Help with SE misfire under load

[CrazyRacer]

I'm looking for help on diagnosing a misfire problem in my SE under load…

A few months ago I bought a 2004 SE from a forum member. Nice car, that ran well but had a spotty service history and a few obvious maintenance issues that needed to be attended to sooner rather than later, i.e. radiator and accessory belts. While I was at it I decided to do the cam belt, seals and water pump too as I had no record of when they were last done. While it was up on the stands did plugs and oil too. In the process of all this the intercooler plumbing ended up fouling on the thicker aluminium radiator I installed so some re-routing was required.

So, all work completed, turn the key and the car fires up and idles nicely. All good…

But out on the road when I try to give it any more than half throttle, it develops a nasty misfire at around 4000 – 4500 rpm.

On light throttle it will rev out all the way to red line.

For completeness of the story, its reasonably stock but with a Nitro Dann exhaust, a Colling Pro intercooler and a manual boost controller.

I did check the cam timing many times, so I'm pretty sure that it’s OK, but if everyone tells me it sounds like that I’ll go back in and have another look.

Just thought I’d check here and see if anyone on the forum can suggest obvious cause or things for me to check before I start ripping it all apart again.

[smy0003]

Have you checked base timing with a light?
How's the BOV routed after you moved the intercooler pipes? Pictures?

[NitroDann]

What plugs and what gap?

Are you CERTAIN it doesnt have an air leak from your intercooler fiddling?

Dann

[bear2230]

Sounds very similar to a problem I had with the white car. It ended up being a coil breaking down under load.
Bought these http://www.ebay.com.au/itm/121191134721 ... EBIDX%3AIT and fixed it straight away.
Of course I didn't discover this until I bought a cam angle sensor, leads and plugs first.
You can get these from Bursons/Repco ect, but I wanted the genuine and these were the cheapest I found. Don't go to Mazda they will charge you more than double.

Richard.

[Todd77]

Sounds very similar to a problem I had with the white car. It ended up being a coil breaking down under load.
Bought these http://www.ebay.com.au/itm/121191134721 ... EBIDX%3AIT and fixed it straight away.
Of course I didn't discover this until I bought a cam angle sensor, leads and plugs first.
You can get these from Bursons/Repco ect, but I wanted the genuine and these were the cheapest I found. Don't go to Mazda they will charge you more than double.

Richard.

Yer I think you are spot on, sounds like a load miss to me, pull coil packs out ( or leads if your car doesn't have coil over plug ) wrap coil tubes or lead ends in electrical tape and take it for a drive, if it helps or fixes the problem, get a new set of leads or coils. Don't get one, because the others have all done the same work! Do one and one of the other three will go in the next month

[CrazyRacer]

What plugs and what gap?

Are you CERTAIN it doesnt have an air leak from your intercooler fiddling?

Dann

Plugs I put in are NGK BKR7EIX-11 at the factory gap of 1.1mm, although they measure up at 1.0mm
The ones that came out were BKR6EIX and measure up at 0.85mm gap.
What plug / gap would you recommend?

Am I certain that there's no air leaks? No, but I'm reasonably confident. I was pretty careful, double checking all of the joins and replaced any jubilee clamps with t-bolt clamps. Although there is still one section I didn't touch that still has the factory spring clamps, but that's as it was before when I didn't have the issue.

[CrazyRacer]

Have you checked base timing with a light?
How's the BOV routed after you moved the intercooler pipes? Pictures?

Base timing is around the 10 deg mark, although it does flick around a bit between 8 and 12. That's on a cold engine with a timing light using an inductive pickup off No 1 spark lead.

BOV is plumbed as before. Its a Turbosmart Kompact plumb-back. Plumbed into the TB elbow, return between AFM and the compressor. Manifold line is to the same spot as before, rear top of the inlet manifold, just behind the tower brace.

[CrazyRacer]

I put the old plugs back in (that it ran on OK before) but it the issue is still there.

[greenMachine]

Sounds very similar to a problem I had with the white car. It ended up being a coil breaking down under load.
Bought these http://www.ebay.com.au/itm/121191134721 ... EBIDX%3AIT and fixed it straight away.
Of course I didn't discover this until I bought a cam angle sensor, leads and plugs first.
You can get these from Bursons/Repco ect, but I wanted the genuine and these were the cheapest I found. Don't go to Mazda they will charge you more than double.

Richard.

$100 each?! Better price than replacing than the NB coilpacks I know, but ...! When I converted the racecar to COPS I used Toyotas from the wreckers, $90 for four. That may not be what you want on a road car perhaps, but it is less than some people pay for plugs. Which reminds me, I probably should go back and get another set for the spares box ...

If the Mazda COPS are a consumable/known problem, I would be looking for a cheaper solution, even if it means changing the plugs on the harness. Is there any other reason not to - maybe dwell times?

[CrazyRacer]

Well I think I've sorted it out.

Crappy quality MBC had the ball valve jammed in the closed position. I think it was over boosting and the factory fuel cut took over.

I dismantled the MBC, had to blow the ball out with compressed air and cleaned it all up and set it to reasonably low boost.

Its not 100% and the problem still occurs occasionally, but its mostly gone.

Plan is to get an ECU and a MAC valve for boost control fitted up ASAP anyway, so I'll just take it a bit easy until I can get that sorted.

Thanks for the help guys.

[NitroDann]

BKR7e, gapped at LEAST as small as 0.8. And you are going to have bent rods at this rate.

Dann

[Todd77]

BKR7e, gapped at LEAST as small as 0.8. And you are going to have bent rods at this rate.

Dann

How does plug gap bend rods?

[NitroDann]

Well I think I've sorted it out.

Crappy quality MBC had the ball valve jammed in the closed position. I think it was over boosting and the factory fuel cut took over.

Its not 100% and the problem still occurs occasionally, but its mostly gone.

[CrazyRacer]

BKR7e, gapped at LEAST as small as 0.8. And you are going to have bent rods at this rate.

Dann

I didn't give it heaps, just edging up on the throttle on progressive runs and backed off as soon as it started to miss, so I'm hoping there's not too much damage done.

I'm planning to get an ECU and MAC valve in it ASAP so hopefully that will resolve the problem completely. Its a hobby car, so I can just not drive it in the meantime if its likely to cause more damage.

I'm intending to do a forged bottom end at some stage, but I'm hoping this one can do me for a while yet.

I've got the old plugs gapped to 0.8ish in it at the moment, but I'll re-gap the new ones to that and put them back in...thanks.

[beavis]

happy to hear the car is in a much better state than it was ~6months ago.