[JMG9350]

I just got my new engine installed and running. I went from a built 306 to a Fordstrokers 347 short block, Edelbrock RPM2 intake, Fordstrokers custom camshaft, 11R 190 heads, Morel lifters. The TB, MAF, 36lb Holley injectors, headers, rocker arms, etc were carried over from the 306.

The previous engine was professionally tuned after I reached my tuning knowledge and ability, and everything was fine. I made some changes in the tune to reflect the new combination and now the idle hangs at 1500rpm after starting it, letting the idle settle, and then goosing the throttle. It won't come back down to 800rpm unless I shut the engine off and restart. There are no vacuum leaks. I'm at 6200ft elevation and I get 17hg of vacuum and a steady gauge. The computer is showing desired idle to be at 800, but it can't get it there. IAC duty cycle is around 35% at 800rpm, and then when it idles high it's at ~45%.

Any ideas? Maybe someone wants to look at my tune?

[JuiceSC]

Sounds like eec not seeing closed throttle after the blip. If you adjusted tps voltage, undo it. No reason to have closed throttle voltsge near 1 Volt.

[JMG9350]

Closed throttle voltage is .91v. I saw a Leech video today during my searching where the TPS was intermittently dropping voltage below acceptable range and causing the computer to default until restart. I'm going to install the OEM part from the original engine and see if that helps. I wasn't sure if I screwed something up in the tune.

[JMG9350]

I also have a 2 step rev limiter wired to my clutch switch using a well know diagram on Corral or Stangnet, including the diode. I'm hoping that doesn't mess with the computer clutch signal. The hanging idle doesn't seem to care if the clutch is depressed or not.

[JuiceSC]

My closed throttle tp volts are about .7v. I just put on a new tps, no mods to adjust. Works fine.

[JMG9350]

By acceptable range it was going below .5v. The computer doesn't care as long as the voltage is between .5v and 1.25v when it's initially turned on.

[cgrey8]

All my experience is with GUFx strats. However I suspect other strats are similar.

Idle has multiple stages, there's Preposition which is where the ISC gets set when the TPS not at closed throttle.

Once the TPS returns closed, the mode transitions to DASPOT which is a decaying of the ISC position from PREPOSITION down. The amount down. However the important thing to note is that DASPOT is an open-loop behavior. It doesn't tweak its position or rate based on RPM feedback.

When things are setup correctly, the conditions are met to enter Closed Loop Idle control which is where the ISC is modulated to maintain a target RPM. However the conditions to transition from DASPOT to Closed Loop are not straight forward. I remember having to read over the GUFx strategy doc and understand the code flow quite a bit to wrap my head around it fully. And that was many years ago, so I don't have the understanding today that I did when I had to do this tuning.

But what I remember is that there was a minimum position during DASPOT while the vehicle was moving to keep the engine in a higher RPM in expectation of having power reapplied. However when sitting still, the position was allowed to go lower. And there was a balancing of tweaks that had to be done to both allow the ISC to close enough to enter Closed Loop RPM control and scalars that would allow for the entry of Closed Loop RPM control.

Like I said, I don't recall the specifics, but it wasn't as simple as just upping numbers high enough to allow Closed Loop control. If you did, you'd enter Closed Loop too soon (i.e. while RPMs were too high above target) and the ISC would be closed off too much causing the RPM-drop during this time to happen too fast and cause the engine stutter or outright conk-out. The intent was for the DASPOT to gradually guide the RPMs into Closed Loop control so the transition happened in a more controlled way.

It also seems there were 2 Closed Loop modes Idle Control could be in, one was just Closed Loop control to get RPMs where the target was, but then there was a learning-mode where the EEC updated its KAM with how much ISC adjustment was required to attain this RPM. And depending on your values, you could have the system transition idle states fine initially (when no learning had happened), but then after some learning, it would learn into a bad state. I don't think this was common, but I do recall it being possible.

Bottom line, I clearly remember idle-tuning was NOT a straight-forward thing. It really required you dig into the docs, understand what settings are available, then start tweaking and getting a feel for how changes to these values affected idle performance. And my ultimate take-away from the experience is there had to have been a better way than what was done in these old EECs. But those sentiments weren't unusual for Ford programming strategies. For example, controlling an engine via Speed-Density is probably superior to MAF in a few different ways, but not when using a Ford SD-based EECs. Their way of SD-engine control was horrible and why people migrated their older SD-systems to MAF back in the day.

[JuiceSC]

The things I found to have the most impact on idle quality & stability:
Idle fueling - maf & injector data. The bigger the injectors and maf, the smaller your 'window' is to get this dialed in.
FN875D, FN875N. Needs to be raised.
FN824. Idle target is set for the stock ~600-ish rpms.

None of this matters if the throttle is not returning to "closed".

[cgrey8]

I failed to also mention those points that if fueling is off during idle, it doesn't matter what your idle parameters are in the tune.

Another thing I discovered and didn't mention since I got so focused just on ISC control aspects is that excessive spark can negatively affect idle as well. While the engine may not be pinging, it is possible to get a surging idle or touchy throttle response if spark advance is too high. Err on the side of less spark at idle rather than too much...at least that's what I experienced.

When I was trying to dial in CNG on my 331, I was told it has a much higher octane rating and it even burns slightly slower than gasoline at any given compression. Both of these I interpreted as I needed to run more advance at all running conditions to offset. But what I found is that at idle and near-idle conditions, this just makes the engine hard to control and predict. The higher advance will make the engine want to run at a higher RPM (regardless of the fuel), but once you start loading the engine, it may hold the load or it may conk with no warning what so ever. With less advance, if you get too quick with the clutch, you'll hear the engine bogging and might can get in on the clutch and push the throttle quick enough to "save" the engine to keep it from conking. But with high advance at off-idle RPM/Loads, it doesn't give you a chance. I think I updated my tune to max closed throttle advance to no higher than 18 when anywhere near idle-RPMs, even with gasoline.

While not related to idle, I did also find that the rattling sound an engine makes when lugged at low RPMs can also be lessened by lowering the advance commanded during really low-RPM-but-high-load conditions. In some cases you may find it is eliminated altogether if you get the advance low enough. And by low, I mean in the 2-6 degree range...significantly less than the base-timing of 10.

[JMG9350]

The fuel was previously dialed in on the last engine. Same MAF and injectors. I changed the CID, and the spark tables are all stock. Base timing is 10 degrees, AFR at idle is 14-15 according to my wideband, and computer lambda LTFT is 1.07. I changed what I think are all the idle strategies according to the Leech Motorsports videos. I'm hoping to start it tonight with the different (original from my stock engine) IAC. And I'll see if the TPS is also having issues. And make sure the throttle is actually closing against the stop.

Thanks for the replies.

Stay tuned.

[JMG9350]

The IAC was faulty. I swapped it out with a known good part (the original IAC from the stock engine) and it fixed the hanging idle. I also checked the TPS and it is at .91v with key on (RATCH). I did need to adjust the throttle blade to get the IAC duty cycle back down to around 35% at idle. It settled at 50% after changing the part. Now im playing with the dashpot function to get the idle to settle faster.