It does sound like a sensor is the issue. Over 3k it will run from maps in the ecu, below that it runs using the sensors.
The main culprits would be the O2 sensor (which you say has already been changed), AFM, Engine temp sensor and CAS.

I think I'd try to find someone else with a Mk1 1.6 locally who will let you try their AFM on your car to see if that fixes it. If it does, get a known-good one from the likes of Autolink.

I didn't know that about running from maps at over 3k. I'd already thought of finding someone local to me to give their AFM a try, I just need to find someone now. I'm around the Hucknall area if anyone fancies letting me try their AFM.

Is there anything I can do short term other than keep the rev's high when pulling away to stop the car dying?

[eltax91]

Are the AFM's interchangeable? I have a mkII 1.8 up on stilts awaiting new wheels/ tyres that you could happily borrow the AFM off to test it. I'm just off J22 of m1, so 30 mins from you.

[CrispyMK]

afm mazda mk91]Are the AFM's interchangeable? I have a mkII 1.8 up on stilts awaiting new wheels/ tyres that you could happily borrow the AFM off to test it. I'm just off J22 of m1, so 30 miom you.
[/quote]
I've just had a look and from what I can work out the design changed from 93 onwards. Mines an 89/90 so I don't think it'll be compatible. Unless anyone knows any different? If it will fit mine I'll drop you a PM.

I've nursed the car home tonight, sorry to anyone I held up pulling away from lights or junctions in Hucknall.

[ronime]

MX-5 Lazza said:

Over 3k it will run from maps in the ecu, below that it runs using the sensors.

The MK1 1.6 ECU goes into open-loop mode above 4,000rpm.

[MX-5 Lazza]

ronime said:

MX-5 Lazza said:

Over 3k it will run from maps in the ecu, below that it runs using the sensors.

The MK1 1.6 ECU goes into open-loop mode above 4,000rpm.

I thought it might be 4k but couldn't remember for sure. I know it's 4k on my Mk2.5. It certainly sounds like whatever is causing the problem suddenly goes away at 3k revs though.

Mk2 and all 1.8 use a MAF rather than an AFM so not compatible at all.

[CrispyMK]

MX-5 Lazza said:

I thought it might be 4k but couldn't remember for sure. I know it's 4k on my Mk2.5. It certainly sounds like whatever is causing the problem suddenly goes away at 3k revs though.

Mk2 and all 1.8 use a MAF rather than an AFM so not compatible at all.

You're right; it is 'kicking' at around the 4k mark not 3k, that was my mistake. You guys really know your stuff.

I've taken the top off of the AFM last night. I was looking inside and the car dies after revving, the little arm/needle is going past the point it idles at. If you imagine it as a clock, so if it idles at 12 o'clock, it'll move to about 9 o'clock when then engine is revved but then drops back down to about 2 o'clock almost killing the engine then coming back to rest at 12 o'clock. The resistor track doesn't look dirty or damaged; it is showing signs of slight wear but doesn't look excessive. Does the behaviour sound normal at all?

I'm still on the hunt for a test AFM if anybody has one that I can use, I'm in the Hucknall area and am happy to do all the work while you drink as much tea or coffee as you can handle?

[snotrag]

Im 99% sure that the Rod Grainger workshop manual will give you resistance figures so you can check yours with a meter.

But yes, if you can get hold of a spare one that will be an easy check. There must be someone on here nearby, if not, stick a post up on MX-5 Nutz, plenty of helpful people there.


(FYI, you can check loads of things like this with Multimeter, Plug leads, coil packs, Temp sensors, O2 sensors etc, very very useful diagnostic tool and can be picked up for about £15, invaluable with old cars!).

[CrispyMK]

snotrag said:

Im 99% sure that the Rod Grainger workshop manual will give you resistance figures so you can check yours with a meter.

But yes, if you can get hold of a spare one that will be an easy check. There must be someone on here nearby, if not, stick a post up on MX-5 Nutz, plenty of helpful people there.


(FYI, you can check loads of things like this with Multimeter, Plug leads, coil packs, Temp sensors, O2 sensors etc, very very useful diagnostic tool and can be picked up for about £15, invaluable with old cars!).

It's something I've considered buying but always been put off any dealings with car electronics putting it down to black magic or witchcraft. I'll pick one up though and see what Youtube can teach me about testing bits on the car. How difficult are they to use?

I'll give Nutz a try, I've spotted quite a few MK1's in the area so hopefully a few of them will be members.

[MX-5 Lazza]

Watching the flap on the AFM won't tell you anything. It's just reacting to the air-flow, not controlling it. What you need to know is the signal voltage from the AFM. But that is where my technical knowledge stops.

[gd49]

CrispyMK said:

I didn't know that about running from maps at over 3k. I'd already thought of finding someone local to me to give their AFM a try, I just need to find someone now. I'm around the Hucknall area if anyone fancies letting me try their AFM.

Is there anything I can do short term other than keep the rev's high when pulling away to stop the car dying?

If you're in hucknall might be worth giving 5-speed a ring, they're literally 5 main drive from you.

[CrispyMK]

MX-5 Lazza said:

Watching the flap on the AFM won't tell you anything. It's just reacting to the air-flow, not controlling it. What you need to know is the signal voltage from the AFM. But that is where my technical knowledge stops.

Ah ok, I knew it couldn't control the air flow but thought that it might be something to do with where the needle is coming to rest, with clockwise leaning it out and anti-clockwise making it run rich. Obviously I'm not on the right track with this.

As a desperate measure I've picked up some electrical contact spray and will give the connectors of the AFM and temp sensor a spray, can't do any harm. I'll pick up a multimeter at the weekend regardless.

GD49, I had considered 5Speed as an option but really want to fix and diagnose this myself. I'm not sure they would fancy lending a part to a stranger just for the love of MX5s. Unless you think it's worth contacting them to find out? Have you dealt with them at all?

[MX-5 Lazza]

5Speed are good guys. I've known Ste for a long time, since he worked down this way with Phil Dixon at Performance 5.
If they are local it would be worth giving them a call, what harm can it do?

[gd49]

CrispyMK said:

GD49, I had considered 5Speed as an option but really want to fix and diagnose this myself. I'm not sure they would fancy lending a part to a stranger just for the love of MX5s. Unless you think it's worth contacting them to find out? Have you dealt with them at all?

They didn't charge me for having a look at my 5 when I first bought it or for hooking up their computer when the engine light came on but I was a regular customer. As Lazza says there's little harm in ringing them and they might be happy to confirm if you're on the right lines, and if you can't fix it at least its not far to limp it there.