Idling on 5. Injector? Exhaust?

[Jonny Hart]

Hello All,

After 60 thousand miles since rebuild (11+ years ago!) I'm giving the SC a bit of love and sorting out a few little niggles.  One of them is the idle, which doesn't seem to be as smooth as it used to be with a rhythmic 'put-put-put' and most would say it's idling on 5 cylinders.   Lift the idle by just 100 RPM and it's silky smooth and the same when driving - no hesitation at all.

As you probably know, I'm pretty handy on the ignition side of things. This is not an ignition problem - there is a spark at all 6 cylinders.   I run headers so I have good access for a thermal camera which does not show a colder pipe.

Could this be an injector with weird spray pattern/drip at low flow - e.g. idle?   I had the injectors cleaned when the engine was rebuilt all those miles ago. 

Also, at this point, the silencer does seems to be lacking in the baffle department.  Could that cause it?  There are no sights of any leaks in the rest of the exhaust system. 

Cheers,

Jonny

 

[Ian Comerford]

You’re about to get a variable list of suggestions Jonny, but I’ll start.  When your engine was rebuilt did you have the metering head refurbished?  When mine was being examined at KMI I recall him saying that, given what he had seen, idle would be inconsistent and hard to set.  Just a thought.

[Northy]

Pull the injectors and pop them into measuring vials, it’s quick and easy to do. Tells you a lot about the injectors and the metering head. You can see the spray patter and check they are all getting equal fuel. 

If you’ve replaced your injector sleeves and o-rings before you won’t need to worry about them being brittle and failing apart either. 
 

 

[flatsix777]

If you find an unequal flow through the injectors then it’s very easy to adjust the fuel flow to each cylinder from the metering head (there is a small allen screw under a cap screw to the left of each fuel line on the head - fractional turn clockwise richens and vice versa). Obviously no good if the spray pattern is the issue. On mine, cylinders 2 and 5 went lean - visible by plug colour. 
David 

 

[Chris T]

As Ian says, a variable list of suggestions; Mine would be to pull all the injectors and take them to an old skool Bosch place that still has the pressure pump to fully exercise the valve/spring and pump cleaning fluid through. You can instantly see the spray pattern and also measure volume dispensed in minutes. Replace the O ring when re-installing, easier with a smear of Vaseline or similar. If no different then before adjusting the metering head, I would send it off to KMI for a check, better to find and fix the fault than adjust to get around it, especially in a stunning car like yours. 

Chris.

[Type911]

Just buy 6 new ones, they are coming up for 50 years old! They are about £30 each

[Jonny Hart]

25 minutes ago, Type911 said:

Just buy 6 new ones, they are coming up for 50 years old! They are about £30 each

Roger that. This was my next move.

After that, will look at the metering head.

Thanks

 

 

[SilverWT]

4 hours ago, Type911 said:

Just buy 6 new ones, they are coming up for 50 years old!

Agree with this, but with a caution.

It is likely they have been on the shelf for a long time and the assembly lube can dry up and make them sticky straight out the box. A clean in suitable solvent and a blow through would be a good idea before installation.

Mark

[AndrewCologne]

On 03/10/2025 at 10:23, Jonny Hart said:

Roger that. This was my next move.

After that, will look at the metering head.

Thanks

 

 

Please note that the failure rate of current Bosch products is significantly higher than it was in the 1980s.
It may be better to inspect the older units and confirm they are truly beyond repair.

Then I’d check the injectors’ flow rate (as others have already suggested above).
Its explained here:
https://nineelevenheaven.wordpress.com/adjusting-the-fuel-distributor/

BR!