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Matt
Matt, Engineer
Category: Mercedes
Satisfied Customers: 20604
Experience:  Mechanical Engineer with 23 years experience in the auto industry, 8 yrs in formula 1 engine testing
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I have a 1998 s420.I was coming home when it started loosing

Customer Question

I have a 1998 s420.I was coming home when it started loosing power.worse on hills. I got it home since then I have changed the feul filter put plugs and coil packs.Now when you first start it,it runs great until it gets hot at that point it misses the oil pressure jumps wildly and gets so hot the muffler system pops and cracks. The car has 170000 on it.Any suggestions?
Submitted: 11 months ago.
Category: Mercedes
Expert:  Matt replied 11 months ago.

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Hello

This could be a wiring issue as things tend to expand when hot and its possible a poor or partial electrical connection is being pushed apart on one of the major sensors so check the connection plug and wiring to the following;

Airflow meter

Crank position sensor

Cam position sensor

O2 sensor

Throttle position sensor

unplug each in turn and use some switch / contact cleaner spray on the pins to ensure its all grease / dirt free

Also worth checking the connections to the fuel pump for the same reason - unplug each in turn and give them a good squirt of switch / contact cleaner - Not WD40

and also unplug and inspect the main power relays and fuses for signs of overheating or corroded terminals - so the ones for the ECU and the fuel pump

And also unplug the wiring loom connections in the engine bay one at time and inspect for corroded or recessed pins.

Check the resistance of the spark plug leads ( 13-15Kohm / metre) and replace if any doubt or high resistance - Also replace the spark plugs if any of them fail this checklist:

http://www.ngksparkplugs.com/tech_support/spark_plugs/faqs/faqread.asp

Also its worth checking that the electric pump that feeds the injectors is working correctly, it should run for a few seconds then switch off when you first turn the ignition on. If the connections to this are corroded or damaged then the pump could stop at any time. Check that its relay switches in and out and the relay contacts are clean and bright – replace the relay if in any doubt and do the same for the fuse, ideally measure the voltage at the pump and check its getting 12V

Expert:  Matt replied 11 months ago.

Hi

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