Red_94Formula

Bank 1 LT Fuel Trim at 139 at idle. 128 as soon as gas is pressed. Bank 2 is 128 constant. Its in BLM cell# 16

This is a cammed LT1 with headers. Any ideas why bank 1 is at 139 only in idle?

I cant find and exhaust leaks. idles around 875 to 900. Has a over the net tune. I think it is Madz28's tune. I do have Lt1 editor version 2.0 and the great Scan9495. Exhaust fumes are noxious while idling.

Thanks for any help.
Derek

Injuneer

How big is the cam? High overlap cams dump air and raw fuel into the exhaust at low RPM. The O2 sensor sees the air, tells the PCM it's lean, and the PCM adds fuel that isn't needed.

Do you have a complete data log for the car, so that we can look at the BLM's in the other 17 cells? The fact that bankk 2 is "constant" at 128 may be indicating a problem with the O2 sensor, preventing the PCM from learning on that bank.

Red_94Formula

The cam is .547/.544 220/229 112 LS.

I will get a data log later today.

Thanks Fred

Red_94Formula

Sorry for the late response.

Here are two logs on google drive in an ancient excel format. The one labeled from 1993 is this mornings log. That fuel trim changed from 139 to 136. Idle is about 100 rpms higher then last log

https://docs.google.com/file/d/0B4m5...it?usp=sharing

This log is from when the trim was at 139. Apparently I pressed the data log not knowing.

https://docs.google.com/file/d/0B4m5...it?usp=sharing



These logs are not under load and do not go much past 2000 rpm. My o2's are all over the place.

Red_94Formula

Those excel data logs spread sheets dont look to well on open office. I also used Scan9495 for the data log. Which does not show the other 17 cells. I gotta dig out my old Lt1 editor.

Injuneer

Opened OK in Excel. The reason it only shows Cell 16 is because it only shows the actual cell the engine/PCM is operating in.... and you are at idle, and 16 is the idle cell. To see the other cells, you have to drive it (in closed loop) and the other cells will show up one at a time, based on the specific combination of RPM and MAP the engine is operating at.

You have to run the engine at least 206 seconds for the PCM to go into closed loop.

Your engine is idling too high, and the IAC valve is closed all the way. Your closed throttle TPS is at 0.82 Volts. Try backing off the throttle stop screw untiil the TPS volts drop to 0.50V (or even lower if needed - don't go below 0.30V). That should allow the IAC valve to open up and actually control the idle speed. Did you have your foot on the throttle... because it drifts upwards to 1.02V in places.

bobdec

I see TPS voltage is at .82 a bit high IAC is 0-1, actual RPM is above desired. Timing looks like it's set higher for cam. First thing I would close the TB blades get TPS voltage down between .45-.75 that will raise the IAC count, I like IAC minimum in the 30-40's. that lets more idle air enter intake idle plenum, helps with split BLM's. Once you get the TB stop and IAC counts balanced , the PCM should dial in the desired RPM w/o and problems. I assume you have double checked for any vacuum leaks at idle. That will also drop IAC and cause split BLM's. By the way O2 voltage dancing between 100-900 Mv as each cylinder fires is normal. PCM averages that voltage to calculate ST blm's which look good.

Red_94Formula

Yes I had my foot on the pedal a few times.

That idle screw is out too much. I adjusted that thing incorrectly ten years ago(car has been off the road for almost 8 years). Il report back soon when I bring it back down.

Thanks for pointing this out to me!

Red_94Formula

I adjusted the idle screw down to .71 but no further until took out the BBK 58mm throttle body and cleaned it. After cleaning I got the tps sensor down to .67 but no further. Its as clean as can be. The blades are closed but they are cocked slightly. So I compared it to a stock TB. The stock TB is almost level vs the BBK which is cocked.

I have pictures
https://docs.google.com/file/d/0B4m5...it?usp=sharing
https://docs.google.com/file/d/0B4m5...it?usp=sharing
https://docs.google.com/file/d/0B4m5...it?usp=sharing

I tested the TPS. It has a range of 0 to 5 volts.

Already this thing is idling better. Fumes dont make me cry. I have a few options go back to stock tb, do something dumb like degree / slot the sensor, or leave it as it is.

Il get a another data log soon.

I did a quick search and did not find anyone else with this problem with BBK TBs. I bought this in late 2000. So its old. Maybe I can get better fitting blades.

As you can see in one of the pictures I drilled a stupid hole in the throttle body. I dont remember why. Must have been a fad. Maybe because the stock unit had it. It can always be plugged up.

Thanks again.

bobdec

The important thing is to get air flowing thru the IAC to the idle plenum in the intake. Just make sure there are no channels between the TB openings and the IAC port on the TB and the gasket is seals the TB ports and the IAC port to intake separately. (no IAC air leaking into main TB ports). Also some OEM TBs had a smaller IAC port hole than the intake hole,fix is drilling the TB hole to match the intake hole. .71/.67 voltages are OK as long as the IAC counts are up. Bottom line is you want TPS voltage between .45 and .75, and IAC counts above 20's. The hole in blades just means you may have to close blades a bit more than normal. If you can't get the voltage/IAC balance then try plugging the hole and/or slotting the sensor to get voltage in spec. You want just a bit of air passing the TB blades and most going through the IAC. That way the PCM is controlling idle, and air is hitting the rear cylinders correctly. That also helps to fix split BLM's on modded engines. Searching on 'LT1 split BLMs' will provide info on this.

Red_94Formula

Here is a idle log run using pcmcomm. The 12th column is the IAC step. Its around 46 47 48. TPS was at .71 volts this time. Columns 16 through 19 are long and short terms. I just dont know which. The car was on enough to get into closed loop. Coolant was 182. I have my idle set to 875 and it is staying just around there (850 to 900). Which is very nice.

https://docs.google.com/file/d/0B4m5...it?usp=sharing

pcmcomm told me that the tps voltage is too high. But did not set a SES light. I think il try slotting that TPS sensor and go from there.

I did not drill into the IAC channel

Side note: When I was porting out the intake manifold ( a year ago ) I noticed one of the IAC channels into the intake runner was nearly blocked off from a casting defect. After clearing up the defect I ran water into the IAC channel and watched the water flow evenly through all eight runners.

Side note #2: Water gets past the drivers side TB blade easier then the passengers. Should I try to correct this?

Thanks Derek

Injuneer

Did it indicate DTC 21? That code sets when the TPS voltage exceeds 2.5V with the engine idling for more than 5 seconds, or the voltage exceeds 4.8V at any time. It also should turn on the SES light. When it sets, it affects the transmission operation with an automatic.

You have to test the voltage on the TPS sensor again, over the full range from closed throttle to open. The factory manual clearly states that a closed throttle reading in the range of 0.30 - 0.90V is acceptable.

Red_94Formula

Oops now I know why it set code 21. I tested the TPS with a screw driver while connected to the car. I was watching pcmcomm on the lap top while I went back and forth between 0 and 5 volts. Its just a stored code. Il be clearing that code.

Today I will add that tube to the IAC vent circuit to my TB. Therefore I can actually utilize the intakes IAC passages. As well as slot that TPS sensor to read .55 volts, when closed, as I think that was stock.

Injuneer

I've looked at hundreds of data logs, and measured a lot of actual setups. The stock, unmolested TPS closed throttle reading is typically 0.65-0.67V.

I'll repeat this one more time, and then drop it - the PCM accepts any value between 0.30 - 0.90V, and uses that at the baseline for 0% throttle position percent (TPP). This is right out of the 1994 factory manual.

Red_94Formula

Well I did not use a tube to correct the BBK TB. I filled it in with epoxy and sanded it to be flat and matched the curves of the TB. Here is a picture of this.

https://docs.google.com/file/d/0B4m5...it?usp=sharing

I did achieve 128 / 128 at idle. The throttle thinks it is 4 percent. I did slot the TPS sensor and set it to .55. Idle is a little higher in this data log. When driving it was close to 900. I also did not plug up that hole. IAC steps are staying at 48. It is also in cell 18.

https://docs.google.com/file/d/0B4m5...it?usp=sharing

The car drives very smoothly. No more violent bucking going slow in lower gears. It hardly bucks at all. Much easier to control the gas while starting off at a stand still.

I will reset the computer. If that does not return the TPS to 0 percent. Then il return the TPS to .65/.67.

Thanks a lot for your help. This car is much more street-able. I never thought this was possible.