Engine hesitant under acceleration - Coil?

[fcd911]

I have tested the coil and my spares (similar results) but the problem occurs when hot and underload so not really representative.

Jonathan, thank you for the endorsement / confidence booster on the silver Bosch Coils – I am now convinced it is the coil so it will be either be one of those or the one that Design 911 sell (specified for CDI and Retrofit CDI units)

Thank you for all you advice and interest – I will report back.

John

[911hillclimber]

You are ahead of me, so deleted post.

[Jonny Hart]

You MUST use a Bosch CDI coil with our ignition boxes. The Bosch CDI coil (p/n as stated above) is unique in that it is very FAST to discharge and recharge. Our boxes exploit that speed in that we fire the coil twice per combustion which offers emissions and performance benefits.

We've actually designed our own coil which performs very well but have struggled to find a company that can manufacture them. After FIVE years of messing about and several false starts, we finally -may- have found a company that can fill and vacuum the coils properly. Bosch can't even do this repeatably anymore which is why the silver ones fail seemingly at random. Given the amount of R&D and outlay, It will probably end up being the most expensive coil in the world!

[Jonny Hart]

duplicate post

[Jonny Hart]

funny biz with the forum tonight.

[911hillclimber]

Ref:
Forum awkwardness, me too.

[Winston Teague]

The following thoughts are not based on any 911 experience. The symptoms of your problem have led you to suspect the coil, I would be inclined to suggest that they lead to a voltage drop in the supply to the coil. Am I correct in thinking the problem hesitation does not exist in daylight with the lights off? It seems possible (likely) to me that once the lights are on, the system is struggling to keep up with the demand, either because there is an inadequately specced cable (s) between the point where the alternator output feeds into the battery/loom or in the cable feeding the ignition system either from the switch or a supply relay. The first test I would try is to put a voltmeter on an ign on feed, coil, pump etc. and get some readings, lights on lights off etc esp when you get the hesitation.........I also feel the the very concept of a second hand coil is rubbish. there are new, branded good quality correct spec coils, and there are the coils on cars that are working properly, only, period. An unknown, secondhand, unbranded etc coil is no good at all unless in a roadside fix and you're desperate, simply to get home....Good luck and keep us posted, W

[Jonny Hart]

^ Sorry Winston, but the info you have given is for Kettering type ignitions and not applicable to CDI systems. Unfortunately there are not 'branded good quality correct coils' available. Both Bosch and the aftermarket alternative both have serious issue with their vacuum process. It is good advice to check all ignition power connections though!

You cannot/should not measure the voltage at the coil - it is a 400V volt pulse for a very short duration and will not show on a meter. Even if you have a scope you need to use a high voltage probe and also risk giving yourself a shock akin to that of UK mains.

As you are running a CDI+, these produce a constant 400V pulse right down to 9v at the battery. Generally speaking a degrading coil will show under engine load as the combustion pressure overcomes the sparks ability to jump the gap. If the problem is electrical it is far more likely to be to do with the points signal (clean them up and set them) or in the supply connections - check the 3 pin plug, clean and nip up the spade connectors a little.

[Winston Teague]

Ah sorry chaps, and thanks Jonny, I really must settle down and understand CDI. W

[fcd911]

Thank you Winston and Jonny for your advice and help.

I have tried two “spares” coils (correct Bosch 0 221 121 001) both gave similar results to the one previously on the car (you live in hope!!). I really like Jonny’s explanation regarding the combustion pressure verses the sparks ability to jump the (spark plug) gap as that is what it feels like.

I should reiterate two things.

1: There is no loss of actual 12V power, the lights do not dim nor take power out of the battery.

2: If I put the lights on main beam, the spotlights on and heater fan on and drive on the level then accelerate away all is fine, come to a hill and put the engine under load the hesitancy kicks in.

I am working through the ignition system checking all the connection and wiring etc. to see if I can make any improvement. I do not want to install a new coil without first going through everything to make sure it is 100%

I have yet to take the plunge on the new coil but common sense (correctly stated by Winston) tells me belatedly that if someone is selling a used coil there is probably a very good reason it is not still on their car!!!!

John

[Nine One One]

Going up a hill - you haven’t got crud in the bottom of the tank that is sliding backwards towards the in tank fuel filter and momentarily blocking off, or reducing the flow of fuel?

[Winston Teague]

Despite my lack of understanding of the CDI ign system, if it's a problem that manifests itself only when there is a high electrical load, voltage drop could still be a factor, if there's an electric pump could it affect fuel pressure or delivery? W

[RobFrost]

Voltage drop at the coil gets my vote.
Caused by inability of the system to deliver the charge which is demanded of it, at the rate it is demanded, at peak demand.

If I understood correctly it happens at the combination of higher revs, higher engine load, and higher electrical load. Is it worst at the engine's peak torque (mid revs) or at peak power (high revs)?

Either you have a faulty component, some unwanted resistance, excessive draw, or insufficient supply. Check earth straps, plug gaps, terminals, ht leads all plugged fully in, points gap, undo, clean and refit all low tension connectors. Have you changed spark plugs recently?

Have you calculated the draw of your lights and made sure it's within capabilities of the alternator?

Sent from my SM-G988B using Tapatalk

[fcd911]

Thank you for all the input.

I have cleaned, tightened, measured and remade (as necessary) all the HT and LT connections plus all the earths, reinstalled the suspect coil and found some improvement – it takes more (electrical) load to bring on the problem and it was not quite as severe as previously when it was trying to give me a whiplash injury!

I then installed the very expensive new coil I had purchased. Testing as above with the same load everything was perfect. However if I further increase the load (wipers, heater fan on full) there was a slight hesitation.

I can live with that and will not need to use the heater fan. An event this weekend with a finish for us about 8.30 Saturday evening so the big test will be how it copes in the dark, fingers crossed

John

[RobFrost]

The reason I asked if you recently changed spark plugs is that some have higher and lower resistance. If you want anotehr option varying the resistance or changing the gap may reduce the demands made by the ignition system.