Fuel injection line pressure

[Mark Aycock]

I've replaced the fuel pump(Bosch),fuel filter, and rear fuel lines on a '72 280sel 4.5. Now I'm doing injectors. What should the pressure rating be for the under hood fuel lines?

[Frank Reiner]

That injection system, D-Jetronic, operates below 40 psi.

[Benz Dr.]

More like 30 PSI or 2.2 bar. 40 PSI would be too much.

[mercmad6.3]

28PSi if you want the exact pressure. At 40PSi the injectors wont open. and i'd hate to think what would happen if a Djet rubber hose burst at that pressure.

[Benz Dr.]

I'm getting conflicting info on fuel pressre. The workshop manual says 28.4 PSI but it also says 2.1 atu, which would be about 30.8 PSI if it was set to bar. Maybe 29 PSI would be just about right or atu is different than bar.

[mercmad6.3]

We are both close.. I find they work better at 28 -29 . any more and the car starts to exhibit excess fueling,pinging etc. Modern fuel with it's aromatics etc doesn't tolerate these older style iinjection units and too much pressuere leads to poor running in hot weather.

Which brings up another issue, the old car network in the UK has found that E type fuels WILL cause corrosion in older fuel tanks. It also causes the old style rubber hoses to swell. I was working on a 350SL which was pumping raw fuel all over the engine as every sigle fuel line had swelled and rotted because the owner had switched to E10. The put those plastic boots on the ditributor and coil for a purpose.

[Graham]

Originally Posted by mercmad6.3

We are both close.. I find they work better at 28 -29 . any more and the car starts to exhibit excess fueling,pinging etc. Modern fuel with it's aromatics etc doesn't tolerate these older style iinjection units and too much pressuere leads to poor running in hot weather.
.

I own a 72 350SL (NA version with 4.5). If pressure is left at 28psig, a hot start problem would crop up from time to time (common problem on D-Jets). When fuel is warm, it will partially vaporize (flash) across the injectors resulting in lower fuel flow than the ECU is calling for. The engine tries to start but is too lean. This is more of a problem here during Spring/Fall when we have more volatile winter fuel combined with occasional warm days but can possibly also happen with regular fuel in hot climates)

One solution is to run at higher pressure. I have chosen 32psig. I then set mixture richer than specs at idle using ECU adjustment and approx per specs under load (using MPS adjustment). Since doing this, I have never had a hot start problem, although I do see the mixture go quite lean on hot starts even still (on built in AFR gauge)

If mixture is not adjusted, only thing I would think slightly higher fuel pressure might affect, is fuel consumption. ECU will still fire injectors just fine (they are flow rated at 3bar by Bosch) , but each time because of the higher pressure, a little more fuel will be injected per pulse. If car pings, surely that would be a timing problem?

BTW - Original spec for fuel pressure was 2.0 -0/+0.1 kg/cm2 (From MB 1972 Technical Data Manual)

or

196136 Pa
1.96 bar
1.94 atmosphere
1471 mmHg
20000 mmH2O
20 mH2O
2 kg_per_cm2
4099 psf
28.4 psi
57.9 inHg
788 inH2O
65.7 ftH2O

[tram]

Originally Posted by Graham

I own a 72 350SL (NA version with 4.5). If pressure is left at 28psig, a hot start problem would crop up from time to time (common problem on D-Jets). When fuel is warm, it will partially vaporize (flash) across the injectors resulting in lower fuel flow than the ECU is calling for. The engine tries to start but is too lean. This is more of a problem here during Spring/Fall when we have more volatile winter fuel combined with occasional warm days but can possibly also happen with regular fuel in hot climates)

One solution is to run at higher pressure. I have chosen 32psig. I then set mixture richer than specs at idle using ECU adjustment and approx per specs under load (using MPS adjustment). Since doing this, I have never had a hot start problem, although I do see the mixture go quite lean on hot starts even still (on built in AFR gauge)

If mixture is not adjusted, only thing I would think slightly higher fuel pressure might affect, is fuel consumption. ECU will still fire injectors just fine (they are flow rated at 3bar by Bosch) , but each time because of the higher pressure, a little more fuel will be injected per pulse. If car pings, surely that would be a timing problem?

BTW - Original spec for fuel pressure was 2.0 -0/+0.1 kg/cm2 (From MB 1972 Technical Data Manual)

or

196136 Pa
1.96 bar
1.94 atmosphere
1471 mmHg
20000 mmH2O
20 mH2O
2 kg_per_cm2
4099 psf
28.4 psi
57.9 inHg
788 inH2O
65.7 ftH2O

The SLs were more prone to vapor locking due to the design of the car- especially the later ones with cat converters. It's not a D-jet problem per se. Mercedes had a fix for vapor lock issues which essentially allowed the cold start injector to fire on start up, which would get the engine firing enough to overcome the vapor lock. There was a fix kit like this for the 6.3 as well.

I've never known a W108 to suffer from this malady- only W107s, and only with the V-8.

28PSI is the only spec on fuel pressure I've ever seen given by M-B. Setting pressure to 28 has never let me down. Higher than that is only a band- aid masking other issues, and this will cause other more expensive injection problems later.

[Graham]

Originally Posted by tram

The SLs were more prone to vapor locking due to the design of the car- especially the later ones with cat converters. It's not a D-jet problem per se. Mercedes had a fix for vapor lock issues which essentially allowed the cold start injector to fire on start up, which would get the engine firing enough to overcome the vapor lock. There was a fix kit like this for the 6.3 as well.

I've never known a W108 to suffer from this malady- only W107s, and only with the V-8.

28PSI is the only spec on fuel pressure I've ever seen given by M-B. Setting pressure to 28 has never let me down. Higher than that is only a band- aid masking other issues, and this will cause other more expensive injection problems later.

No doubt the tight engine compartment on the SL contributes to hot start problems and even more so on the cars with cat converters under the hood. There is no fuel cooler on early cars (but there is on later cars). The vaporization in injectors that I referred to, is specific to D-jets, because only the D-jets had EFI and operated at such low fuel pressure.

The original MB spec of 2.0 -0/+0.1 kg/cm2 from TDM when converted to English units is 28.4 to 29.8 psig. But this was for the fuel that they had back in 1972. The legislated volatility of fuels as measured by RVP (Reid Vapor Pressure), has changed since then for environmental reasons. 28 psig is really not high enough to prevent vaporization under some conditions. Therefore we get overly lean mixtures during starting with high RVP fuels that we get here in winter and fringe seasons.

There is no reason not to increase the pressure on a d-jet modestly, provided the ECU/MPS is adjusted accordingly to provide correct mixture. For those without an exhaust analyzer, I would chose 30psig which is still close to original spec.

The cold start injector normally only operates below a certain temperature, so does not normally come into play on hot starts. I have read on the Porsche forums of adding a switch to bypass the temperature switch so as to get a richer mixture for hot starts. On one model of SL d-jet, there is a relay on wiring diagram that seems to do same. MB also tried several other fixes including bleeding rail pressure down through FPR, so fresh cooler fuel would be available on startup.. I was at one time going to try using the cold start injector idea. But with the slightly higher fuel pressure and richer than spec idle mixture, I have no problems. But it did take 20 years of messing with the car before I got it right!

[tram]

Originally Posted by Graham

No doubt the tight engine compartment on the SL contributes to hot start problems and even more so on the cars with cat converters under the hood. There is no fuel cooler on early cars (but there is on later cars). The vaporization in injectors that I referred to, is specific to D-jets, because only the D-jets had EFI and operated at such low fuel pressure.

The original MB spec of 2.0 -0/+0.1 kg/cm2 from TDM when converted to English units is 28.4 to 29.8 psig. But this was for the fuel that they had back in 1972. The legislated volatility of fuels as measured by RVP (Reid Vapor Pressure), has changed since then for environmental reasons. 28 psig is really not high enough to prevent vaporization under some conditions. Therefore we get overly lean mixtures during starting with high RVP fuels that we get here in winter and fringe seasons.

There is no reason not to increase the pressure on a d-jet modestly, provided the ECU/MPS is adjusted accordingly to provide correct mixture. For those without an exhaust analyzer, I would chose 30psig which is still close to original spec.

The cold start injector normally only operates below a certain temperature, so does not normally come into play on hot starts. I have read on the Porsche forums of adding a switch to bypass the temperature switch so as to get a richer mixture for hot starts. On one model of SL d-jet, there is a relay on wiring diagram that seems to do same. MB also tried several other fixes including bleeding rail pressure down through FPR, so fresh cooler fuel would be available on startup.. I was at one time going to try using the cold start injector idea. But with the slightly higher fuel pressure and richer than spec idle mixture, I have no problems. But it did take 20 years of messing with the car before I got it right!

Sorry, but yes there is a good reason not to do so- the system is then out of spec.

My 1972 280SEL 4,5 runs perfectly with today's gas- and with ethanol, as well, set at 28PSI, just as the engineers that designed the car said it should be.

[Graham]

Originally Posted by tram

Sorry, but yes there is a good reason not to do so- the system is then out of spec.

My 1972 280SEL 4,5 runs perfectly with today's gas- and with ethanol, as well, set at 28PSI, just as the engineers that designed the car said it should be.

28psi is actually below the MB spec of 28.4-29.8 psi (2.0 - 2.1 kg/cm2). But, pressure measurement is not that accurate with low cost gauges (+/- 1/2 psi?)

The specification that really matters is the mixture as measured by %CO in exhaust.

The %CO in the exhaust under idle and the various load conditions MB specifies should be measured. (Full Load, Partial load high, partial load low, idle)

MB say to use the Fuel Pressure Regulator to adjust fuel pressure so as to bring CO emissions within spec.(p199 of 1972 Technical Data Manual)

If you or anyone else has measured %CO at the MB specified load points along with corresponding fuel pressure, it would be interesting to see them and compare.

[Graham]

Originally Posted by Mark Aycock

I've replaced the fuel pump(Bosch),fuel filter, and rear fuel lines on a '72 280sel 4.5. Now I'm doing injectors. What should the pressure rating be for the under hood fuel lines?

Hi Mark,

Just re-read your post and realized that this thread has got way off subject.

Now I see you were asking for the pressure RATING of the under hood fuel lines.

When I first installed new injector hoses many years ago, I purchased MB bulk hose. More recently, when I purchased SAE injector hose, I chose 150psig hose (blue liner). It is well made and certainly strong enough. But it is 5/16" (7.9mm) instead of the proper metric size which is, I believe , 7.5mm. Unfortunately, this hose leaked and I had to do job again.

What I did, is buy another length of hose from the dealer. The part number dealer provided is 230-476-87-26 (number has been changed several times). MB Classic Center confirmed this is the correct hose.

With the SAE hose, I actually had two problems. The MB clamps don't fit properly and eventually the hoses leak because they are oversize for the hose barbs.

Hope this helps.

[Frank Reiner]

"Just re-read your post and realized that this thread has got way off subject.

Now I see you were asking for the pressure RATING of the under hood fuel lines."

Thank you Graham,

Please see 1st reply.

[Graham]

Originally Posted by Frank Reiner

"Just re-read your post and realized that this thread has got way off subject.

Now I see you were asking for the pressure RATING of the under hood fuel lines."

Thank you Graham,

Please see 1st reply.

I did read that Frank. I think you referred to an operating pressure of under 40 psig? This is of course correct, but I "think" Mark was looking for the recommended pressure rating of the injector tubing which should be considerably higher.

One thing I have never been able to find out, is what the relief valve in the fuel pump is set at. I suspect it is at about 40 psig, but that is just a guess. Even still, the rubber tubing which deteriorates over time should have a much higher pressure rating.

[Benz Dr.]

It may not have been the original question but it's useful info just the same.
Question: if the engine is running too rich, which way should you go with fuel pressure?