Backfire Through TBI on Acceleration
#1
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Backfire Through TBI on Acceleration
1984 Crossfire Corvette - Getting TBI backfire with moderate throttle under load. Only a soft gradual throttle will eliminate the backfiring.
Reading it may be valve timing or a bad valve or timing or ECM or..... With the following compression readings can I eliminate valve timing and a bad valve?
Readings cold, throttle closed, all other plugs in.
First Cycle/Fourth Cycle.
#1 110/152
#2 110/152
#3 100/150
#4 97/150
#5 98/151
#6 90/140
#7 100/150
#8 100/150
Other Specs
355 Rebuilt with ~ 300 miles on it
75# Injectors
Renegade Manifold
2" Bored TBI's
SBC Street Fighter Cam (Mild Mod- Have Specs)
9.72 to 1 Compr Ratio
Regulated Fuel Pressure
High Perf Fuel Pump - Not sure of specs
Have EBL Set-up - but have not run VE Learns yet - thinking the the backfire issue needs resolution first.
Reading it may be valve timing or a bad valve or timing or ECM or..... With the following compression readings can I eliminate valve timing and a bad valve?
Readings cold, throttle closed, all other plugs in.
First Cycle/Fourth Cycle.
#1 110/152
#2 110/152
#3 100/150
#4 97/150
#5 98/151
#6 90/140
#7 100/150
#8 100/150
Other Specs
355 Rebuilt with ~ 300 miles on it
75# Injectors
Renegade Manifold
2" Bored TBI's
SBC Street Fighter Cam (Mild Mod- Have Specs)
9.72 to 1 Compr Ratio
Regulated Fuel Pressure
High Perf Fuel Pump - Not sure of specs
Have EBL Set-up - but have not run VE Learns yet - thinking the the backfire issue needs resolution first.
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Car: 91 Trans am
Engine: built 360 TBI
Transmission: built 700r4
Axle/Gears: 10bolt/3.23
Re: Backfire Through TBI on Acceleration
I could not tell from the compression readings. But did you already check the spark plugs, fuel pressure and double check initial timing?
#5
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Re: Backfire Through TBI on Acceleration
I did. Ran through these and others (TPS, Map, O2, Vacuum) more than once. Got frustrating. Why I was thinking valve timing. I'll have to try that accel enrichment adjustment. My laptop battery died so it will be a bit.
Was hoping the compression readings eliminated the potential of a valve issue. Thanks.
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Re: Backfire Through TBI on Acceleration
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Car: 91 Trans am
Engine: built 360 TBI
Transmission: built 700r4
Axle/Gears: 10bolt/3.23
Re: Backfire Through TBI on Acceleration
If no signs of oil combustion on the spark plugs, then it is probably timing and/or fueling related.
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#8
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Re: Backfire Through TBI on Acceleration
A better test of valve sealing would be a leakdown test using compressed air injected into the spark-plug hole. ANY leakage past intake or exhaust valves (hearing air escaping from the throttle body or tailpipe) is not acceptable. ANY air escaping into the radiator (bubbles in the cooling system with rad cap removed) is unacceptable. SOME, minimal leakage into the crankcase (hearing air escaping out of the oil-fill hole in the valve cover) can be acceptable, but too much is not acceptable. A real leakdown tester can put numbers to the leakage helping you determine what's acceptable leakage into the crankcase.
Last edited by Cartrax; 05-05-2014 at 11:40 AM.
#9
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Re: Backfire Through TBI on Acceleration
#10
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Re: Backfire Through TBI on Acceleration
Progress - still waiting on a laptop battery (China express) but I built a manometer and balanced the TB's. In the process I jacked the idle up to about a grand - not intentional. Thought you couldn't do that ... anyway, will have to drop that later when I change the injectors and now I have a balancing under my belt.
During the test drive I noticed less backfiring. Guessing that's cuz I'm pumping more fuel into the manifold with the higher idle - that's probably a good indication it is a fuel enrichment issue. No?
During the test drive I noticed less backfiring. Guessing that's cuz I'm pumping more fuel into the manifold with the higher idle - that's probably a good indication it is a fuel enrichment issue. No?
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Re: Backfire Through TBI on Acceleration
Verify timing.
But with the manifold you use and the 2.0" TB it needs a lot more AE. I would start with 10% more AE for TPS and MAP both. You may see improvement. From there add another 5% and again see how it responds. You may end up with a lot more needed yet.
you have a very large intake and TB's. It all needs to be wetted. When you left off gas or depress the throttle Vac decreases and it goes dry.
Be aware the EBL allows you to add more at certain RPM's. You may find it needs more <2000 rpms but not > 2000 rpms.
I had a similar set up on my 84 CF.
But with the manifold you use and the 2.0" TB it needs a lot more AE. I would start with 10% more AE for TPS and MAP both. You may see improvement. From there add another 5% and again see how it responds. You may end up with a lot more needed yet.
you have a very large intake and TB's. It all needs to be wetted. When you left off gas or depress the throttle Vac decreases and it goes dry.
Be aware the EBL allows you to add more at certain RPM's. You may find it needs more <2000 rpms but not > 2000 rpms.
I had a similar set up on my 84 CF.
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Car: 91 Trans am
Engine: built 360 TBI
Transmission: built 700r4
Axle/Gears: 10bolt/3.23
Re: Backfire Through TBI on Acceleration
Going by the description of the new engine, I would first set up fuel pressure and a new bin with the basic adjustments, according to the new hp output, if it has not been done yet.
These links contain input that may help you if needed, to set up the new bin
https://www.thirdgen.org/forums/post...4-post178.html
https://www.thirdgen.org/forums/post...0-post207.html
These links contain input that may help you if needed, to set up the new bin
https://www.thirdgen.org/forums/post...4-post178.html
https://www.thirdgen.org/forums/post...0-post207.html
#13
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Re: Backfire Through TBI on Acceleration
Thanks for the advice/direction, guys - exactly what I'm looking for. Picked the right forum. Will post results...
#14
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Re: Backfire Through TBI on Acceleration
Took it out today and it seems to be running worse. Missing and backfiring on the slightest load. This without touching anything.
Reading the referenced posts - not understanding much yet - so many options - where to start. Will have to hit the books & Dynamic EFI's tuning intros again. Will keep your suggestions in mind as I'm going through things but I have a ways to go before I can even appreciate "em. Will be back...but not for a while. Probably need to start with some basic VE learns - if there such a thing. Thanks.
Reading the referenced posts - not understanding much yet - so many options - where to start. Will have to hit the books & Dynamic EFI's tuning intros again. Will keep your suggestions in mind as I'm going through things but I have a ways to go before I can even appreciate "em. Will be back...but not for a while. Probably need to start with some basic VE learns - if there such a thing. Thanks.
Last edited by Cartrax; 05-09-2014 at 08:08 PM.
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Car: 91 Trans am
Engine: built 360 TBI
Transmission: built 700r4
Axle/Gears: 10bolt/3.23
Re: Backfire Through TBI on Acceleration
Once your laptop is operational, post a complete list of the engine specs including cam, so we can help.
Once you a have a base bin, set up for your engine, it will be the good time to do VE learns.
Once you a have a base bin, set up for your engine, it will be the good time to do VE learns.
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#17
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Re: Backfire Through TBI on Acceleration
355 Rebuilt with ~ 300 miles on it
75# Injectors
Renegade Manifold
2" Bored TBI's
SBC Street Fighter Cam
SBC Street FighterHydraulic cam 288°/292° .450/.460 lift. Duration @ .050 lift 224/224° degint/exh, lobe seperation 114°, advance 2°
9.72 to 1 Compr Ratio
Regulated Fuel Pressure
High Perf Fuel Pump - Not sure of specs
Have EBL Set-up - but have not run VE Learns yet - thinking the the backfire issue needs resolution first.
Will hook up to EBL later today. Recommendations on where to start? Thanks.
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Car: 91 Trans am
Engine: built 360 TBI
Transmission: built 700r4
Axle/Gears: 10bolt/3.23
Re: Backfire Through TBI on Acceleration
...
355 Rebuilt with ~ 300 miles on it
75# Injectors
Renegade Manifold
2" Bored TBI's
SBC Street Fighter Cam
SBC Street FighterHydraulic cam 288°/292° .450/.460 lift. Duration @ .050 lift 224/224° degint/exh, lobe seperation 114°, advance 2°
9.72 to 1 Compr Ratio
Regulated Fuel Pressure
High Perf Fuel Pump - Not sure of specs
Have EBL Set-up - but have not run VE Learns yet - thinking the the backfire issue needs resolution first.
Will hook up to EBL later today. Recommendations on where to start? Thanks.
355 Rebuilt with ~ 300 miles on it
75# Injectors
Renegade Manifold
2" Bored TBI's
SBC Street Fighter Cam
SBC Street FighterHydraulic cam 288°/292° .450/.460 lift. Duration @ .050 lift 224/224° degint/exh, lobe seperation 114°, advance 2°
9.72 to 1 Compr Ratio
Regulated Fuel Pressure
High Perf Fuel Pump - Not sure of specs
Have EBL Set-up - but have not run VE Learns yet - thinking the the backfire issue needs resolution first.
Will hook up to EBL later today. Recommendations on where to start? Thanks.
Cam intake and exhaust centerline, cylinder head flow numbers and valve size, exhaust, intake manifold flow numbers
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Re: Backfire Through TBI on Acceleration
As for the AE, start by multiplying the MAP & TPS AE PW tables by 1.2 (20% higher).
Then multiply the MAP & TPS AE Filter tables by 0.9 (10% lower). This will increase the duration of the AE event.
But make sure that the fuel pressure is stable, otherwise the tune will go in circles.
RBob.
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#22
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#23
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Re: Backfire Through TBI on Acceleration
Make sure that the fuel pressure is stable and holds under load. Using a pressure sensor and logging it is the best way.
As for the AE, start by multiplying the MAP & TPS AE PW tables by 1.2 (20% higher).
Then multiply the MAP & TPS AE Filter tables by 0.9 (10% lower). This will increase the duration of the AE event.
But make sure that the fuel pressure is stable, otherwise the tune will go in circles.
RBob.
As for the AE, start by multiplying the MAP & TPS AE PW tables by 1.2 (20% higher).
Then multiply the MAP & TPS AE Filter tables by 0.9 (10% lower). This will increase the duration of the AE event.
But make sure that the fuel pressure is stable, otherwise the tune will go in circles.
RBob.
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Car: 91 Trans am
Engine: built 360 TBI
Transmission: built 700r4
Axle/Gears: 10bolt/3.23
Re: Backfire Through TBI on Acceleration
I use the Actron 7817, with the T on the feed line next to the alternator, nothing fancy, but it works.
#25
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Re: Backfire Through TBI on Acceleration
Just replaced injectors today with a set of 90's I bought last year. Started fine - no road test yet.
Actually not real sure now what the old ones where - was told they were 75's.
Part number on one of the old one's indicated it was a 40, the other had no part number. Anyway, planning on re-setting fuel pressure to 18psi - per BPC calc should yield about 400 ponies. Plenty for me - any comments?
Checking timing in the morning then will set up new bins then start working suggestions from there - think this makes sense if not let me know - thanks.
Actually not real sure now what the old ones where - was told they were 75's.
Part number on one of the old one's indicated it was a 40, the other had no part number. Anyway, planning on re-setting fuel pressure to 18psi - per BPC calc should yield about 400 ponies. Plenty for me - any comments?
Checking timing in the morning then will set up new bins then start working suggestions from there - think this makes sense if not let me know - thanks.
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Car: 91 Trans am
Engine: built 360 TBI
Transmission: built 700r4
Axle/Gears: 10bolt/3.23
Re: Backfire Through TBI on Acceleration
I put the gauge under the wiper arm, to verify if pressure drops under load. Pressure must stay constant.
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Car: 88 Firebird Formula
Engine: LO5 5.7 TBI/Ebl Flash
Transmission: 700R4
Axle/Gears: 2.73 stock rear end
Re: Backfire Through TBI on Acceleration
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Car: 91 Trans am
Engine: built 360 TBI
Transmission: built 700r4
Axle/Gears: 10bolt/3.23
Re: Backfire Through TBI on Acceleration
Mine is just long enough to go from the T next to the alternator to fit under the wiper arm, though I bought it around 8 years ago, not sure they are still made the same length.
#31
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Re: Backfire Through TBI on Acceleration
Make sure that the fuel pressure is stable and holds under load. Using a pressure sensor and logging it is the best way.
As for the AE, start by multiplying the MAP & TPS AE PW tables by 1.2 (20% higher).
Then multiply the MAP & TPS AE Filter tables by 0.9 (10% lower). This will increase the duration of the AE event.
But make sure that the fuel pressure is stable, otherwise the tune will go in circles.
RBob.
As for the AE, start by multiplying the MAP & TPS AE PW tables by 1.2 (20% higher).
Then multiply the MAP & TPS AE Filter tables by 0.9 (10% lower). This will increase the duration of the AE event.
But make sure that the fuel pressure is stable, otherwise the tune will go in circles.
RBob.
Changed BPC vs VAC and BST-BPC vs Boost to BPC Calc Values
Added 20% and Reduced 10% Values Noted Per Above
Set AE->TPS% Double Map AE to 100% per Tuning Instructions
Saved values and new BIN
Flashed new Bin
Test Drove....
Still getting backfire on accel with slightest load. Little performance change noted.
How can I verify values were accepted by ECM?
Also, no way of testing stability yet - part on order though.
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Car: 75 Beast
Engine: 383 +EBL Flash
Transmission: 700R4
Axle/Gears: 4.11 with 33"
Re: Backfire Through TBI on Acceleration
In WUD you can always download BIN file from ECM into a new file and perform BIN file compare in the TP.
Just a sanity check - how do you check your base timing???
//RF
Just a sanity check - how do you check your base timing???
//RF
#34
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Re: Backfire Through TBI on Acceleration
Engine at Op temp, bypass disconnected, timing light. Engine starts right up - if that's an indication of anything. Anything else I should be checking? Thanks.
Last edited by Cartrax; 06-07-2014 at 09:46 AM.
#35
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Re: Backfire Through TBI on Acceleration
Thanks will look at this. I'm thinking maybe I should recreate the BIN starting from scratch just to ensure nothing crept in there to create issues. Recommend a base/starting BIN?
#36
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Re: Backfire Through TBI on Acceleration
Additional specs(?) I found this looking through some papers. It was handwritten on the engine rebuild invoice. I did not deal directly with the rebuild shop and parted company with this mechanic not on the best of terms. (He wanted to carb it against my instructions.)
Exactly as written - not sure if any of this helps.
Aluminum Heads
CAM 050 224/224 114° Lobe Sep 2°
In .450 Lift
Ex .460
Heads 245 at .450 Lift
64CC Flat Piston
9.72 to 1 Compression
Another shop pulled the engine and installed a new balancer - they could have distorted the cam timing. Does the engine need to be pulled to check this?
Car has long tube headers installed.
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Re: Backfire Through TBI on Acceleration
When I flash in a new bin I typically name it by that/todays date. Sometimes I add after date ae or pe or propgain whatever the change was. Like 060514propgain.bin. If the flash goes well you will see the green acknowledgement message and the dash light will blink. Be assurred it is running on the new bin.
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Re: Backfire Through TBI on Acceleration
RBob.
#39
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Re: Backfire Through TBI on Acceleration
Did you replace the stock harmonic balancer?
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#41
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Re: Backfire Through TBI on Acceleration
When I flash in a new bin I typically name it by that/todays date. Sometimes I add after date ae or pe or propgain whatever the change was. Like 060514propgain.bin. If the flash goes well you will see the green acknowledgement message and the dash light will blink. Be assurred it is running on the new bin.
#42
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Re: Backfire Through TBI on Acceleration
Yes, there are constants in the conversion formulas that limit the amount of change. VE tables,as an example, must change > .39%. Having said that, I have found that I can 0 out a value, save the bin, then replace the 0 w a new value even though it was < .39% than the original VE. Don't know what the value of this may be.
#43
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Re: Backfire Through TBI on Acceleration
Revving it made it jump initially another ~6 then it stopped moving when revved after that. Shouldn't I see it advance more as I kick up the RPMs?
#45
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Re: Backfire Through TBI on Acceleration
Have you verified that dizzy is positioned correctly? That you are not one or two teeth off on cam drive? Just askin.
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#48
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Re: Backfire Through TBI on Acceleration
Can you elaborate on your set up and how the test is performed or reference a thread? Where does that run-off typically go as you're driving?
Thanks.
Last edited by Cartrax; 06-10-2014 at 07:00 PM.
#49
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Re: Backfire Through TBI on Acceleration
Take the D/S valve cover off, turn crank until balancer mark is at 0 or maybe 6d TDC. #1 I/E valves should both be closed. Now look at distributor rotor. Is it pointing at # 1 cylinder on dizzy cap? It should be right there. If not, you're distributor is not set properly.
#50
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Re: Backfire Through TBI on Acceleration
Going back to basics, I've seen plug wires do exactly what you are describing several times. Try a spray bottle with the engine running in the dark and watch for sparks. Dist. Cap can do that too.