Hunting for consistent idle in LT1

Set up:
LT1 (+.030)
Crane 2033 (227) cam
Ported heads
T56 with 3.73:1 in the rear

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Idle is set at 850. The motor idles at 1100 when cold. After if warms up she hunts between 875 and 950.

Replaced EGR gaskets when I rebuild the motor (4,000 miles ago). Swapped another TPS onto the plenum with no effect. I have heard that if I spray wd-40 around the intake and have a vacuum leak it will stumble, but have not done this yet.

GMTECH this might be for you?

Your hot idle should be a little higher on a T56 car. I can check mine later today for you.

Tim, Welcome to the wonderful world of reprogramming. Step inside.

With a change of cam, you could be seeing issues in three different areas, your lower fuel table, spark table and/or IAC settings, any of which would require a little reprogramming. to correct. Yes, IAC's are kind of like controlled vacuum leaks

Ideally we think of idle as a point, a certain RPM and vacuum (or Kpa) but the reality is different. It is not a point on those charts, it's a small area, on both fuel and spark charts. Any undesired deviation in those charts within that area can easily cause idle issues as the engine ECM sees programming intended for the original cam, but undesired for your cam. It can see MAP and RPM values that change too much for the circumstance. I had to flatten mine out considerably to get away from race - stall - race - stall.

The IAC controls also are a big issue as when too large range of values allow it to pump in and out too much. It effectively bypasses more or cuts it back air, almost like opening and closing the throttle plate too much, too little and keeps cycling back and forth. Generally cutting it's values back 10-20% for initial changes are a big help. Try unplugging the AIC once ir's running to see if it still surges. If It stops then you know the IAC was the culprit. If not then other vacuum leaks or the Tables mentioned can still be suspect.

Also there are usually differences in subroutines for open loop (warm-up) and closed loop operation. Bear with me that I use a different program with TPI, but the basics of how they react to whatever sensor input is available are similar.

Search the big forum for idle surge, and you should get some hits.

As much as I would love to be able to help you out on this one I am at a standstill. I primarily work on stock vehicles with stock components and stock programming. I understand what is going on and what the ECM is looking at, but aftermarket components require special programming to correctly work with aftermarket items.

A higher idle creates a more stable/consistent engine vacuum. A vacuum leak would create a consistent high higher than normal idle (depending upon the size if the leak) with lower than normal IAC readings. Just throwing that out there.

lakeffect wrote:

With a change of cam, you could be seeing issues in three different areas, your lower fuel table, spark table and/or IAC settings, any of which would require a little reprogramming. to correct. Yes, IAC's are kind of like controlled vacuum leaks.

Now I can say this, likely the biggest factor that will effect the idle will be the cam grind. The cam grind will effect the manifold vacuum due to valve overlap and cam lobe separation and this will effect the areas lakeffect mentioned. This is primarily due to what the MAP sensor reports back to the ECM, a lower than normal engine vacuum equates into an increased engine load, as it stands with the factory settings. As MAP Sensors values decrease the IAC makes adjustments in the idle based upon engine load and as this is happening the timing is also being adjusted which also effects engine speed. It's a constant battle and the ECM is struggling to maintain both at the same time.

The ECM only knows how to read the tables in ROM. Fuel mapping, timing curves and load tables all look different now with the new parts and the ECM is trying to adjust the best that it can with only what it knows. (I could go on forever with this, but will stop here.) Now, until someone rewrites the ROM, it is just going to keep doing only what it knows. The ECM uses logic charts and tables as shown below:


I guess what this all boils down to is that long distant troubleshooting is tough. I do all troubleshooting with a scan tool and then interpret the information presented. We can only speculate here, make some adjustments and then report back with the results.

What he said.

The tune is a tune from PCM Performance (Alex). I will check the vacuum lines when I get the chance. Unless it gets worse I can live with it.

My idle is about 950 according to the Z cluster. With a manual, it should be higher than 850.

Tim, do you have a copy of the Bin that was preoared for you. Send me it to my email address.

Maybe I can take a look at it. I'll need to get the correct mask for it so it will open in my tuning program. Tunerpro RT) Do LT1's use a mass air flow sensor?

Yes, they do.

lakeffect wrote:

Tim, do you have a copy of the Bin that was preoared for you. Send me it to my email address.

Maybe I can take a look at it. I'll need to get the correct mask for it so it will open in my tuning program. Tunerpro RT) Do LT1's use a mass air flow sensor?

Uh oh,...

OK Nick, I lied. The sheep want to see it, not me.

Well it was supposidely set to 850 but idles about 950 +/- 25 so I guess I might be to picky. Imagine that?!?!

Bigger cams idle better a little hi9gher anyway tim that part is not unusual. it's tye surge part that needs to get consideration.

lakeffect wrote:

Bigger cams idle better a little hi9gher anyway tim that part is not unusual. it's tye surge part that needs to get consideration.

It does not surge, it starts at 1100 when it is cold or if the AC is on is stays up around 1000. If if is warm and the AC is off it takes a few seconds to settle to 950 and moves a little bettween 900 and 1000. Maybe I am just to picky!?

900 is not a bad idle point for a larger cam.

↑↑↑↑↑ Agreed

should I bump up the program from 850 to 900 or 950?

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Alex already set it higher for you. My 94's idle (227/LE2) is set at 800. I see 775-825 on datamaster.

You didn't mention it, but you did perform the IAC reset procedure right ?

How do I do that?

the [You must be registered and logged in to see this link.] is often overlooked.

jasonlachapelle wrote:

the [You must be registered and logged in to see this link.] is often overlooked.

Easy enough, I will try it on the way out the door today. Headed to Algonquin, Ontario, CA tonight. Paddling with a group of kids from a local church for a week. Looking VERY forward to it.

This might help

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Thank you Mr. Dave.