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Old 01-03-2002, 12:16 AM   #1
john73bb
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Default What causes backfire through throttle body????

I am trying to tune my fuel injection, and sometimes I get fire back through the throttle body. I have checked compression to make sure a valve isn't stuck open, I have separated the spark plug wires to make sure one isn't cross firing another at the wrong time, and I have checked for carbon tracks in the distributor -- none there. So, I assume it must be due to the mixture being wrong. What can cause a backfire through the carburetor or throttle body? Rich or lean mixture?

Thanks,

John.
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Old 01-03-2002, 12:18 AM   #2
lars
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Default Re: What causes backfire through throttle body???? (john73bb)

Lean mixture will backfire through the induction system.
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Old 01-03-2002, 08:28 AM   #3
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Default Re: What causes backfire through throttle body???? (john73bb)

I don't know if this will apply to you, but after I installed my new engine with the stock exhaust that was on the car, it would backfire up thru the carb. I replaced the entire exhaust with 2.5" and Flowmaster mufflers and the problem went away. My car is a smallblock, but I don't know if the other exhaust was too restrictive for the new motor :confused: .
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Old 01-03-2002, 08:48 AM   #4
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Default Re: What causes backfire through throttle body???? (john73bb)

Of course the timing could be off, check it with the plug disconnected...set for 8 degrees....wonder if a timing chain slipped....wonder what the exact effect would be on a computer car, which has a lot of advance at idle cranked in....

GENE
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Old 01-03-2002, 10:03 AM   #5
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Default Re: What causes backfire through throttle body???? (john73bb)

Quote:
Holley Pro-jection 4-barrel throttle body, GM 1227747 ECU
:cool:

I don't know much about the ProJection but I have been working with the 7747

When is it backfiring through the intake? Off idle, part throttle, wot. Has it been backfiring since you first installed the Projection or did it just start? Is it backfiring when in open loop or closed loop?

Check TPS voltage, fuel pump pressure, 02 sensor.

Dan
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Old 01-03-2002, 11:34 AM   #6
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Default Re: What causes backfire through throttle body???? (john73bb)

I'm not sure what is causing it, but my vette is doing it too, but I haven't found out why :confused:
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Old 01-03-2002, 12:50 PM   #7
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Default Re: What causes backfire through throttle body???? (Dan Plett)

Dan,

Are you using the 7747 in your vette or some other car? Are you using the stock throttle body?

As mentioned in sig, I am using only the throttle body from the projection. I am controlling it with the 1227747 GM ecu. It doesn't bacfire with the throttle blades closed. If I open them even a little it will sometimes backfire. It will backfire at part throttle too. If I open it all the way (clear flood) it will send a fireball that will singe my eyebrows :eek: . I believe it is because of a lean condition. That would make sense I guess. What actually ignites the fuel in the intake manifold?

I vary the fuel pump pressure up and down all the time while tuning to determine if I need to change the constants in the prom. I think the O2 sensor is fine, it varies as it should. I have moved the TPS around some, I know it is working. Apparently the exact setting of the TPS is not critical -- the ECM only looks at changes to the TPS to determine what to do with fuel mixture, pump shot, etc.


[Modified by john73bb, 7:52 AM 1/3/2002]
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Old 01-03-2002, 09:05 PM   #8
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Default Re: What causes backfire through throttle body???? (john73bb)

John
I'm using the 7747 ecu upgrade on a 84 Vette with stock TBs bored to 2", ported intake etc.

What you have going sounds like a interesting project. The obvious problems are ignition and vacuum leaks. Sounds like you already covered them.

The next suggestion would be to check the sensors for proper operation. Check the TPS, MAP, CTS, O2.

The ecm likes a closed throttle TPS voltage of .525 + or - .075volts and WOT voltage around 5 volts. The MAP voltage should be around 5 volts WOT to 1 volt at idle. Some other problem might be covering up incorrect TPS voltages. If everything checks out then it might just need prom tuning.

An excellent scantool software is WinALDL. This started out with the 7747 ecu and TunerCat editor in mind. Also there is a very knowlegdable group of individuals on http://www.thirdgen.org that probably has done the Projection/7747.
http://www.winaldl.webhop.net
http://www.tunercat.com/

Is your Projection a 4 injector or 2 injector system? What vacuum do you have at idle?

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Old 01-03-2002, 10:25 PM   #9
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Default Re: What causes backfire through throttle body???? (Dan Plett)

John, it sounds to me like the timing is way off. Retarded spark timing can cause backfiring through the intake with occasional fireballs :) Does it run decent at all?

Matt
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Old 01-04-2002, 04:29 AM   #10
john73bb
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Default Re: What causes backfire through throttle body???? (Dan Plett)

Matt, the timing is OK, I have checked it. I think it is mixture related.

Dan,

I will check the TPS, and set it properly. The MAP is functioning correctly, and the ALDL software I have shows the MAP, the % of TPS, and several other things including O2 reading. If it is backfiring, I just richen it up a whole bunch and it seems to get better.

I'll keep tuning the PROM until it works I guess. It's gonna be a while before spring (and dry weather) comes anyway.

Thanks for the help.

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Old 01-04-2002, 10:35 AM   #11
mrvette
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Default Re: What causes backfire through throttle body???? (john73bb)

You know, just to be sure, maybe you should just experiment a bit, and advance that timing a bit, say move the dist about 1/4 inch in rotation...and try it again....you could have a slipped timing ring on the damper......don't ask me how I know this.....;-)))

GENE
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Old 01-04-2002, 11:26 AM   #12
Ken73
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Default Re: What causes backfire through throttle body???? (john73bb)

John, Dan and I have been working together with the same upgrade - the '7747 on a CrossFire. (2bbl TBI.) I can tell you that running four injectors on the '7747 is really a :nono: - the injectors start to fade out at 3kRPM. (There have been numerous tests to support this.) However, you can build an external injector driver to drive all four injectors without a problem, for very little money. If you've gone so far as to hook up the '7747, this isn't anything beyond your scope.

As Dan said, TunerCat and WinALDL are by far the best tools around. I actually designed WinALDL, and one of our friends (Jonas) wrote it for me. It's made tuning the fuel tables a snap - instead of months (or longer) tuning the VE tables, I did it in 9 chips, (2 weeks!) and that includes changing some other things first.

A few questions though; is the 4bbl TBI you have from Holley the 1:1 linkage, or is it the newer unit with the progressive linkage? Are you running a 4-speed or an automatic? What fuel pressure are you running? What BCC (broadcast code, the four-letter chip identifier) are you using to start off with? Most importantly - are you running an HEI with the vacuum/mechanical advance locked out, and if so - is the ECM controlling it?

Since it's backfiring through the TBI, I'd suspect the distributor of course, which is why I asked about your distributor setup. The ECM (normally) controls all advance functions, but you have to have the distributor set at a certain point (usually around 6-10 degrees) for the base, the computer corrects past that. They have you set it at somewhere around 6-10 degrees in case the computer fails, you can "limp home" with enough advance.

Ken
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Old 01-04-2002, 02:25 PM   #13
john73bb
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Default Re: What causes backfire through throttle body???? (Ken73)

Ken, you have mail. This discussion could get pretty lengthy and is probably not of interest to most of the forum.

Thanks to everyone for all the help.
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Old 01-04-2002, 02:25 PM
 
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