I know there are a million threads on this, but I have not seen an answer to my particular scenario.

'07 5MT GD fit with 122k on the clock, recently had overall mileage per tank drop from 33-35 down to 25. A shop I trust did the valve adjustment, and I did plugs (NGK) and coil packs preventatively. Noticed plug in cylinder 3 was finger tight from the previous owner, so I ran injector cleaner through the tank because the plug was coated in carbon dust. Mileage is still in the 25mpg area.

I also noticed it do a weird thing a couple times recently, if i accelerate in the car up to about 4k RPM and dump the clutch and coast, about 25% of the time the engine RPMs will drop down and the car will just die, like the idle doesn't "catch" it. I removed the IAT and MAP sensors and cleaned them with MAF cleaner, didn't seem to help. Did the idle learn procedure after cleaning the sensors too.

Also done the air filter and just changed the oil. What else could I be missing? Vacuum leak?

Thoughts? Thanks so much all.

 

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You've done well, only things I can suggest are throttle body cleaning for the dropping RPMs, (do the ILP again). Don't just spray stuff in there, wipe the bore and butterfly with a soaked rag to make sure it's clean.

If no joy (and you are in the USA) we could blame it on winter-blend gas and/or colder weather?

 

"Plug was coated in carbon dust."
Did it look like this?

If so was it only the one plug or all plugs?
Carbon fouling is the sign of rich mixture which can be caused by incorrect reading from coolant thermosensor(s) ECT.
Only one black plug is caused by leaking injector or weak ignition (lose plug = weak spark). Consider yourself lucky that you discovered loose plug right in time!
Also, what brand name of coils did you use?
If engine dies on deceleration (coast ) cold, I would re-check the valve clearances.
They may being adjusted with engine still warm (ideal temperature is about 20 to 25 degrees C, less than 30 is acceptable)

 

If this problem occurred after valve adjustment, plug and coilpack replacement then I would stray with all of those. Not done correctly/wrong part, both.

 

my 08 sportie is also getting low mileage, my driving has changed to a lot of highway mileage, I've cruise usually at 65mph, but as high as 80 (short amounts of distances for the idiots that go 60mph in passing lanes), I have never put the car on cruise control at 80mph. I still consider myself a spirited driver though. I'm getting around 275 to 300miles a tank full. mine has 125,000,00 I've replaced the sparks at 100 thousand, not positive of the make of spark plug, Is the type of spark plug that important? if it is , is the NGK the ones I should be using. My sparkies, looked like the ones in the picture above if I recall. I owned my car since New, it has always been fun a low speeds, so I haven't noticed the lack of power at the higher speed, I always figured it wasn't made for speed, so I didn't give it much thought.


I also haven't done the valves yet, they are making noises , and am sure they need to be adjusted, I'd like to do that myself, mostly b/c of lack of $$$ to go to a stealership - but they are crazy for the money they want for a simple tune-up, I'd also like to just do my own work on it. I haven't found a good thread yet on the steps to do it. So if you guys have a suggested threads that would be very helpful My car also has a non-stock muffler on it, but has had that on it since 50,000 I'm guessing.


I just last week. put new snows tires on front (firestoned blasters) and all seasons on back. thought that would help my mileage, but it hasn't. ( I run steelies on it, got rid of the 2008 sport rims). I brought the 2011 sport rims which I use only in the summer (btw they look fantastic on the 08 models) but didn't this year. the car handles better then the sport rims, and I like the low profile look also, and more selections of tires, and of course less money.


I'm concerned about the mileage per gallon, but the car runs like a clock, starts good, heats great, I'm looking forward to winter, I had bridge-stones on it before I replace to these new tires, this car blew me away last year in snow, My car pastED , BMW, AUDI, SUV'S, on snow covered roads and ice covered roads, slowly of course. but still for the price of those cars, I would highly consider another fit, and actually found an 08 with 32,000 on it at our local Honda dealership today!

 

All spark plugs black (covered with soot) means rich mixture.
What is funny that richer than normal mixture reduces demand for the spark voltage and high compression and this makes car to run really great on high speeds. (it also reduces pre- ignition and detonation, so the spark advance can be big which adds to performance as well)
In this case one of ECT's is reporting lower temperature as it is or fuel pressure regulator (in tank) stuck closed.
Since above statements are general automotive chat the next step is to connect scanner with live data and record the following information:
on idle and 50 mph cruising (engine on normal temperature)
LTFT
MAP
ECT
IAT

 

No error codes? - Catalytic converter, O2 Sensor, fuel pump?

 

Thanks for all the help!

In response to the plug questions, I replaced them with NGK plugs, and only the plug in 3 was sooty, the others were white, which (I think) means burning too hot.

Coils were "united motor products" which are labeled as OE replacement on rockauto.

I'm thinking maybe the valves weren't adjusted right? Only other things I can think of would be vacuum leak. I dont have a scanner to plug into the car while driving to take those numbers people are suggesting.

 

No, they aren't. Those coils have a 'heart' which means they are popular. Surely because they are the cheapest.

The Hitachi IGC0053 is the 'actual OE part' and seems to work best according to users here.