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Ask Someone: 2003 Mercury Grand Marquis #1585319128

I have a check engine light on, it doesn’t blink, it’s constant. My car is a 2003 Mercury Grand Marquis and it has 186,000 miles. Recently had a muffler delete on it but the neighbors were complaining so about a week ago I put it back on. Now my car is sputtering and shaking at stops and high rpm’s, I did a scan and it came back as injector 4 but it’s brand new, I even switched them around, still threw the code, got new sparks, still threw the code, got new injectors… still threw the damn code. What could it be really? I even did the whole nine yard on my fuel system, new hoses, new pump, dumped the gas, cleaned the tank, fuel filter, etc. You get the point. Im frustrated and confused, please help me out.

Tell us the codes you are getting… all of them in the form P0123. Post them here and we’ll see if we can help you.

Might be nice if you told us what engine you have and what you mean by “sparks” Spark plugs?

What codes do you have? By muffler delete, I assume you mean straight pipes after the catalytic converter, correct? Were any sensors removed and not replaced when you had a muffler installed?

Yes and no, I kept the front cats on and made it a dual. I didn’t have to mess with any o2 sensors, just some bolts and hangers. And the code is p0304

The code is p0304, and the engine is a V8 4.6l and yes I did mean spark plugs. Thank you, was hoping you’d reply! You seem to be very knowledgeable from other posts.

Thanks for the compliment…

So a P0304… misfire on cylinder #4. Thinking out loud. Spark should be OK, if the wiring harness isn’t damaged… but then swapping the muffler wouldn’t affect the wiring. Fuel has been eliminated with an injector switch and new pump and lines. Again should not be affected by the muffler.

Assuming you have spark and fuel plus no lean codes, I’d be doing a compression check on cylinder #4. You can rent a compression tester for free at parts stores. You can watch how to do it on YouTube, maybe even on a 2-valve 4.6 V8. I’ll bet it comes back low.

You might have created a lean fuel condition on cylinder #4 by removing the muffler that caused a burnt exhaust or intake valve. If the wet and dry compression test show the same low readings, a burnt valve is the problem and the head will need to come off to fix it.


Didn’t even think about that, thank you very much! I’ll get right on it.

Is it ticking at idle? You might have a failing hydraulic valve adjuster at #4. Your compression test will come back low, but not necessarily very, very low. You could try heavier weight oil to see it there’s any change before you dive deep into the valve train.

I wouldn’t think a removing a muffler downstream of the cats would be enough to burn a valve. Open manifolds/headers? Sure I can see that happening, but a good feet downstream(from the manifolds) That would have be a very rare occurrence.

I would agree with that comment, usually, but 189K on the engine, #4 is a corner cylinder, tired fuel pump, maybe it wasn’t happy with the decreased back pressure.

Even if the muffler had nothing to do with it, it is worth a compression test to see in what condition the engine is.

Not entirely

A noid lite kit would tell you if the injector is being properly pulsed

A car this old, it’s possible the wiring is in marginal shape

I’ve replaced a few coil and/or injector pigtails on cars this old, and it was the only cause of the misfire(s)

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