1972 450SL hot start and injectors - PeachParts Mercedes-Benz Forum

erubin · #1 02-03-2004, 01:37 AM

Over the past year my car developped a hot start problem that has gotten worse. I hooked up a pressure gauge with a "T" and measured 28psi on the fuel ring (gauge needle jumps arround about 1-2psi). When the engine is shut pressure drops to 0psi immediately. I pinched off the fuel regulator return line (the one between the regulator and the damper) just when the engine is shut off and then the pressure stays at 28psi for a few minutes before it slowly creeps down (20psi after 45 minutes). Clearly the pressure regulator is shot and i will replace it. While i'm at it i'll replace all the 8 year old fuel hoses since they look hard and cracked.

I'm concerned about the small hoses integrated to the top of each injector. The injectors were replaced 18 years ago (Bosch P/N 280150036, MB P/N 0000783423) and the 18 year old hoses look like they are cracking. Must i buy 4 kits ($16 per pair!) or can i simply replace with 7.5mm hose? the shop manual shows how you must hold the injector in a special vise and then file off the steel cups that hold the hoses on. I'll have to find a gentle way to hold the injectors to file off the cups since I don't have this special vise.

While the injectors are out should I replace the rubber seals as they are 18 years old as well? The online catalog mentions an upper and lower seal. Theye also refer to newer engines than mine (my engine is 11798212-002659) so I don't know if those seals will fit. Should I check if the injectors leak while i'm there, is this an easy test and is any leaking permissable? If leaks should I replace or can they be repaired? i guess I'll run a bottle of Techtron before doing this work.

Since it's a daily driver I have to have the parts before i tear in. Thanks .

afmcorp · #2 02-03-2004, 11:24 AM

hello erubin

first let me say from your description the fuel pressure is doing what it is suppose to do. the fuel pressure first starts out hi 28-32 psi. if you just leave the key on after a second or 2 the pump will shut off. it's not like our domestics where the pressure tends to stay on. if it senses the car running the pump stays on

what you may need to do is to check if the injectors are opening. you should be able to here them clicking on if you turn the engine over by hand on the crank bolt.

i thought the same last year did the same with a pressure gage. tlhen steve B chimed in a corrected me. my car wouldn't start and eventually it had nothing to do with the fuel.

be sure to check your primary and secondary electrical systems.

good luck

ctaylor738 · #3 02-03-2004, 12:02 PM

My math tells me that if the current injectors are 18 years old, then they were renewed in 1986, which means that the original set lasted 14 years.

Not being one to spend another's money wildly, I would have to say that replacing the injectors would be a good investment especially if you are going to keep the car. New injectors made a big difference to my 380SL. And yes, you should replace the seals while you're in there.

Good luck!

bconrad · #4 02-03-2004, 01:41 PM

Replacing the injectors on the early EFI cars is not cheap. Best price I found was over $100.00 per injector. I am going to send mine out for hose replacement and cleaning to www.cruzinperformance.com they will clean and replace the hoses at a much better rate.

WANT '71 280SEL · #5 02-03-2004, 07:05 PM

You can just cut off the lip around the hose and pull the old hose, I belive that is the process you were describing. Just buy (I thought it was 8.5 or 9mm) hose and slide it over the nipple of the injector just like the old hose was and use a hose clamp to hold it. Don't use the kind that most American cars use that have the little slots in the clamp part. Use solid ones like the ones that are elsewhere on the car. You should go to a local indie for those clamps. That is what we did on my friend's 280SEL 4.5 which has the same motor except I believe it may even have a tad more power since it has the older hotter cams.
David

erubin · #6 02-04-2004, 02:41 PM

David,

That sounds great but a buddy of mine told me yesterday not to use regular fuel injection hose on the injectors. he said the hose on the injectors are reinforced and they are only available in the the $16/pair kit. For all 8 injectors my cost would be $64. Seems pricey for ordinary fuel injector hoses and clamps. I already have the smooth injector hose clamps so I'm set there. If any one else can tell me that my friends explanation is bunk I'll go ahead and pull the injectors, grind off the cups and install injector hoses with clamps on the fuel ring side only. Still have to find out if the diameter is 7.5, 8.5 or 9mm.

Thanks,
Elliot

porkface · #7 02-04-2004, 02:47 PM

when volvo used that injection system in the early 70s, we used fuel hose that came off the reel at, maybe, $1/ft. nothing special, but it was injection hose. look at the burst psi rating, should be 100 psi. good luck, chuck.

WANT '71 280SEL · #8 02-04-2004, 10:58 PM

Fuel injection hose is OK. I had an MB mechanic tell me this. If he says it, I take it like the bible, the truth. That is what he does and he is usually the one stuck with the V8's.
David

hill · #9 02-05-2004, 01:17 AM

7.5mm fuel line works fine. If you have the crimp on style clamp a small pair of dikes you can pry them open. I am running 30 lbs of fuel pressure and after sitting for six hours the gauge says 28. Your injectors can be cleaned by soaking them with fuel injector additive overnight. Put the injectors standing up in a can and fill it almost to the plastic part(put can outside). If your a cheapo like me go to a you pull it type yard and you can find 72/73 450 se's with grimy engines and shiny injectors. At $6.50 a piece you can't go wrong.

erubin · #10 02-17-2004, 03:16 PM

I changed the fuel pressure regulator and now i have perfect hot start for the first time in 1-2 years. The pressure stays above 20psi in the injector ring for hours and above 10psi overnight instead of dropping to 0 immediately with my old defective regulator.