1971 280se 2.8L running rich - PeachParts Mercedes-Benz Forum

gocirino · #1 03-27-2009, 03:25 PM

My 2.8 with mechanical fuel injection seems to be running rich at idle and it's missing with the pedal all the way to the floor. Tailpipe is black too. Probably running rich at mid-range rpm's too but seems OK in that range. Plugs are black and sooty also.

Any thoughts on where to start diagnosing?

porkface · #2 03-27-2009, 05:38 PM

buy a fuel pressure tester for the kjet system and read the instructions. the pressures listed with the kit are close enough to let you know if it's control pressure causing the problem. good luck, chuck.

gocirino · #3 03-27-2009, 07:32 PM

Thanks.... anyone else have any theories?

Chas H · #4 03-27-2009, 09:09 PM

The thermosat for the injection pump could be bad. If that's the case the idle should be high and drawing air through the small filter next to it. Or the coolant level could be low, not allowing hot coolant to reach the injection thermo. The cold start squirter could be leaking or it's control circuit holding it open.
Has any work been done to the engine recently, or did this just pop up?

gocirino · #5 03-27-2009, 09:49 PM

Don't know.... I'm merely the new owner. Previous owner did not drive it so I have know history. Idles normal, but rough. WOT the engine misses from being loaded with fuel.

cliffmac · #6 03-27-2009, 10:49 PM

Here's what I'd do (and I have) with this one. I'd forget about overthinking the thing and do the very basic tune-up things....plugs, cap and rotor. Go from there....that's what I'd do.....I just did this with my first Mercedes Benz last year so I have a little bit of experience, not alot, but a little...good luck

cliffmac · #7 03-27-2009, 11:04 PM

but regardless, that was D-Jetronic. The GrandFather of Bosch Fuel Injection systems...the D is large.......I had the same thing on my very first car, a 1973 VW 412 with a Porsche built flat 4 in it. D-Jetronic is very primitave, I am just sitting here reading about it in my manual....

JonL · #8 03-28-2009, 01:59 AM

Quote:

Originally Posted by cliffmac

Here's what I'd do (and I have) with this one. I'd forget about overthinking the thing and do the very basic tune-up things....plugs, cap and rotor. Go from there....that's what I'd do.....I just did this with my first Mercedes Benz last year so I have a little bit of experience, not alot, but a little...good luck

Actually, that is very sage advice. I'd also add valve adjustment, COMPRESSION TEST, check for vacuum leaks, exhaust leaks, etc. Check cam timing/chain stretch. Ignition timing is implied in your post... include centrifugal and vacuum advance function.

hookedon210s · #9 03-28-2009, 08:28 AM

Quote:

1971 280se 2.8L running rich

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My 2.8 with mechanical fuel injection seems to be running rich at idle and it's missing with the pedal all the way to the floor. Tailpipe is black too. Probably running rich at mid-range rpm's too but seems OK in that range. Plugs are black and sooty also.

Any thoughts on where to start diagnosing?

Am I missing something? Isn't your car equipped with the Bosch 6 plunger mechanical injection system? This is not D or K jet. You would probably get better info if this post were moved to the Vintage section. Anyway, these engines tended to run rich when properly adjusted and do not like stop and go, short trip drives. Do a proper tune up including air and fuel filters but also check the coordination of the throttle linkages---these frequently get misadjusted/bent causing a rich fuel mixture (the injection pump lever is opened too much relative to the air venturi lever). Do not adjust the idle mixture on the injection pump until you know what you are doing. $$$ or personal injury. You definitely need to read up on this before proceeding. Mark

gocirino · #10 03-28-2009, 08:52 AM

Quote:

Originally Posted by hookedon210s

Am I missing something? Isn't your car equipped with the Bosch 6 plunger mechanical injection system? This is not D or K jet. You would probably get better info if this post were moved to the Vintage section. Anyway, these engines tended to run rich when properly adjusted and do not like stop and go, short trip drives. Do a proper tune up including air and fuel filters but also check the coordination of the throttle linkages---these frequently get misadjusted/bent causing a rich fuel mixture (the injection pump lever is opened too much relative to the air venturi lever). Do not adjust the idle mixture on the injection pump until you know what you are doing. $$$ or personal injury. You definitely need to read up on this before proceeding. Mark

Mark,
you are correct on all counts. THis post should be probably moved, I suspect that is a moderator function. And it is a mechanical system. I will look at your suggestions,

Thanks

john

Arthur Dalton · #11 03-28-2009, 10:07 AM

The first suspect on that MFI system for your complaint is a a leaking Cold Start Valve [ CSV]
There is a test for this condition:
On the intake you will see a petro valve mounted . It has a single feed wire and a fuel line. On the side you will notice a 6mm hex head plug..that is the test port. Remove that plug and turn the key to the RUN position. There should be NO fuel coming out the port..if there is , the CSV is leaking [ very common] and that extra fuel entering the engine is unmetered..so the engine runs rough at idle and rich thru the ranges. It can be taken apart and cleaned, along with the filter screen. You can also relap the seat if you are handy..they are very expensive, but repairable. If the CSV fails this test, unhook the wire and try the same test. BOTH test results should be NO fuel, as the CSV only operates when the Starter Circuit is engaged.
Unhooking the wire eliminates the possible of a stuck CSV relay.

My suggestion is to go over to the 113 Group, where these procedures are discussed in detail. They have a complete Injection Tour Tutorial over there, from linkage set-ups, right down to complete fuel system testings.
A-Z..very informative .
You can also do a fuel flow test at this same valve by taking the line off and seeing if the electric pump will attain the 1ltr/15 sec spec for the fuel system.
Second suspect is the WRD [ Warm Running Device/thermo]. That device controls the A/F mix thru all ranges when the car is up to temp.
CTH post directs you to both these known MFI common probables.Try 113 Forum is best bet..as they go into the test procedures for both....or use the search feature here, as we have much of that same info, but not as detailed.

gocirino · #12 03-28-2009, 04:39 PM

Arthur,

thanks for the reply. I will be working on it very soon and will report back my findings as I progress in the hopes that it might help someone else in the future.

Regards,

John

Arthur Dalton · #13 03-28-2009, 04:41 PM

You're Welcome..........

porkface · #14 03-28-2009, 09:12 PM

man, i gotta get glasses and read these things. i saw 280se and didn't see 71. sorry for the bad info, chuck.

gocirino · #15 03-29-2009, 12:59 PM

Well, I threw the timing light on it this morning and the marks were lined up 4 degrees ATDC per the hood data plate. That is were I left them following a Pertronix breakerless ignition upgrade I did. The shop manual called for the same with the vacuum hose attached and 8 degrees with the vacuum hose off. When I took the hose off the timing stayed the same.
I brought the timing up to 8 degrees BTDC without the hose, then replaced the hose and went for a spin. Runs like a new motor! Unbelievable!
No pinging or startup problems either.
I am obviously not out of the woods yet because I still have a timing issue to diagnose.. but nonetheless a good starting point.