5.7 hemi rough idle and low backfire out exhaust any ideas

-
1967GTXhemi

1967GTXhemi

Active Member
Joined
Oct 9, 2016
Messages
35
Reaction score
0
Location
louisville ,ky
my 5.7 hemi has been sitting for a month and when i started it it started fine but after a few min of idling it began to idle rough and i could hear a few small pops from the exhaust. i recently replaced the o2 sensors and it ran fine when i covered it up could it be the same issue should i replace the sensors with chrysler brand sensors. also i am using a corvette fuel regulator and it has plenty of pressure any help is appreciated
 
Usually if it pops out the exhaust it's to rich. check the fuel pressure reg, pull the vacuum hose off and see if there is fuel in the hose.
 
brian6pac i thought that might be the issue but i dont find any fuel in the vacuum hose i am using the corvette regulator and it is not adjustable i changed both o2 sensors and it seemed to solve the problem but it has happened once before so i am thinking you are correct do yo have any idea where i could locate an adjustable fuel pressure regulator what brand and such
 
also if the o2's are not getting hot enough that will cause them not to work properly. To rich!
if you get it leaned out the temp will come up, are you running cats?
 
no i am not running cats i am thinking about changing to an aeromotive adjustable fuel pressure gauge and see what i can do with that i dont know how to lean it out i have never done any tuning on the ecm it was reflashed by chris at hotwire when i did the initial install
 
i have a fuel pressure gauge at the fuel rail and it is reading between 57 and 58psi when engine is running and i have read that that is the proper pressure for a hemi
 
pressure is o.k. are you running a maf, were are your O2's mounted how far from the engine
 
yes i do have a mass airflow sensor and the o2 sensors are located just in the front end of the header collectors
 
One cylinder on regular misfire, or random misfires. Hot or cold the same?
I bought a used Charger where No 2 exhaust was not spinning and seems to have the seat and face carbon caked.
It's been spinning now for a few months and is slowly getting better.
When I first got it, it would always start and run and idle smoothly, but after the first time past 1600rpm the valve wouldn't seat right.
Thump, thump, thump out the pass side exhaust.
Shut off and restart and perfect again.
Now it's funny while cold, but at full hot almost perfect. I'm interested in hearing more about your situation.
 
no the plugs look good i guess i should have provided some history when i originally built the car two years ago the motor was a new crate motor and until just a couple months ago the engine ran great no issues then this rough idle thing started and when i ran a scan on the car it showed the o2 sensors were bad i replaced them with sftermarket ones and that seemed to solve the problem but today the idling and popping returned so i changed the sensors with nkg sensors and again the idling was fine and the backfire was gone but i just feel something else is wrong
 
are you running the heaters in the O2's, no blown fuses? I would check the heater circuit, If there not working and the O2's don't get hot enough, short trips around town exc. what was the trouble code for the O2's.
 
well you got me there im not sure about heaters in the o2's i did not know there were heaters for those the harness is from hotwire so i assume they may be there somewhere i checked fuses and do see any bad ones in the fuse block i think the code was po139 bank one sensor 2 or something like that the car had not been run for about two months and this was just after startup
 
So if you are replacing bank 1 sensor 2? you don't have a sensor 2 if your not running cats.
You only have 2 O2's correct, 1 on each side? you should have a 4 wire connector on each.
1 power for heater, 1 ground for heater, 1 sensor in, 1 sensor out. You could have a vacuum leak and the O2's see the extra oxygen and ritchen up the mixture to full rich.
 
hmmm that is an interesting idea possibly the vacuum leak just occurred and thats why the problem just popped up i will have to run thru all the vacuum hoses to see if one is cracked or come off but let me ask you this if there was a vacuum leak and i changed the sensors and the problem no longer was there could there still be a vacuum leak?
 
if im right there are only two vacuum ports on the 5.7 one in the rear of the manifold and one in the top front any others that i should look for?
 
The easiest way to find a vacuum leak is to listen, if you have a stethoscope pull the end off and put on a long vacuum hose and start the engine, put it by the throttle plate or a open vacuum source so you can hear the difference then go all around the intake and see if you can hear anything.
 
When we fired up our 5.7 hemi it had a pop/backfire ,found on of the injectors was stuck closed. The modern gasoline with the ethanol blends are hard on the injectors when they set . You can use your stethoscope on those to make sure they all sound the same running. If you have headers you can feel each tube after it runs for a bit to make sure they are all contributing equally.
 
well i have listened around all the manifold and vacuum and the injectors and dont hear any odd noises and i checked all the fuses and they are ok what confuses me is when this happens if i remove and replace both o2 sensors the backfire stops and the engine idles smoothly but it has happened before and im wondering why it would seem if there is a vacuum leak it would continue until the leak is stopped im kinda a rookie at this type of tuning so im learning as i go hopefully i can solve it.
 
sounds like they weren't in there long enough to get dirty
 
You must log in or register to reply here.
-

Back
Top