Backfiring is killing me!!!

[rang-a-stang]

I can't Kill this backfire and it's driving me BONKERS!!!
AMC 401, warmed over, 454 TBI, Dizzy locked at 0, computer controlled timing, brand new MSD TBI coil, brand new Taylor 8mm spiral core wires, brand new Autolite Copper core plugs (gapped at .0454), New brass terminal Duraspark wide cap, new brass terminal rotor.

My truck runs AMAZING when it is stone cold and going less than 50MPH. If I accelerate to get on the freeway, at about 60 or 65 it starts to backfire out the exhaust. It has been doing this since my rebuild finished (including the installation of my EFI). I have replaced rotor, cap, wires, plugs, and coil. When it is warm, under steady throttle, at 55 or so, it will backfire and run pretty poorly.

1) Could I have a Mechanical engine problem causing this? Cam timing? pushrods too long? obstruction in my intake? leaks out my injectors? Valve issues?
2) I plugged my PCV port for now. I plugged all the other vacuum ports for now, other than trans modulator and brake booster. I watch my MAP sensor and my vacuum is pretty darn steady so I assume I don't have a vacuum leak. Maybe I do?
3) I don't hear any exhaust leaks... My 4 wire O2 Sensor is in my header collector on the passenger side (long tube headers), going to a 3" Y-pipe and out through a single 3" exhaust, no cat, but I do have a Borla Turbo muffler. Could it be an exhaust problem?

I am running out of ideas. I am starting to think I need to pull my EFI and go back to carb... Not sure where to go! This is killing me!!!!

No question/idea is to basic or out of the box. I am not too proud to accept ideas or questions that someone with common sense would have thought of.

[wiley-moeracing]

I would check your valves first, can be a valve hanging up when warm, but it sounds like an electrical issue when it gets hot also, coil pick up , ignition box etc.

[Mikel]

Are you able to get data from your EFI computer?


Am I right to assume that backfiring would only happen if:


a) There are momentary ignition lapses


b) Engine is running super rich and dumping fuel down the exhaust. What's your O2 sensor reading?

[78 WIDETRAC]

Are you able to get data from your EFI computer?


Am I right to assume that backfiring would only happen if:


a) There are momentary ignition lapses


b) Engine is running super rich and dumping fuel down the exhaust. What's your O2 sensor reading?

I agree with this. Good place to start.

[PlasticBoob]

It has been doing this since my rebuild finished (including the installation of my EFI).

Did you run a carburetor after the rebuild, and did it do this when you were running the carburetor but before the EFI was installed?

[wiley-moeracing]

If running super rich this would happen hot and cold, miss on the ignition side a possibility since its only happens when hot( ignition module, distributer pick up, coil) or mechanical, valve sticking when it gets hot. You may need to check your temp sensor, could be reading wrong causing it to lean out severely. Did you run this with a carb before, and it ran ok? then you have issues on the injection side. go back and recheck everything.

[Driftwood]

Has a compression check been done to confirm valves are completely closing?

[levelhead]

If valve train checks out...The easiest next check is to pull your distributor and stuff in an HEI dizzy (its only a 1 wire hook up)....if the problem goes away...you know its ignition related......if not I'd pull only as much of the FI as needed and plunk a known good carb on.

[rang-a-stang]

I would check your valves first, can be a valve hanging up when warm, but it sounds like an electrical issue when it gets hot also, coil pick up , ignition box etc.

Copy that, Tim. I ordered a new adjustable push rod and am going to measure "soon". I just got called out of town for a week so this will sit until I get back.

Are you able to get data from your EFI computer?
Am I right to assume that backfiring would only happen if:
a) There are momentary ignition lapses
b) Engine is running super rich and dumping fuel down the exhaust. What's your O2 sensor reading?

Yep (data from computer). That is how I understand backfires. Backfire in the exhaust is un-burned fuel getting into the exhaust and igniting.
a) In addition to ignition lapses, it could be ignition timing errors (igniting very late, or too early).
b) O2 sensor looks "normal" in my data log. I plan to run open loop (disconnect my O2 sensor) and see if that makes a difference. I also plan to spend a lot of time in my data logs while I am on travel this week to see if I can find anything else.

I agree with this. Good place to start.

concur!

Did you run a carburetor after the rebuild, and did it do this when you were running the carburetor but before the EFI was installed?

Nope. I am a knuckle head. I sold my carb before I finished installing my engine.

If running super rich this would happen hot and cold, miss on the ignition side a possibility since its only happens when hot( ignition module, distributer pick up, coil) or mechanical, valve sticking when it gets hot. You may need to check your temp sensor, could be reading wrong causing it to lean out severely. Did you run this with a carb before, and it ran ok? then you have issues on the injection side. go back and recheck everything.

concur with everything you wrote. My temp sensor looks (in data) to be acting normal. Starts cold and warms up. I will pay more attention to it this week and let you know.
Nope, I was a knuckle head and sold the carb. (more below)

Has a compression check been done to confirm valves are completely closing?

Not yet. It's on my list. I "think" they are closing because it runs pretty well when cold but will check. I have made so many mistakes in this build I am a little scared of what I might find.

If valve train checks out...The easiest next check is to pull your distributor and stuff in an HEI dizzy (its only a 1 wire hook up)....if the problem goes away...you know its ignition related......if not I'd pull only as much of the FI as needed and plunk a known good carb on.

I am almost there. I have a few more items to check. (more below)
*****************************************
I have not touched my rig in a few days (other home tasks and life keep getting in the way).

Here is where I stand.
1) This week I will order a new/good adjustable pushrod and am going to get the correct pushrods. Whether they are causing this issue or not does not mean I should not fix them. They will get corrected.
2) I need a data log in open loop. Going to run the stock 454 tune in open loop (no O2 sensor) and that could tell me if it is my fuel tables are in the ball park. Also, if my O2 sensor is really hosed, it may run better in open loop.
3) I am going to spend a lot of time looking at my logs this week because I am still new at this. Maybe the issue is obvious but I am not seeing it.
4) I ordered a cheapie distributor cap so i can cut a hole in it and verify my Hall sensor phasing (idea from a guy on theamcforum). I have a little hope my issue may be here. When I locked my hall sensor down, it just kind of laid it where the wires wanted it to be and locked it down. I wonder if it is off and when i got to max timing, it skips to the wrong post?
5) I have a carb spacer between the intake and the TBI adapter. Going to pull that out for now.
6) If, after all that, if I am still backfiring, I am going to put my duraspark back in. I still have the module, wiring, and coil so I just need an actual dizzy. I am pretty sure I can find one and stab it in there. I would also check compression at this point.
7) If that still does not work, I will buy a carb, swap fuel pumps, and try that.

I am tossing the idea around of a new hall sensor. The one in the dizzy is the one that was in my truck when I bought it (it looks old and a little crusty. But when I look at my timing and RPM in my data logs, it looks OK. I would assume if it was going, I would see either spikes or something, right?

[PlasticBoob]

Yep (data from computer). That is how I understand backfires. Backfire in the exhaust is un-burned fuel getting into the exhaust and igniting.
a) In addition to ignition lapses, it could be ignition timing errors (igniting very late, or too early).
b) O2 sensor looks "normal" in my data log. I plan to run open loop (disconnect my O2 sensor) and see if that makes a difference. I also plan to spend a lot of time in my data logs while I am on travel this week to see if I can find anything else.


2) I need a data log in open loop. Going to run the stock 454 tune in open loop (no O2 sensor) and that could tell me if it is my fuel tables are in the ball park. Also, if my O2 sensor is really hosed, it may run better in open loop.
3) I am going to spend a lot of time looking at my logs this week because I am still new at this. Maybe the issue is obvious but I am not seeing it.

I am tossing the idea around of a new hall sensor. The one in the dizzy is the one that was in my truck when I bought it (it looks old and a little crusty. But when I look at my timing and RPM in my data logs, it looks OK. I would assume if it was going, I would see either spikes or something, right?

Very good. I was going to suggest looking at your tach input very closely and seeing if you are getting the correct counts just before, and during, the backfire. That's always step #1. Sometimes a lean/rich backfire or misfire can actually cause the tach signal to drop out, which then causes the problem to amplify.

Right now, I'm of the opinion that this is a tuning issue, and your fuel table in the MAPxRPM ranges at these higher steady loads is messed up. You are not seeing the issue below 55mph at the same load because the engine isn't spending enough time in those RPM areas as you romp it. I'd verify the fuel and spark tables are correct for those ranges before moving on.

My truck runs AMAZING when it is stone cold and going less than 50MPH. If I accelerate to get on the freeway, at about 60 or 65 it starts to backfire out the exhaust.

All of this occurs when it's cold? So when it's cold and you get up to 60/65 it still backfires?

When it is warm, under steady throttle, at 55 or so, it will backfire and run pretty poorly.

So whether it's hot or cold, it still runs crappy at 55+? Any video of this happening?

Can you post up some graphs of your timing, RPM, and injector pulsewidth while it's acting up?

Going to run the stock 454 tune in open loop

I can't imagine a stock 454 tune would run anywhere near decently at any RPM on any 401.

[PlasticBoob]

2) I plugged my PCV port for now. I plugged all the other vacuum ports for now, other than trans modulator and brake booster. I watch my MAP sensor and my vacuum is pretty darn steady so I assume I don't have a vacuum leak. Maybe I do?

Probably not related to your issue, but how tight is the fit between the actual PCV valve and the rubber grommet on the intake manifold? Sometimes they loosen up and induce a vacuum leak.

https://youtu.be/AVWyegzD3UQ

[Mikel]

Temp sensor/switch working OK? Maybe the computer thinks the engine is cold at all times and makes it run very rich when it gets warm.

[WT77]

Couple questions about your setup:

1) Are you saying that you are running a throttle body EFI taken off a GM big block 454?

2) Has the fuel mapping been adjusted for use in an AMC small block 401 or are you running stock 454 mapping?

2) Are you sure the backfires are occurring out the exhaust and not out of the EFI air horn?

3) Do you have the ability to adjust the acceleration pump squirt?

4) Are you certain that locking the timing at zero degrees is correct for this application? Are you removing vacuum advance when you lock the base timing? I'm not sure that the timing required by a big block 454 will work for a small block 401. Perhaps you need to adjust the base timing so the computer will produce timing closer to what the 401 requires. I would be concerned about the timing curve as well. I am certain that a 454 has a much different timing curve than a 401. The 454 is a much different animal than the 401 in many ways.

[rang-a-stang]

Probably not related to your issue, but how tight is the fit between the actual PCV valve and the rubber grommet on the intake manifold? Sometimes they loosen up and induce a vacuum leak.

https://youtu.be/AVWyegzD3UQ

I unhooked my PCV for now and plugged the port on the TBI side. But when I hook it back up, I will check! Good lookin out!

Temp sensor/switch working OK? Maybe the computer thinks the engine is cold at all times and makes it run very rich when it gets warm.

Yeah, It seems to be. More below with my history monitors. You can see the purple line where it gets warm, then hangs out at warm.

Couple questions about your setup:
1) Are you saying that you are running a throttle body EFI taken off a GM big block 454?
2) Has the fuel mapping been adjusted for use in an AMC small block 401 or are you running stock 454 mapping?
2) Are you sure the backfires are occurring out the exhaust and not out of the EFI air horn?
3) Do you have the ability to adjust the acceleration pump squirt?
4) Are you certain that locking the timing at zero degrees is correct for this application? Are you removing vacuum advance when you lock the base timing? I'm not sure that the timing required by a big block 454 will work for a small block 401. Perhaps you need to adjust the base timing so the computer will produce timing closer to what the 401 requires. I would be concerned about the timing curve as well. I am certain that a 454 has a much different timing curve than a 401. The 454 is a much different animal than the 401 in many ways.

1) Yep!
2) Both. As a wise man once said (babywag) you can't tune away problems; I started to adjust the fuel and timing tables but stopped because I have problems that need fixing. So I have a set of tables that are rough draft for a 401 and I have stock 454. I used them both and recorded data on both.
2) Yep!
3) Yep!
4) Yep! I have the ability to set my timing wherever I want in the BIN. I have the BIN set up so the Dizzy is at 0 and the computer does the rest based on inputs from the MAP and engine RPM signal from my hall sensor. Yep (vacuum advance removed, also). Good point (454 vs 401 timing). In my first set of logs below I have a set of fuel tables and timing tables I rough drafted and drove on for the 401. The second set are basically stock 454 fuel and timing tables. I can change from tune to tune very quickly and easily. I do not have my knock sensor mounted in the block yet but that is coming. Once it is mounted, I will really dial in the timing. Very true! (454 different animal than 401). My plan is to get it "working", then I can dial in the fuel and timing curves to work for a 401. 454 is the cloesest engine to work with because I am .030 over so it's really 407. 454 is only 47 cubic inches from me vs. 350 being 57 cubic inches. In the MAP I am able to set cylinder volume so my BIN knows its working with 407 cubic inches and not 454.
***************************************
I was on a plane for several hours so I dug into my data logs. Here is my driving route (this is basically cut and pasted from my build thread):

Red lines are 45 or 50 MPH speed limit, black is 65. Run 1 is about 2 miles, run 2 is about 3/4 of a mile, run 3 is about 2.25 miles.

I made one lap with my tables, then swapped to the stock 454 tables. Power enrichment is turned off. I rarely get a flash of Acceleration enrichment here and there but it does not coincide with a backfire.
Green=oxygen sensor
Dark blue= Vehicle speed sensor
Light blue=Engine RPM
Red=Throttle position Sensor
Purple=engine temp
Sky blue=Degrees of timing
Orange=MAP
Here is how it looks as I leave my house on run 1. It runs AMAZING here. You can see the temp is pretty cool and rising. It is open loop for most of this.

Then I sit at the stop light and head up onto the freeway for run 2. Where I have the marker, is right before the first backfire, of 2'ish.

Then I get off the freeway and head down run 3. This is usually a steady throttle, 50MPH run. Where you see me let go of the throttle, that is because it is back firing.


Then I swap over to the 454 BIN.
Run 1, it was stumbling a little but the idle was amazing.

Merging onto the freeway again, run 2 with the 454 bin. It backfired sooner and more often, like 4-6 times. Where the marker is, is about where it was the worst.

Then the last bit as I headed down run 3. It was backfiring pretty bad here.


So I do not see a common indicator of when it backfires. I do not see any of these sensors showing me anything that looks abnormal. The data looks consistent warm and cold.

Anyone else see anything odd in these?

[rang-a-stang]

Very good. I was going to suggest looking at your tach input very closely and seeing if you are getting the correct counts just before, and during, the backfire. That's always step #1. Sometimes a lean/rich backfire or misfire can actually cause the tach signal to drop out, which then causes the problem to amplify.

Right now, I'm of the opinion that this is a tuning issue, and your fuel table in the MAPxRPM ranges at these higher steady loads is messed up. You are not seeing the issue below 55mph at the same load because the engine isn't spending enough time in those RPM areas as you romp it. I'd verify the fuel and spark tables are correct for those ranges before moving on.

All of this occurs when it's cold? So when it's cold and you get up to 60/65 it still backfires?

So whether it's hot or cold, it still runs crappy at 55+? Any video of this happening?

Can you post up some graphs of your timing, RPM, and injector pulsewidth while it's acting up?

I can't imagine a stock 454 tune would run anywhere near decently at any RPM on any 401.

SHoot! Missed this reply.
Looking at my RPM signal, it looks normal to me. No spikes, no flat spots. See graphs above.

Copy (tuning). I am starting to think I may have a distributor phasing issue. I "think" it seems to backfire when timing is max. If my hall sensor is locked down in the wrong spot, I wonder if my spark is jumping to the wrong post. I have a cheapie cap in route so I can cut a hole in it and will verify my phasing (hopefully) this weekend with my timing light. Then I will start looking into tuning more.

Its hard for me to test this cold. There are too many stop lights and cops for me to get up to 65 without getting on the freeway. By the time I am getting on the freeway I am not up to operating temp but I am getting pretty darn close. Does not seem to be any different between below operating temp and operating temp.

I will take some video next drive (hopefully) this weekend. Good idea.

Graphs posted. I did not put injector pulse width but will look at it now. I looked at duty cycle and it never went above 40% that I saw.