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  #1  
Old 07-20-2002, 10:18 PM
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Join Date: Feb 2002
Location: Monroeville, NJ
Posts: 93
Unhappy Hard starting cold/hot/poor gas mileage

I own an 86 560 SEC with 150,000 miles. Starts in one turn cold, 2 turns hot. My 87 560 SL is hard starting cold or hot, and runs lean on three cylinders (compression is fine). Same engine in both cars. After spending well over $ 1,000 on the SL, I'm about to change out the fuel distributor to try to correct the lean condition in three cylinders (poor mileage probably caused by mixture compensation). Based on the threads I've been reading regarding cold/hot starting problems, should I also consider changing:

1. Fuel accumulator ?

2. Vacuum modulator ?

As a former mechanic, I hate taking a "shotgun" approach to a problem, but the lack of diagnostic capability on cars of this vintage leave me little choice.
__________________
M. Sandler

1986 560SEC: 150,000 miles, runs great, but I've got to sell it (too many cars for one man)
1987 560SL: 122,000 miles, used to run poorly, now (thanks to forum), runs great!
1997 GMC Jimmy: Turned out to be a turkey.
1989 T-Bird Super Coupe: 150,000 miles, still runs great. Ford got it right.
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  #2  
Old 07-21-2002, 01:43 PM
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Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150
You can check the accumulator by connecting a pressure guage to the fuel line -- the accumulator should hold some pressure in the system nearly indefnitely. Before you replace it, check the pressure drop on the fuel distributor and feedback frequency valve, because failures here will bleed all the fuel pressure down rather than holding at system pressure.

You can test the vacuum modulator with a MitiVac pump. Check rubber connectors on plastic lines before replacing -- which leads me to ask, are you having late, hard shift problems with the tranny? If so, look for the vacuum leaks!

Usualy failure mode on the fuel distrubutor is to go way rich and stay there from incorrect differential pressure inside. Not fixable by the DIYer (or the MB shop, for that matter!). Can only be tested with the proper equipment.

Get the fuel flow tested before replacing, it costs $1200 or so! Lean on a few cylinders can be a vacuum leak rather than bad fuel distributor, and will give hard starts due to no fuel (the air isn't going through the venturi).

Things to check are vacuum line to brake booster, brake booster itself, vacuum supply line for locks and climate control, and intake manifold gasket. I don't know if the fuel distributor/manifold is a two piece setup like the D-Jetronic or not, but if it is, there is another leak potential. The idle control valve hoses will get hard and loose with age, too, causing intermittant vacuum leaks and unstable idle mixture. If the mixture is adjusted while they leak, the problem gets worse. Check and replace, then adjust idle mixture without the leak. I ended up replacing all the vacuum line rubber connectors on the 88 TE to get a rough idle fixed -- still not done, I suspect the OVP relay and I've still not adjusted the mixture correctly. You can also get al leak at the pressure transducer in the computer (on the fender, small vacuum line from manifold to sensor) -- a leak here will cause mixture control problems.

A vacuum leak will also cause the mixture feedback circuit to mis-behave, since it measures residual oxygen -- excess air going into part of the manifold will cause the feedback circuit to assume the engine is running lean and add more fuel. This will eventually foul the plugs on the cylinders not affected by the leak, causing misfires and hard starts.

And don't forget the cold start valve, either -- if it leaks and dribbles fuel, it also doesn't spray properly. Bad spray will cause cold start problems, and leaks will cause rich running, excess fuel consumption, and flooding on hot starts! On the M103 it goes in the idle valve hose, so replacing the hose may fix some of the problems, too.

And last, what condition is the ignition in? Old tired, burned up cap and rotor, dead plugs, and leaking spark plug cables will cause all of your problems!

Peter
__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #3  
Old 07-21-2002, 05:27 PM
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Join Date: Feb 2002
Location: Monroeville, NJ
Posts: 93
Bless you Peter for all of that very useful information. I changed out the fuel distributor, and driveability improved immediately. I now have a unique paperweight. Should be interesting to see if the gas mileage and cold starting also get better. I still have to check all of the vacuum lines you mentioned. I'm sure there's at least one small leak in one of them, given the age of the hoses.

Also, with regard to the cold start valve, I've been unable to determine exactly how it works.

1. At what temperature does it start to operate?

2. After the engine starts and goes into "warm-up" mode, does it stop operating until the next cold start?

3. Is it worth removing and cleaning it? It doesn't seem to be leaking.

Thanks.
__________________
M. Sandler

1986 560SEC: 150,000 miles, runs great, but I've got to sell it (too many cars for one man)
1987 560SL: 122,000 miles, used to run poorly, now (thanks to forum), runs great!
1997 GMC Jimmy: Turned out to be a turkey.
1989 T-Bird Super Coupe: 150,000 miles, still runs great. Ford got it right.
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  #4  
Old 07-21-2002, 05:58 PM
Registered User
 
Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150
The cold start valve sprays fuel only when the engine is "cold" to the temperature switch, and only during cranking. You have to pull it and check it during cranking with a cold engine -- I don't know the temp, but would guess 50C or thereabouts, not higher. The usual problem is failure to open at all, or leaking during operation. It should not drip at all with a warm engine, or at any time the starter isn't operating. All computer controlled these days, so I can't help much -- most of my experience with K-Jetronic is from twenty years ago on my 1975 Audi Fox. All mechanical in those days. May still be, but I don't know for sure.

The mixure is controlled by a "frequency valve" that regulates the return pressure -- called the electrohydraulic pressure regulator.


I'm glad a new fuel distributor helped, but that is a very expensive "hope it works" kinda thing!

Peter
__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #5  
Old 07-21-2002, 06:03 PM
Registered User
 
Join Date: Feb 2002
Location: Monroeville, NJ
Posts: 93
Thanks for the information on the cold start valve. I'll try to check it out without blowing myself up. As far as the fuel distributor goes, it would probably would have been an expensive roll of the dice, but I bought it used from another forum member. He bought it to try to fix his 420 SEL, but it turned out to be a valve problem.

This forum is the best I've ever been involved with.
__________________
M. Sandler

1986 560SEC: 150,000 miles, runs great, but I've got to sell it (too many cars for one man)
1987 560SL: 122,000 miles, used to run poorly, now (thanks to forum), runs great!
1997 GMC Jimmy: Turned out to be a turkey.
1989 T-Bird Super Coupe: 150,000 miles, still runs great. Ford got it right.
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