Strange Backfire Issues

[Rickyll7]

Here is what I've gathered from the backfire issue :

Only backfires through exhaust. I can hear it poping well from under the car, but not really from above the engine.

It only does it at low load such as when the engine is slowing down or if I hold it anywhere above 2000 rpm without load.

It won’t do it at idle, during acceleration, or while cruising.

It happens regardless of ECT and ACT.

It is random/sporadic - not rhythmic or synced to engine speed.

I've replaced spark plugs, wires, IAC valve, and the throttle position sensor.

Vacuum is at about 15 inches of mercury at idle.

I've checked for vacuum leaks by spraying engine starter fluid around the manifold and vacuum lines and didn’t hear any rpm fluctuations.

It also does it when I run my old injectors with my old tune, which never had problems before the blower install.

The AFR fluctuates between about 13:1 - 17:1 while it's happening and only when it's happening.

Timing advance is about 40 degrees when it's happening at 2000 rpm.

I heard of people having backfire issues cause by a bad alternator. Any one hear of this? Can I test the alternator or would I just have to replace it to see? It's original - 11 years old now, maybe it's just time anyway?

Any ideas?

-----------------

1998 Mustang GT
PI 4.6L Stroker (302ci @ 8.5:1 CR), KB 2.1L IC @ 9psi, Patriot Stg. II Heads, Comp XE268H Cams, SLP Long Tube Headers & o/r X, Magnaflow Catback, 90mm LMAF, JLT CAI, Accufab 75mm TB, FRPP 60lb injectors, KB BAP, Aluminum Flywheel, Centerforce Dual Friction, T56 Trans, Aluminum DS, 31 Spline 4.10 Rear, Tuning Software: SCT PRP, Wideband: LC1, Some suspension stuff…

 

[wmburns]

back fire pinpoint tests

I was reviewing the Pinpoint tests for a back fire condition. Of particular note is the fuel injector leak down test. If for example the injector is leaking fuel when closed, that could create extra fuel for the back fire.

Including excerpts from the back fire pinpoint test.

>>>from Ford service CD.
HC1 CHECK SYSTEM INTEGRITY
Visually inspect the complete fuel delivery system for damage; including fuel lines, connections, relays, fuel tank, fuel pump, fuel pressure regulator, fuel pulse damper and fuel injector areas for leaks, looseness, cracks, kinks, pinching, or abrasion caused by a collision or mishandling.
Visually inspect electrical harness and connectors for loose pins, corrosion, abrasion, or other damage from collision or mishandling.
Check electrical connectors for proper mating.
Verify vehicle has followed maintenance schedule.
Verify inertia fuel shutoff (IFS) switch is set.
Verify battery is fully charged (12.5 volts or greater).
Verify electrical/fuse integrity.
Verify fuel level in the tank is sufficient.
Are any concerns present?

(yes)REPAIR as necessary. (no)GO to HC2 .

HC2 CHECK VOLTAGE AT FUEL PUMP HARNESS CONNECTOR
......
HC3 CHECK FUEL FILTER FOR PROPER MAINTENANCE
.....
HC4 CHECK FUEL PRESSURE
.........
HC5 CHECK FUEL PRESSURE LEAKDOWN
Observe Warning, Caution and Notes.
Fuel pressure tester installed.
Key on, engine off.
Access Output Test Mode and run the fuel pump to obtain maximum fuel pressure.
Key off.
Verify fuel pressure remains within 34 kPa (5 psi) of the maximum fuel pressure for 1 minute after the fuel pump is turned off.
Does the fuel pressure remain within 34 kPa (5 psi)?

(yes)GO to HC7 . (no)GO to HC6 .

HC6 CHECK PRESSURE REGULATOR AND PULSE DAMPER DIAPHRAGM
Fuel pressure tester installed.
Start engine and run engine for 10 seconds.
Key off, wait 10 seconds.
Start engine and run engine for 10 seconds.
Key off, remove vacuum hose from fuel pressure regulator or pulse damper port.
Inspect for fuel in the vacuum hose or regulator port or pulse damper.
Is fuel present in the vacuum hose, regulator or pulse damper port?
(Yes)
REPLACE fuel pressure regulator.
For Mechanical Returnless Fuel System:
Replace pulse damper.

(no)
For Mechanical Returnless:
GO to HC13 .
All others:
GO to HC12 .

HC7 CHECK FUEL PRESSURE, ENGINE RUNNING
Fuel pressure tester installed.
Disconnect vacuum hose at the fuel pressure regulator and plug it.
Drive vehicle with heavy accelerations while observing fuel pressure gauge reading.
Does fuel pressure reading hold steady within 21 kPa (3 psi) during test?

(yes)GO to HC8 . (no)GO to HC13 .

 

[Rickyll7]

Fuel pressure is good... 39 psi with enigne off and it stays there for well over a minute.

 

[trinity_gt]

I don't see how an alternator could be connected to exhaust backfires.

Backfires in the exhaust occur because excess fuel vapor accumulates in the exhaust and is ignited by something, such as an incandescent bit of carbon. The question is where is the excess fuel coming from?

Re alternator: I suppose very low system voltage could cause weak ignition sparks and misfires resulting in unburnt mixture ending up in the exhaust but you'd see bad performance under load. Still, I can't see how the alternator could cause a low-enough system voltage and not be signified elsewhere (e.g. difficulty starting, dim lighting etc.)

Sounds like you're still chasing fuel system and/or tune issues.

 

[Rickyll7]

Found the problem!

The engine misfires and backfires completely go away when I disable the Kenne Bell bypass valve (keeping it closed). Why is that?

My theory is this: The Kenne Bell bypass valve design is starving cylinder number 5 of air at low loads, and the engine is still giving that cylinder the fuel for the amount of air it thinks that cylinder is getting. So, it misfires nearly every cycle and backfires randomly as well. How can that be you ask?

When the bp valve is open and the engine is at low loads, lots of air is going through the blower and little is needed in the engine, so almost all of it ends up going back through the bp valve to re-circulate through the blower and that is exactly what is supposed to happen, right? Of course, but...

Here is the problem: The KB bp valve is located in such a way that it draws air straight out of the intake runner for cylinder number 5. It draws air from that intake runner and that intake runner only because that intake runner is where they put the routing to the bp valve. So what, why is that bad?

Here is why: As stated above, at low loads, very little air is going into the engine, and a lot is re-circulating around the blower through the bp valve. That means that air at a high velocity (relative to that of the air entering each cylinder) is entering the intake runner of cylinder number 5 and running straight past the intake port and into the bp valve. Anyone know what happens to pressure as velocities increase? That’s right... the pressure goes down - vacuum.

What happens then you put more of a vacuum on one cylinder than the others? It gets much less air than the others and the ecu doesn’t know about it, so it puts in the same fuel as all the other cylinders. That cylinder (#5) then misfires, the driver’s side o2 sensor sees excess o2 and adds even more fuel to bank 2 making a bad situation worse because now all 4 bank 2 cylinders are running rich.

Does this make sense or am I crazy? Is there something else I’m missing?

 

[Rickyll7]

It seems to me that I've only got a few options here and they are all going to compromise something (thanks a lot KB).

Option 1:

Tune it with the bp valve closed to get the A/F where it is supposed to be. Then let the bp valve open as it should, but disable closed loop and learning or find some way of getting the engine to ignore bank 2 (as stated above).

Here is the problem with that (Option 1):

If I tune it to within 2% commanded AFR with the bp valve closed, everything works great until I let the valve open. When that happens, the misfire and backfire return and the wideband shows anywhere from 5-10% leaner (the WB is in the driver’s side, bank 2). Now I leave closed loop and learning disabled for now to keep things from getting any worse. 5-10% doesn’t seem to bad right? After all, it's only at low loads, right. Wrong, it's not ok. Here is why (correct me if I'm wrong):

The WB is reading 5-10% leaner on bank 2 (4 cylinders total) due entirely to cylinder number 5 misfiring. That 5-10% extra o2 is coming from cylinder 5 only. That means cylinder # 5 is running off by as much as 20-40%. If it is getting so little air while still getting all the fuel that it misfires 20-40% of the time, it must be running terribly rich and that will eventually wash that cylinder wall, I think, but correct me if I'm wrong.

[1+1+1+(.6 to .8)]/4 = .9 to .95

Maybe I'm wrong, but that’s how I see it.

Option 2: Completely disable the BP valve. KB has a kit for the 96-98 cobras with a 2.1L and no BP valve and no intercooler. They allow it to run up to 11psi at 13000 rpm (blower speed, engine speed is 6500). If it works there, it should work for me. With a stock crank pulley and a redline of 6000, I can run down to a 3 inch pulley before I hit 13000 rpm.

Option 3: Use some sort of mechanical stop to prevent the bp valve from opening beyond some point at which the engine still runs without issue. This still allows some air to bypass, which I'm sure we can all agree is better than none. Also, some OEM blowers bypass air before it can run through the IC, so they are only re-circulating the same hot air. The KB kit re-circulates air after the IC, so If I can get at least some air to bypass, that could be better than the OEMs that are bypassing more air, but hot air.


I'm obviously leaning towards option 3, but I'm open for suggestions as I'm sure I'm not the first one to have this issue as it seems that KB designed this flaw into their 99-04 GT kits. Perhaps they have some sort of way to “tune out” the issue. Although, I don’t see how they could unless they are able to shut the fuel off for that cylinder. Even that isn’t good for the engine.

 

[trinity_gt]

My KB -- and thousands of others -- works fine with that bypass valve arrangement.

 

[Rickyll7]

I just got off the phone with KB and they say the bypass valve design only causes problems on non-stock cars (mostly a cam overlap issue). They say that their kit was designed for stock cars and works well with stock cars. I guess that’s reasonable and is only fair. They suggested I call Tim at MPH and ask him about it. They say he has developed some cams that supposedly work well with the kit.

 

[fast97gt]

wow thats way too much for my lil brain to conceivce, but i i did it. so correct me if im wrong, thats were all the extra fuel is coming from to backfire in the exhaust?

 

[Rickyll7]

Yes, the engine is giving the right amount of fuel, but the engine doesn't know that cylinder#5 is being starved for air. So not all the fuel can burn (when the cylinder even does actualy fire).

 

[Dbane2020]

I'm having a similar issue with my Kenne bell supercharged mustang but mine is lightly modified and I'm having a cylinder #5 misfire as well. Also having a small issue that doesn't affect drivability but is just annoying to me where at low rpms 1500-2300 in 3rd-5th the car will hesitate when trying to drive normally any help would be great.