Engine Misfires on cylinders?

I would suggest switching the coils and plugs one at a time with a cylinder on the opposite bank just to make sure they swap sides. Due to these two cylinders being next to each other we cannot rule out a head gasket issue so we may need to do a compression check if the plugs, coils, or injectors are not the issue.

Here are guides on how to do these tests:

https://www.2carpros.com/articles/how-to-test-engine-compression

https://www.2carpros.com/articles/head-gasket-blown-test

Let me know if you have questions or what you find and we can go from there. Thanks

Please have a look at the lower cowl panel seal. At my dealership we usually have to remove it and replace the one-sided sticky seal because the original is flattened and does not give the upper rubber cowl seal a positive seal against the hood thus making water leak onto the top of the engine, and consequently into the plug holes and onto the coils.

When you changed the plug and coil, did you replace the boot? And was the plug fouled?

By fouled, is the plug black? Wet? Melted electrode? Anything abnormal about it? Did you change all plugs or just #8?. Is it a single coil pack, or coil on plug system? Were the plug wires if coil pack all changed? Coil on plug, where the Boots under the coils changed? And can you feel the injector clicking with the engine running? You may need a fuel pressure and injector leakdown test.

If the check engine light is on let run the codes tis guide will help

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

If not, check fuel pump pressure to confirm it is within spec. If a tune up hasn't been done for a long time look at this guide.

https://www.2carpros.com/articles/how-to-tune-up-a-car-engine

and

https://www.2carpros.com/articles/engine-misfires-or-runs-rough

Please run down these guides and report back.

The injectors in these vehicles are very dependable. Chances are the coil pack is bad. You can check to see if it is getting spark. Also, I have seen the rubber boot actually go bad and cause the coil to short.

Let me know what you find.

Hi,

You have done a lot already. Are you certain there are no vacuum leaks? There is a vacuum hose that runs off the back of the throttle body and will have a 90° bend in it. They commonly dry rot and leak.

If that isn't the case, what code are you getting for the sensor? Also, are you sure it is the engine and not a transmission issue? I ask only because there are no misfire codes.

Keep in mind that the O2 sensor could cause an issue, but I would think you would see a code and the exhaust leak, as you already know, can be the cause. So I would really like to know the code you have.

Next, have you checked fuel pressure to confirm there isn't an issue? Pressure that is too high or too low can also cause a misfire.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Here are the specs for pressure:

1998 Ford Truck Expedition 2WD V8-5.4L SOHC VIN L
Pressure
Vehicle Powertrain Management Fuel Delivery and Air Induction Fuel Pump Fuel Pressure Specifications Pressure, Vacuum and Temperature Pressure
PRESSURE
Engine Running .................... 193-310 kPa (28-45 psi)
Key ON Engine .................... 242-310 kPa (35-45 psi)

Also, a partially restricted exhaust system (converter) can cause a misfire often along with a loss of power.

https://www.2carpros.com/articles/bad-catalytic-converter-symptoms

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

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If you have a live data scanner, could you let me know what the short term fuel trims are?

Let me know.
Joe

See pic 1 for P0133 possible causes. You are very possibly right with the exhaust leak. If you look at pic 2, it indicates possible causes of the P0135. That is different and not related to a power issue. I believe the two are related for the same reason, a lack of power. Check the connector for corrosion, damage, skinned wires, disconnected or pushed in pins or anything out of the ordinary.

Let me know what you find.

Joe

Based on the codes, I feel that is a good place to start. If the computer is trying to adjust air fuel mixture based on faulty readings, anything is possible. If your scanner shows live data, let me know what the short term fuel trims are and see if it is running the fuel system in a closed loop when at operating temperature.

Joe

Sounds like a plan. Let me know what you find or if you have other questions.

Take care,
Joe