Spec me a fix...

Matthew.M

Matthew.M

Matthew.M

Soldato
Joined
18 Oct 2002
Posts
3,325
Location
Birmingham
So, my faithful 1995 1.8 MX5 has finally encountered a problem after 4.5 years and 45k miles in my ownership. I suppose I shouldn't complain!

Basically I noticed an intermittent loss of power when driving, it feels like I have hit quick sand and all progress stops irrespective of the revs of throttle position. This will last for 10-15 seconds then it will "wake up" and be back to full power. I can't reliably reproduce the problem, it happens when it feels like it but is becoming more frequent.

During these loss of power spells the car won't idle correctly, it dips to 500-600rpm and feels like it's about to stall. Strangely, if I walk away from the car and not touch anything, after about 30seconds of almost stalling the idle recovers to it's normal level and it behaves normally! It's truly random and I can't force it to make the same behaviour at all.

What I have tried - new spark plugs, different HT leads. When removing the old plugs it looked like the rear most cylinder (number 1?) wasn't firing correctly - it was covered in soot and oil. But the new plugs haven't changed anything, and I connected the old spark plug and ht lead up for cylinder 1 and ran the engine and it is sparking fine. Without plug 1 connected the engine still idled with the same lumpy sound so I have a feeling whatever is causing this is preventing cylinder 1 from igniting properly. But I may not know what I'm talking about so this might be completely wrong :p

Ideas anyone? My car picks its moments. Luckily the problem started the day before I had a new company car delivered so I am not depending on it anymore BUT I have a trackday booked in a weeks time and I would rather not waste my money by not going or take it on track with a power problem!

Another thing I have noticed is a very faint "popping" sound from the air intake when it is idling, but this might always have been there I've only started listening to things since this started. Besides that it had a basic service 3 weeks ago with oil, filters and these plugs changed.
 
Damaged hose of some sort somewhere maybe. My old Astra did this for a while until I managed to find a tiny hose had come disconnected from something, which was causing erroneous temperature readings and giving symptoms much like yours.

A bit general, but you get the idea. :p
 
Intermittant usually means electrical and temporary loss of power is usually the MAF/AFM (air flow meter), it has been on my skyline, mates Punto and BMW 328.

The engine doesn't know (or gets incorrect readings about) the amount of air it's getting so it sends the fuelling haywire causing loss of power, crap idleing etc which then resets once the electrical issue resets (can be dodgy sensor in the MAF/AFM, dodgy soldering finally breaking down etc).

Usually cheap to replace and 2 mins to do the job - I don't know how much you know about cars so here's a piccy (it's the electric bit right after the airbox)

engine1.jpg
 
Could be a baffle gone in the exhaust, I had the most inexplicable power losses, tried absolutely everything, turned out a loose baffle+Accumulated soot+crud was blocking my exhaust but only on certain conditions, if I drove 'normally' there wasn't enough force to throw the baffle up to block, if I throttled it then the baffle would block..power then would totally drop off and it would unblock itself, so it would cycle erratically through having power, absolute power drop off then power back again, it sent me crazy for weeks until the problem was accidentally found by an emissions probe that was sent in to try and find out what was going on and got stuck! :D
 
After looking at the HT stickers it's number 4 not 1 :o

I'll try the air flow sensor next then and try and hunt down a reasonably priced replacement. I don't think it would the exhaust unless the baffles/cat contents can move around with just idle air pressure without the car moving? It's a stainless system as well. Although I do have a silenced decat section in the back of the garage I could try.

Thanks so far, feel free to add more ideas I hope to eliminate them as I go along this is quite frustrating!

Edit - and the hoses seem fine. There aren't very many because it doesn't even have aircon or power steering.
 
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