head/cam swap idle fine but backfires through intake
03-18-2016, 11:03 AM
#1
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head/cam swap idle fine but backfires through intake ok so I have a problem that for some reason is stumping me. I have a 2000 ls1 (5.7) trans am and I just replaced the heads and cam in it as well as 42lbs injectors. I cranked it over and it would start and run if I applied just a hair of pressure on the gas pedal.so I took it to get dyno tuned the guy put a base tune on it to get it to stay running before we pulled it up on the dyno. after the base tune was installed and it ran for a couple min. we shut it down and went to restart the car when it back fired through the intake after multiple attempts of cranking.so I took the car back home and took the timing cover off to inspect the chain and dot to dot alignment. with my #1 cylinder on comp. stroke the crank dot is at 12 o clock and the cam dot is at the 12 o clock as well. is this correct for timing or do I need to line the dots back up while the crank stays where it is at?(ie cam @ 6 o clock and crank @ 12 o clock) please help cause im stumped.
03-18-2016, 01:15 PM
#2
ok so I have a problem that for some reason is stumping me. I have a 2000 ls1 (5.7) trans am and I just replaced the heads and cam in it as well as 42lbs injectors. I cranked it over and it would start and run if I applied just a hair of pressure on the gas pedal.so I took it to get dyno tuned the guy put a base tune on it to get it to stay running before we pulled it up on the dyno. after the base tune was installed and it ran for a couple min. we shut it down and went to restart the car when it back fired through the intake after multiple attempts of cranking.so I took the car back home and took the timing cover off to inspect the chain and dot to dot alignment. with my #1 cylinder on comp. stroke the crank dot is at 12 o clock and the cam dot is at the 12 o clock as well. is this correct for timing or do I need to line the dots back up while the crank stays where it is at?(ie cam @ 6 o clock and crank @ 12 o clock) please help cause im stumped.
Just a suggestion
03-18-2016, 01:17 PM
#3
#1 cyl at TDC, crank dot at 12, cam Dot at 6. They should point at each other. Keep in mind, though, crank turns 2x the speed of the cam, so it will be opposite once per revolution, aligned once per revolution. Make sense?
Did you check your pushrod length? Valves hanging open just a little due to a long pushrod can cause this.
Did you check your pushrod length? Valves hanging open just a little due to a long pushrod can cause this.
03-18-2016, 02:59 PM
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One thing about putting the cam dot at 6 is it's easier to see if you're a tooth off on the cam sprocket. It's very easy to be a tooth off with the cam dot at 12. I think you're off a tooth
03-19-2016, 12:17 PM
#7
Did you actually make a dyno pull or just idled it? When not under load, an engine with all kinda of issues can seem like it is running ok. Clean your MAF, check your plugs, check your coil wiring, IAC, TPS. Clear the computer(disconnect battery for an hour or so).
What cam and lifters did you install? Are you sure your pushrods are proper length? How long has the car been sitting? Do we know the gas hasn't gone bad?
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03-19-2016, 08:19 PM
#11
Oh I see what you meant. I thought you were talking about the two harnesses that ran down each coil pack bracket lol. (As if you were saying that if you switched the coil packs/brackets around, it would be firing wrong.
The engine harness on the trucks doesn't have a connector to separate the main harness from the section of the harness that runs to each coil pack plug. The only way it is possible to get it backwards is to flip the harness upside down and that would throw off the wiring for the knock sensors.
The engine harness on the trucks doesn't have a connector to separate the main harness from the section of the harness that runs to each coil pack plug. The only way it is possible to get it backwards is to flip the harness upside down and that would throw off the wiring for the knock sensors.
03-20-2016, 11:56 AM
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He has the cam roughly right. On the comp-firing stroke they are both straight up. Top of the exhaust stroke they're dot to dot. It wouldn't run at all 180* out. I would rotate to see if it's the dot to dot though to check and just as importantly degree the cam. Sometimes they aren't right. With different injectors fueling may be a problem too.
03-30-2016, 11:57 AM
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ok here is the update. we got it going after I took it home to confirm the timing and it was fine so I replaced my injectors back out with me stock ones and took it back and haven't had any issues with it backfiring yet which could have been the tune. IMO
the car dyno'd at 432 hp and 386 tq at rear wheels. now after doing some city driving it through a code po106 a couple times and po101 once. and also while sitting at the light I noticed that the voltage gauge jumping below normal and the car will stall out if Im sitting there long enough w/out any pressure on the throttle. ( all this happens just before the car will throw the code. it wont throw a code if it is all hwy driven and will hold idle no prob if not in gear.
the car dyno'd at 432 hp and 386 tq at rear wheels. now after doing some city driving it through a code po106 a couple times and po101 once. and also while sitting at the light I noticed that the voltage gauge jumping below normal and the car will stall out if Im sitting there long enough w/out any pressure on the throttle. ( all this happens just before the car will throw the code. it wont throw a code if it is all hwy driven and will hold idle no prob if not in gear.
03-30-2016, 10:55 PM
#15
ok here is the update. we got it going after I took it home to confirm the timing and it was fine so I replaced my injectors back out with me stock ones and took it back and haven't had any issues with it backfiring yet which could have been the tune. IMO
the car dyno'd at 432 hp and 386 tq at rear wheels. now after doing some city driving it through a code po106 a couple times and po101 once. and also while sitting at the light I noticed that the voltage gauge jumping below normal and the car will stall out if Im sitting there long enough w/out any pressure on the throttle. ( all this happens just before the car will throw the code. it wont throw a code if it is all hwy driven and will hold idle no prob if not in gear.
the car dyno'd at 432 hp and 386 tq at rear wheels. now after doing some city driving it through a code po106 a couple times and po101 once. and also while sitting at the light I noticed that the voltage gauge jumping below normal and the car will stall out if Im sitting there long enough w/out any pressure on the throttle. ( all this happens just before the car will throw the code. it wont throw a code if it is all hwy driven and will hold idle no prob if not in gear.
