Kbird

I'm wondering what influence the MAF has on idle.
I have a 69 camaro with an LT4 installed with a GM performance hot cam. EGR delete. I'm running the stock TB with an aftermarket 90 degree elbow to a stock descreened MAF. Attached to the MAF is a pipe that straight for a couple if inches then has a 45 degree bend and an K&N air filter. This goes to the passenger side like a Corvette. The MAF I purchased used. I have an OBD-1 harness and PCM installed. The PCM has a mail order tune.
When I start the car some times it will meet the desired RPM (850) with IAC counts in the ~30's. Most times IAC will go to 0 and it will stay at 975. When the are engine warms up (ECT ~140) it will start to idle at 1100. I have adjusted the TB plate with no success. I have gone through all the vacuum lines and re-torqued all the intake fittings.
I had a friend over today to help. I went all around the engine with propane. Nothing found. I did disconnect the air filter and pipe connecting it to the MAF. I put my hand over the MAF and started altering the air flow across it. I could get it to idle down by changing (opening my fingers or holding two together and the other two open) the air flow across it. So, now I'm questioning if my air induction set-up is part of the problem or if my MAF is bad?
Here's some data I got using FreeScan.
RPM = 1050, IAC = 0, Desired RPM= 850, Throttle Pos = 0 (0.61 v), MAP = 0.565, BARO = 0.989, Air Flow = 14. Not sure what the MAF units are (g/s?).
RPM = 850, IAC = 33, Desired 850, Thottlw position = 0 (0.61v), MAP= 0.661, BARO = 0.989, Air Flow = 14.
MAF is the same at both RPM's.
thanks for any assistance.

Kevin
69 Camaro LT4 w/Hot cam, headers. T56 9" 3.90, Baer Brakes, DSE suspension, Budnik wheels

Injuneer

MAF flow is grams/second.

Did you verify that the blades are fully closed at idle, by backing off the throttle stop screw? Is that what you mean by "adjusted the TB plate"?

I think your test of placing a hand over the MAF to block flow simply indicates the engine is getting too much air through the throttle body + idle air passages.

Have you verified the IAC is working? Have you checked to make sure the passages aren't plugged up, the bottom plate on the TB isn't loose, the IAC pintle isn't dirty, and the foam seal is in the throttle shaft, behind the TPS?

Kbird

Fred,

Thanks for the input. I did adjust the throttle plate set screw so the blades were closed.
After playing with the air flow via my hand. The vehicle would idle around 650, IAC counts were at 160, and O2's staying at the 0.045ish range. I adjusted the throttle plate screw back to what it was when I originally got the engine. Disconnected the battery. Started the car and still had the same results.
The IAC is new. I did spray the IAC cavity with carb cleaner a while ago before I changed the IAC.
When I had the TB off this weekend. I check the bottom plate screws to ensure they were tight.
The foam seal I have not checked. Can I see that from the throttle blade side or do I need to take the TPs off to inspect?
I assume there is supose to be a hole going straight through the TB where the cavity is for the air to enter the IAC?

Kevin

96capricemgr

That hole is a bleed hole so the IAC does not have to handle 100% of the air, aftermarket TBs lack it and to get them to work right we actually have to drill it. You should never mess with the throttle blade stop.

sharp bends in the induction before and after a descreened MAF can cause issues but I would have expected them at higher rpms. The screen is there to straighten airflow and try to keep it even across the cross section.

Injuneer

If you have 160 BLM's and the O2's (engine hot???) are indicating 0.045V (45mV), you have a serious lean condition. The problem is way more than a small vacuum leak.

The throttle shaft foam seal is accessed by removing the TPS.

Kbird

Fred,
I realized that last night, too. The car was running out of gas. It was probably occuring the same time we were playing around with the MAF air flow. I put fuel in last night and will see what happens this afternoon.
If I have the same high idle situation. I'll take the TPS off and inspect for the foam seal.
How can I check to see if the IAC is fully seated? Remove the cover from the bottom of the TB.

Thanks,
Kevin

Kbird

Same high idle concern with fuel in the tank. I see an orange color seal/foam behind the TPS.
Removed the IAC and bottom plate from the TB. Ports are clean. I did see what I would call wear marks in IAC bore. Though I believe they are manufacturing marks. They are at different depths in the bore. I assume the the lip on the ICA pintle seats against the bore and the IAC doesn actually travel down the bore, correct?
How about the EGR delete plate?
Thanks again.

Kevin

69 camaro LT4 w/GM hot cam, EGR delete. T56 9" 3.90 DSE suspension, Baer brakes, Budnik wheels.........

Injuneer

The pintle moves in and out to control idle air flow. The pintle sometimes needs to be cleaned. It can be cleaned with throttle body cleaner, but spray it with the pintle pointing down so the cleaner liquid doesn't get up into the electric stepper motor that moves the pintle.

There is also an "idle learn" procedure that sometimes helps (courtesy of Shoebox):

http://shbox.com/1/4th_gen_tech2.html#iac_reset

IAC checking:

http://shbox.com/1/iac2.jpg

The EGR delete plate for the EGR valve hole needs to have a gasket and RTV to insure it doesn't leak. That would be a vacuum leak if it was loose or not sealed correctly.

Kbird

Curious if I plugged the thru hole in the TB what would happen. I stuck my finger on the hole. Didn't change much. So, I plug the IAC and thru hole. Engine idles at 850. IAC counts increase. Release and the high idle occurs again. Plug the holes again. Let the IAC counts go to 160. Release my finger and the car idles at 850 on it's own. Rev the engine up and it comes back to 850 idle.
I checked the wiring from the IAC to the PCM. Found pin 1 and 2 at the PCM swapped. Damn aftermarket harness. Haven't tried since swapping the wires back.
Kevin