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Matt, Engineer
Category: Mercedes
Satisfied Customers: 21724
Experience:  Mechanical Engineer with 23 years experience in the auto industry, 8 yrs in formula 1 engine testing
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2005 Mercedes E500, Check engine light come on after hard

Customer Question

2005 Mercedes E500, Check engine light come on after hard acceleration 4000 to 4500 RPM. Code is misfire on cylinder 8. Replaced cylinder 8 coil and wires. Cleared codes. Then again after about 20 miles and several hard accelerations the check engine light is back on with same code. Also after the check engine light comes on the engine runs ruff like it has a misfire but after turning it off and restarting it runs fine until the next hard acceleration. No other codes but misfire of #8 cylinder. Any suggestions?
JA: Does the light stay on, or does it blink?
Customer: Stays on.
JA: What is the model of your '05 Mercedes?
Customer: E500
JA: Are you fixing your E500 yourself? What have you tried so far?
Customer: New coil for #8 cylinder and new plug wires for the two spark plugs on #8 cylinder.
JA: Anything else you want the mechanic to know before I connect you?
Customer: It also has a new camshaft sensor. Car runs fine until hard acceleration.
Submitted: 4 months ago.
Category: Mercedes
Expert:  Matt replied 4 months ago.


I'd suggest to do a compression test across all the cylinders or at least a couple to check that the no.8 cylinder is within 10-15 psi of its neighbours

if this is Ok then I'd next check the state of the fuel injector harness and injector on cylinder 8 and consider moving the injector over to another cylinder to see if the fault moves with it

if it does then you know to replace that injector with a new part

and if thats OK then it could be a lack of fuel and could be a lack of fuel pressure from the electrical pump at the tank.

1st thing to try is to remove the fuel filler cap and then flick the ignition on whilst listening for the brief ‘buzz’ of the pump

If the connections to the pump are corroded or damaged then the pump could stop at any time or could not be running at full speed. 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 pressure at the injector rail (should usually be around 3Bar / 45psi +) and if this is low check the pump as described and also consider changing the fuel filter as if this partially blocked this too will reduce pressure.

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