Spark plugs fouling/backfiring... need tuning help?

harrellv10

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Oct 4, 2011
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I keep having to replace the plugs in my 1994 GT. It runs fine when the plugs are fresh, but after a month or two it starts backfiring, sputtering when taking off from a stop light, and smelling like fuel. The plugs that come out then are looking black and soot-covered. I just thought it was running too rich and causing the carbon buildup on the plugs and then they couldn't ignite the fuel mixture anymore after they got too bad.

I had the car retuned after putting a different intake on it and told my tuner to address the plug-fouling problem while he was at it. At first he said that since it's a new engine (400 miles on it now), that he tuned it a little rich so that when all the seals and gaskets sealed up over time it would lean out a little bit on its own and even out.

OK, well, screw that, it runs like crap and I want it tuned right. I told him that and he retuned it, and it seemed to be running fine again for a month or two but now its doing the same old thing. Now he says that since it sits for about a month at a time between driving session that the plugs are not able to keep the soot burned off of themselves and it is hardening up on them and I just need to drive it more often to keep the plugs in good shape.

I smell BS. Has anyone ever heard of that? Is this possible or is it a bad tune causing it? Should I switch to a hotter spark plug? Is the EEC-IV not capable of handling my combo and I need a stand-alone computer or something?

The setup is this: DSS level 20 347, TFS 190cc heads, TFS R-series intake, 60lb injectors, 90mm Abaco blow/draw through MAF calibrated, 75mm TB, TFS fuel system, MSD 6BTM, coil, 9mm wires, autolite spark plugs (forget the part number), AFM B-41 cam, AFM power pipe, JBA headers, MAC 2.5" Prochamber, MAC mufflers, Vortech V1-B supercharger, 12 PSI boost, SCT software/dyno tune on stock EEC-IV computer.

I just want to play with my car and it always has something wrong with it and I'm about to give up... HELP!!!!!
 
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harrellv10

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Oct 4, 2011
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Elizabethtown, KY
OK, so I called the spark plug experts at NGK tech line and also called my tuner. Guess what? Two totally different recommendations. The NGK guys said to try a colder plug because they think the car might be running lean or detonating. I don't hear any detonation, but I know that doesn't mean it is not there. My tuner said he thinks the plugs were too cold and try one heat range hotter and drive it more often so they can do a better job of self-cleaning. What do I do? I just changed the plugs resterday, with the same part number that it had in it and it runs and idles like a totally different car- perfect. But, I know that a month from now it will be backfiring and acting up again. I believe the plugs that were in it were already 2 heat ranges colder than stock, so I'm going to try one heat range hotter, Autolite 3924, an iridium coated plug that my tuner recommended. I will make it a point to drive the car for at least 15 minutes every day, and we'll see what happens.

Does anyone have any experience with this type of issue?
 

harrellv10

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Oct 4, 2011
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Elizabethtown, KY
The plugs that were in it were Autolite 3923, which are 2 heat ranges colder than stock. I'm switching to Autolite 3924 on Monday and we'll see how it goes. Today I picked the car up from the shop (yes, I am at the point where I am sick of changing plugs and am paying a shop to do it for me now) with a fresh set of 3923 plugs in it, and at first the car ran a lot better, but sounded wierd. The shop also did a little exhaust work and I thought maybe they created a minor exhaust leak, but I can't really tell if it's the exhaust or the engine making the machine gun noise.

Either way, I did the old row through a few gears test and TONS of white smoke came out of the exhaust and even inside the car through the firewall or AC vents or somewhere, but quit smoking when I got out of the throttle. Something is seriously wrong. I'm starting to worry that I have a leaky head gasket or something. White smoke could mean coolant is leaking into the cylinder, and a leaky head gasket could also cause that wierd noise I'm hearing. WTF is going on? Aaaaaaaaaarrrrrrrrrrgh!!!!
 

VibrantRedGT

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When I had an S-Trim on my car I ran Autolite 3923's, two ranges colder with like a .030-32 gap. With the smoke I suggest doing a compression test ASAP. Make sure you're getting 120+ psi in each cylinder. The test is easy to do, take each plug out, put the threaded tube in, make sure the throttle body is wide open, coil wire is disconnected and crank the car over for a few seconds. I also hook up a battery charger during the test. You may be fouling a plug out if water is getting in the cylinder. When one plug fouls out the ECC will try to compensate for this and will turn the fuel wick up. Is the weird noise you may be hearing detonation? What about the O2 sensors? Have you checked for trouble codes? Normally is you're running lean or rich you will see 173 - 177 codes. Do you have a fuel pressure gauge? If so what's it read?

I'll move this into tech, maybe you'll get more replies.
 

revhead347

Apparently my ex-husband made that mistake.
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When I had an S-Trim on my car I ran Autolite 3923's, two ranges colder with like a .030-32 gap. With the smoke I suggest doing a compression test ASAP. Make sure you're getting 120+ psi in each cylinder. The test is easy to do, take each plug out, put the threaded tube in, make sure the throttle body is wide open, coil wire is disconnected and crank the car over for a few seconds. I also hook up a battery charger during the test. You may be fouling a plug out if water is getting in the cylinder. When one plug fouls out the ECC will try to compensate for this and will turn the fuel wick up. Is the weird noise you may be hearing detonation? What about the O2 sensors? Have you checked for trouble codes? Normally is you're running lean or rich you will see 173 - 177 codes. Do you have a fuel pressure gauge? If so what's it read?

I'll move this into tech, maybe you'll get more replies.

This is excellent advice. Pull the plugs and gap them way down. Don't run the iridiums because you can't gap those, stick with the basic Autolite or NGK plugs. I prefer Autolite's myself, but both are good plugs. If the gap is too large, it's going to blow out the spark on a boosted car and cause all kinds of problems. I had my plugs gapped to .025" on my turbo car to keep it from getting the problems you are having. I also agree with the compression test. You should be within 10% on all cylinders.

Kurt
 

harrellv10

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I don't think I have a blown head gasket. The plugs were gapped at .032" and they are Autolie 3923. I checked for low cooland and water in the oil or oil in the water and negative. Oil was low, though, and I think I found the source of the smoke... a pushed out oil pan gasket looks like it was blowing oil out at WOT all over the driver side header and burning off. My dipstick is useless... and I'm trying to figure out a way to run an aftermarket one or fabricate something to work better with my stroker crank/windate tray/7qt Canton pan setup. The stock dipstick tube was bent to fit through the headers and the dipstick I guess was getting hit by the crank and got bent, and the last few times I checked my oil I really couldn't tell what the oil level was. My luck would be that I have a serious oil leak, that does not leave a spot under car when parked, that I don't catch until I hear it knocking from low oil...errrr! But I think that's what the noise was.

I also found a MAJOR fuel leak at a T-junction in the line above my passenger side valve cover. That probably caused a lean condition at high RPMs whick caused the backfire. I do not have a fuel pressure gauge, but I think I'm going to invest in one.

Is it possible that the blown pan gasket was causing an exhaust leak type sound? The car has never detonated before, and I had a compression test run on each cylinder last time is was tuned, which was VERY recent, and all was perfectly healthy. No trouble codes at that time, but since the car has an aftermarket tune and so much stuff has been removed or added, the check engine light has always been on. I tried to have my tuner take care of that, but he said that's just the way the car is going to be. It even tells me the water is low all the time because the coolant sensor was removed when I installed the aftermarket radiator, so I'm not too concerned with that. The tune addresses all of that stuff.

The car is in the shop now getting the fuel and oil leaks fixed and the noise identified, hopefully. It sounded much better after I added two quarts of oil. The dipstick issue is getting looked at, too.

Does anyone know a good way to get my oil dipstick to cooperate without running into crank clearance issues?
Does anyone know if it's possible that too much crank pressure from the supercharger and/or lack of adequate crank case ventilation might be causing the engine to blow gaskets and spring leaks?
Has anyone heard of people fabricating a PCV system that works with their exhaust system - like a draw-through type thing that pull some of the pressure out of the crank case when it exceeds a cerain amount? I have heard that some drag racers do that somehow.
It's still pretty new and has bugs to get worked out, and it's all way out of the realm of stock stuff, so some of you 347 stroker guys must know how to solve the dispstick issue and others. Thanks for your help.
 

revhead347

Apparently my ex-husband made that mistake.
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First of all, the Canton 7 qt oil pan requires a special dipstick. You have to plug your stock dipstick hole, and buy their's. Yes, you can definately blow an oil pan gasket out if the crankcase is not well ventilated enough. Most people either run a catch can system, however there are ways to route a PCV system that will work. There is a whole thread on the corral about different ways to do it.

Kurt
 

harrellv10

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Oct 4, 2011
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Elizabethtown, KY
Thanks for the tips! I called Summit Racing and ordered the appropriate dipstick and tube for my pan. I also asked about the crank case ventilation problem, and the tech rep recommended a kit that utilized a bung that welds into your headers, and excess crank case pressure is vented out through a fairly normal set of valve cover breathers, then through a tube with a check valve going into the header. I think this will solve my problems. We will see in a couple of weeks when I have had time to do the mods and test it out.
 

revhead347

Apparently my ex-husband made that mistake.
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Let me know how the header thing works out. I haven't heard of anyone doing that in years. It actually welds into your headers at the collector forcing the headers to act like a venturi and suck the stuff out.

Kurt