I have a 2001 Monte Carlo ss V6 3800 it has developed a backfire on cold start up.Usually only once on 1st start up. I turn key off and let everything cycle then it usually starts fine.​​​​​​​About a year ago I had the fuel regulator problem where it backfired and blew the plenum apart.​​​​​​​Replace everything that was damaged and has been fine till now.​​​​​​​Any ideas what to test or where to start ???​​​​​​​Thank you ​​​​​​​David​​​​​​​

 

Have you done any other maintenance lately- replaced any sensors / plugs/ etc? Does the car seem to run ok otherwise? When you get the backfire, is there anything odd happening- ie is it seeming to crank for longer than usual / not sparking? Do you have a scan tool?

Several potential causes, but Id say there are two general categories:
-Ignition issues. Something causing the plug to not fire when it should.
-Fuel load up. Something causing extreme amounts of excess fuel. Minor issues don't usually cause it as they still need an ignition source (unless combined with issue 1 of course), but you can get to a point where there's so much fuel that the burn slows substantially or a full cylinder doesn't light off at all.

One other rare possibility- what kind of mileage on it? Its not impossible to jump timing on the chain if it's way up there.

 

When you had the FPR problem, did you replace that?

 

It seems to crank fine when it backfires I stop cranking it.
i have a old brick scan tool.
140,000 miles It runs fine after it starts, no maintance, since plenum blew up, replaced everything than even coils.

 

Does it do it every time you start it cold, or is it sporadic?

When does it do it during cranking- right away, or just before the engine catches to start after its turned over several times?

I'd plug in the scan tool to see if anything abnormal comes up while its running. See how the fuel trims are, O2 readings, timing advance, etc. Before starting it one time - perhaps try popping the intake off, manually opening the throttle body and taking a sniff. See if it smells like built up fuel. Id pop the vac line on the FPR just to double check that's still good too.

 

Most times but not everytime seems worst when its colder out ??
seems like it does it right away , kind of when of would start if that makes sense?

QUOTE=bumpin96monte;731363]Does it do it every time you start it cold, or is it sporadic?

When does it do it during cranking- right away, or just before the engine catches to start after its turned over several times?

Take intake off ?
Or hose to air box ??
And Check
Thanks

I'd plug in the scan tool to see if anything abnormal comes up while its running. See how the fuel trims are, O2 readings, timing advance, etc. Before starting it one time - perhaps try popping the intake off, manually opening the throttle body and taking a sniff. See if it smells like built up fuel. Id pop the vac line on the FPR just to double check that's still good too.[/QUOTE]

Hook up tool after it starts, and right down readings?
Not a expert with this tool used for basics

Thanks David

 

Take the duct off the throttle body itself, turn open the TB with your fingers, and take a sniff inside. Itll of course smell a bit like carbon / oil, but what you're looking for is a strong smell of gas. What Im wondering is maybe an injector is not sealing up and dribbling the fuel rail empty into a port after you shut it off.


Don't necessarily need to write them down, just observe.

Is the misfire counter registering any or is it holding at 0?

Are the STFT and LTFT roughly zero? A little + or - is normal, but if its double digits either way, that could be a sign of an issue.

Is the O2 value roughly centered and bouncing back and forth around stoich?

How much spark timing is it running?

 

Thanks will try this weekend .Thanks Again

 

I tried using the brick I have, and couldn't find most of the things you told me to look for. I'm not a mechanic.
could it be a lean back fire?
A older mechanic told me to cycle the fuel pump 2x then start it.
I have tried it 4x so far and no backfire.
But it hasn't been real cold, it seems to always do it when it's cold.
Any other ideas?
Thanks David
You can always call or text me
​​​​​me 717-572-8292

 

Not to come off as rude, but Id suggest buying one if you plan to continue working on modern cars. They're pretty cheap these days- maybe $50 or so and IMO are every bit as valuable as a ratchet or screwdriver in terms of usefulness (especially since the last 25 years of cars are now compatible after the switch to obd2).

Its not impossible. You could check fuel pressure when starting it in those conditions - maybe the pump is nearing the end of its life and is having a hard time spinning fast enough when stone cold? When starting it normally, if the pressure doesn't come up to spec and you get the pop, then you know to dig deeper there.

I'd think it would be far less likely to be an injector issue as you'd think some kind of partial clog would also show up during normal operation via misfires.

Doing 2 full primes to get enough pressure would likely mean the pump is bad. It should build pressure almost immediately. This goes back to the fuel pressure gauge - you can probably rent one from autozone. Just pops onto the port on your fuel rail. Then you'll have to search a bit for the spec for key on engine off pressure - Id imagine its been posted on here before, or perhaps someone on here could look it up in their service manual. For port fuel injection in general though, it should probably be getting up to at least 35 or 40 though to be ok.

If this is really helping, you'll see the pressure come up part way the first time, and then fully to pressure the second time.

Its an easy / cheap enough check that its worth trying.

Not other than the ones I'd already suggested above. Honestly its hard to diagnose something remotely without being able to get hands on with the data/ testing equipment.