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Discussion Starter #1
Hi guys,

Still finding my way around my 69 396 gen 2 LT1.

Trying to burp the cooling system today, left it running and when running at idle I turned on the air conditioning and put on hot and the revs drop and it dies completely. Last time I tried this, sounded like a screech of a belt as well (which I replaced last weekend).

Any ideas?

The car was originally at 500ft above sea level and I live at 6,000ft above sea level. Should I be getting it tuned for this altitude or should that not effect it?

Cheers
Nathan
 

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What carb are you running? Yes you should definitely jet for our elevation if it hasn't been already. You may need to change jet sizes, but try playing with the air screw first. But turning on the A/C shouldn't kill the engine!

Does the belt screech on start up? The belt may need an adjustment.
 

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OP

Is this a TB/FI Gen2 LT1 using a PCM to control engine or has it been converted to carb & distributor?

If the former it will "self adjust" via the PCM. Also if computer motor your IAC in the TB may be sticking causing the motor to stall when AC is turned on unless the clutch of AC is binding. You will need scan software capable of read IAC counts to see if it is behaving as it should. IAC counts want to be 32 (30-35) at idle, engine at full operating temp

If this is a computer controlled Gen2 LT1 and the motor has a bigger than stock cam and injectors it will need a tune specific for that. There are a few tuners who do "mail order" tunes
 

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It's FI correct? That's a substantial elevation change but the FI should adjust. Is it a factory computer or a custom tune? Tune maybe needs to be adjusted if not factory.

If the idle was marginal I could see AC killing it when it comes on, but it a seems slightly odd especially a FI engine. In the end, probably a tune issue, timing or air/fuel.
 

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Discussion Starter #6
It is computer controlled Gen2 with extra upgrades so sounds like I will need to take it in.


OP

Is this a TB/FI Gen2 LT1 using a PCM to control engine or has it been converted to carb & distributor?

If the former it will "self adjust" via the PCM. Also if computer motor your IAC in the TB may be sticking causing the motor to stall when AC is turned on unless the clutch of AC is binding. You will need scan software capable of read IAC counts to see if it is behaving as it should. IAC counts want to be 32 (30-35) at idle, engine at full operating temp

If this is a computer controlled Gen2 LT1 and the motor has a bigger than stock cam and injectors it will need a tune specific for that. There are a few tuners who do "mail order" tunes
 

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Hi guys,

Last time I tried this, sounded like a screech of a belt as well
Nathan,

Before you get real carried away with the EFI system . . . . are you absolutely sure that the AC compressor isn't starting to fail ? When the one in my 99 Tahoe decided to give up it was doing the same thing and then got to where it actually locked up solid. Then I realized what was going on.

Is the car an automatic trans ? If it is and the ECM adjusts the idle properly when you put it in gear then the problem is most likely not the "tune"
 

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What Bill said.

Especially giventhis "...Last time I tried this, sounded like a screech of a belt as well..."

If it is the proper belt, and if adjusted properly, the only thing that would make it screech is a bad compressor.
 

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Discussion Starter #9
It's a manual.

The belt did screech once (pretty sure it was the belt) so I turned it off immediately. The belt did break previously and I replaced it without 100% finding the cause of the break, other than the alternator had some slightly loose and was moving around under load. I guess the break that could have been caused by the compressor. When I had the belt off if did turn the ac compressor fine by hand fine but is that because the aircon wasn't on?

Any advice on how best to test? It's pretty new but I guess that doesn't really count for much. The front of the compressor rotates without moving the belt drive. Is this normal? It's a Sanden SD505.

I got in touch with the previous owner and he said that he'd disconnected the O2 sensor so I assume this could cause it to run crappy at idle especially given the altitude change?

Sorry for stupid questions - I'm learning this from the ground up.

Cheers!

Nathan,

Before you get real carried away with the EFI system . . . . are you absolutely sure that the AC compressor isn't starting to fail ? When the one in my 99 Tahoe decided to give up it was doing the same thing and then got to where it actually locked up solid. Then I realized what was going on.

Is the car an automatic trans ? If it is and the ECM adjusts the idle properly when you put it in gear then the problem is most likely not the "tune"
 

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So this is a computer controlled Gen 2 LT1 motor, right?

If your AC compressor is locking up for whatever reason than yes that will drag on the engine, thus lowering idle, and can break the serpentine belt

Are you running the donor car the motor came from accessory bracket that holds AC, ALT & PS pump?

Are there 2 additional brackets (bars) that attach to the alternator (one front one rear)?
if not and you have the stock donor car accessory bracket it can flex & crack

If the motor has a aftermarket cam it will need a specific (custom) tune for it to run right, especially idle. Do not take the car to some shop or dealer to tune the PCM if it does have a aftermarket cam. A specialty tuner needs to program the PCM.

PO removing 02's would keep the motor in Open Loop mode, not good.

Take the belt off the car and drive it to see if it still exhibits this low idle/stall when turning on AC. Since AC would be not driven because the belt is off the idle should not be affected. If it is or otherwise stalls because idle dropped than the IAC (idle air control) valve in bottom of throttle body is bad or sticking. You can clean it and the orifice it goes in with throttle body spray cleaner. Do not push or pull on the IAC printle, just clean it carefully or replace it.

Also if you have other than a stock 48mm TB and the car is not tuned for the larger aftermarket TB you will have idle issues
 

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Any advice on how best to test? It's pretty new but I guess that doesn't really count for much.
Cheers!
You might be able to use a clamp on amp-meter and measure current draw bu I am not sure where you would find the spec for what it should be.
 
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