What have I done? Brake bleed = non-starter

[Sorry Pete]

Hello all,

Thought I'd be clever and bleed the brakes on my W124 300D this pm. All went well, brake fluid nice and clean again.
I was using an eezibleed one-man bottle device at 10psi, and it overfilled the reservoir. I syphoned-out the excess, and I mopped-up as best as I could, though the area below the reservoir is still damp. I then started the engine up. All was well for the first 5 mins or so, then it developed a misfire, the ABS light flickered on, and within 30 seconds, the misfire went bad to worse and the engine conked-out totally. I haven't been able to get it started again, it cranks on the starter and feels like it's going to pick-up, but doesn't.

Any ideas?
Has the fluid done something to the electrics?
OVP relay?
I'm really peeved with myself...

 

[panason1c]

I really cant see that anything regarding bleeding the brakes could have caused your problem, more likely you have disturbed something near the fluid reservoir.........a vacuum hose or electrical plug perhaps?

 

[neilrr]

Check the fuse(s) in your OVP.

 

[Sorry Pete]

I really cant see that anything regarding bleeding the brakes could have caused your problem, more likely you have disturbed something near the fluid reservoir.........a vacuum hose or electrical plug perhaps?

Even with my limited technical knowledge, I'm thinking I can't have done too much wrong with a simple brake bleed. Checking fuses and connections tomorrow. The battery's dead and charging tonight. OVP sounds the main culprit.

Time for a beer methinks

 

[Number_Cruncher]

Usually, On an older MB diesel car like this, the engine will continue to run without ANY electrics on the car. While the engine was running roughly, the voltage from the alternator would have been low - possibly low enough for the ABS lamp to come on.

Unless there's been an after-fitted fuel cut off solenoid installed, these engines don't need any electrics other than the starter motor cranking it over to start.

I really can't believe this is an electrical problem at all - the most likely thing this can be is a fuel problem - either you've run out, or the filter is blocked.

 

[markjay]

When I was a teenager I worked at a tractors garage - diesel engined - you could just push-start the beast and drive off with it without even bothering with the ignition key - it really did not need it

 

[Number_Cruncher]

>>you could just push-start the beast and drive off with it without even bothering with the ignition key

Yes!, apart from the vacuum shut off capsule on the injector pumps, older MB diesels are similarly simple.

 

[Sorry Pete]

I was thinking the same thing... Wonderful what recuperative properties a shower and a tipple have.

I had a look at the OVP, single 10A blade type fuse in there. I don't have a spare lying about...

Having heard your advice, and as it's a diesel of a certain era, I had a look at the filters and the pre-filter looks pretty low, and in need of a change. I was low on fuel though not on the reserve, and topped-up after a stroll to the BP down the road, so all set to sort it out.

Procrastinators of the world, unite! (tomorrow)

 

[kth286]

there are numerous vacuum pipes on the petrol cars in the area where you were mopping up the brake fluid - don't know whether relevant to diesel - but have a careful look at the vacuum pipe connections in that area anyway.

 

[Sorry Pete]

there are numerous vacuum pipes on the petrol cars in the area where you were mopping up the brake fluid - don't know whether relevant to diesel - but have a careful look at the vacuum pipe connections in that area anyway.

Thanks, yes, there are around 8-9 vacuum pipes entering the dash just where I was mopping-up, and a heck of a lot of wires. Access is tight and I ended up twisting cotton balls onto the end of a chopstick to dry the area completely. I had a very good look, and I can't see any hoses split or wiring that's obviously loose, so I'm going to change the pre-filter and main fuel filter after lunch.

Did I mention I was a procrastinator?

 

[Sorry Pete]

Okay, so pre-filter, main fuel filter changed. Also did the air filter as that was off the car. No joy.

I did notice on lots of cranking that the ABS light remained on when all the other lights went out. OVP fuse next. Off to Halfords.

 

[Sorry Pete]

OVP fuse was fine.

After getting it running around 7pm, and seeing lots of fuel get to the new pre-filter, idle settled and running sweet as a nut, I thought I'd head for a celebratory drive. I made it 2 miles before it conked-out again.

It turns out I had air in the fuel lines and I was low on fuel. And I had to suffer the ignominity of waiting 4 hours for the AA man to arrive and sort it out. At least the sunset by Battersea Bridge was lovely. 10 litres of diesel in the tank, and slackening-off the lines to the injectors to bleed the system and all is well in 300D world.

I don't know what bleeding the brakes had to do with it, just coincidence?

Anyway, just wondering, how do you avoid getting air into the system the next time (if there is a next time) I change the fuel filters? The big screw-on one is simple, I filled it with fuel. How do I prime the pre-filter?

At least it's running well again, and I was able to make it to the w124 meet today. Mine was the slow and dirty one.

 

[Number_Cruncher]

In this situation, I would go through the whole of the low pressure fuel system, from the tank strainer, up to the engine, and all the way through the system to the return pipe back to the tank, checking the strainers and filters, checking the security of the connections, renewing all of the o rings, and checking the pipes themsleves for damage and splits. It may take a bit of time, but, won't cost much.

 

[Sorry Pete]

Okay, more dumb questions. I've checked the low pressure side of things, and can't see any leaks, the car starts, stops and runs as well as it ever did.

1. What's the purpose of a return pipe? Preventing vacuum? I just don't get why you'd need one.

2. Would the amount of air introduced by not priming the pre-filter with fuel have caused the fuel starvation I experienced? At a guess the pre-filter would take around 50ml to prime, but probably less.

3. The only other explanation I can come up with, is when the car was on ramps when bleeding the brakes, sediment blocked the main fuel tank strainer. Adding fuel freed the blockage enough for fuel to pass. In that case, is replacing the fuel strainer in a saloon a tank-out job?

 

[Number_Cruncher]

The return pipe allows leak off from the injectors to go back to the tank, and most importantly, it allows fuel to circulate in the injector pump, cooling it. It's vital.

On the suction side of the low pressure fuel system, you would not see fuel leaks, as the problem is air leaking IN.

I don't know about the strainer on saloons - I've got an estate. However, you'll need a large hex tool to remove it. It's a robust item which can be cleaned rather than needing replacement, but, I would use a new O ring seal.

 

[Sorry Pete]

The return pipe allows leak off from the injectors to go back to the tank, and most importantly, it allows fuel to circulate in the injector pump, cooling it. It's vital.

On the suction side of the low pressure fuel system, you would not see fuel leaks, as the problem is air leaking IN.

I don't know about the strainer on saloons - I've got an estate. However, you'll need a large hex tool to remove it. It's a robust item which can be cleaned rather than needing replacement, but, I would use a new O ring seal.

Cheers for that NC, there's another bit of tech info I've learnt from this forum: the rationale behind a return pipe!

I've seen others on here with bocked strainers on their estates, but I feel getting to the strainer on a saloon is a little more involved. I'll wait till I get low on fuel before tackling it.

 

[Sorry Pete]

Ok, update, 700 miles later, and passed without incident. Would my failure to pre-fill the small pre-filter with fuel have caused the fuel supply problem I experienced?

Is it standard procedure to have to bleed the injection lines after every fuel filter change? I didn't find any info in the Haynes manual mentioning that as a requirement.

Hypothesis: The car drove fine, the 2 miles down the road on the fuel in the big, main fuel filter. Then air was introduced into the system caused by not priming the pre-filter with fuel. Sound likely?

 

[DrNick]

To add my two penneth...

I notice you say it was up on ramps. Presumably this means that the front was higher than the back, like parking on a very steep hill?

If so then the air in the fuel lines has an explanation (and a cause) although I'm not completely familiar with your model and am assuming its similar to the W210s. Air gets past the seals near the fuel filter, allowing fuel to run back to the tank, emptying the pipes. Car then won't start or will only start for a bit as it runs on fule still upstream in the pipes/filter. Car stops before it refills itself as a big chunk of air arrives. If you have a good battery it is possible to crank the car to refill everything before it goes flat, but a weak battery will go flat before this happens (voice of experience!).

Happens when pointing uphill because gravity comes into play, adding a bit more pressure to allow air in. Replacing the O rings inthe pipes will sort things out, although it sounds like yours is a very mild case so you might get away with it by not parking pointing up hill!