I have search and read most of the posts regarding mods to the TB to correct idle problems. I am not sure if they apply to me. Could use your advice.

I am just finishing a refresh rebuild and would like to correct this issue once and for all while the engine is apart. I can't run the engine now to acquire data so hopefully you can assist without data.

I have the Edelbrock LT1 heads, intake manifold & 52mm TB. I have port matched and polished the heads and intake. I have a crane powermax cam (272/286, .479/.518 with 1.5 rockers) part number 104227. I am using the Comp LT4 stamped 1.6 roller rockers with appropriate length trick flow push rods. Shorty Edelbrock headers & K&N CAI. The engine runs great with this setup except a nagging idle problem.

When it is cold outside it idles too low but if I adjust the idle stop screw to raise the idle, during warmer weather it idles too fast. I did notice that when it is having the low idle problem, the IAC counts are maxed out at 160. The TPS voltage was adjusted to be within spec (I don't recall the measurement).

Before I disassembled the engine, BLM were OK and O2 sensors were functioning properly.

It appears that the IAC is not able to open the idle passage enough when it is cold outside and the PCM is trying to command a high idle. I drilled out the entrance (and exit into the lower manifold passage) of the IAC port but it is still restricted by the actual valve seat which I did not want to touch.

How do I modify this so I can achieve high idle when necessary without causing too high of an idle during warmer weather?

 

What is the programmed idle speed, fully warmed up (stock is 550rpm in gear, 650 in neutral)? In warmer weather, when the engine is fully warmed up, and the idle is where it is supposed to be, what are the IAC counts? And to confirm, with the engine fully warmed up in cold weather, IAC counts max out at 160 and the idle still does not reach the programmed value?

Does the engine experience high idle on cold start, like it is suppose to? For coolant temps 154*F and below, stock programming raises the idle anywhere from 750rpm at 154* to 1,200rpm at 3*. Doesn't matter what air temp is, only coolant temp.

There is no single TPS voltage spec. It needs to be within the range of 0.30 - 0.90V fully closed. If you are opening up the idle stop screw, you are changing the closed throttle TPS voltage.

Sounds like you may need to see if there is a single throttle stop position that allows the IAC counts to stay under 160 in the cold, while in the 20-40 range at fully warmed up idle, while keeping the TPS voltage i the range of 0.30-0.90V. Unless you have already tried that.

 

I am using stock programming but intend to get a tune once I resolve this issue.

On cold starts, the IAC counts are maxed at 160 and the car does not high idle or reach normal idle until I turn up the idle stop screw. Then a few months later when it is warm outside, I will have to turn it back down because it idles too high.

I don't know the counts in warm weather and can't check now that it is apart (and cold).

I initially set the TPS voltage within range but I am sure it is out when I adjusted the idle stop screw to keep the car running in the winter. I have not been able to find a setting that works in both warm and cold seasons. It behaves like the IAC valve does not have enough range for my cam.

 

Shoebox has a procedure to check the IAC motor. Won't prove it's working but will tell if the motor is burned out.

 

I'll run that procedure. The idle does change when ac compreesor kicks in so I believe the iac motor is working. I'll confirm.

 

The idle speed should hold steady as the AC compressor switches on and off.

 

1. you need a PCM tune to accomadate the new cam, heads, TB or car is not going to idle correctly and typically has low idle or stalls
2 Adjust TB stop screw back to stock position and confirm TPS volts are .67vdc blades closed and 4.5 vdc full open
3. Scan IAC counts with engine at operating temp (AC off). They should be 30-35 (32)
4. if they are hi (liked maxed at 160) then you drill the TB where the stock one has the IAC hole 1/8". Then measure IAC counts. If still high drill 1/64" bigger. DO NOT BLINDLY DRILL WITHOUT FIRST MEASUREING IAC AND CONFIRMING TPS VOLTS ARE CORRECT.`

Your new tune would have (if you told your tuner) been modified for the 52. if not transmission issues can shortly follow

 

Just remember... the PCM IAC counts are what the PCM is telling the IAC motor to do, not what the IAC motor is actually doing. There is no feedback as to the IAC actual position. If the IAC isn't responding, and opening up to admit more air, the PCM is going to keep increasing the counts until it hits max.

 

As Chimera96 mentioned, there are a few adjustments in the program that can be adjusted to get it to idle the way you want when it is hot or cold.

But there's also the possibility that 485,000 miles was just too much for it if it is still the original pintle and stepper. When you fitted it to the 52mm throttle body, did you check it to see if it was all coked up in threre? Maybe you can also run the reset routine on it when you get it back together.

 

The engine has been rebuilt a couple of times and the IAC motor has been replaced fairly recently. Right now I am waiting for new rocker arms to come in the mail and then I am going to prime the oil pump and finish off the engine. I haven't checked the throttle body for coking this time yet but this problem occurred with the original rebuild with the new IAC and after everything was redone, including the new 52mm throttle body.

I never had the programming done because I was never sure the mechanical issues were resolved due to this idle issue.

I will run the reset procedure next weekend when I plan to put the engine back in and I will reset the TPS value too. Fred - I am aware there is no feedback loop. fyi, I am an electrical engineer and have had many control systems classes so I understand the theory. I reset those values after the first rebuild and could not find an idle setting that working in both warm and cold weather so I believe either I need to physically modify something (i.e drill mod?) of have the PCM reprogrammed. I don't want to reprogram until I am sure that I have done everything I need to do mechanically.

 

A mail order tune will give you a better baseline for chasing the mechanical problems. That way you know it's not the tune. Not a big cam, but the 110LSA may present a bit of overlap issue. A mail order guy should be able to hit the tune well enough to allow you to chase the bugs.

 

OK. I will make the TBS adjustments and IAC reset mentioned and look into a mail order tune first then. I know there are a million posts on this but is PCMforLess still a good choice? I hear he makes regular trips to the Philly area where I live so I will be able to have him do the follow up dyno tune once I get back on the road.

 

A quick update. I received the mail order tune back from PCMforLess and I tested the IAC using Shoebox's procedure. The IAC tested good and the throttle body was not coked up. I did find the seal around the IAC was hard and loose fitting so I replaced. I thoroughly cleaned the throttle body. Will verify the TPS voltage when I get the engine back in the car. I am very close to doing that.

Thanks for your help so far guys. I am powder coating the valve covers and some brackets which is slowing me down a bit. Should have it running this weekend.

Jeff

 

For initial startup, I will set the TPS voltage to be within range but should that be with the butterfly valves completely closed? Should I back off the throttle stop screw completely and let the IAC control it for initial start up?

 

You need to get the correct idle speed, the IAC counts in the correct range, and the TPS voltage in the correct range.

Back off on the stop screw (close the blades) until it idles at the programmed RPM. Then check the IAC counts to make sure they are in the range of 20-40 counts. If not, back off on the screw until they come up to that range. Finally, check the TPS voltage to make sure it's in the range of 0.30-0.90V.

All that needs to be done with a fully warmed up engine. When the engine is cold, the PCM elevates the idle speed. If you have the hot idle IAC counts in the correct range, the PCM can open up the IAC on cold start to hold the programmed RPM value vs. coolant temp.