backfiring at low rpm

**V8MUSCLE** · 12-20-2010, 01:34 PM

so, if im in the "wrong" gear, say cruising at 45 in 6th and mash the gas, the world ends... backfiring like mad until 2k rpms then all is fine. occasionally under WOT it will backfire around 4.5-5k rpms. this all started about 4-5 months after i put long tubes on. i know it runs a bit rich, i can smell it at idle, and mpg has dropped from about 320 a fill up, to about 280 a fill up, about 14.5 gallons a fueling.

i do add sarges voodoo 2cycle oil 1oz to 5gallons of gas, i stopped for 5 tanks with no change, just lost about 20 miles to the tank. im back on the 2cycle oil now.

a buddy of mine was saying it could be the lack of tune. or bad plugs or wires, i hooked up a scan tool, snap-on-modis with the live data i got it to do backfire and it did not throw any codes, or misfires. so wtf...

since its been doing this it rarely sees idle, i usually keep it under or 2k. when i do go WOT there will be a black cloud behind me. i figure this cant be good, so i rarely do it.

any thoughts?

**V8MUSCLE** · 12-20-2010, 01:37 PM

i pullled 2 plugs, i know it doesnt count for much, but they were both fine. new plugs are probably going in this winter, i have the ngk tr55s gapped to whatever i read on here to gap em at. its been a year. plugs were pulled a week after the backfiring started.

**Wesman** · 12-20-2010, 01:46 PM

I think we first need to clear up what you mean by "backfire".

There are 3 things you may be talking about:

A misfire, or "stumble", where the engine runs rough and has trouble accelerating.

A popping from the exhaust, where unburnt fuel enters the exhaust manifold and combusts in the exhaust.

An actual backfire, where fuel enters the intake manifold due to a hung up exhaust valve or incorrect valve timing. This is very unlikely, especially on a fuel injected car, and would typically blow the intake manifold apart.

**V8MUSCLE** · 12-20-2010, 01:49 PM

i guess it would be a misfire, but why wouldnt the modis pick it up, it cost 5k bucks so id imagine it would be good at figuring that stuff out. it feels like a stumble, under 2k, but the ones at 4-5k are a loud gunshot coming out of the exhaust.

**V8MUSCLE** · 12-20-2010, 01:53 PM

ive only heard the ones above 4k twice since it started, but it was both times i was WOT, so i stopped that. it doesnt matter if its cold or warm outside. the ones above 4k were at operating temp, but the ones under 2k it doesnt matter if it is the engine is cold or not.

**Smittro** · 12-20-2010, 02:02 PM

Black smoke is usually a sign of poor combustion/mixture but.. Could be a weak coil starting, vaccume leak maybe..

**pajeff02** · 12-20-2010, 02:03 PM

Are the plug wires installed correctly -- one "click" on the plug and two "clicks" on the coil?

**Wesman** · 12-20-2010, 02:05 PM

Originally Posted by V8MUSCLE

i guess it would be a misfire, but why wouldnt the modis pick it up, it cost 5k bucks so id imagine it would be good at figuring that stuff out. it feels like a stumble, under 2k, but the ones at 4-5k are a loud gunshot coming out of the exhaust.

Sometimes the Modis isn't good enough.

A GM Tech II is the best to use. When I had a misfire on my Grand Prix, I hooked it up to the Tech II and it told me individual counts of misfires on each cylinder. Very useful info, not sure if the Modis is capable of doing that on a 98 F-body.

I'd guess you either have a misfire or a MAF problem. Either one should throw a code though, strange that you have no CEL.

**V8MUSCLE** · 12-20-2010, 02:07 PM

vacuum leak possibly, i have a mity vac that ive used to check it, when i dont hold the release valve it shakes , but when i hold it in to release it is stable at 17inhg. plug wires are all on good, 1 click for plugs and 2 for coils, checked that already on all 8.

**pajeff02** · 12-20-2010, 02:08 PM

Check the vacuum line on the PCV assembly. Passenger side of throttle body.

**V8MUSCLE** · 12-20-2010, 02:10 PM

i did clean the maf right befor the problem started, i used maf cleaner crc i believe, silver can with red label. the modis claims to have a individual cylinder misfire count, i know it works on a 98 lumina, not sure why it wouldnt on a 00 TA

**Wesman** · 12-20-2010, 02:13 PM

Originally Posted by V8MUSCLE

i did clean the maf right befor the problem started, i used maf cleaner crc i believe, silver can with red label. the modis claims to have a individual cylinder misfire count, i know it works on a 98 lumina, not sure why it wouldnt on a 00 TA

Try cleaning the MAF again with the CRC MAF Cleaner. Maybe when you cleaned it you loosened up some grime and it ended up drying back onto the hot wire.

**V8MUSCLE** · 12-20-2010, 02:14 PM

if your talking about the one thats 3/8" and burns through, i replaced it with fuel line, its doing fine.

**pajeff02** · 12-20-2010, 02:17 PM

Originally Posted by V8MUSCLE

if your talking about the one thats 3/8" and burns through, i replaced it with fuel line, its doing fine.

That's the most likely source of a vacuum leak.

Does it act the same both cold and hot?

**V8MUSCLE** · 12-20-2010, 02:18 PM

yes, acts the same hot and cold. a little bit worse on cold days with cold engine, but still acts up on when it was hot with a warm engine.

**pajeff02** · 12-20-2010, 02:21 PM

Can you see your fuel trims on the scanner?

**V8MUSCLE** · 12-20-2010, 02:25 PM

we could, but we didnt know what we were looking at. and dont remember what it was i know it was never close to 14.7 at idle, i think it was in the hundreds, but it might of been something else we were looking at. since ive read that 17-18 afr is catastrophic to an engine. there was short term fuel trim and long term trim, it also said i was getting feedback from o2sensor 2 bank 2, but there is no plug there, just open wire.

i dont have the modis with me. it is the schools that i went to.

**pajeff02** · 12-20-2010, 02:30 PM

You want your trims to be fairly close to zero. I had a MAF issue one time and the trims jumped up to about +25. The PCM was adding fuel to compensate for a lean condition due to a faulty MAF. That's where the feedback from the O2's comes into play. With everything straightened out, I rarely see my trims stray more than + or -3 from zero.

**V8MUSCLE** · 12-20-2010, 02:34 PM

ill probably end up checking the maf when i get a chance, its cold and dark outside now, and working retail the week before christmas doesnt give me much time out of work, plenty of hours so i should be good paying for whatever part i need, if i need one.

**pajeff02** · 12-20-2010, 06:14 PM

Technically, if the PCM detects a MAF fault it is supposed to ignore it and calculate engine airflow off the VE table using the MAP and RPM inputs. Curious to see what your fuel trims are running.