Ford Mondeo: Mysterious problem, several symptoms

Hello all! I have a 2001 Ford Mondeo (2.0 Duratec HE engine) with 47k miles (yes, that’s low for a 19 yo car!) which I’m trying to fix for the past 4 months. Unfortunately, I’m running out of hairs in my head :frowning:

These are the symptoms:

-At the traffic light, the engine stalls (rough idle) and hesitates when the throttle pedal is pressed, it continues hesitating until ~1500 RPM
-The engine stalls even more if the A/C is turned ON and/or when engine is cold
-High fuel consumption at city
-It gets to the operating temperature too quick
-Fuel trim when stopped is STFT -4% and LTFT +11%
-Fuel trims stabilizes at 0% above 55-60 mph
-When the throttle is briefly pressed and released, the engine takes a couple of seconds to completely reduce the RPMs to idle
-When coasting, if I shift from Drive to Neutral, the RPMs raises to almost 2k and returns to idle only when the car is near to a stop
-The normal RPM for this engine is 720 +/-50, when stopped in Drive, the RPMs stays at ~800 RPM
-Harsh downshift from 3rd to 2nd, unless I brake veeery smoothly
-No error codes

What I did so far:

-Spark plugs changed: little to no difference
-Spark cables: resistance seems OK, all cables below 2k ohms
-Throttle body removed, cleaned and inspected for leaks at the seals and flaps
-Vacuum leak fixed at the injectors (seal changed)
-Injectors tested for leaks
-Vacuum leak fixed at the intake port (intake manifold removed and seals changed)
-PCV hose changed, although it wasn’t broken
-PCV valve inspected, looks fine
-MAF sensor inspected, not dirt or visually damaged
-Thermostat changed, it was physically broken and locked open
-EVAP hoses are not leaking air
-Brake vacuum lines are not leaking air
-Swirl flaps actuator is not leaking air
-Air filter is OK

Where I’m lost:

-I still suspect that there is a vacuum leak somewhere because of the fuel trims and fuel comsumption, but I tried to blow a cigar in the intake manifold and it didn’t leaked anywhere, I put all my lungs on it and nothing. Is there any other way to test for vacuum leaks with efficiency?
-Can it be a fuel pump? It is pretty old despite the low mileage, but at the same time, the car is not hesitating at high RPMs when the pump has to do more effort I suppose.
-I don’t know how to test the coil pack, it can be bad?

Any help/tip is much appreciated!


from how your fuel trims change when driving under load, it indeed looks like a vacuum leak

search YouTube for “DIY smoke machine” instructions, you can make one with a resistance wire, wick and old platic can and few wires/hoses, it will help you to smoke-test your engine bay much better than “cigar method”

Is your speedometer working and showing correct speed?

Thanks thegreendrag0n and CapitalTruck

Yesterday I got my hands on a smoke machine (not DIY machine) and an air compressor, first I pumped smoke through the PCV hose which connects to the intake manifold, (not to the PCV valve) it didn’t leaked anywhere except to a small amount through the throttle body flap. Then I connected the compressor and tried to pump air into the intake, it did not leaked also, in fact when I released the air, I noticed that it had been built quite some pressure inside the intake manifold. I did the same test with the brake hose disconnected and sealed, the results were the same, also, the MAP sensor is measuring 5 PSI at idle. (~10 inHg)

Today I compared the speedometer with the GPS, the car is measuring 1-2 mph more than the GPS.

I checked the spark plugs and cables again. The plugs shows some soot, the cables has 1.8 to 4.7 kohms, depending on the size.

I don’t know, maybe the fuel pump is going bad and the injectors are working more on the open state to compensate the lack of pressure and thus the high fuel trims?

…or maybe the coil pack is failing, however defective coil packs tends to work better at cold and worse at hot temperatures, right? In this case, cold is worse.

Really don’t know…what do you think, guys? Any suggestion?



My friend has a Contour that acted all goofy when speed sensor went out. The fact that speedo works fine ices that idea.

Not exactly my bowl of soup, but let me throw some ideas against the wall…

Check fuel pressure in injectors rail at idle and under load?

Trims are controlled based on the upstream air/fuel ratio sensor (AKA “upstream oxygen sensor”), any chance it went bad?

What about MAF sensor cleaning?

I’m guessing either there’s still un-metered air leaking into the engine or the exhaust system is clogged. What does the warm-idle intake manifold vacuum measure? Use your own vacuum gauge, don’t rely on the car’s MAP. It should be in the 17-22 inches range. If its’ really only 10 inches, figuring out why is where you should focus. Other tests I’d be considering if I had that problem

  • fuel pressure
  • engine compression
  • throttle position sensor

Hi all,

I don’t have a vacuum gauge or fuel pressure gauge, I might be able to get it, but it’s gonna take some time. Meanwhile, I found your comments interesting and will surely test it.

-Will test the TPS sensor

-MAF sensor is not present on these earliers Duratec HE

-Never thought on leaks on the exhaust manifold, I will check that and remove the cat converter to see if it is clogged.

-Upstream O2 sensor is fluctuating in a strange manner, but at sometimes it manages to swing as it should. It is difficult to say that the sensor is giving wrong information or is fluctuating because the mixture is really wrong.

-Downstream O2 sensor is even more interesting, it’s locked at 1.275 V, no matter what, like if was rich all the time. I will investigate this one first.

Thank you!


There’s different technologies used in O2 sensors these days. Some of them , especially the downstream types, don’t oscillate between 0.1 volt and 0.8 volts. Instead they hold to a fairly narrow voltage range.

As I recall the technology that oscillates rapidly are called lambda sensors and the type the hold to a more steady voltage range are called wide-band sensors.

Looks like it’s a heated, finger-type sensor:

Ok, updates:

-TPS sensor is fine, ranging from 2 kohms to 4.8 kohms, smoothly
-Downstream O2 is not present, it seems that it is not required by law in our market, that’s why it shows a fixed voltage
-EGR valve is also not present, instead, there is an EGR delete, OEM from Ford believe it or not:

-Cat. converter is clean and undamaged
-Exhaust manifold is not leaking
-Intake manifold retested, and not leaking

I removed the Upstream O2 sensor and I may have releaved the issue, the fuel trims went to zero, the transmission hard downshift has gone and the engine misfires badly when load is applied to any RPM and the AC is turned on. I alse hear a ‘pop’ from the throttle body indicating a backfire.

Being that, I’m going for the coil pack, already ordered it should arrive tomorrow. I post the results after change. I will also take the opportunity to change the leads despite they are in good shape.

If that doesn’t work, then I’ll go for the fuel pump.



The new coil pack made a lot of difference. The car is not hesitating or misfiring anymore and it runs smoothly at any RPM. After that I also changed the spark plug leads because despite the resistance of old ones are OK, one of them had a crack, so I took the opportunity. Also, with A/C or not, it is not backfiring anymore.

However, the fuel trims went down just a little bit, and my O2 sensor appears to be a little lazy, sometimes it stays at rich or lean for several seconds, and from what I heard, these old sensors may fail at this mileage/age, so I already ordered a new one.

My learnings so far: Sometimes the O2 sensor can mask a problem, like with the coil pack, so disconnecting it can help you to find or get closer to the root cause.

Congrats on getting it fixed!
I was afraid you were going to write it off at some point, due to the age, but you persisted :slight_smile:

Thanks thegreendrag0n yes I’m glad that at least the car is running well, but it’s just half of the journey, my fuel trims are still high and the O2 sensor seems to be bad, I’m hoping to get the new one delivered today so I can change on Sunday and post the results here.

Have a nice weekend!

1 Like