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I have a 2004 Mondeo 2.0 TDCI which has issues when starting from cold: lots of white smoke and rough idling until it warms up.

Had it tested at several garages, who both diagnosed low compression in cylinder 2 and said they thought it could be a bent conrod.

They also informed me that for a sure diagnosis they'd have to remove the head and look inside the engine, which could cost upwards of £1000. My question is, what are the potential consequences of NOT repairing this, and can I 'get away' with not doing so?

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First... find another garage. White smoke and low compression are NOT symptoms of a bent connecting rod. Also, you CANNOT inspect the connecting rods by removing the head - they're on the opposite (bottom) ends of the pistons.

White smoke and low compression are both symptoms of a blown head gasket. That's very classic. It doesn't sound like it's very bad, just beginning... but it CANNOT get better, and it WILL get worse in a hurry.

Consequences? A head gasket replacement is a relatively cheap repair; it should NOT cost you anything like a thousand pounds (that's nearly $2000 to Americans). Ridiculous! But... if you let it go until it gets REALLY bad (it won't take long to do that), then it could easily make the engine unrebuildable and destroy your radiator in the bargain. Coolant will be getting into your motor oil and wrecking its lubricity (besides turning it acidic), and the ruined motor oil will result in catastrophic engine failure. Meanwhile, high-pressure exhaust gases may be exchanging, getting into your coolant, and can very easily destroy the radiator (which is only ever intended to tolerate about 15psi of pressure).

Please - go find a better shop and let them do the work.

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  • "they're on the opposite (bottom) ends of the pistons" - this is my favorite part of this answer. :-) – Bob Cross Jul 30 '14 at 21:34
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    You could possibly infer the state of the conrod by comparing the height of the piston at Top Dead Center against the other three (also at TDC). That would tell you whether or not the rod was bent. A bent rod would mean the piston doesn't move quite as far up. – Captain Kenpachi Jul 31 '14 at 11:46
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The book time for your head gasket renewal is 9.8 hrs. An average sort of hourly rate would be around £95+ an hour(UK). Parts would come to around £150. On these pretty average figures you are looking at something like £1300 with the tax(VAT). White smoke on a diesel engine is symptomatic of the cylinder/cylinders not burning the fuel properly. This could be as simple an item as a glow plug malfunction, especially as you say on a cold engine. The way forward is a leak off test on the fuel side, a cylinder leakage test on the engine, and a scan on the electrics. This will give a good picture of whats going on with the engine before you have any major work done. The other consideration is to use the services of a diesel specialist garage, rather then a garage that 'does diesels'. Your symptoms could very well indicate a bent con-rod. The condition of the engines bore would give an indication, but from having the head off to dropping the sump and pulling the piston and rod would only be another hour or so of labour. It is unusual to have just one con-rod bend.

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A bent conrod will cause low compression in a cylinder, but not white smoke. White smoke is an indication of water being sucked into the cylinder, ergo a leaking headgasket. If you haven't done any performance modifications to your car, it's unlikely you would have bent a conrod unless your timing belt broke or one of the cylinders got flooded. The latter is highly unlikely in a diesel engine because there are no spark plugs that can malfunction and the very act of compressing the air/fuel mix is enough to ignite it. Also, a bent conrod doesn't miraculously get UN-bent when the engine warms up. It's even more unlikely that it will RE-bend itself when it cools down.

Open your hood and have a look at the level of antifreeze in your expansion tank. If it's way below the indicated level, you most likely have a blown headgasket which is leaking antifreeze/water into the combustion chamber. It's free to look.

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