Hi Guys,
Here is my story.
Started a few days ago with very noticeable power loss... Checked errors and it did record miss fire on 6th cylinder. Was not the first time with miss fire problems so I started with coil. Ordered and swapped the coil, reset the error. It never came back...
But... On idle the car is working fine.
On slow speeds and fast/violent acceleration (3-4 k rpm) it is also fine...
However, on highway speeds (like over 50-60 mph, about 2000-2300 rpm), when I want to speed up gently (without gear change), it is shaking and seems to have no power... Same is when going uphill (on the level surface is ok however).. Uphill it is loosing power completely - I have to do the kick down to change the gear down, and then it starts to accelerate again..
There is no more codes recorded.. Thought might be something with the exhaust... Had it checked... No problem. Cats are ok, no clogged pipes or muffers, no high back pressure, O2 sensors also ok...
The most noticeable problem is when going uphill at highway speed, and when speeding up without gear change... No power and bad shakes..
Got any idea what to check now?
Worth to mention that my mpg dropped approx 15-20 % (consumes more)...

 

What brand of spark plugs?
And how many miles on them?

 

My mechanic says plugs are good...
Navi has 120 k miles behind, and plugs are motorcraft (probably still from the factory - did not touch them)..
Do you think such problem can be caused by the plug ? Well, when I press the throttle pedal harder it moves smoothly... The problem is only when I'm gentle... Suppose these are the moments when car is under big load...

 

That would be about when the EGR is active...........the leaner mixture requires more voltage to fire under that condition. Compound that with worn plugs and its likely to misfire.


Maybe your mechanic checked #6 plug when he changed the coil. Just curious, but just how did your mechanic determine that your plugs are good? With possibly 120k on them, what did he say the gap was on them now?


Since your mechanic has determined that the plugs are good, what does he think is causing the misfire?

 

But in such condition - wouldn't the miss error return? There is no miss errors since the coil change..
My mechanic says he doesn't know.... No further ideas...

 

The plugs are designed to last 100K miles and you're 20K past that, of course they can be A problem if not THE problem. That aside, the symptoms you described above indicate low fuel pressure. This could be due to a bad fuel pump, a clogged fuel filter, or a bad fuel pressure regulator.

 

I should have mention I'm on LPG... So using petrol mainly to start and heat up the engine... When hot it switches to LPG... The symptoms are on booth fuels... That is why I rulled out the fuel system problem...

 

My mom's Taurus had the same symptoms you describe here, including no trouble codes. I changed all of the coils and plugs for her, and it cleared up.

For some reason, misfires don't always reach the threshold of throwing a trouble code. But on light throttle (highway speeds, going uphill for example), it feels as if the engine is falling all over itself.

 

I will take a close look to the plugs then... Any good practice or ideas when replacing ? How to get to the deep two (under the wind shield) ?

Also - new symptom... On the highway... it runs much smoother when on cruise control than when pressing pedal myself... weird..

 

Well... still did not do the plugs, as I have to work outside and it is badly cold out there now... But discovered another interesting thing... The shaking and miss firing is worse when it is cold... When it was 0 or a few deg below (Celsius), it was possible to drive... Then when it was about - 15, it was shaking so badly that I was afraid to move it... Now it is about -2/-3 and again.. it is possible to drive again. The miss firing is still out there, and still have problems getting speed..but not that much as when it was very cold... Does it mean something in relation to possible cause?

 

It usually means a weak spark. This could be due to a working yet weak coil, a cracked spark plug, too much spark plug gap, or a bad spark plug boot.

 

Ok so I managed to replace all spark plugs...
The old plugs were in bad shape when pulled out...
I put autolite AP103 as I've had problem to get motorcraft..
It runs better, in general, however my problem is still there...It shakes like it was mis fire, on high gear under load... No mis fire codes... but from time to time lean code on only one bank (174) - on this one where previously was mis fire code (before i replaced the coil)...
I noticed also that although when it shakes, there is no rpm loss (by my ear), however rpm gauge is dropping... So for me it still looks like some sort of electrical problem... hope it is not machanicaly bad cylinder...