w116 High speed hesitation.

[Mad Mick]

Hi, I have a W116 which is the 350se european model that I now drive in Ireland. I recently noticed a serious hesitation when overtaking a row of cars.
On kickdown in second gear at over seventy and nearing maximum revs the engine suddenly seemed to want to die as if out of petrol, on easing the accelorator she changed into third (top) and pulled away like a train. I slowed down to sixty and used the kickdown again to replicate these symptoms and sure enough she wants to die at high revs as if there is no fuel. I did this a few more times and the same happened. Otherwise she starts and drives perfectly with great accelleration until the very top end. I am not sure where to start looking and thinking of HT leads. I have brand new points set to the correct dwell and have set the timing. I recently cleaned the trigger points using info found on this forum, which I must say is excellent.
Much appreciate any help to send me in the right direction re Fuel or Ignition.
Vincent.

[ptoro01]

When was your fuel filter replaced last?
Peter

[Tomguy]

Three things come to mind off the top of my head:

1) The ignition advance is sticky and not working, causing timing to be too late at high revs. Big loss of high-RPM power.
2) The ignition is too advanced at high RPMs. This will almost makes it feel like the engine is stalling almost in the top RPM range. I've seen it firsthand when I set it too high myself.
3) You're too lean on the mixture.

From firsthand experience, I'd suspect #2 most likely.

[Mad Mick]

Thanks guys,

Peter, I don't really know when the fuel filter was replaced as I have only done 2,500 miles since I bought her late last year when she was allegedly fully serviced. I will replace this at the weekend.

Tomguy, My gut feeeling is that you are spot on with your #2 option re; timing as it almost feels as though there is a rev limiter cut of occuring at high revs.

Many thanks.

Vincent.

[Todd Miller]

Good info already posted and I'd also suggest you take out a spark plug and see what condition it's in and what brand. If it's a Bosch Platinum, you've found your problem.

I'd also throw an ohm meter on the HT lead from the coil to the dizzy and see what the resistance is. 10,000ohms is probably fine, but if it's up over 15,000ohm I'd replace it. I prefer solid core coil wires because they don't breakdown like the carbon core resistor versions do.

[Greg in Oz]

These engines usually had a rev limiter in the form of a simple centrifugal switch as part of the distributor rotor. At a pre-determined engine speed the switch trips and shorts the HT to chassis ground via the distributor spindle.

It may be that yours is either the wrong type (eg. for a 450 with a lower rpm limit) or it may be damaged (eg. a weak spring) allowing it to trip below the intended maximum engine speed.

On my 350SLC the redline on the tachometer is at about 6300 rpm, the specs for the 3.5 M116 state 6500 as the maximum recommended engine speed and I have found (when accidently holding low gear too long) that the cutout (which is marked 6650 from memory) operates at about 6800 rpm.

[Mad Mick]

Thanks Guys,

Todd, I have put new Champion copper plugs in and the condition of the old ones were perfect. I will change the HT leads soon as it looks as if two have already been replaced.

Greg, I am sure you are right on the money. I never actually thought to search under rev limiter as I never for a moment thought of there being one on this type of car. I don't have a rev counter on my car so I can't be sure of the revs, but this feels exactly as though this is the problem.

I can assure you I won't be going to the rev limiter very often, still it's nice to have the confidence of knowing where the problem lies. I had a new volkswagon van as a company car around 1975 and the rev limiter would cut in in a very similar fashion in any gear, I just never figured this would be on a 3.5 litre auto merc.

Many thanks to you all. This is a tremendous forum.

Vincent.

[blueeagle289]

These were interesting posts and really closed in on the problem.

I do however have to take issue with the comment about Bosch Platinum plugs! They seem to work well in 3 cars I have installed them in. The only
apparent negative about Bosch plats is that a Resistor plug of any description causes some problems in Mercedes engines. All plats are NOT
resistor plugs and can be beneficial in terms of long life. Some say the extra money is not justified on service life alone, but you can buy Bosch Quad Platinums at a very reasonable price if you look around. I am about to install them in my 99 ML320 at 65K, without any question that they will perform very well. I installed them in my girl friend's ML320 last year and they quickly improved that car's performance over the factory coppers that were in it.

I will let you know if they don't work well in my ML, because that particular car runs extremely well and has very good acceleration when desired - with the current original plugs.

[Todd Miller]

Hi Ben,
I certainly appreciate your opinion on the Bosch Platinums and I'll give you a bit of background to show you where I'm coming from.

The Bosch Platinums are not a "true" platinum plug. Now, if we're talking about Bosch Platines, at $25 or $35ea, or whatever they are now, that's a whole 'nuther story.....but I'm not talking about Platines.

I'm talking about the $1.79ea Platinums that are available at any auto parts store, Walmart, etc.

These simply have a fine platinum wire running down the center porcelain. Once they get some miles on them, that fine wire starts to burn away and receed down into the porcelain tip. What also happens at the same time is they start to crust over with normal deposits. The result? Hard cold starting and misfiring at cruise speeds. They may be great when first install, but that soon changes.

When I was at Bosch Fuel Injection Training School at the west coast Bosch headquarters in Ontario, California, even the instructor there told us not to use them in anything because of the resulting driveability issues.

I can't tell you how many times I've curred driveability issues by removing someone's 'beloved and recently installed BOSCH PLATINUM plugs.'

My plug of choice? The Bosch triple electrode DTC's. A bulletproof plug that's easily good for 60,000mi with no gapping required. They were developed in the 80's for the VW and Audi 1.7 and 1.8 fuel injected engines (W7DTC's fit these engines). I even use them in my 1275 A-series engines.

I don't own anything "modern" (as in, a couple years old) but if I did, I'd only use what the factory put in originally, be they "Silbers" or "Platines" or whatever. There's a reason they're in there.

[Greg in Oz]

Originally Posted by Mad Mick

Thanks Guys,

Todd, I have put new Champion copper plugs in and the condition of the old ones were perfect. I will change the HT leads soon as it looks as if two have already been replaced.

Greg, I am sure you are right on the money. I never actually thought to search under rev limiter as I never for a moment thought of there being one on this type of car. I don't have a rev counter on my car so I can't be sure of the revs, but this feels exactly as though this is the problem.

I can assure you I won't be going to the rev limiter very often, still it's nice to have the confidence of knowing where the problem lies. I had a new volkswagon van as a company car around 1975 and the rev limiter would cut in in a very similar fashion in any gear, I just never figured this would be on a 3.5 litre auto merc.

Many thanks to you all. This is a tremendous forum.

Vincent.

Vincent,

If the overspeed cutout is the cause, it sounds like it may be operating at too low an engine speed. You mentioned the problem at around 70 in second. My 350SLC which should share the same gearing as your 350SE can easily run to around 150km/h (about 95mph) in second. The markings on the speedo are a guide as to the maximum speed for each gear if you don't have a rev counter, although I notice they are actually a bit conservative.

Since spark plugs have been mentioned, I notice you have fitted Champions. I have tried these in my 350 but find that the Bosch Super copper cores work best. You also mention HT leads. Be sure to use non-resistance copper core leads since there is already resistnace in the supressor ends that fit onto the plugs.

Greg

[blueeagle289]

I never realized there were so many Platinum plug types!? The Platinums I use are the Bosch 2 prong or the Quads, which you don't buy in Walmart etc.etc. These are $3.50 apiece at the lowest I can find and seem to work very well; are not resistor plugs either (the ones I get) This is just one of those areas frought with confusion by the word Platinum I guess.

I appreciate the comments about the one with the plat wire running down the center; I have never seen those particular ones.... BC

[Mad Mick]

Thanks Guys.

Greg, I have checked in the Technical data in the owners manual (handbook) and you are quite right as it shows the top speed in second as 93 mph.

I,ll take her for a run this avo and see where she cuts of.

Re; the comments on plugs, this is very useful from you all, as it helps with the learning experience. On this plugs issue I was sold a set of Bosch WR78 SUPER4 plugs when I was in the UK recently, not cheap at over £40 for the set and I am now not sure whether or not I should consider fitting these: This type of 4 electrode plug I found excellent in my 3.0 litre Opel Omega which is a much more modern car and there is no mention of the word platinum on the packaging. If this plug is really not suitable I will return them and get some Bosch super copper cores.

Thanks again.

Vincent.

[Todd Miller]

I haven't tried the latest 2 or 4 prong Bosch Platinums. Nor have I heard anything about them performance or longevity wise. They're probably a nice plug. The fault in those "cheapo's" is that wire, as I mentioned.

Vincent, I personally don't put Champions in anything because they too have such a bad reputation for cheap construction causing driveability issues. Stick with Bosch stuff. You can't go wrong (other than the platinums I was talking about) and it's OEM quality. I think you have a HUGE clue in that you mention some of the HT leads have already been replaced. That to me is a big red flag. I would never replace just one "bad" HT lead. If one is bad, then they all are not far behind.....replace them as a set with the correct Bosch wires.

Also, as mentioned, pay close attention to the details of resistance and supression. You don't want resistor plugs in there if you have either resistor ends, or resistor (carbon core) wires. All that resistance makes for trouble.

[Mad Mick]

Todd, thanks for the info, I'll get new leads and once happy with those I'll change the plugs.

Greg, I took her out yesterday to see at what speed the engine hesitates and it is at 85 mph. I mentioned in the original post I was doing over seventy. Well, with the narrow twisty roads here, when you overtake a row of cars at speed you need to fully focus on the road and not have too much time to look down at the speedo. I had not realised she was travelling this fast as I rarely push her on like this and I am somewhat pleasantly surprised by how quickly she will get from 60 to 85. I think this is, as you say the rotor cut out coming in a little early and I am not really worried as the general speed limit around here is 100kph.

Greg is your timing on your 350SLC normally set to TDC +- 2 with vacuum. and 30 degrees of dwell? I don't have a workshop manual yet and the only one I can see for sale anywhere is the American version for the 350 450 which I believe is set differently to the euro spec models.

Thanks for all the help.

Vincent.