1988 300TE runs fine for 10 min, stumbles for 10 min, then runs fine again...

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Bob - 18 Dec 2006 22:48 GMT

Hi,

I have an 1988 300TE, it's my second car so it's not driven daily, only
on weekends.

Weird problem. The car starts right up and runs fine for ten minutes.
Then the engine starts to idle rough and really shakes the whole car.
In stop and go traffic the car will stall but start up again, but it's
really hard to prevent the car from stalling when accelerating from a
stop. After about ten minutes of rough running, it starts to run fine
again.

When it's running fine, it's really running great. Any ideas? It's
going to the mechanic this week.

Bob M.

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Jens - 19 Dec 2006 00:26 GMT

Does it happen consequently?

In which temperature range does it happen (engine/ambient)?

Bob skrev:

> Hi,
>
[quoted text clipped - 12 lines]
>
> Bob M.

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Bob - 19 Dec 2006 18:35 GMT

It's winter in LA, so it's in the sixties during the day and in the
forties at night.
~Bob

> Does it happen consequently?
>
[quoted text clipped - 17 lines]
> >
> > Bob M.

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Bob - 19 Dec 2006 18:50 GMT

I meant to say that after 10 minutes the engine is warmed up and the
gauge is reading 80 deg C. Locally the weather is in the sixties or
higher during the day, and usually in the forties or fifties at night,
sometimes warmer. I drove the car today and will drive around the
neighborhood for a half hour and see what it does. If I get stranded
I'll only be blocks from work.

~Bob

> Does it happen consequently?
>
[quoted text clipped - 17 lines]
> >
> > Bob M.

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Jens - 20 Dec 2006 10:24 GMT

Anyway, if the problem is intermittant, I would suspect the OVP relay
or some other electrical failure as also indicated by others.

But if the problem is related to the warming up of the engine, it would
probably be in the fuel mixture control. Prime suspects here are the
oxygen sensor or the coolant temperature sensor. Or it could be a
temperature related leakage in the intake manifold system (inkl. fuel
distributor and air mass sensor) or the head gasket (check
compression).

I once had similar symptoms in certain temperature/humidity situations
(the kind of weather you have now). In that case the problem was a
small crack in the ignition coil.

Bob skrev:

> I meant to say that after 10 minutes the engine is warmed up and the
> gauge is reading 80 deg C. Locally the weather is in the sixties or
[quoted text clipped - 26 lines]
> > >
> > > Bob M.

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Take A Guess - 19 Dec 2006 00:39 GMT

Electrical problem; over voltage protection relay the prime suspect.

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Bob - 19 Dec 2006 18:37 GMT

That's interesting. So what is the failure mode. The relay is stuck
or being tripped too often? What should I check for?
~Bob

> Electrical problem; over voltage protection relay the prime suspect.

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wagonmeister - 20 Dec 2006 05:25 GMT

I have an 88 300te also. Had the exact same problem. Put in a new
fuel pump relay, which helped somewhat, new fuel filter..no help. I
looked at the overload protection relay (ovp) but since the fuse was
not blown I figured it was ok. After several days of fooling around
with this problem and before the dreaded visit to a mercedes mechanic
I found an ovp relay at a junk yard and put it in just to see what
would happen.

Immediately, revs when up and engine sounded great. I let the engine
get cold and it started right up, had a smooth idle. A bonus is
better acceleration and even the ac condensor fan which had not be
working sprang to life.

With these type of problems start with the ovp relay even if the fuse
is not blown. Oh yeah, my $120 fuel pump relay was not needed, I put
the old one back in and it still was running great.

To

> Hi,
>
[quoted text clipped - 19 lines]
>
> Bob M.

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