backfire = bad O2 sensors? long

89GTCOUPE · 04-02-2001, 07:00 PM

my 89 5.0 has ben backfiring once or twice under heavy-load wot, but after I cleaned and regapped my plugs, It's been doing it all the time; even under part throttle. the plugs are at .055,timing at 12BTDC, the cap and rotor are fine, the wires have low resistance, and are not crossing. The MIL is turning on after 5 or 10 minutes whenever I drive it, giving codes 11 then 33,41,and 91. my air pump is taken off,so the egr code doesnt bother me, but I just replaced the O2 sensors 2 years ago. Could they be bad already, and causing backfire?
My engine is: rebuilt stock short block, TFS street heads and intake,F303, 75mm MAF, 65 TB, FMS30# injectors(too big, I know),crane FPR set at 32psi(as low as it goes), etc..
Any and all opinions appreciated.

dinomite · 04-02-2001, 07:12 PM

I see that you have the fuel pressure as low as it goes, but it may not be low enough, could still be runing rich. You said you've cleaned the plugs, do you know how to read plugs? cause that would tell you if you are still running rich. That would be my guess. Also, with the stock H-pipe and the cats still intact, but no air pump, the cats will eventually clog, I doubt that has anything to do with this, but it could cause problems later. Your injectors might be to big, I'm assuming your running a 190LPH fuel pump with them, no? To fix it you need to even out your fuel flow with the performance of the motor, get a supercharger! But seriously, you may need smaller injectors/smaller (150lph) fuel pump. Anyone else? seems kind of odd to me.

dinomite · 04-02-2001, 07:15 PM

Looking at the topic of your post again, I guess that the problem could be with the computer being uncalibrated (eh, it's semantically correct), you might try popping off the neg (-) battery cable for 30min, which resets the comp, then put it back on, let the car idle for 1 min and go. That might help it.

89GTCOUPE · 04-02-2001, 07:21 PM

It smells like it is running rich, but my air-fuel guage stays usually in the middle around 14.7:1.My plugs were slightly reddish, which i is from fuel additives, I think. I forgot to mention I have full exhaust including 2.5" off road pipe with plugged thermacor tube. oh yeah fuel pump is 190lph

I live in Wyoming , so I am far away anywhere that can burn chips. Is there any other options besides the extender or epec?

[This message has been edited by 89GTCOUPE (edited 04-02-2001).]

shelby2 · 04-02-2001, 07:54 PM

mine would backfire in mine under part throttle, but only some of the time. i know it is from it running rich. i have long tube headers with the o2 sensors welded in and causes the sensors to read lean because the cannot sample enought of the gases. even tried changing the fuel pressure from 30 to 45 with the vacuum disconnected. when i disconnected the o2 sensor harness it did not backfire at all, but would surge and die at stops. i guess that is because it goes to the stock tables for fuel and doesn't look at the current mixture. i am having a chip burned from JMS and should be here later this week. they claim that they can fix the lean condition caused by the longtubes. i also replaced one o2 sensor, and couldn't replace the other because the harness melted on the header, and it didn't make a difference.

------------------
shawn hill
1967 ford mustang coupe, red w/white stripes, hood scoop, front and rear spoiler, sefi and c4, trickflow intake, 70mm mac tb/egr, billet fpr, 24# inj, c&l maf, heavy duty c4 w/transgo shiftkit, msd 6a box w/tfi blaster coil, flowtech longtubes w/turbo mufflers, t-rex inline pump, ported and polished E7TE heads w/1.94,1.60 valves, lunati 292 cam, harland sharp 1.6 rollers, conventional 8" with 2.79 gears (soon will be gone)

shelby2 · 04-02-2001, 07:57 PM

also, i know mine is running rich because i am lucky if i get 10 mpg on the highway. what's your mileage at???

89GTCOUPE · 04-02-2001, 08:01 PM

Last time I checked mileage was when it was running good(no backfire) I got 21 mpg going over the mountain pass.

smokedchevy · 04-03-2001, 01:21 AM

I know you don't have a blower car but mine did the exact same thing.I had a powerdyne blower on my GT and when the internal belt snapped, my car instantly ran better. It would make it backfire and my gas mileage was horrible.It also smelled like raw gas coming out from underneath the car.I also got codes 33 and 91.It could have something to do with airflow? Hope this helps somewhat.Good Luck!

04-03-2001, 02:50 PM

my car has the same POP (backfire) and my MPG is terrible too.. I was thinking O2's or something too the check engine light comes on after a lil bit of driving but then if i cut the car off and restart the light goes off.. also I have a slight bucking at low speeds with part throttle That might be my CAMSHAFT though. I too have longtubes and a NEW MOTOR i checked CTS ACTS, EGR you name it i am stumped too thanx

------------------
SOLD THE 87GT, .. GT40, 70mmTB, 75mmBullet, 24lbinjectors, cold air induction, Edelbrok 6025 Rpm's, Inch phonolic spacer, tall FMSvalve covers, an SFI balancer, Canton 7quart, the shortblock... Lunati 51025 (224/232 .535/. 544 @112 deg) Ross lightweight pistons (10:1) Eagle H-Beams, Total Seal plasma Moly Rings, Clevite bearings, ARP throughout, FMS main Girdle&Studs, Forged Crank, of course HV-OilPump and hardened FMS shaft, Dual Friction centerforce, T5, and 4.10's Longtubes and 2.25 tail pi

89GTCOUPE · 04-04-2001, 01:42 AM

Well, my problem ended up being no more than a small crack in one sparkplug insulator. hard to believe something that small could cause all that backfire. Anyhow, thanks for the replies!

04-02-2001, 07:00 PM	#1
89GTCOUPE Registered Member Join Date: Nov 1999 Location: Sheridan, WY USA Posts: 10	backfire = bad O2 sensors? long my 89 5.0 has ben backfiring once or twice under heavy-load wot, but after I cleaned and regapped my plugs, It's been doing it all the time; even under part throttle. the plugs are at .055,timing at 12BTDC, the cap and rotor are fine, the wires have low resistance, and are not crossing. The MIL is turning on after 5 or 10 minutes whenever I drive it, giving codes 11 then 33,41,and 91. my air pump is taken off,so the egr code doesnt bother me, but I just replaced the O2 sensors 2 years ago. Could they be bad already, and causing backfire? My engine is: rebuilt stock short block, TFS street heads and intake,F303, 75mm MAF, 65 TB, FMS30# injectors(too big, I know),crane FPR set at 32psi(as low as it goes), etc.. Any and all opinions appreciated.

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04-02-2001, 07:12 PM	#2
dinomite The Dude Join Date: Feb 2001 Location: Arlington, VA Posts: 1,262	I see that you have the fuel pressure as low as it goes, but it may not be low enough, could still be runing rich. You said you've cleaned the plugs, do you know how to read plugs? cause that would tell you if you are still running rich. That would be my guess. Also, with the stock H-pipe and the cats still intact, but no air pump, the cats will eventually clog, I doubt that has anything to do with this, but it could cause problems later. Your injectors might be to big, I'm assuming your running a 190LPH fuel pump with them, no? To fix it you need to even out your fuel flow with the performance of the motor, get a supercharger! But seriously, you may need smaller injectors/smaller (150lph) fuel pump. Anyone else? seems kind of odd to me.

04-02-2001, 07:15 PM	#3
dinomite The Dude Join Date: Feb 2001 Location: Arlington, VA Posts: 1,262	Looking at the topic of your post again, I guess that the problem could be with the computer being uncalibrated (eh, it's semantically correct), you might try popping off the neg (-) battery cable for 30min, which resets the comp, then put it back on, let the car idle for 1 min and go. That might help it.

04-02-2001, 07:21 PM	#4
89GTCOUPE Registered Member Join Date: Nov 1999 Location: Sheridan, WY USA Posts: 10	It smells like it is running rich, but my air-fuel guage stays usually in the middle around 14.7:1.My plugs were slightly reddish, which i is from fuel additives, I think. I forgot to mention I have full exhaust including 2.5" off road pipe with plugged thermacor tube. oh yeah fuel pump is 190lph I live in Wyoming , so I am far away anywhere that can burn chips. Is there any other options besides the extender or epec? [This message has been edited by 89GTCOUPE (edited 04-02-2001).]

04-02-2001, 07:54 PM	#5
shelby2 Registered Member Join Date: Nov 1999 Location: O Fallon, IL Posts: 87	mine would backfire in mine under part throttle, but only some of the time. i know it is from it running rich. i have long tube headers with the o2 sensors welded in and causes the sensors to read lean because the cannot sample enought of the gases. even tried changing the fuel pressure from 30 to 45 with the vacuum disconnected. when i disconnected the o2 sensor harness it did not backfire at all, but would surge and die at stops. i guess that is because it goes to the stock tables for fuel and doesn't look at the current mixture. i am having a chip burned from JMS and should be here later this week. they claim that they can fix the lean condition caused by the longtubes. i also replaced one o2 sensor, and couldn't replace the other because the harness melted on the header, and it didn't make a difference. ------------------ shawn hill 1967 ford mustang coupe, red w/white stripes, hood scoop, front and rear spoiler, sefi and c4, trickflow intake, 70mm mac tb/egr, billet fpr, 24# inj, c&l maf, heavy duty c4 w/transgo shiftkit, msd 6a box w/tfi blaster coil, flowtech longtubes w/turbo mufflers, t-rex inline pump, ported and polished E7TE heads w/1.94,1.60 valves, lunati 292 cam, harland sharp 1.6 rollers, conventional 8" with 2.79 gears (soon will be gone)

04-02-2001, 07:57 PM	#6
shelby2 Registered Member Join Date: Nov 1999 Location: O Fallon, IL Posts: 87	also, i know mine is running rich because i am lucky if i get 10 mpg on the highway. what's your mileage at???

04-02-2001, 08:01 PM	#7
89GTCOUPE Registered Member Join Date: Nov 1999 Location: Sheridan, WY USA Posts: 10	Last time I checked mileage was when it was running good(no backfire) I got 21 mpg going over the mountain pass.

04-03-2001, 01:21 AM	#8
smokedchevy Registered Member Join Date: Jun 2000 Location: ventura,CA Posts: 438	I know you don't have a blower car but mine did the exact same thing.I had a powerdyne blower on my GT and when the internal belt snapped, my car instantly ran better. It would make it backfire and my gas mileage was horrible.It also smelled like raw gas coming out from underneath the car.I also got codes 33 and 91.It could have something to do with airflow? Hope this helps somewhat.Good Luck!

04-03-2001, 02:50 PM	#9
87GTman Guest Posts: n/a	my car has the same POP (backfire) and my MPG is terrible too.. I was thinking O2's or something too the check engine light comes on after a lil bit of driving but then if i cut the car off and restart the light goes off.. also I have a slight bucking at low speeds with part throttle That might be my CAMSHAFT though. I too have longtubes and a NEW MOTOR i checked CTS ACTS, EGR you name it i am stumped too thanx ------------------ SOLD THE 87GT, .. GT40, 70mmTB, 75mmBullet, 24lbinjectors, cold air induction, Edelbrok 6025 Rpm's, Inch phonolic spacer, tall FMSvalve covers, an SFI balancer, Canton 7quart, the shortblock... Lunati 51025 (224/232 .535/. 544 @112 deg) Ross lightweight pistons (10:1) Eagle H-Beams, Total Seal plasma Moly Rings, Clevite bearings, ARP throughout, FMS main Girdle&Studs, Forged Crank, of course HV-OilPump and hardened FMS shaft, Dual Friction centerforce, T5, and 4.10's Longtubes and 2.25 tail pi

04-04-2001, 01:42 AM	#10
89GTCOUPE Registered Member Join Date: Nov 1999 Location: Sheridan, WY USA Posts: 10	Well, my problem ended up being no more than a small crack in one sparkplug insulator. hard to believe something that small could cause all that backfire. Anyhow, thanks for the replies!