legacycentral bbs

I've noticed something as of late. I only seem to get FC during the day. At night when it's cooler I won't get it at all. Like when I go to work I'll get FC even at low boost levels but then at night when I get off to drive home I won't get FC at all. This is leading me to believe that my intake charge is too hot and is somehow causing the FC. And yes, I'm sure it's FC and not a missfire. Vikash and I checked and the injectors are closing when I have the problem. Plus, the other day it was much cooler then it has been and I didn't get FC. But at the same time, I've had FC in the past even on cooler days so my theory may be off.

I figure that I need a bigger intercooler, FM or larger TM, and that this may "solve" my FC issues but here is the question, how would high intake charge cause the ECU to cut fuel? Which sensor would see that, the o2 sensor?

There isn't any sensor that directly measures intake charge temperature.

But, hmm... lots of problem-causing stuff tends to happen more in warmer weather than in cooler weather.

Like, for example, higher intake temperatures do increase knock tendency. It is possible that the ECU is cutting fuel because it sees knock while other parameters are out of spec (like the MAF sensor being railed). But I don't recall us observing any knock activity reported by the ECU...

Or maybe some electrical connection moves just enough as its housing expands in the heat to become an intermittent open circuit.

Or a vacuum hose becomes slightly leaky?

Anyone else have any ideas?

You've angered Apollo

scottzg wrote:You've angered Apollo

Shit, I knew it!

vrg3 wrote:There isn't any sensor that directly measures intake charge temperature.

I know, that's why is seems so strange.

Like, for example, higher intake temperatures do increase knock tendency. It is possible that the ECU is cutting fuel because it sees knock while other parameters are out of spec (like the MAF sensor being railed). But I don't recall us observing any knock activity reported by the ECU...

Ya, it didn't seem like we were seeing any knock but I was wondering about this too.

I've never really had all my sensors all working at the same time either. Before my O2 sensor was fubared and now my VSS isn't workign right. Maybe the ECU is freaking out under WOT because of missing input and cuts fuel because of that. That wouldn't explain the temprature link though.

Or maybe some electrical connection moves just enough as its housing expands in the heat to become an intermittent open circuit.

This was another thing I was thinking about too. I wonder if something in my wiring harness at the ECU or FCD wiring could be getting hot and screwing up.

Are your FCD's sensative to heat at all? What are the circuits heat range?

I'm grasping at straws here but the car would just be soooooooo much more enjoyable if I didn't hit FC all the time.

legacy92ej22t wrote:Ya, it didn't seem like we were seeing any knock but I was wondering about this too.

I suppose it's possible that the ECU could see knock but not report it, if it happens only between samples taken by the scantool. Seems a little far-fetched, though, that it would always only happen at those times.

I've never really had all my sensors all working at the same time either. Before my O2 sensor was fubared and now my VSS isn't workign right. Maybe the ECU is freaking out under WOT because of missing input and cuts fuel because of that. That wouldn't explain the temprature link though.

I thought we had pretty much all your sensors working at the Feed & Speed.

But, yeah, the correlation to weather seems to make this possibility less likely.

What's wrong with the VSS now?

This was another thing I was thinking about too. I wonder if something in my wiring harness at the ECU or FCD wiring could be getting hot and screwing up.

Maybe... Or maybe even at any other connection in the engine management system.

Are your FCD's sensative to heat at all? What are the circuits heat range?

Not especially. All the components are rated to at least 185 degrees, and the entire circuit is potted in epoxy so nothing can really move relative to anything else.

I'm grasping at straws here but the car would just be soooooooo much more enjoyable if I didn't hit FC all the time.

Yeah, that's an understatement even!

how's this for a crazy suggestion:

Your iac is a little messed up so it responds sluggishly. It slowly opens all night, and then during the day, the warm engine and hot weather lets it close. By the time you get in the car to go home, the engine is pretty damn cool, but the gummy iac is still closed.

Of course its more likely a wire or something, im just trying to think outside the box.

Why would a gummy IAC cause fuel cut?

>>>Or maybe some electrical connection moves just enough as its housing expands in the heat to become an intermittent open circuit.

sounds the most plausible. Over the years I have had several electrical issues that tend to occur only at higher temps, either hot out or after warming up thoroughly

I'm not really familiar with vrg's scantool, does it flag an intermittent value from a sensor that is out of bounds ? or do you have to be looking the whole time ?

Matt if your VSS isn't working correctly, the ECU will do some funky stuff. I have first hand experience with this.

Personally, I wouldn't spend any time trying to troubleshoot a problem unless all the sensors are working properly, because you may be trying to hunt a problem that doesn't exist when everything is working correctly.

professor - Yeah, I was thinking that was most likely too... My scan tool's very low-tech; it just displays values as fast as the ECU spits them out. So you'd have to be watching when something went wrong to see the abnormal reading.

But I do believe we've watched pretty much all the data streams while the problem was occuring, and we haven't seen anything unusual.

vrg3 wrote:

legacy92ej22t wrote:Ya, it didn't seem like we were seeing any knock but I was wondering about this too.

I suppose it's possible that the ECU could see knock but not report it, if it happens only between samples taken by the scantool. Seems a little far-fetched, though, that it would always only happen at those times.

Yeah, that doesn't seem very likely.

I thought we had pretty much all your sensors working at the Feed & Speed.

We did, except the VSS.

What's wrong with the VSS now?

The speedometer works but the ECU doesn't read the signal, remember? We swapped in your spare ECU at the end of the F&S and then the VSS worked fine but the MAP wasn't reading and I couldn't go on boost. I know the VSS works, it's just that the ecu won't read the signal for some reason.

Phil's bringing his spare ECU for me to test. Maybe it is all ecu related.

Maybe... Or maybe even at any other connection in the engine management system.

Very possible.

I'm grasping at straws here but the car would just be soooooooo much more enjoyable if I didn't hit FC all the time.

Yeah, that's an understatement even!

I think I'm developing neck problems.

Scott- I don't know if the IAC could cause FC or not. Hmm, I have had FC issues on both motors and I did swap over my IAC to the new one. Then again all the wiring and sensors were swapped over too.

Josh- I agree, that's why I want to try Phils ECU. If I can get everything working and then I still have FC, I can try and go from there.

One thing is that Phil was getting FC too, while his 02 sensor wasn't working right. So maybe the ECU cuts fuel whenever it has a missing signal and the MAF rails out or something like that.

Oh ya, I picked up new plugs yesterday and got a set of 6E's and a set of 7E's. Maybe going to a set of colder plugs will help a little too.

I know you are all saying it is fuel cut, but if I had this problem from the start I'd be looking right at the coil.

A marginal coil will cause the symptoms just like you are seeing, high RMP and high temp problems, and it is pretty hard to feel the difference between fuel cut and spark cut, you just notice no power and missing

That was exactly what I thought at first, professor. But we checked with the scantool, and the ECU is indeed cutting fuel.

but what happens if the coil craps, you misfire badly, and a bunch of unburnt fuel goes right by the O2 sensor. Would that cause the ECU to freak to the extent that it might shut off fuel ?

could the fuel cut be a symptom rather than the cause ?

Oh, I'm sure that the fuel cut is a symptom. The cause is the question. The ECU has to see (or not see) something that is causing it to cut the fuel. The problem is that we don't know what that is.

Mine has been doing something like that lately, at 5psi it feels like it hits the fuel cut. If i keep on it the car will just start jerking badly. Other times the rpms just drop until i let off but it doesnt get jerky. It's been pissing me off cause I like to have the power available.

I suppose that's possible, professor... That didn't happen to me when I had my high-load misfire problem because of a damaged spark plug wire, but that doesn't mean it can't.

And Matt generally sustains his high load and high RPM conditions for several seconds before this phenomenon happens. That doesn't seem very consistent with the idea that a bad misfire is causing the problem.

But it's a good idea to change out ignition parts. Matt, you've already used two different coil packs (your stock one and the boostjunkie one), right? But maybe it'd be a good idea to try a different ignitor or something.

Hm. Hm!

Is your ignitor mounted to a good heat sink?

Did these problems ever show up before you relocated the ignitor for intercooling?

I already had 2 coil packs before the BJ one and I've tried both of them but never tried the BJ one. I've tried different plug wires too.

Hmmm, the ignitor huh? I've never tried a different ignitor. I still have it on the oem bracket, it's just moved over to the side of the oem location for the intercooler to fit. I don't believe I ever had issues before the relocation but I also wasn't running increased boost then. Shortly after installing the TMIC and FCD I started to develope the FC issue though. Maybe the ignitor is getting too hot under the i/c?

Maybe.

I'm bringing some ignitors on Friday.

Cool, man I hope it's that simple.

Lemme know if you find its due to the ignitor's position. I suspect mine is in a very similar location, and while bolted to the firewall, I've been wondering if mine is also getting hot.

I'm not running the chimney, but plan on reinstalling it next week.

I'm not experiencing fuel cut like you are, but instead misfiring at low throttle positions when its really warm out.

Wait, I was thinking about this last night while I was thrashing around in bed with insomnia because of evil visions of Phil passing me while my car bucks with FC, he's laughing as he goes by, like some evil devil man, ripping my thorny crown of fastest SS from my tender head and carefully laying it across his devil man head horns.

The question is this, how would an overheated ignitor cause the ecu to cut fuel? We know the ECU is actually stopping fuel delivery and that it's not just a misfire, so is it even possible?