The car is a 406 fuel injected car with a dfi for tuning. It was running perfectly fine and had no problems, but then I decided to put in a new set of spark plugs since these were a year or so old already. Now when I freerev the car it has a miss and it will pop out of the exhaust only 1 time though. Under load and even wot the car runs great and very smooth with no miss. Idle seems to be fine as well, although occasionally it may seem a bit lower. I checked the plugs again, regapped them and reinstalled them again and they are 100% fine. Wires are also not swapped. The only other thing I did was I wired in my nitrous system to the trigger wire on the dfi, but just to make sure it had nothing to do with that I unplugged the wire from the dfi again and it still does the same thing. Any ideas what to look for?

 

My 350 pops during free-rev as well, not really a miss but it pops out the exhaust. Likely due to a rich mixture and a previously-loved muffler.

 

Try looking for an exaust leak between the header flange and the collector, I had a small leak on a duster and a nova and even though I couldn't hear a leak it would feel like a miss and pop through the exaust especialy on deceleration.

 

thanks for the response, but that is definitly not the problem. It seems as if there is no ignition miss and the exhaust also has no leaks. I'm thinking it has to be a timing issue. It only happens now if you rev it, let the revs drop down to idle and rev it again right away. It will then pop very loudly and backfire through the intake it appears. I'm guessing something is up with the timing?

 

Popping in the exhaust = unburned fuel entering the exhaust and burning

Usually it's a dead hole

Sometimes it's a case of simply being too rich

Since you just worked on something, and it didn't do it before and now it does, I'd go back and check your work.

Popping in the intake = intake valve held open during combustion stroke, or exhaust valve not opening

You may also have a cam lobe gone flat.

Certainly not a "timing issue" (not least, because "timing" affects all cyls equally; not one exclusively, different from the others). And before somebody else pops up with the other newbie "black magic" universal bugaboo scapegoat, it's not EGR either.

 

Oh, c'mon! It's GOTTA be EGR! Everything is EGR (this month). Global warming, high fuel prices then falling oil futures, poorer-than-expected holiday retail sales, democratic senators falling over, it's ALL EGR! Haven't you been reading?

I'm guessing it's your EGR. Don't have one? That's why. Got one? It's bad.

Plug gaps all the same? Did you notice a color differnce in the old plugs between the cylinders? How about on the new ones?

 

Actually, come to think of it I think it may be a valvetrain problem. A while back I went to start the car and it cranked and then did a loud "cha-ching" noise so I stopped trying to start it. Then I cranked again and it was perfectly fine and normal. It did this one other time a few weeks ago as well. Sounds like a valve may be getting stuck.

 

I haven't had a chance to pop a valve cover, but is this something to expect if the valves are too loose due to lack of adjustment? I went home for christmas so I haven't been able to mess with it yet, but once I get back I'd like to take care of this.

If I'm thinking about this correctly, if the valves are set too loose then they will open just a bit late and not far enough, however the cam is driving the dizzy and still firing the plugs at the normal time. Because of this only during freerev I have this problem. Is this a possibility or am I understanding this totally wrong?

 

I should probably clarify, when this happens it's like an explosion from the front of the car, moreso than a conventional misfire. It's very loud and scares the hell out of anyone standing near the car. Almost sounds like someone shot a gun inside the motor.

The only way for something like this to happen is if there is a valvetrain issue or timing is way off right?

 

does it sound like the car in the movie uncle buck ??

 

This is the correct thing to do, you may not accept it, but you are what "happened" because as you said it was not popping before you worked on it.

When working on my engine I lean on it, lay on top of it, push things around, temporarily disconnect things, break old brittle vaccuum hoses without knowing it...etc.

 

I have gone back and checked my work, but all I did was change the spark plugs and I also triple checked that the wires were reinstalled properly. I am 100% sure that it's nothing I did, and the problem may have been there for a while now, I just haven't driven it or revved it up in a while.

I've never seen uncle buck, but basically it revs fine initially, then once the revs drop back down to idle ( set at 1000 ) and you push the gas again it sounds like a gunshot inside the engine.

 

Just for S&Grins put an $8 set of walmart plugs in it. It may be that one of the new plugs is defective. I've seen some very strange symptoms from new plugs that were internally defective. It happens. Don't assume that just because they are new, and expensive, that they can't cause a problem.
Here's a scenario: plug gets dropped on it's nose and the electrode closed.
Dude with a pocket knife pries the electrode back up and puts it in the box.
Engine with that plug <----------->owner

 

I was thinking about that too ... but wouldn't I have a constant ingition miss then, not only under no load when revving?

 

if there's no miss under load even at wot, i'd imagine it's fuel puddling or other issues with your tune, but you said all 8 plugs looked fine when repulled and spark was getting to all 8 as checked by a timing light or something...?

how's orlando treating you by the way?

 

You'll probably need to scope the ignition system, but misfires in general tend to cause driveability problems and that's not what you're describing.

 

I'd like to take the 50/50 here:
Backfiring out of the exhaust or intake?

 

From sitting inside the car I initially though it was out the exhaust, but when I had a friend rev it and the car was pulled into a garage it definitly came out the intake, not the exhaust.

Yea I checked for spark on each wire by using a timing light and when I pulled the plugs they all looked the same and normal. Under load and wot the car runs great, it's only under no load. The tune wasn't messed with other than the NOS settings menu, but that menu only comes into effect when a 12V signal is sent to the computer so I unhooked that wire entirely as well just to rule that out.

I love orlando so far .. no V8s though that I've seen. It's 99% hondas with body kits ad fart cans.

 

Any other ideas? Still trying to figure this out!

 

A timinglight won't tell you if a cylinder is running lean, an Oscilloscope will.

 

right, but my dfi fires numerous injectors at once, not just 1 at a time so it would be safe to assume that not only 1 injector wouldn't be working since they share the same wires and the car would definitly not run right under any type of load. They all ohmed out the same, so they are all probably working.

 

Did you try 8 new el-cheapo sparkplugs?

 

No I didn't, I figured if it was a bad plug I'd have problems under load as well.

 

My Trans Am ran like crap when the 355 was first fired up. Smelled real rich, popped through the carb, was hard to start, had low vacuum, couldn't tune the idle circuit at all, etc. Turns out I had cracked one of the plugs when installing them causing an intermittent miss that eventually became constant.

I had relied on the parts guy to give me the correct plugs rather that do some work myself and look them up first before going to the parts house. I was sold the wrong set of plugs and not knowing this I installed them and had nothing but troubles with them until just the other day when I realized the plugs were the wrong ones.

The guy sold me ".R45T" and I should have had a set of "R45TS". Big difference.

Like everybody else I guess am just saying "check your plugs".