olchickenbeard

Hey folks, so I just bought a 1995 M-Edition last week that’s been giving me some trouble. For starters, the PPO boosted it, but removed the turbo kit and reverted it mostly back to stock before selling to the PO I bought the car from. There are still some remnants of the kit though, most notably a MSPNP gen 1 ECU still installed, as well as AFR/boost gauges, wideband O2, and some various disconnected vacuum ref lines in the bay. Beyond those, a crankcase breather, and EGR “delete” (just welded the exhaust bung closed and removed the lines/connector on the intake, no block off plates), everything else is basically stock. PO said he dailied it for a while, then one day it started blowing white smoke and he decided to sell it.

When I went to look at it, coolant was completely empty, so it wasn’t really blowing much smoke anymore. We both assumed head gasket, so that was the first thing I replaced. After a few miles all the smoke cleared up, but it was still running rough. Tons of hesitation, backfires, way low on power.

Here’s a quick timeline of what I’ve done so far:

• After replacing the head gasket, it still had a rough idle. The car sat for several months, and the fuel didn’t smell too great, so I drained most of it and put fresh 93 in. I discovered cyl 3 injector leaking fuel at the fuel rail. Got all new injector seals, no more leak. Injectors are stock btw.
• Obviously timing belt was R&R for the head gasket, timing is good, but idle still wasn’t quite steady enough to precisely adjust timing, but did my best to adjust to 12-14°.
• I got the AFR gauge working and discovered some pretty scary AFRs - idle was at around 11.6-12.4, but if I held the gas at any RPM it would misfire and shake like crazy, leaning out at 18+ AFR, and quickly get the exhaust manifold glowing bright red. That confirmed to me that the ECU was still tuned for boost, so I swapped the stock ECU back in.
• With the stock ECU in, idle was much smoother, AFRs in a much safer range, and I was able to more precisely adjust timing to 14°. Although my tach no longer worked (from what I understand the tach needed to be rewired for the MS, which means dead tach with stock ECU). But on the test drive, it was still bogging/hesitating like crazy, AFRs at about 14.5, until about 4-5k, then it would smooth out and AFRs would drop to 10.5-11.5. But it was still low on power. And the hesitation would also improve a little once the car was warmed up.
• Fuel filter also looks pretty new, and checked fuel pressure at the rail feed line, steady 45psi.
• Compression tested at about 190psi, +/- 5.
• Spark plugs looked pretty new, so I didn’t replace them at first, but the base/threads were black and had some deposits, and the side electrodes were all white. They were also slightly wet with fuel, and there was more fuel sitting in all of the cylinders. I figured it running so lean before might’ve fouled them out. Replaced plugs and wires, gapped to 0.043, and it did improve it a bit, but not enough.
• After a few more test drives I rechecked the new plugs, still black base/threads, white tip, and still slightly wet with fuel, with more sitting in the cylinders. It’s also still going through fuel like nothing. I’ve probably only driven 40-50 miles with test drives, but it’s gone through 3/4 of a tank within that time.

At this point, from what I can gather, it seems to be pushing too much fuel, choking the plugs and causing hesitation, then once revs get high enough it’s able to catch up a bit. Then once it is fully warmed up, the extra heat helps a bit in igniting more of that extra fuel, so it’s not hesitating as bad or for as long. But even at its best, it still feels like it’s at least 20% down on power.

I’m also wondering if there’s anything else in the wiring that was modified for the MS ECU, that could be causing this. I did order a tuning cable for it, so I’m considering throwing the MS back in and retuning for N/A as a last resort, but if anyone has any additional input it’d be greatly appreciated.

Rrrracer

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SimBa

You might post some detailed photos of the engine bay. Someone might spot something you overlooked.

This thread has posts about AFRs on stock ECU's. Your's doesn't seem far off from what others have reported
Stock ECU AFRs

What are you comparing the missing power to? Another Miata, this car at a different point in its life, just what you expect it to feel like?

olchickenbeard

Thanks, I’ll take a look at that thread, and I’ll get a few photos. Any areas in particular to get pics of?

And I’ve owned two 1.6s previously, both stock motors. This one is struggling to even get up a slight incline when it’s hesitating, and even when it smooths out it’ll still stumble quite a bit. I can’t even get this one to break loose in the slightest.

SimBa

Anything that doesn't look stock. If it's been turbo'd I'd look for modifications around the fuel rail, coil pack, AFM or other places that would be commonly changed to accommodate a turbo.

sixshooter

iTrader: (12)

You've seen evidence of injector problems and someone saw fit to change the filter recently. I would get them cleaned and tested before proceeding. If they are leaking or squirting instead of spraying a nice fan of mist then it would explain some of your issues.

Uneven flow between the injectors can really screw up trying to diagnose anything through the AFR readings.

Things in the exhaust can get red hot because of a lean condition or a retarded spark condition. It can also happen if you have a bad spray pattern for your fuel and it is still burning when entering the exhaust manifold. When a fuel injector squirts instead of sprays you get incomplete combustion within the cylinder and it may still be burning upon exit.

olchickenbeard

Yeah, I’m thinking it may be the injector spray pattern. I pulled them back out and did a bit of a ghetto rig to test them and spray cleaner through, and they all sprayed pretty evenly but had more of a conical spray/stream than a mist. Of course that might just be because it was at lower pressure, but either way I’m just gonna order new injectors to be on the safe side. I called around about getting these serviced and it’s cheaper to just get new ones.

In the meantime, I did decide to try hooking the MS ECU back up to fine tune it for N/A. Reflashed with the base map and I got my buddy helping me fine tune it, and we’ve definitely made some huge improvements. It’s still a bit on the rich side, especially under heavier load and/or more throttle, but in the areas where it is smoother I can definitely tell a huge difference in power, it feels like it should finally. We’ll be making some more adjustments today, then when the new injectors come in we’ll see what sort of difference it makes.

olchickenbeard

Looks like I may have found my “issue” lol, fuel pump was upgraded to a Walbro 255 and never got swapped back to stock. So I guess it was overpowering the FPR/injectors enough to choke the plugs. Gonna try to compensate for it when we get back to revisions later today!

sixshooter

iTrader: (12)

That'll do it

olchickenbeard

Update, tried adjusting the tune for the larger fuel pump, still running rich no matter what we did to the fuel tables. Tested the fuel return pressure, only hitting about 2-3psi. Thought it was a bad FPR, replaced it, no difference. Also tried disconnecting the return line at the hard line to confirm there was flow, and there definitely is. When I first started it with the new FPR idle AFR was better at 14.5, but as soon as I gave it some gas it dropped back down to 10.5.

What am I missing here?

curly

iTrader: (25)

The return line has no pressure, you’ll read system pressure in the rail or feed line.

a 255 isn’t a problem on a naturally aspirated car. Over kill? Sure. But when I’ve gone from stock to 255, my idle afr goes from 14.7 to maybe 12.5-13, because of the higher pressure, stock FPR can’t dump enough volume to maintain 3 bar, and a simple tweak to the tune fixed it. You have something else going on.

olchickenbeard

It seems I found the last piece of the puzzle. A bit of a stupid oversight on my part… taking a better look underneath I found that all of the nuts on the bottom of the intake manifold were either loose or missing. Previous vacuum leak tests seemed to show no leaks, so I was more focused on chasing a fuel issue, but after getting the manifold tightened down fully my AFRs were finally looking better and were more responsive to changes being made in TunerStudio.

Still needs a lot of fine tuning, it’s still got a bouncy idle/idle AFR, and still slightly rich at high load/WOT, but power-wise it finally feels like it should, sounds way healthier, and no more bogging or choking plugs.