: Backfiring through intake causes????



eldorado1
11-20-04, 08:45 PM
Stock engine, aftermarket ECM.... Edit - engine is a 96/VIN9.. When I start the engine, 8 times out of 10, it will backfire. It used to just happen when it's warm, and been sitting for an hour or two. Now, it's happening all the time, cold, warm, hot... I tried turning down the timing during startup to 5*, and it stopped it for a day, but now it's acting up again. Interestingly though, it doesn't sound as loud at 5* advance as when it was at 16* advance. Any ideas on what I should be looking for??

edit again - The PCV outlet just steams when the engine is warm... seems to be a combination of water and gas... don't want to lead the witness, but should I be on the lookout for a stuck open injector? Also - I've been told having the wrong spark plugs could cause this... I DID just change the plugs to those double platinum autolite plugs not too long ago......

eldorado1
11-21-04, 11:31 PM
Well... I think I may have 2 problems...

I pulled my rail, and discovered all of my injectors are leaking... albeit very slowly. After pressurizing the fuel rail, and waiting 20 minutes, all of them were "moist". 3 of them were more than moist... one of them had almost enough liquid fuel on the tip to make it drip. Total, there might have been 2 drops of fuel spread out among the injectors. Is this enough to cause a backfire????

Also, I kinda noticed all my plug wires were practically bundled together. Could the wires being this close together cause a spark on a cylinder that isn't closed yet, causing the intake backfire????

Again, I appreciate your prompt responses......

CadiJeff
11-22-04, 01:27 AM
Is it possible one of your intake valves is sticking open a bit? Also ditch the autolites and get AC Delco OEM plugs, something to do w/ heat range of the autolites being too high

eldorado1
11-22-04, 11:16 AM
Is it possible one of your intake valves is sticking open a bit? Also ditch the autolites and get AC Delco OEM plugs, something to do w/ heat range of the autolites being too high

I suppose anything's possible...... I'll switch to the delco plugs as soon as I get money for them :(

dkozloski
11-22-04, 02:25 PM
Ordinarily, backfiring in the intake is an overlean condition. The lean mixture burns so slowly that it is still burning at the beginning of the intake stroke. The intake valve opens and ignites the new intake charge. This is what launches the blower on a fuel dragster. Look for all the things that make your engine run lean like intake leaks or low fuel pressures. Occasionally ignition crossfiring can cause this.

eldorado1
11-22-04, 07:16 PM
Hmm... well I separated all the plug wires away from each other, and the last 2 starts have been backfire free. It's too early to say if it worked or not, but <knock on wood>, it's working so far.

I don't know that the injectors have much (if any) effect on the problem. The way I see it, there's always going to be some fuel vapors in the intake, depending on where the engine stops in it's cycle (I'm pretty sure there's almost always one cylinder open to the intake... So whether it's coming from the cylinder, or from a leaking injector...

Basically what I think is happening is at startup, the plug wires would trigger sparks in adjacent wires, igniting the fuel in the intake. Sooooo........ I'm crossing my fingers this solves it, cause replacing the spark plugs and fuel injectors will force me to declare bankruptcy :bonkers:

Insomniac
11-22-04, 08:14 PM
replace the spark plug wires, and get a job ;).

Spyder
11-23-04, 03:01 AM
hmmm...I don't get the get a job comment...maybe i'm just a bit dull tonight...anyways, I do know what backfiring through a carburetor can do... :):):) Hehehe, damn near lost my eyebrows the other day...working on my old Ford with a 390...running the choke and the throttle by hand, with head under the hood and an overly rich carb and two plug wires on backwards...not the smartest thing I coulda done...Probably should have checked the firing order before I just threw the two loose plugswires on the two spots on the distributor. :)

eldorado1
11-23-04, 01:23 PM
well... past 4 startups have been backfire free!! :bouncy:

Ranger
11-23-04, 09:40 PM
I just had a similar problem with a misfire. After changing plugs (which needed to be done anyway) the problem persisted. A new set of AC Delco wires solved the problems. I noticed your wires seem to be routed wrong. That said, the 4 wires off the rear bank run together thru at least two looms. If your wires are leaking then they are getting weak and will need replacing soon. Buy Delco when you do it as I have heard others say they had problems with some cheaper aftermarkets. The Delco's come with a lifetime warranty. Dealer wanted $186 for a set. I found them at http://www.rockauto.com for $122 incl shipping. Ordered them in the evening on Thursday and had them Saturday.

eldorado1
11-29-04, 06:29 PM
I just had a similar problem with a misfire. After changing plugs (which needed to be done anyway) the problem persisted. A new set of AC Delco wires solved the problems. I noticed your wires seem to be routed wrong. That said, the 4 wires off the rear bank run together thru at least two looms. If your wires are leaking then they are getting weak and will need replacing soon. Buy Delco when you do it as I have heard others say they had problems with some cheaper aftermarkets. The Delco's come with a lifetime warranty. Dealer wanted $186 for a set. I found them at http://www.rockauto.com for $122 incl shipping. Ordered them in the evening on Thursday and had them Saturday.

Thanks for the information. I got through a good 15 starts, and then last night, it backfired again. Last 2 starts have been good though, so now it's just an intermittant problem. How thick is the insulation on those Delco wires??? Is it larger in dia than my generic 7mm ones?? Also, you mentioned that my wires are routed wrong, do you know of the correct routing? I got my engine used, and it had no plug wires installed, so I just kinda slapped them on. :annoyed:

ellives
11-29-04, 07:33 PM
the plug banks are labeled as is the coil pack (not sure what the GM term is for this module.) It's simply a matter of identifying visually and tracing the wires.

Ells

Ranger
11-29-04, 08:21 PM
I believe the OEM Delco wires are 7mm. I am not sure if the routing of your wires would actually cause any problems or not but I think the factory routing is in the opposite direction of your picture. That is, from the coil tower toward the drivers side along the rear cam cover, then over the initake for the front bank. The rear bank runs in the same direction, then doubles back to the plugs. Be sure to keep the all the clips on them for seperation and keep them away from the blower motor it you reroute them. Also make sure you have the plastic conduit on them to help preven leaking or grounding out when they come near metal.

BeelzeBob
11-30-04, 04:40 PM
Since the 93-99 Northstar uses the waste spark technology two plugs will fire off each coil and the plugs fire every revolution of the engine (at TDC firing AND at TDC exhaust). Because of this, if there is a "stray" spark issued from the coil pack due to interference on the crank sensor wires, there is a high likely hood of a spark issuing to a cylinder that has an open intake valve (open thru normal operation) thus causing the "backfire". There is logic built into the production PCM to prevent this but if an aftermarket PCM is being used this is likely not there so the propensity for the start up backfire is much much greater.....

Best thing to do is to shield the wires for the crank sensors from any magnetic field induced by secondary ignition leads , other power wires, the battery cable to the starter, etc..... This will minimize the likelyhood of any stray signals induced on the sensor leads and minimize the chance of any stray firing of the ignition module during crank. Things are happening relatively slowly during crank and with the high current flow to the starter it is much easier to induce stray firing of the ignition system. The production PCM is designed to eliminate this but......

Or....you can put a delay relay in the power feed to the coil pack. Just cut the 12 volt feed to the coil pack and put a 2 or 3 second delay in the circuit via some electronics and a relay. The idea is that you want the engine to turn over several cylinders so as to pump any combustable mixture out of the intake manifold before the spark is enabled. This way, even if there is a stray spark to a cylinder with an open intake valve it will not flash back into the intake.

Sounds like you are putting the little intake pop-off valve (overpressurization device....LOL) to the test......good thing it is there or the plastic intake would have fragged by now.


BTW....the PCV sounds normal. You will always get some vapor issueing from the PCV. Blowby, byproducts of combustion, water vapor, fuel vapor, some minimal atomized oil, etc....that is what the PCV is for...to eliminate that from the crank case.

eldorado1
12-01-04, 04:48 PM
Since the 93-99 Northstar uses the waste spark technology two plugs will fire off each coil and the plugs fire every revolution of the engine (at TDC firing AND at TDC exhaust). Because of this, if there is a "stray" spark issued from the coil pack due to interference on the crank sensor wires, there is a high likely hood of a spark issuing to a cylinder that has an open intake valve (open thru normal operation) thus causing the "backfire". There is logic built into the production PCM to prevent this but if an aftermarket PCM is being used this is likely not there so the propensity for the start up backfire is much much greater.....


Oooo... That's an interesting point. When it was backfiring constantly (about every start), the wires were all laying on each other, DIRECTLY above the cam sensor wiring.



Or....you can put a delay relay in the power feed to the coil pack. Just cut the 12 volt feed to the coil pack and put a 2 or 3 second delay in the circuit via some electronics and a relay. The idea is that you want the engine to turn over several cylinders so as to pump any combustable mixture out of the intake manifold before the spark is enabled. This way, even if there is a stray spark to a cylinder with an open intake valve it will not flash back into the intake.

Good idea! It's too bad my starter will wear that much faster.... But I might give it a shot.



Sounds like you are putting the little intake pop-off valve (overpressurization device....LOL) to the test......good thing it is there or the plastic intake would have fragged by now.

Psht... and it'll prevent me from building boost :burn:

I wonder if the 93-94 intake is quieter during a backfire (through the intake)... considering the sound doesn't have this gaping hole to go through, but rather minute spaces... I bet it sounds more like a "puffft" than a "BANG!"

eldorado1
12-01-04, 05:13 PM
I am not sure if the routing of your wires would actually cause any problems or not but I think the factory routing is in the opposite direction of your picture. That is, from the coil tower toward the drivers side along the rear cam cover, then over the initake for the front bank. The rear bank runs in the same direction, then doubles back to the plugs. Be sure to keep the all the clips on them for seperation and keep them away from the blower motor it you reroute them. Also make sure you have the plastic conduit on them to help preven leaking or grounding out when they come near metal.

Don't suppose I can persuade you (or anyone else) to snap a pic of the routing? :D And the locations of any wire separators.... :worship: