300SEL 3.5 bad fuel injector... - PeachParts Mercedes-Benz Forum

sreams · #1 10-11-2006, 06:58 AM

I've been having a problem with one cylinder missing (3.5 V8 in a '71 300SEL). I've narrowed it down to the injector. No fuel is entering the cylinder. Is there a way to test the two wire connection at the injector with a voltmeter? Should there be constant voltage there with the car running? Or should voltage only be there to trigger the injector once per rotation?

If it does turn out to be an injector, does anyone have a good source for the yellow injectors this motor has (not the blue ones the 4.5s have)? Thanks ahead.

nhdoc · #2 10-11-2006, 07:13 AM

The injectors are either open or closed. Open when voltage is applied, closed when not. The amount of fuel depends on how long they are allowed to stay open.

I believe if you connect a test light to the wires you should see it pulse on and off if it is receiving voltage. I am not sure whether the voltage at the injector is the full 12V or not but if not it should be at least 6V and that would be enough to see some light on it. You can also connect a VOM to the injector and see what its resistance is.

http://witchhunter.com/injectordata1.php4

This site has a table listing the injector's flow rates and resistances. If the coil is open on yours you don't have to bother testing with the light, it is the injector that is bad. I just sent all 8 of mine to witchhunter for test/cleaning. They do them for $15 each which seemed to be reasonable compared with others and they seem to know their stuff but I haven't gotten them back yet so can't comment on their work.

I suspected one or two of mine were not getting full flow causing a misfire under load so I decided to have them all cleaned and tested. I already cleaned my trigger points and those were fine and I can tell that each is firing at idle since when I pull the plugs off one at a time the speed drops by about 50 RPM or so for each.

If you need a replacement I'd bet you can find a replacement on ebay or post in the parts section a WTB ad and see what happens...

FWIW I found that www.********.com is the only place online that sells both new and rebuilt injectors for our oldies. The rebults seem reasonable at about $30-$40 each. Again, I have no personal experience with them except that they offer them for sale on their website.

Tomguy · #3 10-11-2006, 01:23 PM

A while back I got 8 3.5 injectors from a local member here (I actually got the whole top end of his engine, including heads). They are rated for the same flow as a 4.5, FYI. The only difference I know of is the color.

The injectors are ALL missing the tips - hence why I haven't sold them or done anything with them. If you want to send me $10 to cover shipping, all 8 are yours presuming I can find them all. I know I have at least 4 in one box.

cth350 · #4 10-11-2006, 03:48 PM

The simple way to test the electrical function of the injector is to put a screw driver up against it and then put your ear on the screw driver.

Try it with one of the working ones first. You'll get a characteristic buzz.

A dwell meter will tell you if the problem is the wiring rather than the injector. Again, compare the reading to a working injector.

Note that the injector wiring on an EFI v8 is in four sets of pairs, not that I recall the pairing info.

When pulling the injectors, be sure to replace the TWO rubber seals on each injector.

-CTH

Tomguy · #5 10-11-2006, 04:07 PM

CTH: The top one isn't actually a seal, it's used to retain the injector. Mine are as old as the car itself and they all are fine. Only the lower ones need to be replaced honestly.

Pairing is 1-5, 4-8, 6-3, 7-2 (just like firing order). Plugging a nearby wire into the suspected bad injector will make it fire if it's a bad wire. It doesn't have to have the same pairing. But if the other injector on the pairing is firing and that one isn't, it's most likely a bad injector.

david s poole · #6 10-11-2006, 04:41 PM

those injectors are powered by 4 volts dc

#7 10-11-2006, 11:20 PM

The blue top injectors flow more fuel than the yellow one's. I learned the hard way on my 1971 300SEL 3.5, it had one cylinder that always fouled the plug with fuel. That was the one cylinder that had a blue topped injector. I called Bosch and a German fellow with a very heavy accent got real upset that someone had put a blue injector in the car. He claimed the raw fuel would hurt the piston rings. I bought a yellow one and the car ran much better, no more fouled plugs either. They are rate in cc's of fuel per minute, and the blue injectors flowed way more fuel than the yellow one's. Just my .02.

sreams · #8 10-12-2006, 05:17 AM

Quote:

Originally Posted by Tomguy

A while back I got 8 3.5 injectors from a local member here (I actually got the whole top end of his engine, including heads). They are rated for the same flow as a 4.5, FYI. The only difference I know of is the color.

The injectors are ALL missing the tips - hence why I haven't sold them or done anything with them. If you want to send me $10 to cover shipping, all 8 are yours presuming I can find them all. I know I have at least 4 in one box.

Thanks Tom, I'd appreciate that very much. Let me know how to contact you.

-Scott