m20 stroker stumbles and quits (long, help)

[N2MYE30]

There is a lot of good info here but let me just say that when this car is working at 100%, this M20 was moving! Very impressive.

[nando]

yeah, I was getting ECU resets. Well, what I thought were resets.. it was really the ECU shutting off from the bad relay. Felt like a rev limiter.

[mattdk318i]

cats will usually smell like rotten eggs

since the relays are original I'd replace them anyway. I chased a problem for almost 2 years that turned out to be a bad main relay. It wasn't until it got really bad that I figured it out.

This, i had the same problem with a fuel pump relay. Tested it and tested it. The contacts would release intermittantly making it stall like a misfire..

[nando]

cats will usually smell like rotten eggs

since the relays are original I'd replace them anyway. I chased a problem for almost 2 years that turned out to be a bad main relay. It wasn't until it got really bad that I figured it out.

[NC325iC]

The relays are not brand new. Neither is the pump. The for and filter are both new. I will crank it up and check the fuel pres gauge, but it seemed to still be holding ~36psi after shutoff. I didn't run it on a stock computer, and can't do so without changing harnesses or undoing some wiring id rather not undo. I have an open downpipe I can try out later this week.

Also I was getting a plasticish smell that I attributed to the new muffler, could that be the cat?

[CorvallisBMW]

How about your fuel system? FPR? Clogged filter? Failing pump?

[e30rapidic]

what exhaust do you have? I have a complete M20 system in the garage, sans manifolds that you can test out. I'll need it back pretty quickly though.

[nando]

a clogged cat shouldn't affect the AFRs. it will get very hot and probably smell though. if you had a test pipe it would be easy to check.

To me it sounds electrical. How old is your fuel pump relay? did you ever try it with the stock computer/AFM?

[NC325iC]

Nando? Jlevie? Anyone?

The more I think on it, it makes sense. If the exh gasses backed up into the cylinder enough that air was not being pulled past the maf (which is indicated by dropping vac) then the fuel would gradually be cut, which would lean and kill the motor.

[NC325iC]

First background on the engine.

has been running in my coupe for a couple weeks now
stock i exhaust barring stromung muffler
stock i intake manifold and throttle
Miller PSIK and war chip with tune

Last week being about the first time id really got to drive it i put it through about 30-40 miles of test runs just cruising with a little spurt of speed every now and then. It was running magnificently, absolutely amazing (ask n2mye30). Put it away for the week as my commute doesnt warrant the power. took it out friday evening for about a 20 mile drive just wandering around for fun, was running quite well, a little stumble here and there on my way back home. Made note of it and contacted brody with the issues as he asked me to, thinking it might be a little thing in the low throttle range of the tune. Yesterday didnt get much driving in, just a quick run around with my dad to show him how it was going, no issues very short drive. Today i take it out head up to my uncles place to watch the LFC-AFC game with him, running fine on the way there (with the now known stumble at low revs and low throttle) runs fine at the half when we go get food. Leave his place after the game and head to my dads place, running fine still with the occasional stumble at low throttle, but still not significant enough to be worrisome. Im at dads for about 10 minutes then im heading out towards school, get on the highway to head out there and it starting to hesitate a bit. I see on my AFR gauge huge swings from way lean to way rich and i back off down to about 50mph. after about a mile it stabalizes and i get enough courage to get back up to cruising speed. So im at the Wolfpack Motorsports shop checking out the weekends progress on the Baja SAE car. Im there for about 10min and i take off back towards home, on my way back to the highway im getting some hesitation similar to what was on the highway so i back off and try to hold fairly constant rpm. I approach a stop light push in the clutch and it slow dies out, im watching the afr and it is fully lean. I manage to get it started and into a parking lot where it again dies out. After letting it sit for about 15-20 min i decided to give it another chance fired it up, now i can keep an eye on both AFR and manifold vac, at first it fires up just fine vac jumps to ~15in (where it is when idling normally) afr is a fairly calm. Then after about a sec of idling alright the vac starts to drop off gradually and AFR leans out and engine dies.

Things i tried in the lot:

re seat many connections
unhook o2 from computer (results in much worse idling for that 1 sec)
unhook ecu to reset

nothing had any effect, so i got a tow home

I did a quick google on vac gauge reading which suggests this reading to be a clogged exhaust. This would fit as i bought the cat from a junked car, who knows how bad it is. I want to know though a) if this fits with the AFR readings b) if there are any other likely scenarios.

For me best case is clogged exhaust as that can be easily fixed, what else is it likely to be (keep in mind that it was running strong recently with no indication of any major problems until todays events) almost everything is new within the past 80 or so miles (on the engine)\

thanks