I have been living with this for a few years, but it is really getting on my nerves.

Basicaly I have a Bridge Ported FD with a TO4R, lots of mods that I don't feel are to important to the problem. It seems like I have a vacume leak, but I have been through two motors and have checked both motors several times for vacume leaks and have found nothing. When I start the car cold I get low idle around 900-1K rpm, but as it warms up the idle steadily increases up over 2K rpm. When it starts cold I have to pedal it to keep it running. If I set the idle on cold start to 1200 or so, when it get hot it will idle around 2200rpm. Pretty annoying. Sometimes I can start it up hot and it will idle normal for a few minutes, but it always creeps back up. I can also drag the idle down with the clutch and it will stay for a minute. At this point I am at a loss for how to fix. Any ideas?

 

I suppose I should give a little more info.

I am running a power fc with the IAC active. I have tried it both ways and still does it either way. Idle is set to 1400rpms, but it doesnt seem like it pays much attention to where is it set most of the time. I show no sensor errors on the TPS in the sensor check part of the power FC. I have the air screw under the elbow all the way closed.

 

First, there is nothing wrong with the engine. It's what they do. As the engine warms the idle will come up. Everything is OK in that regard.

Second, I have a customers car right now that has the same problem with the PFC. The BAC is not functioning at all. I haven't put any time into diagnosing the issue but you're not the only one.

Third, use timing to help stabilize the idle. Lower the timing is the area you want it to idle. Where you want it to idle (say 1100 rpm) the engine might want 10-15* timing but try setting it lower, say around 5*. The engine note will flatten out and you may lose a slight bit of responsiveness but lowering the timing in the idle area and just above that area will help keep the engine from wanting to accelerate as it warms. Then at revs below where you want it to idle, say 0-800 rpm, bump the timing up 10* or so above what you have the idle timing set at. This will help keep the revs up under cold start conditions. The idea is simply to add timing at low revs to stabilize the cold start and take it out right at the desired idle range and just above to keep the engine from accelerating as it warms up.

To do all that you may need to change some of the rpm ranges in the PFC so that they are spaced closely enough to properly effect the idle. As an example we've gotten a BDC half-bridge to idle cold start and hold a consistent idle at 1100 rpm @ 14:1 a/f ratio without a BAC valve. This is with a Haltech E8 which has some more resolution to play with so we can stack load sites around the idle area. But the same principle applies to any engine.

Make sense?

 

Ported engines need more timing if you want a clean burn with less fuel but will be less smooth.

Less timing requres more air and fuel and burns richer even though smoother.

This all assumes that the ISC solenoid is BYPASSED as it does not use the timing maps. Also: O2 FB has to be turned off and the three idle speeds set to 0.
This is also with normal split.

My mild port engine idles a lot smoother with negative split idle.
Try this as a starting point: 12L 24T.

 

Thanks for the info guys! I will give that a try. I did get it to idle lower by setting the decel fuel cut to around 1600 vs the 1900 where it was, but then it would just bounce for a min before it would settle at 1400, but it did settle after 20 seconds or so. I will keep you updated. Right now I have to figure out whyI am dropping fuel pressure. This E85 switch has been a PITA!