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Thread: back fire:

  1. #1
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    back fire:

    I just bought a 1997 trans am with a lt1 motor, when I go to start it, it back fires and there is pressure on the radiator cap that when I open the cap the antifreeze comes out and there's no water or antifreeze in the oil. Does anyone know what would cause this? thanks,starr0630

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    Spaz is My Mentor SMWS6TA's Avatar
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    yes I did thanks, I tried to post it but don't know if it did.

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    Spaz is My Mentor SMWS6TA's Avatar
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    I'll just post it here....



    I don't have too much experience with the LT1's. We do have a section that if you post this in, a few members will be able to help.


    Now when you say back pressure is this after it's been running or right away?




    Backfires are the results of a few things. Bad spark, extra fuel, MAF issues. Sitting for long periods of time.

    Bad Spark - You have the dreaded Opti-spark system. Big, big problem with the design and location. In fact it's why a lot of ppl hate the LT1's. If it get's wet (usually from heat cycles cracking the case) it starts effecting the motor.

    MAF - if dirty this will give the ECM improper readings.

    Extra Fuel - this can be caused by a lot of things. Bad O2's, leaky fuel injectors, etc..
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  5. #5
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    when I try to start it, it backfires and will not start.

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    Did you try to pull codes from the history ?
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    back fire:

    sounds like plug wires are crossed or opti is shot. how many miles? Has it ever been changed?

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    I agree -- check the plug wire and ensure that they are going where they should.

    Was the car running when you bought it? Any work done to it recently by the seller?

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    The person I bought it from said he was driving it then it just started to back fire, but I know the battery is no good because he would have to put a charger on to try to start it. would a bad battery do this? and he ran it out of gas because we had to go get a couple of gallons of gas. Also what is the right thermostat for that LT1?
    Last edited by starr0630; 01-28-2015 at 07:49 AM.

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    Best bet might be to scan it or codes. You have a combination of probable issues and the scanner may narrow it down a bit. Could be a faulty crank sensor or something. The Opti is always a prime suspect. Since the car was running and it was a sudden issue, the plug wires are probably in the correct order, although it can't hurt to check them for both order and connections.

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    Thank you I will have this done asap.

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    Let us know what you find.

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    Quote Originally Posted by starr0630 View Post
    The person I bought it from said he was driving it then it just started to back fire, but I know the battery is no good because he would have to put a charger on to try to start it. would a bad battery do this? and he ran it out of gas because we had to go get a couple of gallons of gas. Also what is the right thermostat for that LT1?

    This tells me depending on how long the fuel has sat you could have bad gas or condensation. Both will cause miss fires. Simple solution would be to pour 5 gals of good gas, high octane and HEET or some similar product let sit for IIRC 15-30 mins (read directions) and run it at idle for 15-20 mins. This should clear up any condensation issues and if bad gas the fresh will dilute it . Also a change in the fuel filter couldn't hurt.


    Just how long was it sitting?


    Getting a fuel pressure gauge and check pressure reading wouldn't hurt also.
    Last edited by SMWS6TA; 01-28-2015 at 09:16 AM.

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    Quote Originally Posted by SMWS6TA View Post
    This tells me depending on how long the fuel has sat you could have bad gas or condensation. Both will cause miss fires. Simple solution would be to pour 5 gals of good gas, high octane and HEET or some similar product let sit for IIRC 15-30 mins (read directions) and run it at idle for 15-20 mins. This should clear up any condensation issues and if bad gas the fresh will dilute it . Also a change in the fuel filter couldn't hurt.


    Just how long was it sitting?


    Getting a fuel pressure gauge and check pressure reading wouldn't hurt also.

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    Any updates?

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    I got it to start, but it is not hitting on all cylinders I will check wires and etc. thanks

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    Makes it a bit easier to diagnose with it running. Scan it yet?

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    I'm betting it's opti.....

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    Quote Originally Posted by SMWS6TA View Post
    I'm betting it's opti.....
    I've been staying out of this thread because it looks like you and Jeff have him pointed in the right direction, but as soon as he said it fired up and isnt hitting on all cylinders first thing I thought of was opti spark. I've been there and done that.
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    Someone once told me that with LT1's - "it's always Opti" Out of gas = check the opti, Dead Battery = check the Opti, Raining outside = it's the opti's fault.

    I saw him changed his Opti 6x's in a yr period. He ended up doing a LS1 conversion to quit dealing with the opti and has been happy ever since.

    I know there's a 24x conversion but in the end the cost of doing it and getting it right, a LS k member and LSx swap could be done for a little more.

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