I recently swapped the head on my 1995 Miata from a 1999 head, which was working without issue. However, when I took it to a machine shop to have it redone, they discovered bad scoring on the cam journals from some latent debris from the car it was pulled off of (thanks Alex Todd). As such, I've since swapped that out for a 2002 head and have retained the parts from the 1999 head swap (NB1 fuel rail and NB1 VICS intake manifold). All of the other parts of the car are stock, including the ECU. VVT isn't hooked up, but the oil tubing is. The VVT head was freshly rebuilt by a machine shop using OEM seals and parts.

With all that preface, my current issue is since the recent NB2 head swap, the car will not run properly or even idle. It will stumble and requires throttle to maintain a shaky, misfiring "idle." In this state, it's not really pulling vacuum, and it's running seemingly lean (according to my real wideband). Giving it throttle to go up to 2k+ RPM corrects the misfire, and the engine seemingly runs without issue, but it's difficult to maintain this state by throttle. Not change is observed with the stock narrowband unplugged.

With the MAF unplugged, the car will shoot up to 3-4k RPM at "idle;" however, it's running extremely rich (10.5:1 according to my real wideband (yes I have a stock narrow band plugged in properly as well)) and bellows brown-ish smoke. It does not smell like it's burning oil or anything else, just running wildly rich.

I've double checked all the vacuum ports to make sure there's not a massive leak, and I don't see anything out of place. I double checked the the CAS and timing, and both seem to be set without issue. The sparkplugs are brand new NGK units as well, and I put fresh 93 in the tank after draining it and flushing the lines. I have an OBD1 car, so I haven't been able to retrieve any useful diag information from the computer. The car ran perfectly with the NB1 head prior to all this, but that was about a year ago.

Here are some videos of the issue:

Any thoughts, help, or suggestions are welcome. I'm at a complete loss as to what may be wrong.

Thank you in advance.

 

This may help, it is for BMM ecu but look into this: https://www.miataturbo.net/attachmen...233d1653015415
Also try this thread: https://www.miataturbo.net/engine-pe...gine-na-65172/

 

Have you done a compression and leak down test? We’re valve clearances checked?

 

thanks for the links. I'll dig through the wiring a bit and make sure everything is connected properly. Guess I'll verify the sensors are within spec as well while I'm at it.

Valve clearances were checked and adjusted according to fsm spec by the machine shop. I have not yet done a compression or leakdown test. What should I be looking out for with those in particular?

 

Leakdown is kinda the sequel to a bad compression test. Compression should be middle/upper 100s (150-190ish) and within ~10% highest to lowest. But done stress overly hard about the specific number, IE if the service manual says everything should be 170 +/-5psi and you get an even 160 across the motor, it’s fine. Every gauge reads a little different unless you have some big titty freshly calibrated lab grade comp tester. Compression test should generally be done engine warm, all spark plugs out, throttle held wide open while cranking. Make sure it’s cranking at roughly the same speed for all 4 cylinders, and for about the same amount of time per cylinder. Generally the gauge stops climbing after 4-6 compression strokes (so ~8-12 revolutions). Also make sure your compression tester has a Schraeder valve installed at the engine-end of the comp tester adapter tube. That ensures you’re actually measuring compression of the cycling/camber volume, and not cylinder, chamber, plus the volume of the comp tester tube, and also makes sure the gauge actually holds pressure once you’ve passed the compression stroke. If you have a weak battery you may wanna add a jump pack. (Apologies if this is a redundant/overly granular explanation, idk how familiar anyone might be with this procedure, and there’s a lot of misconceptions around it out thwre)

If you have one or more cylinders that are way out, leakdown test will let you diagnose whether it’s a valve sealing issue, a piston ring problem, or a head gasket issue. Google can probably a more concise job than me rn with finer points of interpreting leakdown results.

If compression is happy and healthy across the board, leakdown isn’t really necessary.

I know the VVT phasers can develop problems (I’m not very well versed in exactly what issues they present but I’d assume poor angle control and/or not returning to fully retarded position) —So it’s possible that while the belt and cam pulley are timed correctly, the cam itself may be advanced to some random degree due to a malfunctioning phaser. Maybe post a photo of the cam lobes with cyl 1 at TDC? Could be somewhat apparent in a pic if it’s sitting at full advance. The cam phaser is controlled via oil pressure, so also if the phaser solenoid has **** the bed it might be randomly advancing the cam with oil pressure ***** nilly.

Do you have plans to eventually control the cam phaser? If not, why not refurb the bp4w head or replace it with another?

 

First an foremost, did the car run properly with the original head? That is your first data point.

Disconnecting the MAF won't tell you much other than with it disconnected, the stock ECU doesn't know how to figure out air flow so engine control goes berserk. You say that you're using the VICS manifold - are you using the NB1 TB? If so, how are you controlling the idle air control valve?

My guess is that somewhere between the head swap you've introduced a (hidden) vacuum leak. I'd start with the old "spray brake clean around the IM and see if the idle jumps up" trick.