Vapor lock with 107's

[John S]

I thought that this comment on the USA Mercedes List was worth passing on.

The fuel header pressure on the old D-Jetronic engines used on early 70's MBs, Porsches, Volvos, VWs etc is about 28 psig. Hard starting when hot and left to stand for say 1/2 hr or so is a common problem. If car is left for longer, say an hour or two (long enough to cool down), it will start just fine.

One theory is that the heat from the engine with the fan not running (no electric fans at that time) cause the temperatures to rise. You can see this happen on the gauge. My gauge has very poor graduations - When running it is about 3/16" above 175F. Next mark is midway beween 175 and 250F - 212F?
Gauge hovers just below this 5 min after turning engine off.

I have not found a vapour pressure vs temperature chart for our current (USA) winter blend gasolines, but have seen that the RVP (Reid Vapour Pressure) which is normally about 6psi, could be as high as 15 psi at 100F in winter.
This is apparently due to addition of ethanol (and butane to increase volatility).

I found a formula on the net to calculate True Vapor Pressure of gasoline from RVP:
TVP = 6.9 + (0.14x(T-60)) - where T is the temperature, presumably in F.

This would say that the TVP of winter gasoline is 28.2 psi at 212F. And my fuel header actually measures about 27psig. The gas would start to boil.

It would seem then, that increasing the fuel pressure to say 30 psig might help? So would lower RVP gasoline, which should show up here soon.

But, I think I had the hot start problem last summer too, so maybe this theory is not the only cause of the hot start problem!!

Posting this to the list too - I am sure some will be interested and maybe shoot my theory down . I would love to hear any other suggestions - I have checked temperature sensors, fuel pressure etc. I did find a temperature switch wire disconnected (on coolant return), but this did not change anything. I am currently "talking" to 3 or 4 others who have similar problem, and it is the most common web post regarding these old 107 SLs.

Regards,

Graham Moss

 

[Styria]

Hi JohnS, is D-Jet really that troublesome ? Unfortunately, I know nothing about it except for what Oscar has been dealing with over a period of some two to three years. One of the things with D-Jet that would worry me is the cost of various spare parts for the system, and whilst engines fitted with D-Jet are more powerful, I'd still be reluctant to take one on.

I guess it's like with everything one knows and is familiar with - probably half the problem. Regards, Styria

 

[B13]

is D-Jet really that troublesome ?

Interesting comment, Styria.

For years I was put off D-jet. My 1st W116 was a D-jet 350SE with LPG and was basically a bad purchase... do bad it should have put me off W116s and MBs for life. However 3 months later I bought a K-jet 450SE, and both my 6.9s obviously have K-jet. I was a little reluctant to buy another D-jet car but decided to anyway and thats the 450SE TeamW116 car I talk about so much here and at the other place.

There was lots to learn about D-jet, but its actually a fairly logical EFI system and once you learn the basics its an easy system to be comfortable with, so I can see why Oscar and others stick with it. Right now, and this will sound strange, I reckon I have a better working knowledge of the D-jet system than the K-jet system in my 6.9 Its just fortunate that K-jet is a little more robust, I'm not expecting to have to do much on the system on my 6.9 for a long time.

I.