Merc V8 Question.

[tigran]

Right en, i know there are quite a few merc tech heads about - and general mechanical whizzkids so here's one to pick your brains.

The V8 in the merc is 90% of the time silky smooth - there are a lot of ponies left and it pulls like a fugging train on kickdown. However for the other 10% there is an awful 'wobble' from the engine. It's as if there's a misfire in one of the cylinders which is unbalancing the whole engine - it only happens after the engine has been warmed up a long time and after a lot of driving. But once that happens unless you let the car cool down it will keep happening as soon as you switch the ignition on. So anyone got any ideas what that might sound like and why it doesn't do it unless the engine gets a lot of use/driving.

Any ideas appreciated. Gonna have a weekend of poking about with the merc so would like to get some ideas in my head that i can investiage.

[Lewis]

What's the ignition system on these like, is it a cap and rotor affair, optical?

I'd give it a full ignition service - plugs, leads, cap, ignition consumables - at least you know what you're working with then - just to eliminate it being something simple like that i.e. a knackered plug, lead or the cap having a crack that expands when hot.

Are there any other symptoms, like rich running, or running overly hot?

Once you've gone through that I'd start looking at the fuelling system - Maybe a duff coolant temperature sensor, or dry joint somewhere throwing stuff off. Pull all the connectors off and reseat 'em too (hell, at least if it doesn't fix it it'll feel damn productive!)

[tigran]

Yeah I'll go down to euro car parts tomorrow and get all the ignition shizznack and reaplace the lot. It's not running hot - the guage is behaving impecably, not running rich - just burning the usual bit of oil from worn stem seals.

Oh yeah and the bit that bothers me - it's not a constant thing, it's directly related to the ammount of use the engine has had.

*prays it's nothing to do with ally and cracking*

[magnetteman]

tigran said:

Yeah I'll go down to euro car parts tomorrow and get all the ignition shizznack and reaplace the lot. It's not running hot - the guage is behaving impecably, not running rich - just burning the usual bit of oil from worn stem seals.

That could be your issue, a build up of oil getting past the stem seals and fouling a plug or two during a long run.
Pull the plugs and you'll be able to see if there's any unusual burn on the tips.

[dollywobbler]

Lewis suggests wise things. Could be a dizzy issue so an ignition service is well worth while. Its the sort of thing that gets worse as the engine warms.

[BenzBoy]

Aye I'd suggest an HT lead breaking down or a fouled / knackered plug.
Does the 420 have the same ignition system as the 450 - breakerless dizzy? I think so. Wouldn't have thought it'd be that though.

Oh, just to add... the HT leads are copper, rather than silicone on these puppies I think. £££££££ for a full set. I've been told that it's not wise to swap them for silicone ones, someone might be able to clarify this though.

[tigran]

I've had a look at the plugs and one of them is absolutely soaked in oil after a long drive. I've ordered the head gasket sets and stem seals (along with other service bits). So will get the heads off on monday and replace the stem seals. Hopefully this'll cure it - it's gotta be done anyway as it's being a bit of a smoker.

Oh and i checked the ht leads and they look brand new.

[BenzBoy]

Good luck with the heads - they can be a curse word! Buy a good Hex key socket (Snap-On or something) if you don't already have one. And most conventional valve spring compressors won't work. You can either buy a made-for-the-job one from Samstag, or do what I did and make a bench-mounted one.
The stem seals are pressed in, so you'll need to press 'em out, and press the new ones back in.

This is going on working on my early M117 V8. I'm pretty sure it'll be the same for the later ones.

[tigran]

benzboy said:

Good luck with the heads - they can be a curse word! Buy a good Hex key socket (Snap-On or something) if you don't already have one. And most conventional valve spring compressors won't work. You can either buy a made-for-the-job one from Samstag, or do what I did and make a bench-mounted one.
The stem seals are pressed in, so you'll need to press 'em out, and press the new ones back in.

This is going on working on my early M117 V8. I'm pretty sure it'll be the same for the later ones.

Cool thanks for the tips. I'll blag some time down the workshop when it's not too busy - so should have all the tools i need about.

Here's to hoping anyway.

[Lewis]

G'luck with sorting it out!