Cascade Crew

fiveoformula · Posted: Sun Jan 23, 2011 1:59 pm Post subject: Battling intake back-fire

You guys may remember my "Pop" thread.

http://www.cascadecrew.org/phpBB2/viewtopic.php?t=7401&postdays=0&postorder=asc&start=0

Well im still having intermitten problems after 4K RPM, sometimes it won't do it at all and pull great to 5K.
I have the W/B on there, it's seeing plenty of fuel, 10.X:1, my timing is set up at 0* like stock.
Yesterday I went out and bought a vacuum gauge, taped it to the windshield and made about 5 WOT runs, only 1 time I got it to happen but when it did, the gauge stayed at zero. Does that disprove a mechanical problem like an intake valve not closing all the way?
Idling it moves from 20' to 21' and you can hear a miss when it goes back and forth.
The last big thing I did before this problem developed was change my old leaky valve stem seals, then it was a good month or two before it started doing this. And at that time leakdown was all good on every cylinder.
Thinking that my valves were misadjusted, I re-did them at 1/4 turn passed 0 lash, still no change.

I really hate throwing parts at a problem but I did... Tried a coil, ICM, and a pickup coil.

Now Im looking for other things to check or recheck...

Twilightoptics · Posted: Sun Jan 23, 2011 3:58 pm Post subject:

Try dataloging and see what the knock sensor says.

The vacuum gauge should read zero at WOT. There's no vacuum when the throttle blades are all the way open. If there is a vacuum there, it means there is a restriction in the intake. IE too small air filter, too small tb, etc.

How violent is the pop? Is it a single pop or multiple?

10x:1 AFR shouldn't be too rich to cause a pre ignition problem.

How do your spark plugs work. What plugs are you running?

I had a similar problem. Car seemed to run pretty okay then when I hit about 4500rpm it'd pop studder etc. Turns out I had an exhaust lobe on my cam go flat. Shouldn't be the case with your roller but it's possible. Typically if you get an intake backfire, you see an exhaust valve staying closed. You say the lash is okay. Keep an eye on it.

How many miles are on the motor? If there's alot, I might think about changing the valve springs. It's possible you have a weak one and when conditions are right, you get some valve float which can cause backfires out both ends.

Think about running some fuel injector cleaner for shitsngiggles too.

BluFbdy · Posted: Sun Jan 23, 2011 10:55 pm Post subject:

Don't know if this is helpful but my friend jason on tgo was havin this issue when I was down in az, he tossed a bunch of parts at it, I checked his timing, reset his IAC and TPS and adjusted minimum air, so far it hasn't done it again but its only been a week, it scared the crap outta me cuz my hand was on the plenum when it happened lol

fiveoformula · Posted: Mon Jan 24, 2011 7:01 pm Post subject:

Twilightoptics · Posted: Mon Jan 24, 2011 7:22 pm Post subject:

lol Crusty? Since the valve seal replacement?

You're TBI so injectors are out, or you'd have multiple holes having issues.

Super hard to advise more than this, since we haven't personally heard it.

DO NOT TRY TO TUNE AROUND WITH IT WITH A CHIP.

Unless you have heads/cam/intake the stock chip will work fine, just needs refining. Your issue isn't in the tune.

fiveoformula · Posted: Mon Jan 24, 2011 7:35 pm Post subject:

That plug seems to get more junk attached than the others, like grayish junk... lol
I know it's hard to diagnose over the web, just wish there was a sure-fire check to see if its mechanical or ignition. I stopped myself from throwing a hundred dollar distributor at it, and just got the pickup coil. But that made no change.
My thinking with the vac gauge was that if it was a valve acting up, the needle would have been erractic when it did it "pop" thing. But I guess not.

blue89 · Posted: Tue Jan 25, 2011 1:55 pm Post subject:

My v6 did the exact same thing. I unplugged the maf sensor and the problem went away. Could have been a bad one? I went to speed density before I solved it so I didn't care. Figured I would give you my 2 cents worth.

fiveoformula · Posted: Tue Jan 25, 2011 6:54 pm Post subject:

fiveoformula · Posted: Sat Jan 29, 2011 6:32 pm Post subject:

Could false knock cause a pop if it rapidly pulls timing? like if the knock sensor was too sensitive?

Dewey316 · Posted: Sat Jan 29, 2011 7:08 pm Post subject:

fiveoformula · Posted: Sat Jan 29, 2011 7:15 pm Post subject:

No more than before I guess, just thought it could have been pulling a bunch of timing when the pop happens, just a thought. Because I don't think I got it to pop while we were datalogging.

Dewey316 · Posted: Sat Jan 29, 2011 7:22 pm Post subject:

I honestly think Paul is on to something, with a slightly weak valve spring, or something causing the exaust vavle to float just enough every now and again, to get an backfire through the intake.

Really, the motor seems to run really well, other than the popping every now and again. (which isn't every time, we ran it up above 4k a couple of times). I am thinking, maybe give it a seafoam treatment, and toss in some normal plugs (not the ngk v-groove things), and see what happens. Maybe a can of injector cleaner, or something like that, and a fuel filter to make sure things are good there. Just the normal tune-up things.

Maybe try a different ignition coil, and a new cap/rotor and maybe plug wires?

If doing a fresh tune up on all that stuff doesn't work, i think the next best possibiltiy is going to have to be, looking into something like a weak valve spring on that exhaust valve.

fiveoformula · Posted: Sat Jan 29, 2011 7:49 pm Post subject:

I suppose it's worth a shot, you think im over-thinking it huh?

Dewey316 · Posted: Sat Jan 29, 2011 7:53 pm Post subject:

Twilightoptics · Posted: Sun Jan 30, 2011 8:55 pm Post subject:

Nothing wrong with the NGK V-Power. I've had great great luck in v8's with them.

I'd run them now if they had a long enough reach for the Pro heads.

fiveoformula · Posted: Mon Jan 31, 2011 9:52 pm Post subject:

So I just finished up putting new plugs in. Yesterday I stuck a new cap, rotor, and wires on it, then used 1/2 a can of seafoam in the vacuum lines, other half in the tank. Runs really smooth now.
I did WOT a couple times down the road while I was seafoamin' Twisted Evil

Smoked out a BMW pretty good! Cool

and didn't pop at all. But tomorrow I'll drive it more to be sure.

fiveoformula · Posted: Wed Feb 09, 2011 11:15 pm Post subject:

Well I have good news! Finally I figured out this pop. Tuning it up, made the pops fewer and far between BUT still there.
So I thought I would try something different, went back to tunerpro and burned 2 chips, 1 with double the fuel in every cell after 2K RPM, and 1 with half the fuel.
Basically when I pulled half the fuel out, my AFR at WOT was about 13.4, and it no longer popped. So problem was that it was much to rich in those high RPM's I guess.

Twilightoptics · Posted: Thu Feb 10, 2011 1:00 pm Post subject:

Bad tuneup + extra fuel = fuel left in the cyl

Heh didn't I say to get your AFR up from the get go? :OP

fiveoformula · Posted: Thu Feb 10, 2011 6:56 pm Post subject:

Twilightoptics · Posted: Thu Feb 10, 2011 6:57 pm Post subject:

Gotta remember, stock tune encompases how many thousand engines that aren't perfectly the same? Fuel pumps that aren't the same, fuel pressure regulators with an acceptable "range." Add a few hundred thousand miles and it all changes!