Car slow to start and high fuel consumption on idle

My diesel car is slow to start in the morning AND when I let the car sit for hours even if its really hot outside.It takes about 4 seconds after I press the START button for the engine to start.
Only when I start the car minutes after I turned the engine off,it starts imediattely.
Another problem I face is a really high fuel consumption on idle.It shows about 20 Litres/100km although when I drive it ,the fuel consumption is normal (4l/100km on highway)
Other than than,I dont have any fault lights on the dashbord and my car drives fine.
I used a car scanner app and no trouble codes were found,no missfires ,although when I investigated some sensors I found some strange values when the car was stationary with the engine turned on:

MAF air flow rate:11 grams/second
Throttle position: 59%
Fuel rail pressure: 3500-4000 psi
turbocharging pressure: 990 mbar
rail pressure: 260 bar
hydraulic unit temperature: 0 degress Celsius

I did some research and what strikes me most is why the throttle position is at 59% on idle when i dont touch the gas pedal?I will try to make a test with gas pedal close to the ground and see the difference.

I know little to nothing about diesels, but the 990 mbar turbo pressure strike me as odd. My gas engine reads zero cold then goes to vacuum as the idle drops (~1500 RPM drops to 600 RPM), does not go to positive manifold pressure until I accelerate briskly.

The zero degree Celsius would seem to be a bad sensor, that might be causing the other symptoms, what does your coolant temperature read?

My guess on the throttle position is part of the engine warmup. Especially if the computer is receiving a zero degrees reading.

When I started the engine in the morning,the coolant temperature was around 40 degrees Celsius.In 10 minutes while the car was stationary,it gradually increased to 60 degrees.
I just changed the thermostat because it was stuck open because I rarely got to 83 degrees celsius even after driving 40 kilometers.
The new thermostat seems to be working because after I drive a couple of minutes,it opens at 83 degrees and stays around there.

That would help your fuel consumption right there. A cold engine uses more fuel.

Basing fuel consumption from the cars instant consumption display is wrong. Ignore that. Proper readings for consumption are calculated when you fill the fuel tank. liters to full divided by the km from the last fill.

As for the slow start, I’d guess a dirty fuel filter, dirty injectors or both.

Just changed all the filters(fuel,oil,polen,air) one month ago.Concerning the injectors,I dont know much about their condition but I am using from time to time some fuel additives to help clean the injection system.

You may also be having a problem with your lift pump, the low pressure pump that brings the fuel to the high pressure pump. The check valve in it may be leaking or the pump may be failing.

I assume this is a modern direct injection diesel, you didn’t tell me what year Renault this is, how many km it has, or the engine or really anything other than it is a Renault and it has on-board diagnostics.

The car is a 2010 Renault Megane 3 ,1.5 dci,110hp,185 000 kilometers.

Glow plugs? Valve adjustment? Compression test?

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Ok, direct inmection diesel with lots of kms. Potentially everthing I mentioned. Given you have used injector cleaner, probably not dirty injectors. I would check the lift pump pressure and if it holds.

No experience direct- injected diesels , but for a port fuel injected gasoline engine, coincident problems with poor mpg and taking a lot of cranking time , my first guess would be a leaking fuel injector. The injectors should completely shut off when the engine is turned off, which keeps the fuel rail pressurized. If an injector leaks fuel into the engine when the engine is turned off, the fuel rail pressure goes down, and this causes it to take longer cranking times to build up the fuel rail pressure again.

I also did a second scanning test having the car stationary with a bit of throttle(1500 rpm).The results were:

MAF air flow rate:5 grams/second
Throttle position: 59%
Fuel rail pressure: 4800 psi
turbocharging pressure: 1000 mbar
rail pressure: 316 bar
hydraulic unit temperature: 0 degrees Celsius

I see than the throttle position is stucked at 59% even with a bit of throttle(1500 rpm).

I’ve never heard of a diesel with a throttle.
Maybe that throttle position # doesn’t mean what we gassers think it does.
1000 mbar or so is atmospheric; what I’d expect in a throttle-less manifold idling and the turbo near rest.

This is just a maximum indication.
If you’re standing still the Litres/100km is actually infinite, AKA 0 mpg.

Sorry, I can offer no help on the hard starting.

Why is the MAF air flow sensor showing 11grams/second at idle speed(1000rpm)?I did some research and it should be around 2 times the engine displacement which means around 3 grams/second.It is strange because with some throttle(1500 rpm) it shows around 5 grams/second which is ok according to my calculation.Shouldn’t the airflow have a steady linear rise?

Quote:A rule of thumb on a MAF’s air flow rate at 500 rpm is 1 gram per second per liter of engine displacement

You may want to post this on a Renault forum, this is a US forum, no Renaults sold here for decades. You’re welcome to post, just saying that posting this where Renault owners gather might be more productive.


so, the coolant temperature was reported at 40 and then up to 83 when thermostat gets opened, but this one stays at zero?
why is that?
bad sensor or broken wire?

if this thing has glow plugs, and is like other diesels with glow plugs (sorry, zero renault experience here,) the glow plug controller works off of engine temp to know when to fire them. If GPC is getting a faulty engine temp signal, then it may not be powering up the glow plugs, leading to harder start.

if no glow plugs, then check compression.

A faulty throttle position sensor can certainly cause problems in gasoline engines. OP could use the forum search feature link here to see what others have said, upper right on this page. I’d guess solving the throttle position sensor mystery is where to start. Those sensors are often (potentiometers) variable resistors, which means they have something inside that wipes along the length of a fixed resistor as the throttle position changes. The interface between the wiper and the resistor can degrade w/use and make for erratic operation. If yours works like that, and you have a resistance meter, measure the resistance as the throttle position changes. It should change smoothly, and in the same direction, either up or down. The engine should be off, and the battery disconnected. If the resistance jumps around randomly, that’s likely the problem.

back in the days when mechanically controlled injection was the name of the game for diesels, it was much more simple like “check your fuel is there and no air pulled in, make sure it did not turn jelly from the cold temperature, check your glow plugs work… if still no… go for compression… if no - go for the pump and injector testing/calibration”

now, with electronically controlled diesel… well, the basics still have to be covered, but number of things which can go wrong definitely increased

one more thing - the throttle position for diesel has quite different role on the engine control if compared to gasoline engines, so I would not consider the fact it is reported as half-open as “aha, gotcha!” thing