German Roads Need This Mustang – Help Me Fix My 4.6 GT!

[Chabes]

Hello everyone,

I'm reaching out for help and advice.

I own a 2005 Mustang GT 4.6 V8, manual, totally stock, with around 280,000 kilometers on the engine.
I'm located in Germany, and unfortunately, it's very hard to find qualified mechanics here who are familiar with this kind of car — especially when it comes to proper diagnostics.

Here are the issues I'm currently experiencing:

1. Engine stuttering on the highway
Occasionally, at around 80 MPH (130 km/h), the engine starts to stutter.
Shortly after, the check engine light comes on and stays on.
A mechanic once read out a misfire code related to Bank 2, but I don't have the exact code anymore.

2. Throttle hesitation at low RPM
The engine doesn't respond properly to throttle input between 1,000 and 1,800 RPM.
At idle, the engine runs smooth — no rough running or shaking.
However, there are occasional misfires at the exhaust.
This might be normal, but I have no other stock Mustang nearby to compare.

3. Overheating after shutdown
When shutting off the engine, the cooling fan does not continue running.
A few times, after turning off the engine, the coolant temperature briefly spikes into the red for about 3–4 minutes, then drops back to normal.
I'm not sure if this is an issue with the cooling system, a sensor, or fan control.

4. Soot marks near the fuel rail
I've noticed two soot streaks, each about 1 cm wide, near the gasket under the fuel rail.
Could this indicate a leak or combustion residue?

My goal:
I truly love this car and want to keep it on the road here in Germany.
I want it running properly again — not just for reliability, but because it still turns heads and makes BMWs and Mercedes look silly.

Can anyone give me some advice on what to check or what common issues could cause this combination of symptoms?
What tests should I have done, or what components should I look into first?

Thanks in advance for your time and help!

Best regards,
Jens

 

[Dino Dino Bambino]

Welcome to the forum Jens.

Issues 1 & 4 could be related. You might want to check if any of the spark plugs are either loose or have been cross threaded into the cylinder head. Your model will have the break-prone two-piece plugs unless someone has changed them previously. Therefore you might want to buy a Lisle plug extractor tool and have it handy. It's also possible one of the coils is arcing on the valve cover. The best way to find out is to start the engine when it's dark because you'd then see a small spark.

Issue 2 definitely isn't normal but this might be resolved easily by thoroughly cleaning the inside of the throttle body and gently cleaning the MAF sensor element. You could also add a bottle of fuel system cleaner (Wynn's is really good) to your next tank of fuel. This will clean the injectors and make the engine run more smoothly.

Issue 3 is a difficult one to troubleshoot without further information. The fan is supposed to shut off anyway when you switch off the engine. However you might want to check if the fan switches on at all. It should switch on in low speed mode when the coolant temp. reaches 216*F, and the gauge shouldn't move above halfway. If the temp. continues to rise, the fan should go into high speed mode at 228*F. You might want to buy an infrared temperature gun, take some measurements when the engine's idling (AC, defrost, or heater off), and report back. Finally, let us know if the fan DOES switch on when you use the AC.

 

[Chabes]

Thanks for the warm welcome and a lot for your helpful insights!
I have the same red as you (without the stripes though). =)


Regarding Issue 1: I'm currently running Motorcraft SP547 spark plugs. These were installed back in 2021, which interestingly is around the same time the issue first started. Do you happen to have a recommended alternative to these plugs?


As for Issue 2, I'll go ahead and follow your suggestions – cleaning the throttle body and MAF sensor, and adding a fuel system cleaner – and I'll report back with the results.


On Issue 3: The fan definitely does come on, and the temperature stays stable during highway driving. I've only experienced overheating in specific situations: either after briefly shutting off the engine (for about 10 minutes) and restarting it, or when I've been stuck in traffic for around 20 minutes following a long highway drive. As long as the car is moving – even at a steady 80 MPH – the temperature gauge stays right in the middle.


One thought I had: could there possibly be air trapped in the cooling system? Might that explain the symptoms?


Thanks again for your support!

 

[Dino Dino Bambino]

The SP547 plugs are the ones I recommend, and they're the updated versions that have spot welds between the two pieces to ensure they don't break. It's possible whoever installed them applied too much anti seize compound on one of them and contaminated the electrode.

If you've recently done work on the cooling system, it's possible there's some trapped air remaining. Burping the system is easy. It helps if you have the car parked on an incline with the nose up but if that isn't possible, jack up the front end. Start the engine from cold with the cap removed from the expansion reservoir and keep an eye on the coolant level. It should drop initially and if you squeeze the radiator hoses, you might see more bubbles. Top up the reservoir as required and put the cap back on when you see the coolant level rising with no more bubbles. Then go for a minimum 15 minute drive to get the engine properly warmed up. After you return home, shut the engine off for at least four hours and then recheck the coolant level in the reservoir Top it up again if required. You should then be good to go.

 

[GriffX]

Welcome!

yea, finding a workshop in Germany for our cars is a challenge.

The Motorcraft spark plugs are fine, did you change them? I would take them out and inspect them by myself. And, measure the coils with a multimeter (when I remember correctly around 4 kOhm).

Did you checked the thermostat? Cold engine, 1 km drive, feel the heat in the upper radiator hose, it must be cold, if not you need a new thermostat. But, overheat in heavy traffic can be a problem with the fan. There can be an issue with the connectors of the fan relay in the fuse box.

The Ford coolant filling procedure is heating up on idle and drive some minutes, don't think that is a problem, but why did you change the coolant?

Good luck

 

[Gladams]

Welcome to the club from Missouri.

 

[Chabes]

The Ford coolant filling procedure is heating up on idle and drive some minutes, don't think that is a problem, but why did you change the coolant?

Thank you too for your input.
I'm sorry for the misunderstanding but i didnt change the coolant nor was it changed as far as I know. Airbubbles in the system was just an idea.

 

[GriffX]

Thank you too for your input.
I'm sorry for the misunderstanding but i didnt change the coolant nor was it changed as far as I know. Airbubbles in the system was just an idea.

ok, so that is not a problem, to get the right coolant in Germany is also fun, I used the yellow from Ravenol. BASF sells the probably OEM Motorcraft type (MSDS is equal) in 20l barrels only, the imported Motorcraft Gold is very expensive.

If the thermostat is still the first, I would change it anyway, they die slowly.

I guess you know https://www.iihs.net/fsm/ already?

 

[DieHarder]

Thanks for the warm welcome and a lot for your helpful insights!
I have the same red as you (without the stripes though). =)


Regarding Issue 1: I'm currently running Motorcraft SP547 spark plugs. These were installed back in 2021, which interestingly is around the same time the issue first started. Do you happen to have a recommended alternative to these plugs?


As for Issue 2, I'll go ahead and follow your suggestions – cleaning the throttle body and MAF sensor, and adding a fuel system cleaner – and I'll report back with the results.


On Issue 3: The fan definitely does come on, and the temperature stays stable during highway driving. I've only experienced overheating in specific situations: either after briefly shutting off the engine (for about 10 minutes) and restarting it, or when I've been stuck in traffic for around 20 minutes following a long highway drive. As long as the car is moving – even at a steady 80 MPH – the temperature gauge stays right in the middle.


One thought I had: could there possibly be air trapped in the cooling system? Might that explain the symptoms?


Thanks again for your support!

Issue 1: Check the coils. When people replace the spark plugs it's often the case that the end of the coil gets pushed up too far inside the boot and while the engine will idle fine; one or more coils no longer make good contact resulting in a lugging feeling under acceleration as the spark now has to jump a gap. You can get an entire set of coils inexpensively on fee bay. Note: Buy OEM coils only. Best if someone can hook up a scanner on your car and ID exactly which ones may be acting up. Then move them; if the miss follows that's the culprit/s.

 

[GriffX]

That's a good price

Zündspulen Ford Mustang GT , 8 stück | eBay.de

Ford Mustang GT Zündspulen , ab 2005 bis ?

www.ebay.de

No, even with shipping and tariff, import from US is better.

 

[DieHarder]

Try this: https://www.ebay.com/itm/1463182577...Sf6gXOwvZC559TQ6OXwox7LFry0c|tkp:BFBMmqKBsdll

 

[Chabes]

Hey guys,

I just wanted to leave a quick update as you all were so kind to help me out.
I got it diagnosed again and it error code told cylinders 1 and 2 missfiring.

A close look at the ignition coils showed some kind of hairline crack. So as you told me i changed all coils as well as the spark plug connectors (hope that translation is right) and the car is now running like new!

thx a lot!