Help me diagnose my engine problem.. please
 

Set up:
90 miata. 1.6L Greddy intercooled at 5psi. I'm still using the vortech 12:1 on a stock pump and tan top injectors. Iv been running this set up for a solid 5-6 thousand miles. No issues until last week.

Here's the problem. On a cold start, the car would idle fine. If I rev it up to about 4000rpm using 10% of the throttle, my car would sound like its running on three cylinders.

If I rev it up using 100% of the throttle, she would sound normal. I took it out for a drive and the same would happen. Low throttle was bad. Full throttle, she will pick up like normal. Also, at full warm up the idle would change. It no longer idled like it did at cold.

It sits at a 1000rpm then about every 5 seconds, there's a sudden drop of about 300 rpm. That cycle repeats.

Iv changed the plugs and got no improvement. I also changed the coil pack (new) and still no improvement. My wires only have about 10,000 miles. So I didnt change it because im sure they're still good and i don't want to waste anymore money on parts that aren't fixing my problem. UNLESS, its just my luck that these wires went bad already.

Any thoughts??

I would replace your fuel pump and fuel filter. My car did that years ago and was a sign of a failing pump.

I would expect failing fuel pump/filter to cause a lean/knock/misfire condition under full throttle long before causing a lean/misfire condition under light throttle. I've had a failed fuel filter before, and it would only cough and sputter if I had the pedal to the floor. I would be suspect of a sticking injector first, followed by a failing head gasket. I've no experience with sticky injectors - I DO have experience with a failed head-block seal.

That's what you'd expect, but when my pump went bad it did the exact opposite. Slight throttle would cause massive stumbling and misfire, but full throttle would be fine.

Now that I think about it, I can see a scenario where this would be the case. :bigtu:

You sure you're not pissing off the cold-start maps in the ECU? But given the age of the pump, I wouldn't be surprised if it needed a new one.

A missfire would be more noticeable at a lower RPM - at higher RPM the pulses thend to blend and disguise the problem.

In no particular order...

What did the plugs look like when you went to put in the new ones ?

Check the wires for high resistance, fouling arching of the boots in the wells.

How old is the fuel filter ?

Did the car sit for a long time ( water in the gas ) ?

Is the Vortech giving you the correct fuel pressure ?

I appreciate everyone's input. Fuel pump and filter are up next! I hope this is it. (Crossing my fingers)

A local gave me the same advice. Thanks! Im on it

What's your afr say? That's how I knew my pump was "bad".

I put bad in quotes because it ended up being a bad hose between the fuel pickup tube and the fuel pump.

And miatas do eat spark plug wires, turbo heat doesn't help.

Unfortunately,I dont have a wideband. My plugs are pretty black though. Tomorrow, im going to hook up a fuel pressure gauge and see whats going on. My COPS just showed up in the mail. Iv been reading brains write up on the harness. Hopefully I know what im doing and will have it installed this weekend:jerkit:.

Ill give an update with psi readings in about 24 hours. I believe im suppose to read 80psi? If i can recall correctly.

I had this problem back when I was running a stock setup.
I swapped the PVC valve, plugs and wires, and it was fixed.

When I installed the turbo kit, I also got the problem back, but then it was an injector with a poor plug because of a custom harness.
The problem with these issues, is that it can be almost anything..

I recommend ohming the wires. You can also take out one plug at a time to check which one is failing, if it's the plug.
These cars eat them for breakfast, so my first response to issues like this would be swapping wires and plugs.

It's 1 of 3 possibilities - It lacks fuel, spark or air, and taking out a plug at a time would identify which cylinder is the problem.

Black Plugs = "Usually" too much fuel or poor ignition.
If all the plugs are more or less equally black it would sort of eliminate the wires since it is unlikely that they all go bad at once.

Based on the above I'd blame the AFPR regulator.

If the obvious recently messed with things don't do it start checking other things like AFM if you have one, O2 sensor and even the Cam angle sensor.

The vortech always runs rich at 12:1. I'm hesitant to say that's his problem.

OP, get an AFR setup.

I am assuming that you are still running the AFM. If so, swap it out with a known good one to see if it improves.

Can you explain to me why it would be the AFM? Im just curious

Wideband is definitely coming next. Iv only been running 5psi with no tune. So i wasnt even looking into it since 5psi is for :greddy:

The AFM sends a variable voltage signal to the ECU. It generates this voltage by a brush moving across a contact strip tied to a gate that moves based on the amount of air passing through the AFM.

This brush weakens over time and depending where it is on the strip it can loose contact and send a false signal.

Update: I got a steady 38-39psi on the fuel pump. Its on the low side and I'm sure it can use a replacement. Tomorrow I'm going to install the COPS and see if that helps.

FIXED!!!!!!! fucking, finally