mx5cartalk.com

The Problem

• When driving the car at WOT and going above 5k RPM, the power will suddenly halve. Idle will drop from 900-1000RPM to 300-400RPM. The car will struggle to make any power.
• Took car for a drive to the Hunter Valley - because YOLO, you know? - And the constant 110KM/H fueled the fire so to speak. When I got back to the hunter, the car was barely making any power however it was practically drive-able. It felt a little hotter than normal but temp gauge showed that it was running at about half. Idle was at about 400RPM again and the car ran like a dog.
• Car doesn't splutter or misfire when revving it out. Runs a little rough at idle which is to be expected.
• Turned car on this morning to go to work and it wouldn't rev beyond 1000RPM until it warmed up for about five minutes. After that, it was very slow to rev.
• Car will struggle to make power even if in neutral. The engine will just rev up slower than normal.
• This morning - while the car wouldn't idle above 1000RPM - there was a sound that sounded like tappets and the engine was shaking a bit. Not so much a misfire, just like it was struggling to idle.

Diagnosis tried

• Did a basic throttle body and intake manifold clean out with a proper cleaner as per recommendation of a mechanic mate. Didn't do anything to help unfortunately.
• Did a basic compression test on each cylinders. There was next to no deviation across cylinders. From memory the readings were 138-138-136-138

I will update this list as I try any recommendations that you might throw out

Thoughts from research

• Dodgy CAS - I read this thread and will try this on the weekend.
• Just discovered that I have posted previously about this. I will trial some of these out this weekend.

If you have any thoughts about what might be wrong or what I could test to try find the culprit, I would really appreciate it.

Engines need fuel, air and fire. Sounds like your car is struggling with one or more of these.

Have you tried the usual culprits, plugs, plug wires, AFM connected properly?

Is it getting fuel properly? The EGI main relay is often a cause of faults like this.

Is it running rich? (like the fuel is being supplied but not burned properly)

Starting at the easiest to check of the air/fuel/spark/compression checklist.

Compression you've checked, 138 seems a bit low, but then I can't remember what a normal NA6 has (I seem to remember a friend's being at about 180psi?) either way, it's all the same so should be ok.

To check air, check the air filter is clean, how long since it was replaced? maybe it's choked with sh*t?

Check the crosspipe and any other hoses, if there's a split etc it will create a vaccuum leak, which can cause serious idle issues etc.



For fuel, first off check the fuel filter is clean, or just replace it ($40 or so, and about 30mins work if you dont rush)
Then start the engine and listen to each injector (rest a screwdriver on each one with your ear on the handle, you should hear a TIKATIKATIKA as they open and close. all 4 should sound similar.

Spark i'd remove the plugs, and rest each one on the rocker cover while cranking the engine over, they should spark. (do this with the new plugs suggested below)
I never actually check my plugs, I just bin them and buy a new set (they're $4 each or so)
new plugs also mean you can drive for a while before removing them and checking the air/fuel mixtures (black soot = rich, white crust = lean)






as for the CAS, i'd be strongly suggesting you put a timing light on and see if the timing is out.
That would potentially cause the issues you're describing.

Thank you for the suggestions guys!

My check list for this weekend;

• Check air filter - replace is needed. IIRC i changed this not long ago but I'll double check it.
• Check cold side for possible air leaks. Soapy water should do the trick, right? And just check for bubbles?
• Check injector noise. These are a couple of years old and they're the blue ones from MX5 Mania but I'll still listen for them non-the-less.
• Recheck the compression (I agree that 138 is pretty damn low, but I was just going from memory. I'll get a proper test in - with pictures - and report back!). If this fails, forget the rest I'll just bin the engine.
• Find out about timing lights and check the timing is good. If this fails, forget the rest.
• New fuel filter - I also have a feeling that it's fuel related and haven't gotten around to changing this. Is there any other fuel-related component - like the FPR - that I should be checking? It doesn't appear to be leaking fuel, so there's a plus.
• New spark plugs - why not.

Unfortunately Vacuum leaks are hard to find.

Since the engine sucks air in through them the soapy water wont show anything (it will jsut suck it in the engine)



From the symptoms I would check the timing first, since it's free to check, and free to fix. (if you don't own a timing light try and borrow one, if not you can buy a basic one for about $50)

HT leads. Common issue.

I have no idea what model?
But I will say Spark Plug Leads.