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Username Post: TBI problems        (Topic#350219)
persh 
Frequent Contributor
Posts: 1491
persh
Loc: Tucson, Arizona
Reg: 07-19-00
03-21-18 11:12 AM - Post#2728414    

When cold the system work well. When warmed up it will backfire occasionally from a stop with sudden wide-open throttle. It looses power as rpm increases. The system is from a '91 Camaro with a 7747 ecm from a truck.

Things I've tried to fix problem:

1. Checked timing (4 degrees BTDC)
2. Swapped out ESC module with another one I had.
3. Tried two different ECMs
4. Checked the TPS with .67 to 4.05 volt results. Voltaage seems to increase and decrease slowly with throttle movement.
5. Change ignition module with another one I had laying around. Dist cap and rotor looked good.
6. Injectors recently professionally serviced.

So, I'm out of ideas on what is causing the problem. One speed bump is that my service engine light will not work, so I'm not getting any help or clues there. The light used to work fine but recently just wouldn't light up. I've changed lights and know that it will shine when it sees 12v; I've pinned out the wires and they seem okay; I've plugged in different ECMs.

Any help would be greatly appreciated.

'60-'66 Idea Page or Corvair Stuff


 




acardon 
Ultra Senior Member
Posts: 11262
acardon
Loc: DFW TEXAS
Reg: 03-25-05
03-21-18 01:43 PM - Post#2728431    
    In response to persh

I would suspect a O-2 sensor problem. Sounds like it's leaning out on acceleration, after it goes into closed loop.

Don
66 Corvair (driving)
57 2dr HT (driving)
56 2dr HT (waiting to be restored)


 
persh 
Frequent Contributor
Posts: 1491
persh
Loc: Tucson, Arizona
Reg: 07-19-00
03-21-18 04:12 PM - Post#2728443    
    In response to acardon

I forgot to add that to my list. The O2 sensor was replaced as one of the possible culprits just a couple of weeks ago.

'60-'66 Idea Page or Corvair Stuff


 
1983G20Van 
Super Senior Member
Posts: 3762

Loc: Bedford, Texas, USA
Reg: 11-13-02
03-22-18 04:48 PM - Post#2728550    
    In response to persh

Injector size, Prom Chip, Knock sensor, and ESC module need to match the engine size. You said the system was off a Camaro. That would be a 305. Most 7747 truck applications were running 350s.

1983 G20 Van, 350 TPI, Ported 906 Vortecs, Edelbrock 3817 Base, ASM oversize runners. Reed Custom Roller cam, 700r4, 12 bolt with 3.08 gears, Doug Thorley Tri-Y headers, true duals


 
acardon 
Ultra Senior Member
Posts: 11262
acardon
Loc: DFW TEXAS
Reg: 03-25-05
03-23-18 08:15 AM - Post#2728648    
    In response to persh

The fact that it runs well in closed loop, running on the ECM set parameters, but not when using the O-2 sensor still indicates a problem there. Is the O-2 heater working. How far is the sensor from the heads?
Another though just occurred: could it be a knock sensor activated by vibration retarding the ignition?

Don
66 Corvair (driving)
57 2dr HT (driving)
56 2dr HT (waiting to be restored)


 
persh 
Frequent Contributor
Posts: 1491
persh
Loc: Tucson, Arizona
Reg: 07-19-00
03-23-18 03:46 PM - Post#2728685    
    In response to acardon

That's for the tips, Don. I'm going to check out the knock sensor and make sure the O2 heater is working.

In the meantime, I did a pretty bone-headed thing when I reset my timing and checked the functionality of the TPS just before I started this post: I forgot to reconnect the reconnect the timing jumper that goes to the distributor! Holy cow, what a dummy.....

Anyhow, with all the other stuff I did, it runs okay, but just okay. I'm still getting a back fire through the throttle body when give a sudden WOT and it doesn't move out as crisply as it should.

I'm starting to think that the mods I did to the wiring harness I got from the donor car may not be the best and I might have some problems with one or more circuits. I may just buy a wiring harness from one of the aftermarket guys and call it good. I hate to do it and then have everything be exactly the same as it is now. I



'60-'66 Idea Page or Corvair Stuff


 
persh 
Frequent Contributor
Posts: 1491
persh
Loc: Tucson, Arizona
Reg: 07-19-00
03-28-18 10:42 AM - Post#2729226    
    In response to persh

It seems like I changed everything the MAP sensor so I thought, what the heck. I had one laying around and threw it on. It made all the difference. The old 350 woke right up. It's not perfect yet; it still backfires when given sudden WOT, but we're making some good headway.

'60-'66 Idea Page or Corvair Stuff


 
someotherguy 
Senior Moderator
Posts: 28980
someotherguy
Age: 49
Loc: Texas
Reg: 08-01-03
03-31-18 11:20 AM - Post#2729493    
    In response to persh

The mention of the CEL no longer working concerns me. Sounds like you're missing at least one ground, if not more? I wonder if you're getting a reference voltage issue on one or more sensors because of insufficient grounds. This is a retrofit of a TBI engine into an older vehicle right?

Even just weak battery voltage can put a TBI system into despair. When I was piecing back together a stolen/stripped '92 C1500 I thought I had it 100% back together; I'd had to use an older throttle body which meant splicing on the larger TPS connector. When I'd go WOT it would bog down badly, but seemed to drive OK otherwise. I blamed it on the splice and let it go like that because I was selling the truck cheap to a friend that just needed a basic work truck.

It also seemed to need a battery as it would start most of the time but every now and then would act like it needed a jump. Again, let it go like that because selling cheap; all issues disclosed to my buddy before he bought it.

He ended up replacing the battery (dead cell) and the WOT problem went away, too.

Richard

06 Silverado ISS / 06 Silverado SS / 06 300C SRT8


 
persh 
Frequent Contributor
Posts: 1491
persh
Loc: Tucson, Arizona
Reg: 07-19-00
03-31-18 03:45 PM - Post#2729522    
    In response to someotherguy

Yes, this system is on an '83 Suburban 350/700R4 that been transplanted into my '65 pickup. I used the wiring harness that was in the '91 Camaro donor and there were lots of splices involved. It never occurred to me that there might be a ground problem. The CEL used to work just fine but who know what might have happened in the meantime. I'm thinking seriously about buying a basic aftermarket harness from, maybe, Affordable Fuel Injection just to get rid of any gremlins that will probability haunt me later and maybe get my CEL functioning again...plus have the added bonus of getting rid of the messing looking job I did in the beginning. It all worked pretty well for the first 15 years of use but now it's starting to cause a little grief!

On a brighter note, I changed out the temp sensor, which did nothing, and added a brand new coil. I've only driven it a few miles but the coil seems have ended my backfire through the throttle body problem. I hope, I hope, I hope...

'60-'66 Idea Page or Corvair Stuff


 




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