Frustrated with rich/stumble/backfire issue
Thread Starter
On The Tree
Joined: Feb 2011
Posts: 118
Likes: 4
Frustrated with rich/stumble/backfire issue I have a 67 Chevelle with a 5.7 ls1 in it. It has 243 heads, small cam, and a tune and has been running great for years. I have been working and searching with no luck. Recently noticed the car had been running rich and started running rough on acceleration (just not smooth). It is not showing any codes. I started going through what I thought could be the problem and nothing has fixed it. I've done the following:
new air filter
cleared maf
cleaned throttle body
new o2 sensors (found the old ones were very charred)
gas is good
just put b12 fuel system cleaner in tank
new plugs
I just changed the plugs and the car is now backfiring which it has never done before. I swapped the coil packs from side to side in case I mixed them up and it didn't change anything. I am thinking of getting new plug wires. I am now thinking maybe a vacuum leak or bad maf?
Any ideas? I keep making it worse.
Thank you!
new air filter
cleared maf
cleaned throttle body
new o2 sensors (found the old ones were very charred)
gas is good
just put b12 fuel system cleaner in tank
new plugs
I just changed the plugs and the car is now backfiring which it has never done before. I swapped the coil packs from side to side in case I mixed them up and it didn't change anything. I am thinking of getting new plug wires. I am now thinking maybe a vacuum leak or bad maf?
Any ideas? I keep making it worse.
Thank you!
TECH Regular
Joined: Dec 2022
Posts: 444
Likes: 158
Yes that obd scanner will do.
If there is a vacuum leak your fuel trims will be positive(adding fuel to account for the unmetered air)
Should also be able to monitor cylinder misfire counters to see which cylinders are missing out
If there is a vacuum leak your fuel trims will be positive(adding fuel to account for the unmetered air)
Should also be able to monitor cylinder misfire counters to see which cylinders are missing out
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Verdad Gallardo Thread Starter
On The Tree
Joined: Feb 2011
Posts: 118
Likes: 4
Pulled and regapped all the plugs this am. She fired up nice and idled for 15 minutes without a backfire or pop.
afms were all negative.
took it for a test drive and within 1/4 she was backfiring again
im looking for cylinder misfire in the apps but I’m ot sure where to find them yet
afms were all negative.
took it for a test drive and within 1/4 she was backfiring again
im looking for cylinder misfire in the apps but I’m ot sure where to find them yet
TECH Fanatic
Joined: May 2016
Posts: 1,292
Likes: 436
From: St. Louis
I like the CRC maf cleaner also It's really good stuff I use it on my cars and my kids RC electric motors too it shouldn't hurt anything
I think we're all interested to see what plugs look like and what some scan tool diagnostic data would see?
I think we're all interested to see what plugs look like and what some scan tool diagnostic data would see?
Thread Starter
On The Tree
Joined: Feb 2011
Posts: 118
Likes: 4
Plugs all looked fine. The #2 plug was a little tough to get in and out and I'm wondering if it wasn't fully seated and needs to have a thread chase etc. I'm going to mess with it again tonight when I get a minute to see if that's the issue
TECH Fanatic
Joined: May 2016
Posts: 1,292
Likes: 436
From: St. Louis
TECH Fanatic
Joined: May 2016
Posts: 1,292
Likes: 436
From: St. Louis
I really wish I could help you more But you say It's stumbling and running rough But not setting codes and it's modified with a camshaft swapped in a vehicle so must be tuned to some degree? You need to look at it with scan software to see how it's misbehaving So you can see what's happening with the timing and the fuel during the stumble Even if it's not hotter than usual, humidity can be a bother sometimes too
Depending on what could be done in the tune, certain things could be turned off and could make it harder for you to diagnose the problem and It's also possible The tune wasn't close enough to begin with
Depending on what could be done in the tune, certain things could be turned off and could make it harder for you to diagnose the problem and It's also possible The tune wasn't close enough to begin with
get a spray bottle. put water in it. crank engine. spray the exhaust manifolds right by the head for each exhaust port. or if you have headers, spray each tube by the exhaust ports. the water should evaporate very quickly. If it evaporates noticeably slower on one, then theres your problem cylinder. move the coil to another cylinder and test again. If problem moves..its the coil. if not, then dig deeper. May have a broke valve spring or collapsed lifter etc.