car backfires when letting off gas

[disfoolritehere2]

Hi,

 

I noticed that whenever I let off the gas pedal after getting to speed, ie 40mph at 3500rpm, the car sputters and backfire's right around 2500 rmp and as the car sputters, I see that the tach moves rapidly back to 1000 rpm (idle). Does any one have any idea what would cause this. I have an 87 conquest. Stock turbo with OVHP and BOV (Generic one from 101propose)and 3" exhaust from 101 as well. I also have an 88 ecu and a 1gMAS installed. I have just recently replaced the Fuel filter as well. I haven't done anything else to the car. Do the injectors/clips need to be repaired or replaced?

 

Thanks,

 

Edwin

[Technology]

By back fire do you mean a real back fire where there is fire in the intake, or does it pop out the exhaust?

[JoeyLeet2]

He probably means out the exhaust and I'm pretty sure it's inevitable on these cars if you have a modded exhaust system.

 

Sometimes when my car is running rich, I wouldn't be surprised if there was some flame-age going on if i gutted my precat

[Indiana]

Too much fuel, leaky injectors, improperly set ignition timing, leaky crap chinese BOV

[disfoolritehere2]

I hear it coming from the exhaust. When I take my foot off the gas, it will backfire a couple of times and then quickly go to the idle rpm. Another thing is that when I start from a complete stop, it will stutter all the way up until I get to 2500 rpm.

 

thanks for all the help.

 

Edwin

[mikec]

If the "idle switch" (aka "nose switch") on the end of the ISC mechanism doesn't work, the ECU won't know "throttle plates are at idle" which will screw up the deceleration fuel cut: when you lift off the throttle, while the engine RPMs are well above idle, the ECU knows no fuel is necessary so it cuts the injectors off until the RPMs are near idle. Without the idle/nose switch input though this never happens.

 

Command fuel into into an engine with closed throttle plates but well-above-idle RPMs leads to a lot of misfiring: there isn't enough air to actually burn the fuel really. So you get misfires and gas going into the exhaust... the hot exhaust pipes though can make the gas burn, leading to the backfiring sounds.

 

Does your StarQuest idle at 1500 RPMs or so when the engine is stone-cold, slowly dropping to the normal 950 range as the engine begins to heat up? If not, that's more evidence the ISC assembly is screwed up.

 

mike c.

[Indiana]

Another thing is that when I start from a complete stop, it will stutter all the way up until I get to 2500 rpm.

Sounds like a dirty injector. Too much fuel and it won't run, puddles up, gets the spark plugs wet, barely runs until the rpms get high enough to try and burn it up and when it builds boost it might get a little bit better and it won't rev up quickly

[disfoolritehere2]

If the "idle switch" (aka "nose switch") on the end of the ISC mechanism doesn't work, the ECU won't know "throttle plates are at idle" which will screw up the deceleration fuel cut: when you lift off the throttle, while the engine RPMs are well above idle, the ECU knows no fuel is necessary so it cuts the injectors off until the RPMs are near idle. Without the idle/nose switch input though this never happens.

 

Command fuel into into an engine with closed throttle plates but well-above-idle RPMs leads to a lot of misfiring: there isn't enough air to actually burn the fuel really. So you get misfires and gas going into the exhaust... the hot exhaust pipes though can make the gas burn, leading to the backfiring sounds.

 

Does your StarQuest idle at 1500 RPMs or so when the engine is stone-cold, slowly dropping to the normal 950 range as the engine begins to heat up? If not, that's more evidence the ISC assembly is screwed up.

 

mike c.

 

is there a link for checking the ISC assembly?