High idle when hot

[RJK91]

Hi everyone

My F4 van has an issue where the idle increases when the car gets hot and I'd like some help diagnosing the issue.

I had problems with a stuck choke cable and slack in the throttle cable. I replaced the choke cable and took up the slack in the throttle. These solved the high idle issue generally because the choke valve now opens up, but when the engine heat increases after a few minutes of running (particularly in traffic), the engine idle increases with it. The mixture seems far too rich when this is happening and it's like I'm Saddam Hussein gassing the Kurds with my exhaust gasses in traffic.

I've seen various threads about similar issues - it seems it can be due to fuel getting too hot before entering the engine and/or due to vacuum leaks. I think the engine may be getting a bit too hot generally so I'll change the coolant just in case. I'd appreciate any feedback because carburettors remain something of a mystery to me.

Thanks

 

[André4Renaults]

If your exhaust's producing too much unburnt fuel either the mixture's too rich or the sparking plugs aren't fully burning the fuel that is being pumped into the engine. Do you need to use the choke to start the car when it's cold-really cold-as it would be on a morning after the car's stood in the open overnight? If you don't, and the engine starts easily, then I'd suggest the mixture setting is too rich, since it's normally necessary to pull the choke to start a cold engine.

The problem might be in the ignition system, and a 'dying' capacitor will function correctly when cold, but will fail to function as it become hot, when the internal insulation breaks down. Perhaps change the capacitor for a new one first and see if this cures things. It'll save your hands becoming unnecessarily dirty!

Report back your findings and thoughts. I'll suggest a few other ideas if needed!

 

[RJK91]

The car actually came with a spare capacitor in it but I didn't know what it was until reading your post.

I installed the capacitor today and it resolved the issue - it now idles correctly even when warm. Thanks!

 

[André4Renaults]

Glad to have been of assistance!

 

[RJK91]

Hi again - on closer inspection, I think that the resolution was caused not by the fitting of the capacitor but rather by the fact that the subsequent drives were made in cold weather with the lights on. It seems that having the headlights on pretty much solves the issue of the rising idle.

I would therefore like to ask if anyone has any suggestions as to why this might be and how I might be able to change it so that the car idles correctly without the headlights on?

 

[André4Renaults]

Yes, there are a number of possibilities:
1/ The car's battery is not in the best of health. In very cold weather the chemical reaction within the battery that generates the electricity slows down and the battery therefore produces less power. In these circumstances when the car's running the electrical load (lights, heater fan, wipers, heated rear window etc.) is more and if the battery's not in good health the electrics are more reliant on the dynamo. If more load is placed on the alternator/dynamo it will have an increased resistance and this will slow the fan belt running over the pulley, which will, in turn, slow the main engine crank, thus slowing the engine.

2/ If the car's fitted with a dynamo, as opposed to an alternator it's possible that the voltage regulator is incorrectly set, although this is unlikely, in my opinion. The voltage regulator compensates as the battery needs charging and reduces this charging rate as the battery reaches full capacity. It also compensates for the effects of low temperatures, and it's possible that this aspect of the regulator needs adjustment. Check your battery's capacitance first: this measures the battery's health and not merely the nominal voltage it can hold. If it's poor or weak, buy a new battery, as you're going to need one soon. In the summer a dying battery might hang on for a few months, but in very cold weather-forget it-you'll need a new one!

In my humble opinion (although based on more years than I care to remember spent repairing these systems!) you should set the car's tick-over speed with the engine warmed up and with no electrical load (lights etc.) switched on. Setting the tick-over's important, as too low a rate will cause stalling when moving off and too high a rate makes manoeuvring difficult.

More advice if you need it!