Troubleshooting help request - lots of lag after trackday
 

Hi all,

Been lurking and reading lots on here, but as my chequebook build NB2 has been performing faultlessly, I have not been posting much. I am hoping for some troubleshooting help though now.

The car has been perfect on road for a year, and for its first turbo'd trackday session over the weekend. The car was not as happy in the second session though, I was getting significantly more lag than normal and the car felt slightly down on power even when reaching full boost.

Datalog graph below shows the difference between session 1 and session 2 on the same 6 second slice of corner entry to exit.

You can see how much more throttle I needed to use in session 2, and how much slower the boost builds in this session.

Any tips on where to start looking? I cannot see any leaks in the exhaust manifold, nor in the plumbing from turbo through intercooler to intake. Maybe the wastegate is staying slightly open, but cannot think how to test that?

The engine bay was getting quite hot, not sure if that is relevant. Coolant temp reached 228F (109C), oil temp was reaching 262F (128C) but manifold air temp was steady at 105F (40C) in both sessions.

Thanks all,

Martin.


https://www.miataturbo.net/attachmen...ine=1434976143


Full spec for reference:
- 245bhp at the hubs, at 16psi.
- Standard VVT engine.
- e_1 manifold for Tial V-band housing and Tial MV-S external wastegate
- e_1 3" down pipe with re-introduced wastegate dump
- Garrett GTX2860RS with uprated metal bearing carriers and stainless heat shield
- Tial MV-S 38mm external wastegate with .4bar spring fitted
- Pierburg high-frequency boost solenoid
- Custom centre section reduces to 2.5" for Flyin' Miata catalytic converter
- Stock rear exhaust section (cat-back)
- Vibrant intercooler from Fab9
- Fully TIG welded hard-piped intercooler run
- Tial Q50 BOV
- Flyin' Miata Stage 1 clutch
- EV14 620cc/min fuel injectors
- Vishnu dual feed fuel rail
- MsLabs Basic MS3 with closed loop fuel and knock control enabled (and setup on dyno)

Sounds like you've pretty much covered the suspect areas. I'd go back over the exhaust side and charge piping again. Either of those could easily cause the issue, especially with extended periods of boost/heat, and they are issues that are easy to miss if you're not going over it very thoroughly. You want to make absolutely certain you didn't overlook a loose manifold stud or coupler before you even start to think about bigger problems.

Another possibility, I wouldn't think too many turbo+track guys run a cat. I'm not sure how well they would handle that environment, but if it's starting to die the extra backpressure could explain your symptoms.

Thanks pdexta. I had not thought about a blocked cat, good call.

Is there a DIY way of testing for a blocked cat, or is it just a case of getting a cat bypass pipe made up and see if that fixes it?

Martin

Had a cat bypass pipe made and installed yesterday, and no difference. Still takes 5-6 seconds to build up to the full 12psi.

In the absence of any other areas I could check, I thought I would pull the plugs and check compression. Got 159/155/168/172 psi which I think is OK. Not great, but probably normal on a 113k mile engine.

Cannot see any exhaust or boost leaks by feel or sight, but have ordered a kit that allows me to pressurise the whole intake tract via a compressor and listen for leaks.

Anyone got any other ideas of things I should check?

Martin.

More dismantling/diagnosing done. I think I may have finally found the problem.

Wastegates should not be open 6mm should they?

Martin

Nope. That looks to be your issue.

In the future, a few tips to make diagnosing easier (i dont mean to insult your intelligence if you already know this) :

1. Slow spool + RICH AFR = leak post turbo (assuming you still have a draw through style MAF). The air gets measured at the MAF, fuel gets added, and the air never makes it to the cylinder which causes you to run rich. Don't see AFRs in your datalog, so i wasn't sure.

2. Slow spool / low boost and normal AFR = an exhaust leak pre turbo since the turbo is struggling to spool, and it looks like you hit the nail right on the head with that wastegate. If i'm not mistaken, that WG has ports for coolant correct? May not be a bad idea to plumb some coolant through there to maybe prevent it from sticking again.

Nice, that's definitely the problem. Did the wastegate just get stuck there? Were you able to get it working again? Any idea what caused it?

Thanks for the confirmation guys. At least I now know what the problem is.

Yes, the wastegate just got stuck there - no idea what caused it. It had been working perfectly for the 9 months since turbo install, then failed on my first trackday.

Coolant is already plumbed in and cooling it.

Time to see how good Tial's warranty and customer service is.

Martin.

<br />
<br />Except no one has this.

I've got a big back ground in turbos but never on this platform, so i wasnt sure. Thanks for clearing that up.