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Hello guys, I've spent some time reading through sites and the workshop manual and cannot work out the problem, other than a weak ignition.

The car has been maintained through Mazda its whole life, is at 115,000km and due for a minor service.

I replaced the old denso plugs with new NGK BKR5E-11, went to start it up and nothing. Assuming it couldn't be plugs, I went through all the online advice, checked pump, relay etc and all ok (I don't own a fuel pressure tester but will look at this during the week). So I gave up and put the old plugs in, fired first time after one rotation. I let it warm up and did a comp check, came in at 180psi. The car seems to run ok, idles smooth and free revs fine.

What are the chances of all 4 new plugs being faulty? My only other guess is that there is a weak spark and the old plugs have a slightly lower resistance giving enough spark to keep it running.

I don't have a lot of experience with MX5's, or Mazda for that matter, but it seems pretty weak. Given my previous experiences, I put it down to being "drivers car" and having a flatter torque curve than I'm used to; Plus it's my partners car, so I'm not overly fused if it's not quick.

Any other ideas? Bad pump, clogged fuel filter, bad crank sensor, bad other sensor? No idea why old plugs work and not brand news.

Cheers, Russel

I'm thinking of replacing the coils and leads anyway. I can get the lot for $170 from amazon (Beck Arnley coil and NGK leads). Does anyone know of a better price for coils and leads? I've used amazon for all my recent car projects without any problems and haven't been able to beat their prices but they've been older cars and more common in the states.

Sounds like the plugs were seated on the leads/coils properly or something was seriously wrong with all plugs.

Did you buy them from a reputable source? I know there's fake NGKs out there.

I removed the plugs and coils several times so its unlikely they were fitted incorrectly with the new plugs but fine with the old. I purchased the plugs from supercheap. I check the engine earth and its fine too. I ran two of the new plugs dry and the first spark looked good but subsequent sparks looked weak, though still sparking.

The fact that it starts and runs fine on the old plugs but wont even kick once with the new ones has me completely baffled.

Gaps?

They're brand new plugs, gapped from the factory, that is what the -11 means on the plug number (shouldn't be playing with pre-gapped plugs). They are all exactly 1.1mm.

Always check plug gap. They're usually off due to bumps in transport, etc. Get a feeler gauge out and check them. Also, did the car start before, and if you put the old ones back in, will it fire. What else did you do?

The gaps are all exactly 1.1mm, they're all exactly as they should be.

The car starts and runs fine on the old plugs. My Mrs has been driving it since I put them back in and it's firing better than most people are describing as "normal".

I just changed the plugs, I went to start the car to warm it up for the oil change and nothing. I tried jumping it off a 4x4 battery and it cranked really well but no joy. I played with the leads check the the resistance, retorqued the plugs and tried again, still nothing. I checked the fuel pump, fuel relay and checked for fuel passing into the cylinders, all ok. I tried push starting it, nothing. I pulled the plugs out and tried dry firing them and they're all sparking. I tried a few more times with no joy. I decided to try a cold compression check, not good, 70psi. I replaced the open cylinders with 2 old and 2 new, it had a gentle kick but no real indication of a start, I put them all back to old and fired like normal. Let it warm up to running temp and did a hot compression check and it was 180psi.

I've never experienced anything like it and I'm not convinced its just plugs. All of my Toyotas will run on 2 cylinders, I'd have thought with 2 old plugs and 2 new plugs I'd get something, even a minor kick or half fire or a splutter. I'm taking the plugs back and getting a new set to try again but like I said, I'm not convinced. It reads very similar to what a lot of other guys have experienced but no one has tried replacing with the old plugs, but the answer has always been varied from fuel delivery or spark control.

Cheers,

Seems strange to have four dodgy plugs from new.

If the gaps are ok, then are you sure you have the correct spec plugs? Do the new plugs have the same 'nuts' for the COPS to plug into as the old plugs?

I am betting that it is something so obvious that you will kick yourself for not spotting it (especially if the old plugs are still working fine).

As a last resort, take them back and get a replacement set.

Good luck.