400E/V8 starting problem - PeachParts Mercedes-Benz Forum

peterhardie · #1 02-28-2007, 03:10 PM

This is a VERY rare problem, maybe 3-4 times a year on a daily driver. My wife's car will flood (I am guessing) badly, and will take a minute or so of cranking with the accelerator floored to get it to start. It will bog a bit when pressed for a while driving, then return to smoth running.

It happens so rarely, I can't pinpoint the exact circumstances. Yesterday, the car had not been driven much this week, moved it briefly to clear snow from the driveway. Weather was damp, above freezing.

Any ideas???

sunman · #2 03-03-2007, 12:28 AM

could be an eigine wiring harness if you have never replaced it.

peterhardie · #3 03-05-2007, 08:13 AM

The harness has been replaced recently. As has the crank sensor.

It is tricky to diagnose, it won't repeat the behavior.

Weeks and months later the car runs and starts just fine. It will only happen once in a great while.

Larry Delor · #4 03-05-2007, 08:04 PM

When it does happen, it might help to note the temperature and whether it was humid or not, and when the car had been driven last. (and maybe even what brand of fuel you used).

myarmar · #5 03-05-2007, 08:46 PM

My car (92 600SEL) has a similar problem. It always starts fine. If I did not let it to warm up to about 60C and shut it down, as if move it out of the garage on the driveway, next time it would not start and would flood as you describe. I had happened in any weather, hot or cold. The way to start it is to remove fuel pump fuse and crank until it starts, then reinsert the fuse. Everything I can think of was checked - sparks, fuel pressure, harness replaced. One last thing I suspect is extremely high oil pressure during start gets hydraulic tappets pumped-up causing partial loss of compression. That would require oil pressure test during start. For now I've learned to live with this and avoid shutting down cold engine.
Mike

peterhardie · #6 03-06-2007, 11:01 AM

Thanks, Mike, that does correspond to the most recent occurence. I had moved the car a carlength to shovel the snow. The next day, I had the problem.

Why would the car start without the fuel pump? Are you burning the excess fuel out? Do you reinsert the fuse as soon as the engine catches? Explain the procedure a bit more, I would like to try this next time.

myarmar · #7 03-06-2007, 11:41 AM

Yes, I think you burning or most likely pumping exessive fuel out of the engine. You need somebody to help you. As you crank the engine it would start, and at this time you need to reinstall fuse. If you get this timing just right the engine would keep running with heavy gas smell. After a minute or two everything is back to normal. I was never able to start it the normal way - the battery would go dead cranking this big V12.

400E · #8 03-08-2007, 10:10 PM

Quote:

Originally Posted by myarmar

My car (92 600SEL) has a similar problem. It always starts fine. If I did not let it to warm up to about 60C and shut it down, as if move it out of the garage on the driveway, next time it would not start and would flood as you describe.

Mine has ALWAYS (from an early age) done exactly this as well. I have learned NOT to start it to back it out of the garage to wash it, unless I plan to allow it to warm up as you describe.

On the occasions where I forgot to do this, I was always able to get it started by flooring the accelerator while cranking - this seems to clear the flooding. It will run rough for a while once it starts, but then level off eventually.

Seems to be one of the M119 "features."

wbrian63 · #9 03-09-2007, 08:42 AM

Quote:

Originally Posted by myarmar

One last thing I suspect is extremely high oil pressure during start gets hydraulic tappets pumped-up causing partial loss of compression. Mike

In general, if functioning properly, hydraulic tappets don't lose an appreciable amount of their "pump" from one start to the next, even over several days. Regardless, when they "pump up" - they never inflate enough to cause valves to stick open when they should be closed. Most, if not all MB engines are interference design - a stuck open valve is going to kiss the piston.

Of course, I suppose a lifter could potentially fail and cause a valve to float, but we're talking failure conditions, not inflation due to high cold-start oil pressure.

The problem with your car is fuel related, as evidenced by your solution of disabling the fuel pump to prevent additional fuel from being dumped into the system until the motor fires - this means there's already too much fuel there. Cutting off the pump still allows some fuel into the engine, as the injectors are going to open when they should, there just won't be much, if any, pressure behind the gas in the lines.

Any car, injected or carbureted needs an enriched fuel mixture to aid in cold startup. Cars with carburetors use a butterfly flap to reduce the volume of the air coming in, which correspondingly increases the air speed, which pulls more fuel through the venturis = richer mixture. Fuel injected cars solve the problem in several ways - some use an additional injector in the throttle body, some just increase the injector duration to provide more fuel.

Cars that exhibit hard-to-start/flooded behavior after a short start-then-stop cycle have some part of the cold start circuit confused about what is required of the system. When you start a cold engine, all of the heat is concentrated around the cylinders at the top of the block, just where the heads meet the block - the second area to heat up is the exhaust manifolds. When you run the motor briefly, that area gets nice and toasty very quickly, and then that heat slowly spreads to the rest of the engine after shutdown. Possibly when you restart it a short time later, the areas of the engine that benefit from enriched fuel during cold starts are actually warm enough to not need it, but the sensors that control the cold start cycle are still seeing a "cold" motor, as they get their readings from the coolant. The fuel injection computer calls for enrichment, which that actual combustion areas of the engine don't need - and you get a flooded condition.

This behavior, I wouldn't think, is normal - there's something wrong somewhere that's causing it.

myarmar · #10 03-09-2007, 10:26 AM

I agree it does sound like a fuel problem. The advantage of troubleshooting V12 engine is that it has two of everything. So if the fuel problem happends on both banks, it got to be some common component. I can't imagine both LH modules are going bad same time.
The reason I'm thinking tappets pump-up was this thread:
http://www.peachparts.com/shopforum/showthread.php?p=1376945#post1376945
Next time I'd be ready with oil pressure gauge.
Thanks, Mike

peterhardie · #11 03-09-2007, 11:01 AM

wbrian63,

I don't doubt, like you say, there is some problem that can be figured out. The problem with your analysis is that it is not a short run and a no-re-start later. My car will start up warm or not-so-warm, even after a quick on-off in the driveway.

I had not run my car for a couple days this past time, and it sounds like a few others have had the same issue. The car is cold when it happens. It might be that a too quick run does produce some "stuck" problem as myarmar and 400E suggest.

My tech says he has noticed these engines seem to do this a lot for no apparent reason (no codes, problem not persistent enough to tear a bunch of things apart). I guess that none of his customers (including me!) have asked him to hunt down the problem as infrequent as it happens. I actually tried to recall, and I think this has happened three times in the six years we have owned this car. I posted hoping someone had traced the issue to something I could fix and brag to my tech about!

We really ought to follow the conditions a bit, 119 owners marking the circumstances that produce the condition, and compare notes.

david s poole · #12 03-09-2007, 12:29 PM

have you checked the distributor caps for "THE GREEN CRUD" add moisture and green crud together and you have problems.