I have a 1986 CJ-7 with a 258 engine. It overheated a few weeks back (my temp gauge is broken) and ever since makes a knock/rattle when I start it cold. It will usually go away after being driven for a few minutes but always returns after it's cooled down. The engine is OK otherwise (no oil burning or smoking) although it's runs a little sluggish sometimes. Any comments/ideas what it could be??

You can't judge a Jeep by its paint job!

Could it be that your engine is not pulling enough vacuum when it is cold in the valve cover to lubricate your rocker arms? This would cause some valve slap, I have the problem when my oil pressure drops too low at an idle (I also have some other problems in the engine that are contributing to it) and I get valve slap. It is possible that overheating has overstressed the system, and killed the vacuum until it heats up enough to expand everything.

When you say "returns when it cools down" do you mean ...... that you drive it ...... it makes the noise .... you stop and it sits for a half hour and when you start up again the sound returns .... or do you mean "when the engine is stone cold it makes the noise untill the engine is somewhat warm ....... it's quiet the rest of the time ....but every cold (fairly cold ...sitting for several hours) start yields the noise"? Due to the overheating that you describe ...... I wonder if what you hear is "piston slap".....perhaps the piston skirts got damaged ........ a simular condition ...... minus the overheat thingie ..... exists in some engines from new ..... American manufacturers played with using looser tollerances for less internal friction to achieve better fuel economy..... resulted in a mild, but noticable, knocking until the engine warmed up.....my '89 GM 2.0 L4 for one ....my 75 GM 350 for another.... sounded like a slight rod knock ......

GeeAea

[img]/wwwthreads_images/icons/tongue.gif[/img] OK, how about this...the oiling system usually has some means of holding the oil in the engine after shutdown so it won't flow back through the clearances in the pump and have to start empty each time. Could it be that this little check valve..or whatever they used...is damaged and no longer holding? [img]/wwwthreads_images/icons/wink.gif[/img]

CJDave

O.K. How about this for a guess, You warpped your head and have a small headgasket leak into the cylinders. When it cools down, that cylinder becomes liquid locked from the pressure of the cooling system forcing coolant into the combustion chamber. When you start it cold, it hammers, and after it warms up, the leak is too small to cause any problems.

Carefully note if you get a puff of white smoke for the first couple of minutes after start-up. Also note if you are loosing small amounts of coolant. You could also pull the plugs before cold starting, and note if any coolant is sprayed out of the holes during crank.