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1988 300CE
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7 Posts
Discussion Starter #1
Hey guys,
Have an interesting problem with my 1988 300CE m103 idle.
it starts fine and idles around 1000 to 900 rpm in park when cold.
BUT...when fully warm the idle goes down to around 600 in park and 500 in drive
drives ok and idles nicely in drive but if i shift it back to park or neutral the idle stays around 500 as if nothings changed! (lots of shuddering and lumpy idle)
I have changed all the vacume lines including the boot for the air flow meter,
have tried a couple of well cleaned ICV valves (new vacume lines) the cap rotor and wires are around 2 years old. the coil is brand new. Temp sensor is brand new.
have checked the OVP relay and since cold start is fine i assume its ok.
Crank shaft sensor gives me a reading of 947ohms which is within range.
I'm at a complete loss as to what is causing the problem. Any ideas?
Thanks in advance.
 

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1988 300CE
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7 Posts
Discussion Starter #3
sorry the coolant sensor is new (two pronged green thing). The idle in park when warm goes up to about 750 when first shifted from drive to park/neutral but then gradually goes down to about 550-600 if thats any indication? the O2 sensor is about 2 years old as well.
 

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W124 Moderator
86 190E 2.3L 16V, 2 95 320TE's, 02 S500
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12,822 Posts
550-600 in park is normal. It's also normal when in gear. The computer compensates for idle adjustment when in gear, A/C on etc. etc.

Complete your profile. Give us miles on the car, when the O2 sensor was changed and so on. More information means less guessing. No one wants you to throw money at a problem.

Good Luck,

Jayare
 

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(lots of shuddering and lumpy idle) Hummm…. It probably looks like your car’s tranny experiences hard time with pumping transmission fluid. The main pump can load the engine when something blocks on the way of the transmission fluid flowing. The main cause can be old radiator. When it warms up, some deposit inside begin blocking free fluid flowing and car behaves like a mustang even in neutral position. Complete blockage leads to gasket blowing and loosing the fluid, as it was in my 88 Sterling 825. Certainly, computer will notice it as the presence of the load on the engine. To check it, make a temporary bypass to eliminate the radiator and check if (lots of shuddering and lumpy idle) disappears, The Neutral position or Park usually is absolutely free of load to the engine except running A/C/.
 

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1988 300CE
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Discussion Starter #7
damn! that sounds like a biggie, if its a tranny problem! but will do the bypass trick this weekend and see how i go. The PO changed the radiator and water pump (about 4 years ago).

some additional info :
the car is a 1988 300ce with m103 motor
has 240K on the clock (km not miles, i'm in australia)
The vacume gauge in Park hovers around 1/4 and at the start of the red part in drive. (only pulls about 13-14hg with external vacume gauge hooked up)
compression is even throughout all the cylinders.

The valve stem seals were done about 6 months ago. although i do have a valve tick on startup not sure if thats relevent?
 

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1988 300CE
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7 Posts
Discussion Starter #8
you might be right mate might be a warm only OVP problem? when unplugging the OVP car idles at around 800 and is rock solid! no rattle from the OVP but that probably means nothing! hope its not the ECU! merc parts are stupid expensive over here in Aus!
 

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There shouldn't be any rattle in OVP performance, because rattling means frequent disconnecting inside the OVP after warming up and can simulate the problem described above. Try to replace it (just borrow it from somebody who has a similar car) to see the difference. OVP might be worn out inside.
 

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1988 300CE
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Discussion Starter #10
yep might have to go to the local merc wreckers and see if i can try a few OVP.
Is there a auto transmission shift position sensor or switch on these cars? what tells the ECU that the car is in PARK?
 

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W124 Moderator
86 190E 2.3L 16V, 2 95 320TE's, 02 S500
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12,822 Posts
Your transmission isn't the problem. Proper tune up, new ovp, new filters etc. will do wonders.

Good Luck,

Jayare
 

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1988 300CE
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7 Posts
Discussion Starter #12
thanks mate, yep sure point to a bad OVP... will try to source on and see how i go. Thanks all for the help.
 

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1988 300CE
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Discussion Starter #14
bad news :( it wasnt the OVP relay! tried a known good one from another car, but the problem remains! anything else i could check on?
 
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