New MX-5 owner need help to diagnose engine (misfire)

[patmahe]

Hi All,

Just wondering if anyone else has had a similar problem and how they solved it?

I recently bought a 91 MX-5 as a bit of a project (so I've got an early 1.6 engine), I got it through the NCT (our equivalent of the MOT), but there is what seems like a misfire from the engine at the moment, the engine shudders side to side when idling and a noticeable stutter at the exhaust exit, there is also a noticeable vibration through the gearstick when idling in traffic.

I guess the reason I am posting is because I want to avoid is a situation where I throw money at the problem if its possible for me to narrow it down first. (Various Google searches range from misfire to crank wear) would checking fault codes be a good idea?

What I have done so far:
- Changed plugs and leads - this resulted in slight improvement but the problem still persists. I'm thinking coil pack next but thought I'd ask here first.

Other info:
- Shudder is always there but seems to worsen as engine warms up.
- The car seems likely to have been standing for a while before I got it, needed a good clean

Feel free to ask any question or offer any advice to help narrow it down.

Cheers

Pat

[cianha]

Congrats on the 5 purchase, I always knew you were a man of excellent taste!
Does anything look out of place around the airbox? Loose wire bad connection on the MAF, or a bad earth?

[Cyder]

If you haven't already I'd start by changing the plugs, air filter and HT leads.

Mine was a bit rough when I bought it but changing the above along with the oil has completely transformed the behaviour of the engine.

[renaultgeek]

sounds like HT leads. Are you on mx5ireland.com? Good community there.

[patmahe]

Cheers all - HT leads, plugs and air filter have all already been replaced. As I said they improved things but did not cure them.

I will check the MAF and change the oil. I am on MX5Ireland so will follow up there too if I make no progress.

A question to the Irish lads that posted, are there any 'go to' mechanics when it comes to MX-5s? Guys who know them inside out?

[SeeFive]

Not a whiz on mx5s but had a recent similar issue on a streetka.

An intermittent misfire became permanent. So I picked up a set of leads and a coil pack as that is the usual fault on streetkas. Like your situation, it improved, but was still not quite right in my mind. Popped it on a computer and injector 3 was low. Replaced that and all was well. I guess the stronger electrics from the coil pack and leads helped make a bang of sorts on that cylinder, but it was still weak due to the mixture problem.

When you changed the plugs, did your notice any different colours on the ceramic around the electrodes on a specific cylinder? My guess is that it might be worth investing a few quid with your local indie garage with a Bosch computer to diagnose the fault rather than keep spending on a trial and error basis.

Hope it turns out to be something cheap for you.

[renaultgeek]

patmahe said:

Cheers all - HT leads, plugs and air filter have all already been replaced. As I said they improved things but did not cure them.

I will check the MAF and change the oil. I am on MX5Ireland so will follow up there too if I make no progress.

A question to the Irish lads that posted, are there any 'go to' mechanics when it comes to MX-5s? Guys who know them inside out?

where are you based?

[patmahe]

renaultgeek said:

patmahe said:

Cheers all - HT leads, plugs and air filter have all already been replaced. As I said they improved things but did not cure them.

I will check the MAF and change the oil. I am on MX5Ireland so will follow up there too if I make no progress.

A question to the Irish lads that posted, are there any 'go to' mechanics when it comes to MX-5s? Guys who know them inside out?

where are you based?

Athlone/Tullamore kind of area, but willing to travel to someone who knows what they are at when it comes to MX-5s

[oilslick]

Could be worth having your timing checked, read of a similar situation on MX5Nutz where the belt had jumped a tooth.

[renaultgeek]

patmahe said:

Athlone/Tullamore kind of area, but willing to travel to someone who knows what they are at when it comes to MX-5s

give this guy a call. He's a breaker but he's done work for me before and he can swap out parts to find the issue for you since he has a load of bits. He's based in swords, but he's known all over the country by the mx5 crowd.

http://cars.donedeal.ie/carparts-for-sale/seats-fo...

[reaper668]

Sounds like a worn keyway on the crankshaft. I've had the same symptoms. A very common problem with early cars. Luckily there is a guide on alleviating the problem.

http://miata.net/garage/hsue/crank/loctite_1.htm

[dblack1]

The first task is to figure out if you have (a) specific cylinder(s) misfiring or if the misfire is random. As stated previously, this could be an issue with the crank key. Honestly, it kinda sounds like a coil.... I'd replace the crank key and bolt either way just to be safe (You never know who serviced it last).

[MX-5 Lazza]

Coil pack or CAS are likely to cause this sort of thing too. Both easy enough to swap over if you can find someone locally with known good ones you can try before you buy (i.e. off of a working car - if it fixes it buy one).

[cianha]

If it's an early engine, then it could be the short nosed crank. My 91 has a LNC, I'm not sure when they changed.

You might even be able to check it yourself? 4 bolts indicate a SNC.

https://www.google.de/search?q=short+nose+crank&am...

Edited by cianha on Thursday 15th May 11:13