[coyoteboy]

Hi all, my previous request was lost in another thread title, lets see if anyone can help out now I've actually made it obvious

Does anyone have the coolant temp sensor (EFI mainly) resistance curve for the 1.6 8v petrol megane 1? I'm trying to identify the cause of a misfire without replacing every sensor one at a time, this is one of my suspects but I can't find the curve to test in either the haynes or online?

Any help appreciated!

 

[roby37]

Coolant and air temperature sensors use the same materials and all have the same temperature/resistance characteristics. The precise readings aren't that critical. Just so long as it is in the thousands cold and the hundreds hot.
The Renault advice is "Replace the coolant temperature sensor if the resistance is not 2252 Ω ± 112.16 Ω at 25°C."
More often rather than a CTS failure the problem is dirty/corroded connections.
When are you getting a miss-fire?
A broken CTS circuit gives a rich mixture causing hot starting/running problems and a shorted CTS circuit gives a weak mixture and cold starting/running problems.

 

[coyoteboy]

She has some issues with cold start occasionlly not working first time round, hot starts sometimes also questionable and general misfiring under load (particularly taking off from junctions, which can be a dicing with stalling at times). It started off at lower RPM but now pretty much across the range until about 4000rpm when it seems to clear up (but it's hard to tell by that point). I've checked HT lead resistance and plugs are new, MAP hoses seem fine and coils resistances check out fine. Leads track when I pour water over them, but that's a bit of an extreme test! Checked the TDC sensor too, clean and happy.

Thanks for that figure, I'll whip it out and test it to see if it's close. They don't ALL have the same characteristics though, different vehicle manufacturers use different sensors - I presume you just mean within the renault range. That could be a handy diagnostics tool- I'll bring in the air temp sensor too!

I've checked over the connections and unless I have a loom failure somewhere (possible) they're all fine.

 

[roby37]

These are the figures for the ATS.
7419 Ω ± 365 Ω at -5°C.
5887 Ω ± 282 Ω at 0°C.
4707 Ω ± 220 Ω at 5°C.
3791 Ω ± 173 Ω at 10°C.
3075 Ω ± 137 Ω at 15°C.
2510 Ω ± 113 Ω at 20°C.
2063 Ω ± 87 Ω at 25°C.
1716 Ω ± 71 Ω at 30°C.

 

[roby37]

Have you had the plugs out?
At least they would tell if it is one or more cylinders and whether it is rich or weak.
If the ecu goes into limp mode due to a fault being detected it often assumes the temperature of a stopped engine is 0°C and a running engine 118°C, not sure if this applies to your particular engine.

 

[coyoteboy]

That's a handy set, cheers. Printed these off and got them saved in a file now!

Had the plugs out - yep, very clean but nothing particularly "anything" about them - they are not sooted but they don't show classic signs of either lean or rich, but seeing as it idles and cruises fine I think it would have to be pretty far out to show signs? The miss is initiated at tip-in generally (hiccups and never recovers until you let off the throttle), or if the engine is loaded while at constant throttle and some unknown load level is overcome, again throttling back stops the miss but often is too low to get going so a gear change is needed and the tip-in misfire hits . Very odd. It had a new head gasket not so long ago but it had the misfire before this and afterward, just the same.

Misses seem to get slightly less harsh and frequent as the engine warms, but are always present (which tilts me toward the ECU thinking it's too warm and leaning out so hotter actual temp is closer to imagined temp).

No error codes/lights are flashing so it's as though it's not reading a fault, but stranger things have happened (out of range but not dead = no fault?). Other previous suggestion on here was o2 sensor. It's speed sensor keeps failing but I'm not sure if that is fed back to the ECU and the misfire was present long before this developed.

I suppose one final issue could be the ECU/fusebox. It has had a fusebox fault (caused havoc) about 18 months ago but this caused distinctive and definite failure which has not been repeated after I checked the whole thing over for dry joints, the misfire never altered through this period and the car continued running even when the fuseboard had failed (providing you kept the ignition on!).

You can understand why people slag off french cars because of electronics!