classic FAST system with the IAC set to 32/8, standard setting. When cold starting the IAC goes loopy and will almost die, rev up to ~1200, almost die, etc. If I put it in gear while it's doing this to load the engine, it straightens itself out and is ok, unload the engine and it hunts and pecks again. Will do it for a minute or 2 till it warms up, then is perfectly fine. This started maybe 6 weeks ago or so, I changed the 32/8 values try to to get it to respond quicker and it did this. I reset the values and it is still doing it. Ideas?

Thanks

 

Re: IAC going loopy (classic FAST)

Most likely your IAC settings are not correct.
The throttle follower should be set to 10 across to where your pedal is at its lowest position then curve up to the right.
The start IAC position may not be correct
Are you using timing trim? you should be
The after start fuel may not be enough or come in quick enough when it starts. That will also make it move around a bit.

 

Re: IAC going loopy (classic FAST)

I should have said it had been acting normal, albeit a bit slow reacting, before all this. The throttle follower and start position are good I think. Follower behaves fine and I have starting position set full open to flare up (to try to energize my GM 1 wire alt without me having to rev it)

I'm not using timing trim, my impression was that would most likely mask other problems I might be having with the tune?

The car will start and die 2 or 3 times if I don't give it any gas. After the 3rd time it will generally stay running but will hunt and peck. This all got much worse when I finally added tail pipes to the car early spring. I had to take a surprising amount of CTS enrichment out after the tail pipes were added to keep it from burning my eyes. It was rich enough it wouldn't reach target rpm with the IAC open. You're thinking that maybe I need to give a little more after start fuel though....will have to try that.

 

Re: IAC going loopy (classic FAST)

Timing trim is a tool you should be using. It only is active until you are past the max tps for idle.

The start and die 3 or 4 times then hunt is not enough after start and maybe not going into or staying long enough in after start.

The removal of CTS enrichment would be correct after adding pipe to the exhaust system which also made more back pressure and gave you a better reading at the O2 sensor I bet.

The fuel smell isn't always the eye burning reason, it can also be timing at idle too hi.

Check your throttle follower at idle in park or neutral. it should be about 10. if not then fix it, and re-adjust your throttle.

 

Re: IAC going loopy (classic FAST)

I probably won't get a chance to mess with it till the weekend, but will let you know how it goes, thanks for the help!

 

Re: IAC going loopy (classic FAST)

To your last point: When I first got it running in 2009 N/A it did take a while for me to dial in the right throttle position to get high and low idles within the IAC's range. I can double check this again.

I doubled my afterstart fuel and decay and nothing changed. I was looking at the throttle follower while it was hunting and pecking and the IAC was wide open at that point per the graph, but when I put it in gear the idle went down slightly and the IAC went down into the middle of the graph. That was odd.


I need to run a log while it's doing the hunt and peck to analyze more closely. As soon as I put a load on the engine by shifting into gear it straightens out, and then put it back in park it starts hunting. I would guess I had at least a minute of run time on it at that point, so I should be way past the afterstart tables at that point, yes? How long should it be in afterstart? Maybe I took a little too much CTS fuel out....again, need to log.

 

Re: IAC going loopy (classic FAST)

After Start should only be active for a short time.

 

Re: IAC going loopy (classic FAST)

a short is 1 second, 15 second? Sorry to ask so many questions, the FAST manual doesn't give a lot of guidance about typical values for the afterstart tables like it does from some of the cranking tables. Is there a typical shape to the afterstart fuel and decay tables? Does the starting point correlate to idle fuel use like the cranking tables?

 

Re: IAC going loopy (classic FAST)

no longer than 15-20 seconds. just enough to get the engine running stable. After that it should be only adding fuel for the enrichment vs CTS

 

Re: IAC going loopy (classic FAST)

Making some progress on this for anyone interested. I dramatically added afterstart fuel and slowed decay rate and the hunt and peck is much better. Still working on the crank and die. I added more cranking fuel and set more delay for afterstart to take over. Not sure if that last adjustment helps, since that's supposed to help burn off cranking fuel and I seem to be lean.

 

Re: IAC going loopy (classic FAST)

After start should start almost immediately. Then time out around 5-20 seconds later. Cranking fuel is just for getting it to fire.

 

Re: IAC going loopy (classic FAST)

Ok, so here's where I'm at after messing with it a bunch:
it will start right up and will stay running, barely, catches itself and is then ok. No surging and goes through a nice warm up if you just sit there in park. To get that I have added more cranking fuel, more afterstart fuel, and longer decay of afterstart. IAC is full open to flare up as stated before.

Trouble is, in order to get the no load, in park warm up sequence steady, it's too rich when under load. If I wait for a minute before I take off, I'm fine. But if I immediately turn on the a/c and hit reverse to back out of the garage, the car loads up real bad. It stays running, but I have to pop it into neutral and clean it out. I've got the no load tuning pretty near to as lean as I can I think, as there are some warm restarts where there's a slight hint of hunt and peck. There's maybe a little more work I can do in the water temp range of 80-120 to see if I can squeak any more fuel out of it and maintain steady idle, but the car acts like I need an 'afterstart vs MAP' table, which of course doesn't exist.

Further advice would be appreciated.

 

IAC should be set near 100 at startup. Depends on the size of engine and injectors. Smaller engines with large injectors like more.

You should be able to crank the car with your foot off the throttle and it will start right away... at least in 4-5 revs of the crankshaft.

The afterstart then takes over and adds a little bit of fuel for no more than 20 secs or so.
Then the warmup starts.. it will add fuel until around 150 in most cases.

For you to tune the VE table is has to be done in gear, under load after the engine is warmed up first. Tuning in park is useless. The engine will sometimes be a little leaner in park but really wont matter.

If it hunts and pecks even though you got everything else filled out correctly then you idle P and D values are not right. I would start there with 32 in the P and 8 in the D this works well with most cars if you have the idle screw and throttle follower set up properly.

Afterstart vs MAP is not needed. You just don't have it right yet.