jarrett2k

a couple of days ago i noticed the miata was trying to cut out at 5k, no backfiring, no sputtering and it acted completely normal up to that point. but at 5k it would just act like it couldn't even pull its own weight. so i post up on miata turbo about it, only one guy replies thinking it's a spark issue.

i call a buddy of mine, he says it could be many things, told me to check the throttle cable, it's fine. told me to check plugs/wires, and that it could possibly be a head gasket.

okay, today i changed the plugs and wires and i noticed that a vacuum line going from the turbo piping to the boost controller was cut, so i replaced it too. i drove the car a while. it felt fine during normal operation (always did) but once i got into it it proved that it wasn't fine.

at about 3500 rpm, it starts building boost, gets to 1-2psi and falls at around 4-4500 rpm, but it will keep revving, just VERY slowly. and i noticed slight smoke at idle, and a slight teapot-like whistle under the hood, i'm assuming coolant, but you can only notice this after driving it a while, stopping and revving slightly.

i'm guessing it's a blown head gasket, but i hate to order all the stuff, waste the time replacing it all only to find out it wasn't the problem at all.

i don't know all that much about boosted vehicles, but i know how to work on stuff and change shit and whatnot, but trouble shooting is a whole different ballgame. any input will be appreciated.

Laserjock

You checked the plugs when you pulled them? You can tell a lot about how the motor was running based on how the plugs

Check fuel pressure

actionpaintball

Compression check, leak down check and coolant system pressure check first...as mentioned above-check your plugs when you take them out for the cylinder compression check.

b18b

Sound like you have a boost/vacuum leak. Check all couplers for rips, gasket areas on intake manifold, and hoses.

jarrett2k

plugs were a dark grey, they were only about 6 months old.

car's on the back burner for now, weather is horrible and i pulled a muscle in my back today at work.

jarrett2k

data logged it today. boost cuts hard after 3psi and about 4krpm and revving dramatically slows

since i have no wideband it's no way of me telling if something's up with the fuel.

ProbeGT-ZE

Sounds like a boost leak of some kind. Are you still running a MAF?

jarrett2k

yeah, i'm running a MAF unless i'm mega mistaken.

i changed out the actuator today, didn't help. checked for leaks in the vacuum lines, couldn't find any just looking by eye. my uncle was over (GM tech) he said that it may be the internal wastegate itself but he's never seen one fail hiself.

ProbeGT-ZE

When you say the boost falls, do you mean you lose all boost or do you just stop building boost? Turbo itself might be have a blown seal

jarrett2k

it looses boost. according to my latest datalog, it hits 3psi and falls rapidly. it goes into vacuum, even at WOT