Misfire at start up

[sterl]

Can anyone help with a misfire problem at start-up on my 250SL? The engine, hot or cold, seems to run on 4 cylinders for the first few seconds, then 5 for a few more seconds then finally on all 6 after about 15 seconds total. The following have been checked:

spark plugs - new BP5ES (slightly hotter than std), gapped 0.035
plugs have good coloration
points & condensor new
dwell 40 deg
timing 30 deg at 3000 rpm (vac advance disconnected)
distributor weights moving freely
all plug wires and caps have correct resistance
no vacuum leaks
even compressions, 140 psi
new throttle linkage rod ends and cross shaft bushes
throttle linkages adjusted per method described elsewhere in this forum
idle mixture adjusted as recommended in this forum
Warm running device operates correctly

It seems like a couple of injectors are not getting fuel initially. If I just turn the engine off for a few seconds and then restart it, I still get the same misfire. I've read that it can be difficult to track down such a problem, but it's annoying and I'd like to get to the bottom of it.

Other than this it seems to run fine, hot or cold.

Thanks,
Mark

Billericay, Essex, UK
Late 67 LHD 250SL

[DaveB]

Could be the start valve/injector on the manifold?
Does it start any better if you let the fuel pump run for 10-15 seconds before starting?

DaveB

[al_lieffring]

quote:Originally posted by sterl

Can anyone help with a misfire problem at start-up on my 250SL? The engine, hot or cold, seems to run on 4 cylinders for the first few seconds, then 5 for a few more seconds then finally on all 6 after about 15 seconds total. The following have been checked:

spark plugs - new BP5ES (slightly hotter than std), gapped 0.035
plugs have good coloration
points & condensor new
dwell 40 deg
timing 30 deg at 3000 rpm (vac advance disconnected)
distributor weights moving freely
all plug wires and caps have correct resistance
no vacuum leaks
even compressions, 140 psi
new throttle linkage rod ends and cross shaft bushes
throttle linkages adjusted per method described elsewhere in this forum
idle mixture adjusted as recommended in this forum
Warm running device operates correctly

It seems like a couple of injectors are not getting fuel initially. If I just turn the engine off for a few seconds and then restart it, I still get the same misfire. I've read that it can be difficult to track down such a problem, but it's annoying and I'd like to get to the bottom of it.

Other than this it seems to run fine, hot or cold.

Thanks,
Mark

Billericay, Essex, UK
Late 67 LHD 250SL

Mark

I have similar symptoms on my 66 230sl. for as long as I have owned the car (since 1976) when I restart the car with a warm engine there is a miss on one cylinder. If I rev up the motor it will clear out in about a second or two. At one time many years ago I think I may have used an engine osciliscope to determine it was cylinder #3. I used a diesel injector nozzle tester to see if the nozzles might be dripping off the pressure, Nothing conclusive, so I tested a bucket full of used take-out injector nozzles and use the best 6 out of the bunch. A little improvement,  I came to the conclusion that the check valves in the injection pump might be bleeding off pressure when the motor sits. My pump has the early pintle and spring valves, I don't rember if a 250 has that style or the later style, ball check valves. At one time I had gotten a set of the ball style check valves but was told that they didn't interchange so I never put them in.

The strange thing is that It always starts up on all 6 cylinders when cold, I would think that if the pressure bled off when warm that there would still be a miss on cold start up.

Hope I didn't add to your confusion.

Al

66 230sl
113-042-10-014715
904/396 blue, Ivory Tex
condition: not-as-rusty-as-before-bucket

[Rolf]

I have the same problem with my 1966 230sl. It starts on all cyclinders when cold. When warm, it starts one cyclinder at a time. I even let my fuel pump for a several seconds to no avail. After all the cylinders are firing it runs fine,

Rolf
1966 230SL Auto
1992 300D
1991 FLHS

[sterl]

Here's a little more info: letting the fuel pump run for 15 secs before starting makes no difference.

DaveB - are you referring to the CSV? I tested it this morning and the electrical and leak tests show normal behaviour.

The injection pump looks to be the early style with the upper starting solenoid.

Any other suggestions?

Thanks,
Mark

Billericay, Essex, UK
Late 67 LHD 250SL

[Naj ✝︎]

Mark,
What ign coil and ballast resistor have you got on there?

naj

68 280SL

[philmas]

I have been undergoing this issue for years, but only on cold start.If the engine's stopped for more than an hour or so, it occurs again.Looks as if there were some loss of pressure in some fuel lines.
Isn't there supposed to be some kind of valve system built in the f.i pump to prevent fuel lines from "draining" ?

Philippe
'71 280 SL

[sterl]

Naj, the coil could be the original. It's a Bosch with a black body and the light brown coloured top. Does that help or do I need to remove it to find some numbers? It measures 0.9 ohms. The ballast measures 0.8 ohms.  I have tried substituting for another coil (admittedly of unknown history) to no avail.

Because it runs OK as soon as it clears itself surely this is a fuel issue?

Mark

Billericay, Essex, UK
Late 67 LHD 250SL

[Naj ✝︎]

Hi, Mark,

In my view, 0.035 spark plug gap is too big for the black coil and 0.9 ohm ballast combination. I beleive the original spec calls for 0.028" and new Bosch plugs come gapped at that.
Some people using the red coil with 1.8 ohm ballast do use bigger gaps.
Just a thought...

naj

68 280SL

[ja17]

Hello,

Sounds like a leaky injector or check valves in the injection pump.

Joe Alexander
Blacklick, Ohio

[sterl]

Thanks for the info.

Joe, what do I have to do to investigate the check valve?
Can a leaky injector be fixed by cleaning?

Mark

Billericay, Essex, UK
Late 67 LHD 250SL

[ja17]

Hello Mark,

I think I would have the injectors checked first. Those early check valves in the injection pump are a little more involved.



Joe Alexander
Blacklick, Ohio