Reply With QuoteI don;t know why everyone get hung up on the values, Have you calibrated your gauge recently?
The concern is the difference between the figures
How many kays since rebuild?, could lift the heads, but there must be a reason? runs like crap? heating?
Master
SupporterWell, I finally did a compression test on my 3.5L V8 today. And the results are pretty well everywhere... I did some work to the engine when rebuilding it to bring the compression ratio up to somewhere around 12:1 but in most cylinders i'm not achieving that.
1- 157psi 2- 174psi
3- 174psi 4- 167psi
5- 148psi 6- 181psi
7- 162psi 8- 203psi!!!!
Yep all over the place, I couldn't do a wet compression test as I didn't have anything to squirt oil down the plug holes. But i'm pretty sure it's the valves since when I did the rebuild all I did was grind the valves onto the valve seats.
Does anyone else have some suggestions as to what could be causing this? I suppose there is only two... rings or valves eh? But overall the values are too low for the compression ratio i'm running.
Stirling
OldBushie
I don;t know why everyone get hung up on the values
The concern is the difference between the figures
How many kays since rebuild?, could lift the heads, but there must be a reason? runs like crap? heating?
Master
SupporterThe gauge is brand new. I only just bought it today.
The engine doesn't run like crap or over heat. A little under powered and yeah the difference in the readings is the main concern here. I was told to aim for 10% Certainly isn't 10%.
As for how many K's... hmmm 50,000 in four years I would approximate
Stirling
OldBushie
I've never had mush luck with "wet tests", I figure your going to pull the heads anyhow
Master
SupporterHmm... could be rings... I really should just get in there and change the rings and head gasket. I'm changing the exhaust manifolds on it anyway.
Stirling
Moderator
get a leakdown tester from somewhere (beg/borrow/steal) and do a leakdown test, you'll then know exactly whether it's rings or valves, and which valves (inlet or exhaust) just by listening.
ChatterBox
What sort of running do you do in it short distance??
If so give it a really good flogging for 1000 ks or so it may come good either that or it will finish it.
3.5 l Rover motors dont mind a bit of hard work try it you may be suprised by the result.
I say this of course for an engine that the only evidence of a problem is a compression test if there are other signs then dont waste your money.
Ps was the engine hot or cold when you did the test??
TopicToaster
thats what i was gonna say!Originally Posted by rick130
if your gonna rebuild motors at home you need more than spanners.
ive been in this situation b4, hence now i have a leak down tester, valve spring tester, bore gauges, hones etc.
if your freindly with anyone with a leak down tester or even know of a mechanic that has one, pay the money and get them to do the test, at least then you will know where to start.
cheers phil
Master
SupporterSnap! - I did the same test today to my EFI 3.5 (9.35cr) and got 145-150 on 6 cylinders, 80 on one and 40 on an adjacent cylinder! Wet test didnt improve the low readings so I think I've got head problems - probably burnt valves.
All plugs looked OK - which I think is further evidence for the head being the problem, as I imagine the plugs would be oiled up if it were rings.
Master
SupporterThanks for the replies guys. Yeah naturally when I did the rebuild I didn't do it to the book. You know how these things are. 145-150 from a 9.35:1CR engine? For a 12:1 I should be getting much much higher than this. I would say that Number 8 is the only one that is working properly all the others are leaking somewhere.
The reason I suspect it is the heads is because the cylinders and pistons were done properly, New 30 thou 10.5:1 pistons (we got 12:1 by machining the block down 0.7mm) and rings and the cylinders were honed out professionally.
Where as all we had done to the head was get them skimmed, then just used valve grinding paste on the valves to seat them. That was all the attention the heads got.
Sigh... I don't know if I can be bothered opening the engine up again. So much time effort and money has already gone into this car....
Ever since the rebuild the engine has consumed oil, about 1L per 1000km. And where it goes is puzzling, because for this sort of consumption there should be a hell of a lot of blue smoke, but there isn't any. It's possible it might be getting past the rings and then being burnt properly without producing smoke due to it being LPG and high compression engine? Really don't know.
Just another thing I noticed on my plugs. 1 to 7 all had this brittle white material buildup on them which had a very rough appearance (number 7 had very little of this buildup). Number 8 was very clean with a slight brown tinge on it.
Stirling
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