Bad idle, barely runs, and backfires WTF???
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Registered User
Joined: Mar 2019
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From: Raleigh, North Carolina
Bad idle, barely runs, and backfires WTF???
My truck started backfiring bucking and would shut down and sometimes even cut off completely. I installed a brand new distributor, cap, rotor button, cam censor, ignition coil, fuel pump and relays. I also put a brand new PCM and a msd 6AL. It stumbles so bad it'll barely move. I can let it idle for a few seconds then nail the gas and it'll pull good for about 2 or 3 seconds then stumble again
Champion
Joined: Jan 2013
Posts: 4,737
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From: Northwest Louisiana
What shape is the catalytic converter in? I notice "cam sensor", that implies it's a Magnum - did you do the crank sensor? And ditch that MSD 6AL - there's a reason why MSD is known as "My Spark Disappeared". It's not necessary on a stock motor.
But yes, you're describing a plugged catalytic converter there.
RwP
But yes, you're describing a plugged catalytic converter there.
RwP
Thread Starter
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Registered User
Joined: Mar 2019
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From: Raleigh, North Carolina
What shape is the catalytic converter in? I notice "cam sensor", that implies it's a Magnum - did you do the crank sensor? And ditch that MSD 6AL - there's a reason why MSD is known as "My Spark Disappeared". It's not necessary on a stock motor.
But yes, you're describing a plugged catalytic converter there.
RwP
But yes, you're describing a plugged catalytic converter there.
RwP
Champion
Joined: Jan 2013
Posts: 4,737
Likes: 374
From: Northwest Louisiana
HOWEVER.
I'd check the plenum gasket, check the TPS to make sure it varies reasonably, check the MAP sensor to make sure it's sane with the real world (the ECU only can tell it's sane or not within its limits of programming; it can't tell that it's measuring off by .5 bar or so ... )
Also, check all the vacuum lines. "Which one?" ALL. They're ALL about 25 years old and rubber's rotting out at 25 years.
RwP
Record Breaker
Joined: Apr 2010
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my 92 and my 93 acted like that when I put a 94 or 95 computer on them..... I dunno why. put a 92/93 brain back on either truck and it ran ok. seems like with the 94 brain it took 2 warmups/shut down/cool/warmup before it came out of it.... never did come out of it with teh 95 brain.
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Registered User
Joined: Mar 2019
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From: Raleigh, North Carolina
Not until it warms up and goes closed loop, but yes, it can.
HOWEVER.
I'd check the plenum gasket, check the TPS to make sure it varies reasonably, check the MAP sensor to make sure it's sane with the real world (the ECU only can tell it's sane or not within its limits of programming; it can't tell that it's measuring off by .5 bar or so ... )
Also, check all the vacuum lines. "Which one?" ALL. They're ALL about 25 years old and rubber's rotting out at 25 years.
RwP
HOWEVER.
I'd check the plenum gasket, check the TPS to make sure it varies reasonably, check the MAP sensor to make sure it's sane with the real world (the ECU only can tell it's sane or not within its limits of programming; it can't tell that it's measuring off by .5 bar or so ... )
Also, check all the vacuum lines. "Which one?" ALL. They're ALL about 25 years old and rubber's rotting out at 25 years.
RwP
Champion
Joined: Jan 2013
Posts: 4,737
Likes: 374
From: Northwest Louisiana
What year is your truck? We may have the factory service manual linked in our FAQ.
But basically - measure the voltage across it while it's operating. The TPS has a minimum and a maximum, and should transition smoothly between the two ends (that can be checked with a ohmmeter, no need to have the ignition on.)
The MAP sensor can be tested with a vacuum pump (a hand operated one is fine for this), and apply 5V or 8V (depending on the year!) to the outside and make sure it's varying with the vacuum presented to it.
But I'd hit the vacuum lines first; a major vacuum leak can cause these symptoms also.
One more thing - connect a vacuum gauge (hey, they're cheap at Harbor Freight, or use the Loan-A-Tool type program at your Big Box Auto Parts Store to rent one for a day!) and see what kind of vacuum you're getting.
RwP
But basically - measure the voltage across it while it's operating. The TPS has a minimum and a maximum, and should transition smoothly between the two ends (that can be checked with a ohmmeter, no need to have the ignition on.)
The MAP sensor can be tested with a vacuum pump (a hand operated one is fine for this), and apply 5V or 8V (depending on the year!) to the outside and make sure it's varying with the vacuum presented to it.
But I'd hit the vacuum lines first; a major vacuum leak can cause these symptoms also.
One more thing - connect a vacuum gauge (hey, they're cheap at Harbor Freight, or use the Loan-A-Tool type program at your Big Box Auto Parts Store to rent one for a day!) and see what kind of vacuum you're getting.
RwP
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Thread Starter
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Registered User
Joined: Mar 2019
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From: Raleigh, North Carolina
What year is your truck? We may have the factory service manual linked in our FAQ.
But basically - measure the voltage across it while it's operating. The TPS has a minimum and a maximum, and should transition smoothly between the two ends (that can be checked with a ohmmeter, no need to have the ignition on.)
The MAP sensor can be tested with a vacuum pump (a hand operated one is fine for this), and apply 5V or 8V (depending on the year!) to the outside and make sure it's varying with the vacuum presented to it.
But I'd hit the vacuum lines first; a major vacuum leak can cause these symptoms also.
One more thing - connect a vacuum gauge (hey, they're cheap at Harbor Freight, or use the Loan-A-Tool type program at your Big Box Auto Parts Store to rent one for a day!) and see what kind of vacuum you're getting.
RwP
But basically - measure the voltage across it while it's operating. The TPS has a minimum and a maximum, and should transition smoothly between the two ends (that can be checked with a ohmmeter, no need to have the ignition on.)
The MAP sensor can be tested with a vacuum pump (a hand operated one is fine for this), and apply 5V or 8V (depending on the year!) to the outside and make sure it's varying with the vacuum presented to it.
But I'd hit the vacuum lines first; a major vacuum leak can cause these symptoms also.
One more thing - connect a vacuum gauge (hey, they're cheap at Harbor Freight, or use the Loan-A-Tool type program at your Big Box Auto Parts Store to rent one for a day!) and see what kind of vacuum you're getting.
RwP
