f7p engine issues

[number1]

hello. i have a kind of knocking coming from my engine, it sounds like big end but also i was thinking maybe headgasket has blown between two cylinders, when you rev it when it's stood still you cant hear the knocking, only when you drive it, it doesnt do it in the mornings and doesnt do it when warmed up it's only when driving under load, oil pressure seems to be fine. it also has lost all power
did a compression test earlier and it struggled i got from the crank end to the cam end: 150, 150, 100, 87 psi.. :S going to pull the head off tommorow after work

how much fun are they to do the headgaskets? ive only ever done one which was out of a car and that was literally head and block.

Dave

 

[Dan@SJM]

well knocking has nothing to do with the headgasket. if its knocking underload deffo sounds like a big end issue.

 

[Dan@SJM]

headgaskets arent easy, just very very fiddley.

 

[stevo172-RWD]

prob big end bearing no?

 

[number1]

well it sounds like big end but that is the only symptom it has good oil pressure dosent knock on the spot just when driving but it doesnt seem anything like the last time i had a f7p big end failure, i also forgot to mention that it sounds like a scooby, dont seem like it's firing on all 4 all the time.. seems real hesitant when pulling away and definatly doesnt have a great deal of compression on number 4 or 3
i was driving normally up a hill when this noise came in, enough oil in it etc

 

[stevo172-RWD]

my 172 had lost loads of power and the warning light came on
before i got her turbod!
thought id blown the head gasket or burnt the valves!
all it tur4ned out to be was the spark plugs lol!
too much go go juice!
stupid octane booster per tank of fuel!

 

[stevo172-RWD]

check the plugs?

 

[Dan@SJM]

well it will sound like a scoob running on 3 cyls, but any knocking onless yuours has dropped a valve will be b/e related.

 

[number1]

plugs arent that old might have to change them again tho. tbh i think it's going to at least have the head pulled off as pot number 4 has half the compression of 1 and 2.

 

[stevo172-RWD]

so ur loosing compresion somewhere!
head gasket,valves,piston rings?

 

[number1]

just been out to have a listen, under extreme load nothing, as in 3rd gear handbrake on pointing slightly uphill bring the clutch up to the point the revs start to fade and no knocking or rumbling, drive it up the road, anything of 2-3k rpm up and its knocking away, im putting my money on compression issues before big end

 

[number1]

While the heads off might as well sling a new set of shells and rings in me thinks going to start pulling it apart tommorow will take pics

Dave

 

[stan*]

big-end bearing shells by the sound of it....hence dropped compression also.

 

[Jones]

my 172 had lost loads of power and the warning light came on
before i got her turbod!
thought id blown the head gasket or burnt the valves!
all it tur4ned out to be was the spark plugs lol!
too much go go juice!
stupid octane booster per tank of fuel!

Why were you putting octane booster in with a standard lump??

Stan why would knackered big end shells cause a drop in compression? surely the tolerance being that far out to cause a knock wouldnt cause a drop in compresion also, would it?

 

[Dan@SJM]

higher wear means that the squish is larger so a lower static cr.

 

[Chris_H]

you will find with bugggered big end shalls squish is very small as the piston smacks into the head Dan.

 

[BenR]

If anybody closes up 44 thou quench clearance from buggered big ends, they will bloody well know about it long before anything touches the head lol.

But yes, chris is right, increased oil clearance if not supported with oil will bring the piston closer to the head.

Why didnt you do a leakdown test to isolate where your loosing compression first?

 

[Chris_H]

nah ben sdeen a few where the pistons smashing the head, got one in the shed, the pistons are nice and shiny and the head has the marks as well. They can run surprisingly well.

This engine drove for 10 mins then siezed then driove for anothe r10 mins

heres a dead big end next to the next cyls good shell, poston was hitting the head