Random lack of boost?
 

Long story short: my girlfriend bought a 1990 Miata off of her coworker about a year ago- it was her coworker's late husband's car, and it was just sitting. She needed transportation. 2+2=4.

Turns out it had an aftermarket turbo setup installed, sometime around 1993. After doing a little research, found out it's an Aerocharger (no longer in existence), but since it ran and drove just fine (it's a total blast of a car, even at a minimal ~6psi), we decided it would work out great, and have been driving it since.

Lately, however, I've been running into an issue. It will occasionally fail to produce boost at open throttle. That is, the boost gauge will read pressure, but no extra power will be delivered to the engine for a good 3-4 seconds. Then, it will suddenly come on, albeit heavily delayed and all at once. It does this intermittently, and sometimes doesn't do it at all, and performs normally.

Any thoughts on what could be causing it? Based on the on-off nature of the problem, I was thinking it could be a wastegate problem, but I figured I'd run it by you lovely people first. If it is the wastegate, what can I do to fix it? If it isn't, what else should I be looking for?

Thanks for your help! The Miata is a fantastic little car, and I just want to make sure my girlfriend can keep driving it problem-free!

where is boost gauge reading boost from? turbo or manifold? Probably boost leak.
Otherwise gauge lies.

Not sure, I'll have a look at it when I get home tonight.

I'm reading that to say, "The boost gauge shows boost, but the car doesn't accelerate as exected." Is that what you're saying? If that's the case it doesn't sound like a boost leak or defective gauge to me. I was leaning toward something like plug wires.

Any sputtering or changes in the engine sound while this is happening?

This is correct. The car accelerates on boost, it's just delayed. Although sometimes boost does not kick in at all, despite showing boost pressure on the gauge.

The turbo seems louder when I'm getting boost but no additional power, which is what leads me to believe it's the wastegate, but it's inconsistent enough that it's hard for me to tell.

Can you get a good pic(s) of the engine pay and the setup and how its all routed?

This to me says you are venting the boost somewhere, and not into the engine. Does the AFR stay reasonable when this happens (if you don't have an AFR guage in it, get one). Where does the boost guage read from?

Boost leak
 

Very likely a boost leak, google checking for boost leaks to find how to make a cap you can put on the intake of the turbo, and pressurize it with an air compressor. Makes finding them very easy and you don't need to put too much pressure either about 5 psi should be it. If the turbo kit has been on awhile, it is very likely leaking at a connector.

Chris Rummel