Cylinder #6 misses at idle (sometimes)

[SkopeMan]

So, it's a 1968 250 SL.  It runs fine, except when at idle after
the engine is warmed up.

While idling, it goes through a cycle, it will run on all six for a while,
then cylinder #6 stops firing for a while, then it runs on all 6, then
cylinder #6 stops firing, and repeats this, over and over.  It's very
consistent.  I would estimate about 1 minute running fine, then 30
seconds of missing, then 1 minute running fine, 30 seconds missing, etc.

Here's what we've done so far:

I swapped the spark plug between 5 and 6.  The problem stayed at #6.
BTW, the spark plugs looked great.

I swapped the spark plug wire between 5 and 6 (both at the
distributer and the plugs).  The problem stayed at #6.

Swapped the injector at the cylinder for #5 and #6. 
The problem stayed at #6.

Did a compression check, it was fine.

We adjusted the valves, to be sure it wasn't a tight valve.

Replaced the points.

Watched the spark by lifting the #6 wire from the
distributor.  There is no difference in the spark
when it is missing or running fine.

Once you rev it up, everything runs just fine.



WHAT'S GOING ON!

Any help you can give, would be greatly appreciated!

Thanks,
Dan Gray, Portland, Oregon

[ja17]

Hello,

The 19mm fitting on the injection pump should be torqued down at 21 psi (280sl) if it is too loose the pump will not build up enough pressure to open the injector at ilde. The frirst cylinder (frontmost) on the injection pump goes to #6 injector on the engine. Remove the frontmost line and torque the fitting first.

If this does not work the check valve in theinjection pump can also be bad.

[SkopeMan]

Thanks, ja17, what you say makes a lot of sense.  I'll try this out later this week, I'll let folks know if this solves the problem!
Dan

[geezer]

ja17,

I have a similar problem though mine is rough running upon hot restart for about 15 seconds.  I have suspected a check valve in the inj pump.  Are those check valves available?

[reggie]

have you tried blocking the vacuum pipe for the brake servo - if its leaking it will have that effect on one cylinder ?
REG

[ja17]

Hello,

The vacuum for the automatic transmission modulator picks up off of the intake manifold at #6.   Check the line for leaks, also check to see if the modulator diaphram is bad. If so you will find ATF fluid in the metal vacuum line.

[jack66]

Hello,
Interesting thread. I have also problems with my 230 sl 1966. I have changed my ip with a reconditioned, replaced all the ignition parts, checked nozzles and compression. I have altso adjust the valves, and checked the fuel reply and return. Everything seems okay when cold. When the engine is hot, there is no effect on cylinder # 2 and # 5 at idle until 2500 rpm when everything seems okay and the car is running fine. I will check the torque at the 19mm fitting on the ip, and the vacum pipe for the brake servo and see if that will help.
Jack