Smoking NB - Might need bottom end rebuild?

[booki89]

Hey all,

Have a 1998 NB MX5. When I purchased the car, it had been puffing out some blue smoke on acceleration, was most apparant when the car was cold. With this, I figured it was stem seals.

Long story short - took the head off, replaced stem seals, head gasket, lapped the valves, port and polished the combustion chamber and ex/int ports, put it all back together with a new water pump, cam seals, front main, timing belt. Fresh oil and coolant.

Started it up and....its exactly the same

Tested compression just now and got (Throttle wide open)

Dry
1: 190
2: 190
3: 190
4: 190

Wet (Cap full of oil)
1: 240
2: 240
3: 240
4: 240

All cylinders tested the same.

What are normal compression results for a 1998 NB? I didn't think these results were overly bad, then again im not sure about the wet test as there is a huge difference (50psi).

Any idea's why my engine is still smoking?

If i do need to replace rings or do bottom end work, is it doable in your own garage? The head was all done by myself and i was happy with the way everything went.

Any help much appreciated!

[manga_blue]

Tested the PCV valve? Your compression readings look pretty good.

[booki89]

Removed it from the cam side, and ran the car. Still smokey

[project.r.racing]

blowing smoke using the go pedel when cold isn't valve stems. blowing smoke on first start up is. it will only continue when cold if seals are really really bad.

you have an oil seal problem in a heat related area, hence is dissappearing when hot.

[booki89]

The old stem seals looked pretty worn, they had cracks in them.

The only other then i can think of is oil control rings? Going out on a limb here, they don't effect compression?

Unless there is another way for oil to enter the combustion chambers, I am out of ideas.

[Rolley]

Just spit balling ideas,
What weight oil are you using? Too thin and it could be getting past the control rings.
Have you tried a heavier oil or an additive to see if the smoke is reduced.
How many k's are on the car? How long have you owned it and how many k's has it done while you have owned it?

The previous owner my have really babied it (continuous low rev driving and over frequent oil changes) and cylinders are glazed, how did the cylinder walls look when the head was off?
A couple of months of "sprited" driving or a couple of full intervals between oil changes may just bed things in.

If you were to rebuild the bottom end it can be done at home but you would need to invest in an engine stand and crane and depending on how much of the rebuild work you wanted to do you would also need a piston ring compressor, ring file, good set of feeler gauges, some plasti gauge and I'm guessing you already have a decent Torque wrench if you have had the head off.

It's all doable it just depends if the investment in tools/equipment is matched by the savings in labour if you don't use those tools again frequently.

Good Luck and keep us posted.

Luke.

[manga_blue]

My guess is that you have gummed up oil rings. Hopefully some hard work and thin oil could clear them. Otherwise it could be broken oil rings or glazing and you're up for a bottom end rebuild. Personally I'd be loath to jump into it until I've got to the source of the problem. If you try revving the motor with the oil cap off and your hand over the hole is there a lot of crankcase pressure build-up? Is oil being pushed out through the seals? The trouble with engine replacement is that it would be a shame to waste all that head and front of engine work to swap in an unknown.

DIY re-build at home is doable if you have the space. If you're doubtful about the knowledge then find a mentor: there'll be someone who is willing to pass on skills. If you did the head OK then you should have most of the skills for the block. At least you have time to source the parts.

When I re-built mine it took me about a day to get it out and a day for the missus and me to get it back in and going. Engine and gearbox together is easier than engine alone if you have a good engine hoist (and easier still with an NA8 if you take the CAS and coilpacks off first). I borrowed a hoist and stand in Melbourne - ask around and you'll find one. For a minimalist job I'd plan to strip the block bare, get a pressure clean and very light cylinder hone at a reconditioning shop, put in a new set of rings, do a plasti-gauge check of main and big-end bearings, install new welch plugs, check the clutch plate and replace all the hoses, seals and gaskets. At the end of that you'll have a complete motor of known condition.

[booki89]

Thank you for the practical advice, I also agree that I have gummed up oil rings.

As far as tools goes, I am a bit of a tool addict, and have pretty much everything that is required (Engine stand, engine crane, hoist, feeler gauges, torque wrenchs, impact tools etc).

I do not how ever have a micro meter or "plasti-gauge" (never heard of those before!).

Time, space are not issues. This car is not my daily (I have a NA that will be going through the same thing at a later point) but this car is a semi-project car, I originally bought it for my dad to start racing with me on club days

The car has 27x,xxkm's on it, the oil that was used in it after i purchased was a 10w-40, but after the head work I threw some 5w-40 into it. Running thicker oil may or may not fix the issue, but i would rather fix it once and for all and avoid putting additives into my oil. It doesn't need it from the factory so it just seems like a band aid fix.

I am actually looking forward to taking the engine out, as this will be my first engine out project (And I know dad secretly is too!)

Now its just a question of what parts to get, and where to get them.

[project.r.racing]

You get better answers and solutions if you put all the info into the first post.

[booki89]

I can't edit my first post.

[manga_blue]

270k and trackwork changes everything.

I wouldn't source rings or bearings until after a full inspection of pistons, bores and crank journals.

You'll need to figure out what sort of engine you need for the track. That means working out what sort of competition and class you're targeting. e.g. if you going to run sprints with the Vic MX5 club then stock internals with ACL race bearings will do for standard NB class. You'd add full engine balancing for clubman class. You'd have ARP studs and think about higher compression pistons for modified (<90Kw) class. You'd throw in serious rods and pistons for the higher N/A classes. You'd have some long discussions with an engine builder before going turbo. FInally you'd think hard again about finding a good second hand engine and keeping your old head as a spare.

[booki89]

Just to update this thread - I ended up taking the motor out, and stripping it down.

Was indeed stuck oil rings.

So now, it has all gone back together, with

2001 Pistons and rings, STD Size
fresh main and rod bearings, STD Size
New Water pump, timing belt, tensioners, oil pump
New Exedy clutch, new rear main, throw out bearing and spigot bearing
Port and polished head (very slight, just removed casting marks and polished)
Reseated the valves

All seems well now. No more smoke!

Just done a mildly warm compression test (car had been sitting an hour or so)
This is what I got
1: 185 psi
2: 185 psi
3: 190 psi
4: 190 psi

Numbers might be a little off, because I was doing it by myself.

Have dríven the car approx 120km-140km's since everything has been done, and honestly. I thought it would go a little better. I have checked my timing, incase I was out. But it doesn't appear I am.

[manga_blue]

Sounds like a good approach and probably a good result ultimately.

You need to bed the rings in properly before you can assess how well it's performing. Use mineral oil to bed in, never synthetic, just do a series of full throttle pulls in the highest gear you can legally use to maximise heat, fuel and pressure on the rings for as long as you can. It really only takes a few minutes to bed them initially.

Just google bed-in procedures and ignore any bull about nursing them for hundreds or thousands of Ks. That nursing technique almost guarantees that you'll glaze them and you'll have to pull it all down and start again.