can a problem with the ecu/maf sensor cause no.4 cylinder to be low on a compression test.reason i ask is i have an A160 with misfire and hesitation and lumpy idle.put in new plugs and no difference.had star test done,not by mercedes people though and no faults found.did compression test and no1 2 and 3 cylinders read 210lb sq in no 4 read only 50lb sq in.decided to have a go at removing head and fitting new inlet and exhaust valves to no4 cylinder but before starting priced up parts with local merc dealer.prices not too bad but when he told me they have not sold inlet or exhaust valves for this model nor have they sold head gasket or bolts i wonder if something else could be causing no.4 to be so low on compression.should i have it diagnosed by local mercedes dealer or has anyone else out there come across a problem the same or has anyone removed the head and valves with engine in situ.some say you cant do it in situ but some say you can.if you cant why??? or is it just too involved.all help much appreciated with this matter.
A160 misfire hesitation
- Thread starter chipsraff
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Dieselman
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The compression is a function of pistons and valves and not a lot else. No engine control can influence it.
Perform a leakdown test to see where the air escapes from.
Perform a leakdown test to see where the air escapes from.
Perform a leak down test on the cylinder while the cyl head is off (it requires a special tool). That will tell you if your piston rings are bad. Also look for scoring on the cylinder wall before you re-install the head. You can use the same tool to check the head to see if there is a bad valve seat or a crack in the aluminum. I have seen a short circut in the harness at the fuel injector, which will cause the oil to be washed away by too much fuel (rare), causing damage to the cylinder wall and the rings.
