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engine backfires when i let off the gas

I love my Chevy Chevette!
Posts: 7
Joined: 04/11
Posted: 02/09/13 03:07 PM

engine backfires when i let off the gas. and i cant get it to idle at spec rpm 450 please help?  

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My first time was in a Chevy
Posts: 181
Joined: 04/12
Posted: 02/09/13 04:58 PM

i am taking that you have a C4 corvette of some kind...

might want to check out my post on scan tools for the C4..  

backfiring in the exhaust system is kinda strange on a C4..

that could be caused by a bad diverter valve or a leaking check valve in the air injection system..   even exhaust leaks...

what happens is when you take your foot off the gas.. the manifold vacuum goes up.. and the high vacuum draws extra fuel from the injector tips .. this and air from one of the leaks allows combustion in the exhaust system...

you could also have an ignition issue...  where the coil is leaking high voltage from the sides..   or the ground strap that goes into the cap before the coil got left out..  oops.. that will cause a backfire sound almost as loud as a shot gun going off under the hood..  as the high voltage builds up until it can flash over like a lightining bolt..

bad spark plug wires also..  if the ends are pulled loose... if the wire has too much resistance..  even cross firing between the wires..

the idle part i can also help with... i am taking its a TPI L98 motor..  not a crossfire motor..  

first... engine off.. grab the throttle lever.  open the throttle slightly... see if the lever and shaft wiggle around when its partially open...

a few things have to be perfect.. including the throttle shaft in the throttle body housing.. the bore wears because the spring and the cables are all pulling in one direction..  what happens is the shaft can then come closed against the stop but at a slightly different angle each time.. the Throttle position sensor at the other end of the shaft  reads angle and converts it to voltage..  this is critical as the TPS voltage with the throttle closed lets the computer revert to IDLE programming..

with a scan tool... or if you cannot find one.. with a digital volt meter... look to see if the minimum TPS voltage is exactly 0.53 volts..   work the pedal several times.. engine off and engine running.. if it  comes back to a slightly different voltage each time.. you have a worn throttle body.. there are rebuilders who can remove the shaft and rebush the housing. warning.. don't try this your self.. the tiny screws will break off..

next.. after you verify the TPS and the throttle shaft bore is not worn...

fire it up with the scan tool plugged in... get into the live data stream... with the engine at operating temp.. you will need to see these values..
ECT..  190 to 230F  coolant temp..
TPS... 0.053 volts..
IAC 20 to 50 counts..   IDLE AIR COUNT of the idle air control valve..

post what values you get...  

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V-6 Camaros rule!
Posts: 83
Joined: 01/12
Posted: 02/17/13 08:09 AM

Not knowing exactly what type of engine back-fire your having, is it in the exhaust or the intake?
  The EGR valve, and/or the exhaust heat riser, not functioning properly could do this.
  Also sometime as the throttle is being closed its not a slow action. If its sharp closing the air fuel back-fire could be taking place in the exhaust system.
  Run the engine at night or in a dark garage. Please Leave the doors cracked open, to ventilate the area. Set the E-brake. Block a wheel.
  Now with the engine running try to duplicate when and where the back-fire occurs. Rev the engine and watch the exhaust especially. Do you see any flames, or do you hear the back-fire. (It may only back-fire with a load coming off the engine).
  Sometime you can have a blown exhaust manifold gasket and it will suck RAW AIR (making the air/fuel mixture lean) then you hear the back-fire....Anywhere in the exhaust system you can watch could be the problem area....Try to tighten the exhaust manifold bolts.


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