Megane Mk1 bad starting and cutting out - bad spark plugs?

BrianD3

Megane 1 1.4 16v petrol - has been starting poorly for a while. It was OK if I charged or boosted the battery so I assumed that the battery was on its way out. But today things are worse, starting is weak even with boost and once it starts it cuts out almost immediately or else runs lumpy for a short period and then cuts out. After cutting out it won't restart. If the revs are kept up it runs ok but once they are let drop back to idle it cuts out.

So far I have
-Put in a fresh battery, made no difference. Starting still very weak, as though the fresh battery is near flat. PS I do not have a battery tester but do have a battery charger.
-Swapped out the plug coils one by one for a known good coil. Made no difference
-Cleaned the TDC sensor. Made no difference.

The next thing I'm going to do is get new spark plugs. The fact that the car is starting and running badly makes me zone in on the plug coils or the plugs.

I don't think it is the battery seeing as I have now tried two of them. Even in the unlikely event that the fresh battery is weak it shouldn't cause cutting out after starting?

Could it be something like a corroded earth?

Thanks for any suggestions

slimjimmc

The throttle body may need to be cleaned.

kev1.3s

I have a 2.0 petrol avantime and have a similar issue, car is quite spluttery and almost cutting out on start up which goes away after a couple of seconds and a good shot of throttle, I changed the spark plugs and also bought a new coil pack and changed them out one by one to see if one was faulty which there didn't seem to be. Last port of call I'm my opinion now is the injectors which may need a clean with some redex or something similar.
Interestingly I changed the battery recently and the problem seems to be vastly improved but not gone away.
For what it's worth I did mention it in passing to a mechanic when he was doing something else and he said that its running well........for a Renault! So it must be a common problem.

gk5000

Had a similar problem with a 1.6 mk1 megane - had to floor the throttle to get it started, and it was very bad until it got warmed up. Turned out to be the temperature sensor - there's 2 on mine, one for the ecu which is the important one (and a separate one for the temp guage).

Check the resistance across it - new one was 2.7 k ohms, while the broken one read 21k ohms which makes the car think its already hot.

BrianD3

Thanks for the responses - I don't think it's temperature sensor or temperature related as it does the same thing hot and cold. When I got it started I kept the revs up until it got hot then reduced the revs, it cut out immediately and wouldn't start again. Same behaviour as when cold.

I am moving away from the spark plug theory too as I don't think a bad plug or plugs would produce similar symptoms to a flat battery. With a bad spark plug it would turn over spluttering and trying to start. In my case, with a fresh battery, there is just some weak clicking but it will start (sometimes) with a boost.

I also don't think that it's the throttle body.

My latest thinking is voltage regulator or earth, does anyone agree that these would cause the symptoms of very poor starting with a fresh battery then poor running, cutting out and not starting again.

I don't have a tool to check the voltage across the battery terminals, must get one.

These are all the thoughts of a non expert so I may be way off. This car has been extremely reliable over the 17 years we've owned it. The only time it has ever failed to start or run poorly was when a battery was old/flat. Also, until I started swapping today, the plug coils were the original ones.

Waterson

Do you have acess to a multimeter? If so you can check the earth connections by doing a voltage drop test. Connect one lead to battery negative and the other to the various earth points in he engine bay ( bulkhead, under battery tray, chassis members etc) any reading gram eater than 300mv would indicate a poor earth. Also check for a 5 volt reference signal to some of the major sensors on the engine, (coolant temp sensor, MAP sensor). A diagnostic scan would probably give you a direction also.

BrianD3

Car is starting and running again. I got a multimeter in Maplins and now think that the issue all along was that the "old" battery was flat and the "fresh" battery was flat or dodgy. I didn't have a multimeter and was making wrong assumptions on the condition of both batteries based on what the battery charger was telling me.

I gave the old battery a slow charge and it is reading about 12.5 volts out of the car and holding that over time. It is reading 14.5 volts with the car running and 14.1 volts with the car running and the heated rear window and headlights on.

I have not slow charged the fresh battery yet but after fast charging until the green light comes on on the charger, the battery is reading 11.5 volts and this drops to 11 and then 10.5 after a few minutes off the charger. I think this battery is bad or needs a long slow charge. I can't test it in the car for now as it won't start the car.