Intermittent Bucking
 

I've got a 1990 with a greddy manifold/turbo, BEGI exhaust, DIY intercooler, injectors, fuel pump, Megasquirt PnP, and LC1.

Last saturday I went to the track and drove my M3 while a friend of mine drove the miata.

Before the track day we topped off the tank with 1/2 tank of 100 octane unleaded.

First session, car ran great.

Second session he came in afterwards and said that the car was hesitating a little bit.

I took it out, car ran great in the paddock, after the first corner, the car went ape shit. Just bucking like mad, not wanting to run, felt kind of like a clogged fuel filter.

Went back into the pits, shut it down, started it up....perfect.

Back out on the track, went through the same corner, started bucking again.

I looked at the vacuum lines, any obvious wiring issues, and the only thing I saw was my CAS bolt was a little loose.

The car sat for a few hours, got in to drive it home, and it was perfect for about 10 minutes, then was just horrible. At first it only seemed to be happy in boost, while in boost my AF was 12:1 so that was telling me it wasn't a clogged fuel filter.

The problem would come and go. Later in the drive home it would stutter under boost as well.

When I got home I hooked up the laptop and drove around a little bit, my IAT was going nuts. Toggling between 60 F and over 200 F. When it would toggle, the car would buck. So I'm thinking that the sensor must have an intermittent short or open. I will replace it tonight.

Anything else to check?

post logs and msq...sounds like it was going lean.

but it could be something like the boost cut, accel enrichments, spark blowout, etc.

I will post more after I check/replace the IAT tonight.

The wideband was not showing a lean condition.

My issue sounds exactly like this as far as what the IAT reads.

https://www.miataturbo.net/forum/t38460/

So if that is the case, then that is not why my car is going ape shit.

I need to go through everything and see if the previous owner screwed anything up when he installed it.

Okay, I just took the car out and did some datalogging....haven't gone through it much though.

I was wrong about the IAT. I was working correctly when the car was first started. When I started it up, it ran perfectly for about 5 minutes, then started going ape shit as seen here.

http://img192.imageshack.us/img192/2972/datalog.jpg

You can see when MAT goes nuts, it's reading 215, which is invalid. It was really bad, I could barely keep the car moving.

I pulled over and unplugged the IAT to see if it would stay constant at 215, or keep going all over the place, and it kept going all over the place.

So that could mean it's a wiring short, though it seems like it could be something that craps out with temperature. So I need to try and put a fuel pressure gauge on it.

And here it got worse, it was really bad.

http://img682.imageshack.us/img682/2333/datalognuts.jpg


One thing that I noticed was that the IAT did go more than between 60 and 215....even when it was unplugged.

Go waaaaay back in the wiring, as close to the ECU as you can get, and cut it. Or unplug it. See if you can get a constant reading. My guess is the wiring was shorting on something metal when you'd go around that turn, and eventually it stuck.

Yeah, that's what I think too.

The funny thing is that it ran perfectly for the first few minutes I drove it, then it was absolutely horrible.

Coolant/intake sensors are weird things. The actual sensor went out on my '88 chevy pickup. It ran fine for 5 minutes or so than would start doing really weird things. I'd have to wait hours for the coolant to cool back down for it to even run again.

I think the fact that the sensor was still outputting something other than invalid when I unplugged it tells me that there is a short in the IAT.

Found it....fixed it.

What this picture show is the OEM MAF plug, with the IAT wires connecting into it VIA spade connector.

The problem is that the guy just plugged two spade connectors in and set it to the side and didn't insulate it, so it's free to flop around and make contact with the frame and make the car go ape shit.


http://img69.imageshack.us/img69/501...connection.jpg


So I'm going to tape it and then secure it. It makes sense that I saw the problem on saturday because at the track the connector is more likely to flop around and make the sensor short.

I cut my AFM plug off and soldered the two IAT sensor wires directly into the factory harness. I cut enough out that I could solder it back in if I REALLY wanted an AFM again, but I probably won't. Much more reliable and trust worthy than that. Especially for a track car. It already ruined one track day, don't let it do it again.

Yeah, that's my plan. For tonight I just thoroughly wrapped it in electrical tape so it's not going to short on me right now.

I just went and beat on the car with no issues.