Firstly, this is in a Westfield!

It's an MX5 Mk2 1.8 engine (140 bhp).

Took the car for an emissions test before taking it for the £450 IVA test.

The inlet manifold is shorter using silicone hose and an alloy elbow with an aftermarket filter. EGR system is removed and blanked off, everything else is standard. Car has one lambda sensor after the manifold, well before the cat.

The engine was hot, fan was cutting in and out, fast idle approx 2500rpm.

The Lambda reading was approx 1002 and over a period of 20 seconds, climbed to 1014ish with everything on the sensor machine in the green. Then the Lambda dripped to 995 and everything went red (rich?). 2 or 3 seconds later, it climbed back to 1000 and everything was green. It kept doing this for the ten minutes or so, we tested it.

There is no hesitation in acceleration, no air leaks, no lumpy tickover or poor starting.

Any ideas?

Are the silicone hoses collapsing under vacuum and strangling the airflow??

I wouldn't have thought so as they're only about 100mm long each, but I'll look at it when I get home.

It's odd, it runs weaker and weaker then richens up too much.

someone on here had a problem similar to this, and for them it was the lamda sensor not being directly in the flow of the exhaust gasses

quote:

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Originally posted by gremlin1234
someone on here had a problem similar to this, and for them it was the lamda sensor not being directly in the flow of the exhaust gasses

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heres a link for that thread
http://www.locostbuilders.co.uk/viewthread.php?tid=208078

Thanks.

Had a look, but mine is only a single Lambda and it's sat in the exhaust stream about 18 inches from the cylinder head.

Have another emissions check at a different garage (the first garage the rollers were too deep to test the brakes!), so will film it if it happens again.

I disconnected the MAF, throttle body, coolant sensor and EGR valve and they all made a difference to the engine, so I don't think they are at fault.

Todays' garage is about 5 miles away, so I hope to give the car a good run to warm it up and get the cat hot!

Ian.

1 - are you measuring tailpipe lambda, or the lambda sensor's signal?

2 - what ECU are you running?

Stock exhaust, tail pipe lambda.

Just got back from garage. Car really started misfiring on the emissions machine. Went on the brakes machine and it had a 16% bias to the left, but passed as the limit is 25%. Think they need bedding in.
We noticed the car was running better, so tried the emissions again and it passed. There's still a misfire, that seems to have appeared in the hot weather!

I'm thinking coil pack?

At least if it keeps misfiring, I can swap parts until I cure it.

What do you think?

quote:

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Originally posted by Ianboom
Stock exhaust, tail pipe lambda.

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Lol! Stock ECU not exhaust!
Exhaust is wunoff with a cat.

Then I suspect what you are seeing (misfiring this afternoon aside) is a perfectly normal calibration.

After your lambda reading goes lean (i.e. above 1.002) how long until you go rich (i.e. below 1?)

It seemed to be approx 20 seconds where it leaned off to 1.014ish, then dropped to .995ish for 2 or 3 seconds where it went back above 1 again.
I think it had a slight misfire that caused the richening, but was hardly noticeable at 2500 rpm.

Today I think the heat may have made (possibly the coils) break down a bit more so the misfire was more noticeable. The emissions readings were going red/green all the time and you could hear and feel it miss fire.
There're no fault codes.

quote:

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Originally posted by Ianboom
It seemed to be approx 20 seconds where it leaned off to 1.014ish, then dropped to .995ish for 2 or 3 seconds where it went back above 1 again.
I think it had a slight misfire that caused the richening, but was hardly noticeable at 2500 rpm.

Today I think the heat may have made (possibly the coils) break down a bit more so the misfire was more noticeable. The emissions readings were going red/green all the time and you could hear and feel it miss fire.
There're no fault codes.

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BP and Texaco 95 Ron. 10 litres were put in at Xmas and 10 were put in this week.
Don't think it would affect it more today than yesterday. Bio attracts water, could be this but it wouldn't be so intermittent.

High under bonnet temperatures could well cause your symptoms, I had similar which I cured by lagging the exhaust manifold and cutting vents into the bonnet.

It has been very hot, I'm chuffed the car hasn't boiled over!
If it cools down (it's a bank holiday so I expect it will) I'll run a similar test and see what happens with the missfire.

I would suggest a replacement coil pack and new leads especially if they are the ones from the donor car or a few years old, for some reason mx5's eat through coil packs and leads, possibly excess heat with being at the back of the head on the cylinder that is known to get the least cooling.

Bloody things, still love em though

Tom

[Edited on 30/5/17 by tmay1991]

Hi

Plugs, leads and (cheap) coil pack on order.

I'll post back what happens (especially with the cheap coils), if it stays cool, I'm not sure whether the fault will be replicated.

Ian

Well, number 1 coil connection looked burnt out.

Fitted the new coil, leads and new plugs and.......it's exactly the same!

It appears to be getting worse as on tickover, it kept dropping revs and nearly stalling.

So it's not coils, plugs or leads. I put a multimeter on the MAF sensor and got 2-3volts, measured the throttle body and the resistance changed with rotating the throttle.

Tempted to buy a used maf, throttle body, crank and cam sensor. I have a spare ecu and immobiliser so will try these too. Only other thought is a duff injector.

Ian

Borrowed some data logging equipment from work.

I can't post the screen shots as I'm not sure how to on this site.

Basically, the cam sensor looked as how I expected it to, the coils seemed fine too.

Checked the crank sensor and on tick over it measured 0v to 2v and when the car went over 1000rpm, the signal inverted at went from 2v to 5v, missing a pulse.

I also had spikes on the output which seems to coincide with the car 'missing' and looking closely, again there was a pulse missing.

Will report how I get on when the sensor arrives and is fitted!!

Ian.