Coughs and Splutters

[davidsl500]

Thinking caps on gentlemen and ladies please.....I want to tell you a story

MY 500SL R129 is playing up a bit, here we go....

The car is garaged and usually only comes out weekends. Last saturday it was booked in for a pre-mot and started up fine, first go as is it always does, drove about 4 miles to the station and parked up for about 20mins whilst waiting for the MOT guy to arrive. On starting it, it was running roughly - missfiring badly (never ever done this before) and it stalled as it went onto the ramp and wouldnt restart, turning over fine with the occasional "kick" and slight backfire, but nothing else. Plugs out and dried, bit of WD40 on the leads and left it 20 mins - starts up okay with a slight misfire which cures as it is left idling during the MOT Test, emmissions fine.

Drove it 20 miles to work and back again - no problems at all.

This morning i had to take it for its new set of tyres so I thought i would give it a bit of a run first this time to get it thoroughly warmed up ( i read somewhere that the dizzi caps were prone to condensation?). Drove it 15 miles or so - perfect as usual then as i approached the tyre station it started misbehaving and eventually spluttered to a halt as i slowed for a traffic island. It refused to restart, turned over fine - occasional kick and backfire. Left it 20 mins or so and it started again - not perfect, drove it a couple of miles and it was back to normal. Got the tyres (£547 - ouch !) and dropped at the MOT station for a "rubber stamp" and drove another 20 miles to work - everything fine but maybe a very very slight uneveness when idling at 500rpm in drive waiting at the lights - but no sign it was going to misbehave again.

I know that when i start it tonight to go home it will be spot again. It had a full engine service last October - leads, plugs, filters, dizzi caps and rotor arms and oil by Mercedes ( full Merc Service History) and I have done about 4,500 miles in it since.

I'm puzzled that it should play up after the longer drive, i put the first time it happened down to a short drive with weather change (it was quite chilly) and surmised that this was probably just condensation, but wouldn't have thought that i would get the problem after a good warm up whilst the engine was running. I didnt refuel at all so can rule out dodgy petrol. Unfortunately I didnt have the opportunity to check the plugs or caps second time as i wasnt suitably dressed ( or rather i was and didnt want to ruin my clothes!).

It will be a plugs out and caps off tomorrow - any other suggestions ?

 

[Mid_lfe_Crisis]

could be loads of things but my stab in the dark would be fule pump realy

 

[paulcallender]

The fact that you have replaced the plugs, leads, cap and rotor arm should eliminate most of the ignition system components. It would be worth taking out one of the plugs and inspecting it, for condition. Funny things, such as fitting the wrong heat range plugs, or having the gap wrong, can affect new components. Also, you will get an indication if your engine is running excessively rich, lean, or hot.

I agree its fuel related. The fuel pump relay is a simple, albeit not so cheap, thing to replace (maybe you have a spare one? Or could do a test where you make a direct switched connection into the fuel pump, bypassing the relay).

Other fuel-related checks would be checking if you've filled up with poor quality fuel; a fuel pressure and quantity delivery test (see Haynes manual?); CO gas analyse or preforably, 4 gas analyse the running.

 

[davidsl500]

The fact that you have replaced the plugs, leads, cap and rotor arm should eliminate most of the ignition system components. It would be worth taking out one of the plugs and inspecting it, for condition. Funny things, such as fitting the wrong heat range plugs, or having the gap wrong, can affect new components. Also, you will get an indication if your engine is running excessively rich, lean, or hot.

I agree its fuel related. The fuel pump relay is a simple, albeit not so cheap, thing to replace (maybe you have a spare one? Or could do a test where you make a direct switched connection into the fuel pump, bypassing the relay).

Other fuel-related checks would be checking if you've filled up with poor quality fuel; a fuel pressure and quantity delivery test (see Haynes manual?); CO gas analyse or preforably, 4 gas analyse the running.

Thanks for that Paul. I filled up a couple of weeks ago when petrol came down again so i am on the same tankful with 30 litres to go. Plugs are to the correct spec and have been in 10 months or so without a murmur. The mot station took them out last week after the the first non start and they looked fine then. I will take them out again tomorrow and have a good look.

As an update, It has been standing since 10.30am this morning at work and I left at 6.45pm, started first time and didnt miss a beat on the way home. So if it is the relay then it must be pretty intermitent. I really need to get it totally cold and take it out for a run and drop the plugs out when/if it packs up and then drop the plugs out to see if they are wet?

The gas tests last week were

CO 0.52
C02 14.48
HC 142
02 0.78
Lambda 1.015

 

[paulcallender]

Those gas results seem normal, and the other troubleshooting efforts are logical too. What I'd do, is replace the fuel pump relay and see what happens....

Its difficult diagnosing intermittent faults, you really need the car to go into the 'error condition' and then run your bunch of tests.

If you don't want to spend the money on the part (I am unsure, but on a W124 this is a double relay with the kickdown and is quite pricey) you could temporarily wire up a wire with a switch to the fuel pump directly from the battery. When it next goes funny, flick the switch and see what happens!

 

[davidsl500]

Looks like it was the distributor caps. Gave it a quick run sunday morning and then switched it off and it wouldnt restart. First plug was wet on removal so turned my attention to the electrics. First cap off and the inside was wet, second cap exactly the same with both caps pretty marked on the contact terminals. After a thorough clean and a bit of WD40 the engine started fine from cold and ran it for a few miles, stopped it, restarted it and all appears well. I also have a feeling that the coil lead to the cap on the right hand bank was not pushed home fully - the outer terminal was not a bright as the rest and i am now wondering whether it was working loose sometimes...

The real test will be saturday morning after it has been standing all week, fingers crossed.....

 

[Mid_lfe_Crisis]

It had a full engine service last October - leads, plugs, filters, dizzi caps and rotor arms and oil by Mercedes ( full Merc Service History) and I have done about 4,500 miles in it since.

So the new dizzy cap was faulty or they didnt fit one as said in post???

 

[davidsl500]

The two distributor caps were definitely changed for new ones they were just coated in water and the terminals inside were not worn as such - just dirty. Its amazing that with the various covers around the caps and the cap design itself that any moisture can get in there.

Anyway, started perfectly saturday, drive it 4 miles and switched it off and restarted perfectly. Today I took it for a decent ride and it behaved exactly as it should so problem over.....

 

[shayes]

I've lost the plot a little on this one - dizzy caps (plural?) but had similar problems with rough idle and lately complete dying on me and AA unable to start the Car with fluid via the Air Filter. Having just got half a mile i recokon I used up fuel in the system pipes.

I have replaced the OVP Relay with a brand new one and although tried a s/hand Kickdown Relay cannot get car to start.

Took out three of the plugs and they looked like they were brand new.
Left and Right Front and right back as you face the Engine - No deposit on the circumferance and only light soot on the pin.
I have tried the bridging of the number 7 and number 8 positions on the Kickdown Relay. I hear a click sound at the engine but not continuous noise. (I did not have the ignition switched to "On") IS THIS THE CORRECT WAY TO TEST? If so, is the part at the front of the engine at fault, or possibly the Petrol Pump is not pumping and I should be hearing a continuous sound as it builds up pressure in an obviously empty (I say obviously as I do not even get the odd firing of a cylinder) syatem?

Only thing done in the past with my Distributor is to have a damp cover fitted over it. Done several hundred miles since. Also, AA man said bags of spark.

New Kickdown Relay about to happen irrespective, but unable to get car to a Garage. Petrol half tank full as i know the question will be asked.

So, vis a vis originators problem, was it all down to dampness in spark plug caps?? or distributor Cap or was it poor connection at Coil. Can't imagine how it could have been dampness if car garaged. Had no problem with my SEC which was never garaged.

 

[davidsl500]

I've lost the plot a little on this one - dizzy caps (plural?) but had similar problems with rough idle and lately complete dying on me and AA unable to start the Car with fluid via the Air Filter. Having just got half a mile i recokon I used up fuel in the system pipes.

Only thing done in the past with my Distributor is to have a damp cover fitted over it. Done several hundred miles since. Also, AA man said bags of spark.

So, vis a vis originators problem, was it all down to dampness in spark plug caps?? or distributor Cap or was it poor connection at Coil. Can't imagine how it could have been dampness if car garaged. Had no problem with my SEC which was never garaged.

V8 M119 engine - 2 distributors, 2 caps and 2 coils - one for each bank of 4 cylinders. In my case it was deffo moisture in the distributor caps. There is a big seal on the distributor body, sticking up an inch or so from around the rotor arm that engages in a deep groove in the cap as you fit it. My seals had gone rock soild like plastic whereas they are supposed to be flexible - thus allowing moisture to creep in. Cost about £1.75 each for the seals, Andy Gayle dropped a pair in when I had my Disks done. Perfect since. The loose coil lead on one bank I think was a problem waiting to happen but a red herring for the present problem i think. The car is always garaged when not in use.

Inevitably this wont be your problem I suppose since it is a different engine but hope it clarifies what my problem was. Yours is going to be fuel or electrics, you will just have to work through it logically and dont forget the obvious - like blocked fuel filter. i'm afraid i have no knowledge on your particular vehicle, sorry!

 

[shayes]

Thanks for clarification David. May have a look at my Distributor cap thou' AA man said good spark at plug lead so should be sound. Worth a simple check as rubber cap only recently fitted to it and maybe something has crept in during these dewy mornings on the Seafront.

Had a brand new Fuel filter fitted only month or so ago, so that not the problem.

Won on e-bay a brand new FUEL PUMP SHUTOFF. KICKDOWN relay £26 + £5,20 postage which should come tomorrow and hopefully will fit, (update on original part number I am guessing). Guy had three other new ones to sell if anyone is looking for a replacement ref A0035452505 - feed in item 4603260274 and if can't contact the seller (as bidding over) let me know and I will send your e-mail address to him.
Not going to check out the Fuel pump myself but took off my Rear bumper to make job easier and also check out state of rusting - may have just saved myself from large expense as rusting at corner points underneath quite severe. Jenolited, and will spraypaint and underseal the Chromes to hopefully get a few years more life out of them before they become paper thin. Probably would'nt have bothered to look if the problem hadn't occured.

 

[television]

I think that you will find that the condensation comes up from the sump, you only have two bearings and no seal. This brings up a normal houshold problems, these guys that go around fitting double glazed plastic windows, seal them both outside and in, should any moisture get in, it cant dry out, In other countries you are only allowed to seal one side, not both, my point is do not try and seal it up too well. The guys in the USA get this all the time with the 119.

Malcolm

You could try a PCB laquure

 

[tom7035]

Good that you have it sorted. I was going to chip in with a little disagreement about 'definitely fuel related' (but didn't have the bottle!). The clue I picked up was the 'slight backfiring' when cold mentioned. This points to a mistimed spark - hence dizzy problem.

 

[shayes]

New Kickdown Relay failed to solve problem of not starting - towed to the Garage, I looking on forlornly at the sorry sight!

 

[shirubaby]

Does your problem become more apparent when the atmosphere is damp...like in the mornings and not when it's raining?

 

[shayes]

Certainly there has been a lot of moisure on the engine etc these dewy mornings but it never affected the SEC I owned, and the recent addition of a rubber dissy cap cover tended to make me think it can't be the problem.

Anyway, the news this evening is, the mechanic phoned me on his dying battery mobile, and said that he thought he had located the problem to the dissy rotor arm. He was cutting out on the phone but said there was no spark and he was waiting on a replacement arm to be delivered.

This has astounded me as the AA man said that there was a "good spark" and i trusted him to tell me what was going on whilst I turned the key!
I'm simply gobsmacked to use the colloquiel, as i never looked into the Dissy because of what he said!!

Will find out tomorrow if the Mechie has made a wrong diagnosis or discover how a Rotor arm can cause total failure to spark, even intermittantly.

 

[shirubaby]

This has astounded me as the AA man said that there was a "good spark" and i trusted him to tell me what was going on whilst I turned the key!
I'm simply gobsmacked to use the colloquiel, as i never looked into the Dissy because of what he said!!

AA men dont always get it right. I remember with my first car ( a 1.3 litre d reg mk2 golf) had a bad diagnosis from a AA mechanic. Told me it was the head gasket. WRONG.!!! And I found that one out to my great expense.

 

[JB direct ( wigan )]

re

dont know if this is relavent to your problem but i spent an absolute fortune once with a motor i had ... similar things happened like start first time every morning drive locally all ok ... drive that bit further started to run slugish ... pull up turn off wont start leave a while start but miss ... all in all it was a faulty petrol cap tank was'nt breathing and pulling fuel back after so long

 

[shayes]

Now thats an interesting point you make, because I find there is no vacuum sucking when I remove the Petrol Cap as there was with my SEC.
It has the rubber and seems to fit snugly enough, so i have not really worried about it. I did mention it to my Mechie when he recently was going to change the fuel pump, as I recall he got covered head to foot in petrol when he changed my SEC pump a year or so previous, and I had remarked that perhaps he had overlooked the cap release. I believe his reply was that it was not necessary.

Anyway will update later today when I hear from him.

Interested to know if anyone else has a lack of suction when removing fuel cap and thanks for your input?

 

[shayes]

Update on events.
Car back, running fair but not perfect, possibly missing intermitantly or need a tune ( I did adjust mix ever so slightly and then put back to former position with an allen key ).

Cause of problem? - Absolute fact that the Rotor Arm is not carrying any current! Replaced and also Distributor Cap, which has whiteish marks around several lead push in points, which i am told may be arcing marks.

Unbelievable, I would never imagine that a Rotor arm could literally pack up.
Mechie suggested may be wise to buy new set of leads. (costly).May just check spark plugs before that expense, after Tuning results.

Its also a moot point that I sort of favour Copper HT leads in plastic as less prone to the moisture ingress than the things on the car.

Anyone want to strip back the rotor arm out of curiosity? If so i'll post it to them.

AA not my favourite organisation at the moment "loads of Spark!!!" Yes Mate?