Just wondered if anyone might have a suggestion. I went away for 2 months and when I came back the 351 started first time (great), and all went well. In driving the car I noticed, however, my mpg is much reduced. It was very good before I left. Can anyone think what might have caused this by the car sitting in a garage for two months? (it's a 3.0 litre) My only thought is something might have deteriorated by sitting? Or could it just be spark plugs or air filter? But if those I would have thought the drop off in mpg would have ben gradual, but this was quite a sudden and dramatic drop in mpg.

 

One suggestion I will try - https://www.lucasoil.com/product/fuel-treatments/

 

Hi QP7,

A common cause of sudden decreased fuel economy is a stuck-open engine coolant thermostat or a defective engine coolant temperature sensor. If you have access to a scan tool that can read Live Data, you can monitor the coolant temperature to see if the engine is reaching full operating temperature. If the coolant never gets hotter than, say, 180 degrees F, the Engine Control Module will continue to apply cold-start fuel enrichment even after the engine should be fully warm, leading to rich running and excessive fuel consumption. If you don't have a scan tool, look at the exhaust tailpipes to see if the deposits appear darker/blacker than usual, which could indicate rich running.

Cheers,

Don

 

Thank you very much, that is a very good idea.

If that is the cause, is it expensive in terms of labor to get to it to possibly fix it, or replace it?

 

I would think that if it's a thermostat (and it's housing), replace it with a new aftermarket aluminum one for less than $100 plus a couple hours labor. I'm guessing your 2013 3 liter does not have the sensor in the thermostat housing, but check it. Might as well get rid of that plastic failure prone part.

 

Check if one of brakedisks are warmer than others after short run. Brakes typically start binding when vehicle is not in use.

 

Thank you Vasara and Freddy J - much appreciated.

I have some homework todo now.

While I am at it what else might boost mpg generally? - new spark plugs, new fuel filter and new air filter? Anything else?

 

Spark plugs wont impact your MPG like that. You'd be misfiring all over

 

OK, thanks - understood.

 

Problem appears to be solved - it seems to have been water in the gas as when I used "STP Water Remover" earlier today, the problem resolved.
https://www.stp.com/product/all-season-water-remover/

The product says especially in warm wet weather and my car was left in the tropics for last 2 months. Warm and wet - you think!! Even in my garage it was beyond warm and humid.

Thank you everyone - much appreciated.

 

QP7, take a lesson from us guys who own boats and lay them up for the winter. Always fill the gas tank all the way up to prevent condensation that forms on the inside of the tank. And if you want to be safe about ethanol fuel separation (more water) you put a fuel preservative product like Stor-N-Start into your tank as well. Unlikely that you bought gas contaminated with water but it is possible in humid areas where the gas station gets condensation in their storage tanks.

 

Thank you very much, Freddy - great points; I had not thought of those excellent suggestions. I am learning a lot from all of you and it is very much appreciated - thank you. As Thermo knows, I really can't stand noise and so keeping my X351 running well, rather than going back to my old MB wagon (which is great, but darn noisey by comparison) is key for me - so thank you for that !!