Big problem with primera

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richy_boy · #2 Post by **richy_boy** » Thu Jan 05, 2006 11:20 pm

Hi Tony, I don't know all the answers but I'll throw a few suggestions at you. I'm aware of some cars in my car club that have done 150k miles+ and have been using LPG for AT LEAST the last 40-50k miles without a problem. (Nissan 200sx Club). It's quite well known that LPG burns at a higher tempreture, and doesn't contain the 'lubrication' or cooling effect normal LRP petrol does, so additional wear is possible, although this depends from engine to engine (well head to be more precise). end of part 1

richy_boy · #3 Post by **richy_boy** » Thu Jan 05, 2006 11:21 pm

From what you've said, you are seeing poor compression across 3 of your 4 cylinders, which does sound like bad news. I'm assuming your mechanic did both a 'wet' and 'dry' compression test? A dry one first will indicate the compression loss, the wet one is done by putting a bit of oil into the cylinder.. if you still see the loss then it's likely to be a valve, if you see LESS or no compression loss then it's going to be your piston rings. This is standard mechanic know-how.. so I'm assuming he did this? For your RINGS to be warn in 28k miles something is seriously a miss, especially for a non-turbo car and normally only wear and tear (i.e. lots of mileage) would wear them as they don't 'lubricate' in oil as such. Now, your valves are a bit different as they are usually cooled by the effect of petrol being injected into the cylinder, which obviously when running on LPG they don't have. This coupled with the lubricating effect of petrol can put them under a great deal of stress, which they're not specifically designed to handle, especially if revved hard. MOST valves are fine though (although proof of this is pretty scarce!). So, the bottom line is: a) find out if he did a 'wet' test. b) if the wet test didn't change the results then it's highly likely it's your valves and quite an easy repair job (although I hate to think of the Nissan cost!) c) If the wet test DID change the results then your rings are fooked and you can either rebuilt that engine or get a new replacement in. end of part II

richy_boy · #4 Post by **richy_boy** » Thu Jan 05, 2006 11:21 pm

I'd be interested to know what your mixture (LPG) is like too, as a leaner mixture will create extra heat again in the cylinders.. so maybe the combination of LPG and a lean mixture has created just too much heat and damaged bits. Unfortunatley, unlike with petrol you can't run the engine rich and get additional cooling <IMG SRC="http://www.my90.co.uk/lpgforum/clipart/sad.gif" ALT=":-(" BORDER=0> Anyway, hope this helps a bit, but I'm afriad I don't have the answer as such. Rich

richy_boy · #5 Post by **richy_boy** » Thu Jan 05, 2006 11:23 pm

Oh (sorry about this long post!!) it could also be your head gasket which would occur from overheating too. You would be likely to get a mixture of oil/water or water being blown out from your expansion tank if this were the case.. I'm sure he would of checked for this too! <IMG SRC="http://www.my90.co.uk/lpgforum/clipart/happy.gif" ALT=":-)" BORDER=0> Rich

steve_96 · #7 Post by **steve_96** » Sun Jan 15, 2006 1:45 am

I'm always curious to know why many think that LPG burns at a higher temperature. It is widely known that LPG has a lower calorific value than petrol, which relates directly to its ability to release heat when burnt, i.e. less calorific value, less heat produced. So how can it burn at a higher temperature than Petrol? Answer - It cannot! The only way it can differ is in how quickly it burns. If LPG burns faster, a greater proportion of the total heat released will be in a higher position up the cylinder bore, which may (and I stress MAY) result in a slightly higher local cylinder head temperatures on gas as opposed to Petrol. But that would depend on many things, not least engine design..... In general terms higher temp.s when running on gas are not a thing to worry about. Soft Valves are another can of worms.

classicswede · #8 Post by **classicswede** » Tue Jan 17, 2006 8:44 pm

Its not just the seats you need to worry about. Most ohv engines will suffer acellerated valve guide wear due to side loading and lack of lubrication.