Mechanics

CAR WON'T START SECOND TIME.

Renault Other

Renault Megane Scenic 1998 191 000 km
K7M motor

Car always starts first time and runs like a dream. EXCEPT :
When cold, if I switch off the engine a very short time after starting, it won't restart. For example, if I reverse out of the drive then switch off, it won't restart. Or if I try to start in gear and it jumps and stalls, then it won't restart.

If I reset the computer by disconnecting the battery, it will then restart first time. I'm baffled.

When the engine is warmer after about a minutes driving there is no restart problem. (Have replaced water temp sensor)
AD
Avatar
Roger44
June 3, 2007.




Have you checked to see what is missing when it wont start? (Spark or fuel) Is or has the check engine light been on recently? If so have the PCM scanned and we should get a code, that will give us an idea of what direction to head in
AD

Tiny
Jack42
Jun 4, 2007.
Hello. I've made progress.
The problem is the caculator is not opening the injectors long enough during a cold restart. On the graph you can see that average voltage on the injectors rises as coolant sensor temperature falls (the blue line I've added) but below 16°C the calculator only lets in a miserable amount of fuel during a re-start.

In fact the engine just needs to turn without firing(injectors, fuel pump and ignition disactivated) and the next time injector opening voltage will be miserably too low if the coolant sensor says less than 16°C. He knows "engine has just turned" via the


http://www.2carpros.com/forum/automotive_pictures/77479_restart_graph_1.jpg


TDC sensor.

The fix is to shunt the coolant sensor with a 6.8k ohm resistance to fool the calculator into thinking that engine temp > 16°C during a cold restart. But then when the engine fires, idiot calculator recovers its intelligence, sees a not-cold engine and doesn't deliver enough fuel for a cold engine, which runs down and stalls. I've got about 10 secs to remove the shunt.

I've inserted a list of all experiments.


http://www.2carpros.com/forum/automotive_pictures/77479_starting_excel_table_2.jpg


Tiny
Roger44
Sep 19, 2007.
Have you tried a coolant sensor? Yours might be out of range?

Tiny
Jack42
Sep 20, 2007.
The first thing I checked was the coolant sensor, it's OK. Anyway it can't be that for two reasons : 1. The car starts fine first time cold. My problem is when I switch off the engine after a few seconds and try to restart it.

2. If I replace the coolant sensor by a fixed resistance corresponding to the temperature it starts cold and won't restart as usual. It's baffilng me.

Tiny
Roger44
Sep 21, 2007.

AD