88 5.0 backfiring and missing.

[suddenchance]

Hey everyone,

Update! Bronco is running rich! Fuel injectors are new and fuel pressure is at 33lbs and holds. Spark plugs are fouling.

I have a 1988 Bronco with a 5.0 engine that's experiencing some issues. It’s backfiring through the intake and missing, making it feel like it's only running on six cylinders. The engine is new and was tested before installation, showing positive results. The ECU is also new and has been tested.

The engine starts easily, but after 2-3 minutes of running, it starts to miss, and I hear backfiring at around 1200 RPM and above and the check engine light stays on. There are no vacuum leaks, and the fuel pressure at the rail is holding steady at 30-33 psi. I’ve replaced the O2 sensor and changed the throttle body, the idle air control (IAC) valve, and the manifold absolute pressure (MAP) sensor.

After running the diagnostic codes, I found that both the IAC and MAP sensors are out of range. I tested the voltage on the MAP sensor, and it read correctly. The engine is running rich, and I’ve also installed new injectors, spark plugs, and plug wires. In fact, every component on the truck has been replaced. The truck is displaying the same symptoms it had with the old engine, so I know the issue isn't due to the new one.

It's acting exactly like the timing is off but the timing is set perfect and the firing order is also correct.

Any ideas or suggestions would be greatly appreciated!

 

[L\Bronco]

Hey there sudden, welcome to the zone!
That sounds like exh restriction, (maybe one side?)
Does it still have cats?
Hook a vacuum gauge to a manifold source, read it at idle, then 2000 rpm.
It should be 18ish in hg at idle and 2” higher at 2000 rpm (in neutral)
If it drops, it usually means a plugged cat.
The other thing is get it to backfire and miss for a few seconds and feel the intake runners. Any warm/hot ones have an exh restriction. (The exhaust gas backs up into the intake and lights the fuel behind the intake valve.
It might help you narrow down which side. Ive even seen a plugged exh manifold port or 2. (But they misfire immediately)
Hope that helps!
Cheers!!

 

[Tiha]

That is a great idea. I was gonna say pop an O2 out and start it, or pull the EGR and start is. Let the exhaust leak and see if it runs any better.

 

[suddenchance]

Hey there sudden, welcome to the zone!
That sounds like exh restriction, (maybe one side?)
Does it still have cats?
Hook a vacuum gauge to a manifold source, read it at idle, then 2000 rpm.
It should be 18ish in hg at idle and 2” higher at 2000 rpm (in neutral)
If it drops, it usually means a plugged cat.
The other thing is get it to backfire and miss for a few seconds and feel the intake runners. Any warm/hot ones have an exh restriction. (The exhaust gas backs up into the intake and lights the fuel behind the intake valve.
It might help you narrow down which side. Ive even seen a plugged exh manifold port or 2. (But they misfire immediately)View attachment 32575
Hope that helps!
Cheers!!

Appreciate the info! I did check and the exhaust and it did fix the backfiring issue! The engine is still misfiring. I checked codes and it's pulling 31 O, 81 O and 82 O. Would Appreciate any info!