1991 Chevy 454 backfires, runs good, backfires, sometimes cycles

I have a 1991 Chevy 3500 454 with auto trans. It was running fine, the next day running rough. I changed plugs, wires, dist., coil.. Didn't help..I started it yesterday the engine ran good. A couple hours later I started it again, ran smooth when I first started it. After it warmed up it started running rough, when giving it gas it would backfire. Today I changed the 02 sensor, started running rough when first started, then it would cycle run smooth, run rough. Then it would continue running rough.. Engine light has been on. I borrowed a OBD scaner and no code would come up. I am not sure if scaner is any good. The guy I borrowed it from says its good. Maybe I need a different kind. Everytime it runs rough, it will backfire when stepping on gas .This motor is TBI....HELP

check the fuel pressure should be 45+.if pressure holds at 45+ .how long since the fuel filter was changed? when a bad fuepump backpressures it will pump 45+ pressure then the pressure will drop down to as low as 10 pounds .if you could rig a pressure gauge in where the fuel filter is then you can run the motor and monitor the fuel pressure when it acts up. this is the way i found out my fuelpump was backpressuring.some fuelpumps die quick but you have some that want to be a pain in the *** as they get ready to quit.

Rockwood84....Thanks for the information. The fuel filter hasn't been changed in a long long time. When I bought the 02 sensor I also bought a fuel filter but have not put it on yet. I will put the fuel filter on and if that doesn't work I will get the fuel pressure. THANKS AGAIN....Petecheck the fuel pressure should be 45+.if pressure holds at 45+ .how long since the fuel filter was changed? when a bad fuepump backpressures it will pump 45+ pressure then the pressure will drop down to as low as 10 pounds .if you could rig a pressure gauge in where the fuel filter is then you can run the motor and monitor the fuel pressure when it acts up. this is the way i found out my fuelpump was backpressuring.some fuelpumps die quick but you have some that want to be a pain in the *** as they get ready to quit.

i was just informed that an '88 tbi is only supposed to have 9/13 pounds fuel pressure so a '91 is still a tbi set up so check for 9/13 pounds fuelpressure instead of 45 pounds.

I installed a pressure gauge and had 13 pounds of pressure. thanks....i was just informed that an '88 tbi is only supposed to have 9/13 pounds fuel pressure so a '91 is still a tbi set up so check for 9/13 pounds fuelpressure instead of 45 pounds.

do you put any injector cleaner in the gas tank as the tbi is an injector and the injector cleaner will keep the tbi clean. is the ses[service engine soon ] light on?

Rockwood, Yes, I have used injector cleaner and Yes, the service engine light is on. I need to know what scaner I need to read the faults. Tried a jumper on A and B terminals, tried a cheap plug in scaner. Have had no luck with either one. Any suggestions. Thanks for all your help..Petedo you put any injector cleaner in the gas tank as the tbi is an injector and the injector cleaner will keep the tbi clean. is the ses[service engine soon ] light on?

I'm going to move this thread to the C/K forum.

Onto your problem. If it always runs smooth on a cold start then your pcm must be working ok since its operates off of predetermined information stored in the pcm. That is rather than taking data from sensors on and around the engine it is running in closed loop until the engine coolant reaches a specified temperature. Thus the engine should be mechanically sound and there is a faulty sensor sending bad information to the pcm once the system goes into open loop and begins to use information from sensors to adjust fuel ratio, timing ect.

There are lots of things it could be. A dirty IAC valve, this is mounted to the TBI with two torx screws with a little pintle that controls airflow in the TBI. It could be injectors with a bad spray pattern.

When you jumped the ALDL did the ses flash a code 13 and then some other codes?

One other thought and its a longshot. Have your timing checked. there is a connector under passengers side of dash where the wiring harness comes through firewall and you will see a tan with a black striped wire with a connector connecting the wires together, this is the est bypass wire, you need to unplug this connector and set timing to 0 degrees then turn off engine then reconnect the connector.

Hope this helps.

Shortbus, Thanks for the info. I replaced the distributor so I did time the engine. Not sure if I disconneted the right wire. I will time engine again. Truck will run rough and backfire through the TBI when its cold sometimes. Sometimes it will run great when cold. Sometimes it will cycle. Run rough and instanly smooth out. There's no pattern. The service engine light stays on. It will go off for a split second. When it goes off the engine instanly runs smooth. It might do this three or four times. The light will then stay on. Sometimes the engine will continue to run rough, sometimes it will run smooth with the light still on.. I have to check for a broken wire on the plug where you connect a scanner so I can get a code. Will check and let you know how I make out. Thanks again, This site has really helped. Pjksr1955 One other thought and its a longshot. Have your timing checked. there is a connector under passengers side of dash where the wiring harness comes through firewall and you will see a tan with a black striped wire with a connector connecting the wires together, this is the est bypass wire, you need to unplug this connector and set timing to 0 degrees then turn off engine then reconnect the connector.

Hope this helps.

Shortbus, When I jumped the ALDL... A and B terminals nonthing happens. I have to check for a broken wire. Even when I disconnect battery for a couple days, the service engine light will not clear. It stays on...Thanks, Pjksr1955 I'm going to move this thread to the C/K forum.

Onto your problem. If it always runs smooth on a cold start then your pcm must be working ok since its operates off of predetermined information stored in the pcm. That is rather than taking data from sensors on and around the engine it is running in closed loop until the engine coolant reaches a specified temperature. Thus the engine should be mechanically sound and there is a faulty sensor sending bad information to the pcm once the system goes into open loop and begins to use information from sensors to adjust fuel ratio, timing ect.

There are lots of things it could be. A dirty IAC valve, this is mounted to the TBI with two torx screws with a little pintle that controls airflow in the TBI. It could be injectors with a bad spray pattern.

When you jumped the ALDL did the ses flash a code 13 and then some other codes?

Make sure that the wires going to one of the injecters is not pinched or has a necked place in it,, tbi are bad about getting between the breather and throttle body,,, tuned look along where the wires run down along the intake or head,, ALSO SOME VEHCILES HAVE 2 OXG SENSORS AND THE WILL NOT ALWAYS GIVE OUT CODES IF THEY ARE BAD

Did you replace the entire distributor? Just wondering since you can put them in 180 degree's off. Or even one gear tooth off and that can cause the problems you describe.

If you did you might double check by bringing number 1 cylinder up to TDC on the comp stroke and make sure the rotor is pointing to number one on the cap.

Rockwood, Yes, I have used injector cleaner and Yes, the service engine light is on. I need to know what scaner I need to read the faults. Tried a jumper on A and B terminals, tried a cheap plug in scaner. Have had no luck with either one. Any suggestions. Thanks for all your help..Pete the cheap scanner just do the a b terminal short and beep the codes. when you jump the a b terminals did you use a piece of wire ? i always cut a paper clip and it is just the right size to plug in a b terminals with. when you put the paper clip in turn the key on do not crank it. the ses light will flash a 12 3 times then it will flash the codes 3 times each then when there are no more codes it will flash a 12 3 more times to tell you there are no more codes. it will flash a 12 by flashing a 1 then 2 flashes and so on.if it does not work try rubbing the terminals in and out with the paperclip to clean the terminals using the paperclip.could be the terminal[a b] are dirty and not letting it jump.

I use a large staple for this, but any U-shaped wire will do. If your engine light is on, you should be getting codes. If not, something is up. If the engine light is coming on even after the battery is disconnected, then I would suspect that whatever the problem is, it is triggering a code as soon as the truck is started. I'll leave it to the experts on which problems would instantly trigger a code.

Shortbus, Yes, I did change the entire distributor. I will check TDC this weekend. I changed dist. because truck was running bad. I thought it might be the ecm ? When I put new one in it ran the same.. thanks again, pjksr1955Did you replace the entire distributor? Just wondering since you can put them in 180 degree's off. Or even one gear tooth off and that can cause the problems you describe.

If you did you might double check by bringing number 1 cylinder up to TDC on the comp stroke and make sure the rotor is pointing to number one on the cap.