I would describe myself as an experienced homemade tinkerer, like many on here I think
I know my way around the car and engine, do my own service, and have done a tonne of mods and upgrades over time, but I’m not a trained mechanic, just a humble hobbyist. This one has me completely stumped. I’ll try to describe everything I know about it in the hope that someone much cleverer than me can piece together the mystery.The car - 2003 MR2 MK3 with a 1ZZ engine, K&N induction kit, and Cobra Sport cat back quad exit exhaust. A few other mods but not relevant here.
The Problem
From cold, it starts up first time every time, purrs nicely, and performs well. This continues until the engine is fully up to temperature and I’ve been driving for about 20 minutes. At that point, it starts to run rough, rough idle, an unpleasant acceleration noise, and a super laboured acceleration between 800 and 2500 rpm. This only happens as I say after about 20 minutes of running at operating temperature, and is more likely if I’ve driven it hard. Sometimes it seems to start happening after I switch the car off, then back on a few minutes later and it is straight in to ‘rough mode’. It only ever happens when hot though, never cold.
When it is in ‘rough mode’ and I stop, the engine drops to about 700-800 rpm and has a notable ‘different’ noise to it, no longer purring but more like a sore throat trying to purr. Sometimes the rpm will bounce slightly when in this state, like it’s about to stall, but it doesn’t. If I sit still in that state for a while, the ‘check engine’ light will come on. The OBD codes read random cylinder misfires, though you can hear the problem well before any codes show and most of the time the check engine light never lights even though the problem occurs on every drive.
If I try to pull away whilst it is doing this, it will have extreme hesitation, like there’s a heavy trailer on the back or something stuffed up the exhaust, and lots of loud vibrating from the engine. Again hard to describe in words, but imagine trying to clear your throat, that noise. It will do this between 0% to about 80% of the accelerator pedal range. Now, if I floor it whilst it is doing this, it will ‘punch through’ - the acceleration will jerk back to normal, the car jolt forward, the engine sound lovely, and accelerate normally. If I lift off again, vibration and hesitation.
What I’ve Done So Far
I suspected the ignition system. Initially I targeted the spark plugs, as I hadn’t changed them in around a year. When I changed them, I noticed a lot of white crust on them (they had only done 4k). I thought this could have been down to the E10 petrol in the UK, which as I understand has higher ethanol content, which absorbs water and corrodes the pipes on these old cars. I switched to E5, and ran a fuel system cleaner through the tank as well as changing the plugs.
For a while (maybe 1 month) it ran a little healthier, not 100%, but then started getting bad again.
Reading up on ignition problems, next up I changed the coils as it was still running originals, but no change. Next up I changed the fuel injectors, again as they were original and if the sparks had the corrosion I thought maybe they would be clogged up too. A little better again for a month or so, then started declining again. Finally I changed the vacuum hoses for new silicon ones as the originals were a bit cracked. I changed all except the water hoses and 1 vacuum hose behind the fuel rail that I couldn’t quite get to, swore too much and gave up on.
Now, hands up - at this point I left the plastic top cover off my engine, and after a very bad rain storm I was horrified to lift the boot to find water pooling in the well around the tops of the coils. I mopped it all up, ran the car for a while to evaporate any last bits of water, and put the plastic lid back on. Nothing to see here, think I got away with it. Don’t leave the plastic cover off, kids. At least not in the UK.
Second hands up - At some point (I don’t know where in the grand timeline), my soft top drains had become blocked and in another very bad rain storm (UK yay) water had pooled about an inch deep around the top of the fuel pump cover and electrics in the compartment behind the driver’s seat. I drained it all, dried it, unblocked the drains, etc. and everything seemed to work okay. No idea if that could cause a problem but mentioning it here in case it helps piece this puzzle together.
Now after changing all these (specifically after changing the vacuum hoses), something even weirder started to happen. I would drive somewhere and park up for maybe 30 minutes to 1 hour, then when I get back to my car and turn the key - BANG from behind me. Quite a big bang, like someone had reversed in to you but without the movement. This would happen when turning the key to the FIRST click, i.e. not even trying to turn the engine over, just the electrics on. Sometimes it wouldn’t be a bang but instead a kind of ‘wheeee’ like a motor spinning for a half second then stopping. I have no idea what that was, and it stopped doing it after a few times which is quite worrying.
I didn’t use the car much over winter, maybe 1-2 times per week for a few miles, so never got really bad symptoms. They were still there but generally I was home by the time they were starting to occur so the engine was switched off and I was in having a cup of tea with my slippers on pretending everything was fine.
A couple of weeks ago was time for my service, so I took it as an opportunity to investigate a bit more. I changed the oil filter, flushed the engine, changed the oil, changed the sparks again, cleaned and re-oiled the K&N filter, and changed the fuel filter (which feels bit like trying to defuse a bomb as you lift it carefully out of the open tank). When changing the spark plugs (only 8 months since the last change) I was again horrified to see the right hand two (when looking from the back of the car) had rust and brown gunk all around the threads and the tips were blackened. Along with this, the right hand spark chamber had lots of white dust-like corrosion down inside it. My guess is I didn’t quite get away with the rain disaster like I though. So I took all the plugs out, took the rocker cover off, gave it all a good clean to remove all the corrosion dust, fitted a new rocker gasket, put it all back together and new plugs in. The inside of the valve chamber looked in good health whilst I had the rocker cover off.
I started it back up hoping I’d finally solved it. Up to operating temperature, all good, took it out for a 20 mile drive, all good. Next day, I went for a 10 mile drive, parked the car for 10 minutes, got back in and turned the key, and it was straight in to rough idling, vibrating, and hesitation under acceleration again.
My Hunches
I’m now at the limit of my guesswork I think. I’ve ruled out spark plugs, coils, injectors, fuel filter, air filter, vacuum leaks (almost), rocker cover leaks, and service items. Pretty much all the standard ignition system problems you find on the internet. I’m wondering if it could be a dirty maf sensor, but there are no maf error codes (though to be fair it’s only reported misfire codes twice so I don’t exactly trust it). I’m also wondering if it could be fuel pump or even ECU. I’m almost at the point of thinking I need to take the engine out and strip it down, but I’d really rather not as I need to use the car weekly and although I think I could probably do it, it would likely take me weeks.
Hopefully I’ve tried to describe all the symptoms in detail in case anyone can see a pattern or something I’ve missed. I’ve got an OBD reader and can map the sensor graphs but I haven’t included any because there are a lot and I’m not entirely sure what ‘normal’ is. If you want me to grab any specific graphs please let me know and I will be happy to post in the thread.
So, thank you for reading, and where do I go from here?
