'88 Euro 635 CSi M30 engine - spark plugs

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plip1953
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'88 Euro 635 CSi M30 engine - spark plugs

Post by plip1953 »

Is there any clear preference between the Bosch W8DC and W8LCR spark plugs?

Sorry if this question has been asked many times before.
Phil
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Re: '88 Euro 635 CSi M30 engine - spark plugs

Post by Pod »

Bosch specify the latter for '86 onwards, so why not follow their recommendation?
plip1953
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Re: '88 Euro 635 CSi M30 engine - spark plugs

Post by plip1953 »

Pod wrote: Thu Sep 20, 2018 9:20 am Bosch specify the latter for '86 onwards, so why not follow their recommendation?
Real OEM seem to list both for my particular car.
Spark plugs for BMW.png
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Phil
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Re: '88 Euro 635 CSi M30 engine - spark plugs

Post by Brucey »

the B35 motor ought to have the LCR spark plugs in it. The DC plugs are for the B34 motor. RealOEm ain't gospel; it is for 'acquaintance purposes only', so take anything it says with pinch of salt.

BTW the LCR plugs also fit the earlier B34 motor too; at least they were OK in mine..... that is what was fitted (incorrectly) when I got the car. Despite the smaller combustion chambers in the B34 motor the longer reach plug didn't hit anything..... :shock: :shock: :shock:

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plip1953
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Re: '88 Euro 635 CSi M30 engine - spark plugs

Post by plip1953 »

I'm glad I asked. So I might as well chuck the set of W8DCs in the bin!!

The last time I changed the plugs was probably around 15 years ago, and I'm intrigued to see which ones I used back then.

The only reason I'm thinking of changing them is because I've recently found that the engine is a bit reluctant to restart when hot. Any thoughts on whether this is most likely down to plugs, injectors, spark plug leads, rotor arm, dist cap or coil, or some combination of them? Apart from the plug change(s) and having had the injectors cleaned back in the 90s, I'm pretty sure the rest of the items are original. The car/engine has done 73k miles, but only around 5k miles in the last 20 years.
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Re: '88 Euro 635 CSi M30 engine - spark plugs

Post by Brucey »

cold starts use the cold start injector on early cars and later ones go 'double duty' on the injectors during cranking; this basically guarantees that the engine will fire briefly if there is any fuel in the fuel rail even if the pressure is not enough. Bad hot starts on an 'open loop' car can indicate (amongst other things) that

- the AFM is bad (reading zero airflow on cranking, so you get zero fuel.... :wink: )
- the throttle butterfly setting is wrong (not enough air getting into the engine (with the ICV closed), so no fuel either)
- the fuel has boiled in the fuel hose under the bonnet, and the check valves didn't stop the fuel line from emptying itself
- that the fuel hose has an air leak in it; on cranking the injectors are injecting vapour until the fuel hose is refilled with liquid fuel.
- that the coolant temperature sensor is faulty and needs replacement
- that the 'start' signal into the ECU is bad
- that the fuel pumps don't want to run for some other reason.

If there is a tiny leak in the fuel lines (very common) then you may see a delayed cold start; in perfect health an M30 should start sort of "zikka-brum" and "zikka-zikka-brum" may mean something isn't quite right and any more than three zikkas means there is a fault of some kind, probably with the fuel supply.

The fuel pumps only run on cranking until the engine runs properly. If you force the fuel pump to run for a few moments (eg by jumping the relay) before trying a start and this causes the car to start more promptly, you have an answer; there is probably a tiny leak in the fuel hoses, usually under the bonnet somewhere. If it stinks of fuel then the leak is pretty bad and has been there a while. A method of finding the leak (on a warm engine) is to touch each flexible hose/joint in turn and then to sniff your fingertips after each one; if you touch near a leak your fingers will immediately start to smell very badly of stale petrol.

If you have not reset the throttle body as per the manual, I would check that first; the linkage wears in such a way as the throttle plate closes more than normal as time goes on.

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Re: '88 Euro 635 CSi M30 engine - spark plugs

Post by plip1953 »

Thanks Brucey for your very comprehensive reply :D

I found the point about a possibility of a fuel line problem to be especially interesting as I have been getting an occasional whiff of fuel when working on the car from time to time - but usually when from underneath the car rather than specifically the engine bay.

The first ever occurrence of any kind of issue was 4 years ago when (after a long break from when it had previously been out on the road) I used the car for my daughter's wedding. In the period leading up to the wedding ie getting the car MOT'd etc, there was no issue, but on the day, just at he vital moment of bride and groom being whisked off to the reception venue it took two or three goes of extended cranking before catching, but was then fine.

There have been a couple of similar occurrences when I've used the car again a few times this summer, both shortly after the engine had previously been run long enough to be fully warmed up, but on a third and most recent occasion it tried to fire up (after several seconds of cranking), but when it did eventually catch it was clearly not running on anything like 6 cylinders. I tried to coax it with a bit of throttle, and was pretty confident it would then clear and run cleanly, but it wasn't having anything of it and stalled! I wasn't in any rush to get the car running so left it for around an hour, and when I went to start if after that it fired up straight away and cleanly.

I think I will probably change the plugs anyway (for W8LCRs) and will also check the fuel lines, throttle body etc as you suggest.

Just one other thought - would old fuel be another potential explanation? I tend to keep the tank at around half full during lay up periods, but clearly that means there's always gong to be a fair amount of stale or partially stale fuel to clear through the system even, even if as in this instance I took the first opportunity I could to fill the tank with fresh fuel. Maybe I might need to consider having the injectors checked/cleaned?
Phil
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Re: '88 Euro 635 CSi M30 engine - spark plugs

Post by Brucey »

the fuel filter should (in theory) take out anything that is too large to pass through the filter screens on the back of the injectors. However that is theory only; reality is different to that and anyway crud can be generated inside the fuel lines in various ways.

Stale fuel never burns so well but I have never had a starting problem with it despite doing everything wrong, i.e. leaving the tank with little fuel in it for months on end.

What did cause starting to be slow was when I got a tiny leak on one of the flexible hoses under the bonnet. Cold starts were slightly delayed and hot starts suddenly became much more difficult too.

As I mentioned before running the fuel pump before a starting attempt is a good diagnostic tool; if there is air in the fuel rail you should hear quite a lot of gurgling from the fuel rail as it fills up. This takes a few seconds (at least) and can result in a no-start for at least as long as the fuel rail takes to purge; there may still be air on the back of the injectors not fuel for a few revolutions of the engine.

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