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Thread: Low/no compression on rebuilt engine

  1. #1
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    Low/no compression on rebuilt engine

    So Iíve been trying to diagnose why my freshly built engine is having trouble starting. After lots of issues, Iíve managed to get spark and fuel, so the last thing to do was compression test the engine. Cylinder #1 was 75psi #2 was 25psi #3 was 30psi #4 was 40psi. What could be causing this low compression? I canít even get the engine to start, it cranks and make slight splutter noises as though itís going to start but wonít fire up.

    Any ideas on how to diagnose the issue would be much appreciated.

    The car has been off the road now for 2 years as 3 months, and I just wanna get her fired up!

    This is my first engine install and so far everything that could have gone wrong already has happened and been sorted 😂😂. I just have no luck with my S14

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    nothing is really bedded in yet. spray something flammable down the intake as you crank it over and it might just kick into life, keep cranking whilst its spluttering, hooking another running car up via jump leads will help give the starter motor a boost. keep an eye on your oil pressure gauge.

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    Timing could be way out, or something hasn't been built right. Did you refresh the head? Who built the engine, as I'd be straight on the phone to them!

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    that's probably more likely. take the cam covers off and check the static timing. id just assumed it would have been built right.

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    It was a mate of mine that rebuilt the engine, It has been timed correctly at TDC. The Valves weren’t changed, it may possibly be them. I tried something flammable in the intake and it didn’t fire up. The company that was building my engine lost their garage, and I ended up with so many lost bits it was unreal, I’ve had to spend about £500 just on random bits to even get the car back to standard, let alone all the money that was spent on new bits for the engine and the skimming and re-lining of the cylinders.

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    What exactly was done? In all honesty such a poor compression result sounds like something has been done wrong as apposed to just because something (such as valve lapping) wasn't done.
    Have you had the cam cover off and checked the timing yourself? Very easy to do

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    Yeah I think the timing had been off, took the head off today, seems to me like the exhaust valves have touched the pistons, I’m guessing they are bent, so will be ordering some new valves once I’ve inspected them.

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    I'd be having strong words with the person who built it, SR's are possibly the easiest engine to time!

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    Quote Originally Posted by Slamz View Post
    I'd be having strong words with the person who built it, SR's are possibly the easiest engine to time!
    Ls engines look simpler but yeah the sr doesn’t look hard.. Did they not spin it over by hand first to check.
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    So I’ve taken the valves out today. And every single one is straight....... so I have no clue what so ever why I have little compression!

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    I see in work done to engine you mentioned relined cylinders, a big drop in compression on all four,not equal but all for have something
    suspect's could be, piston to bore size is incorret,all four pistons have broken rings, if head was dismantled all the shims and rocker arms are mixed up
    and at least one valve on each cylinder is slightly open, is the cylinder head known to be good ?,or is it from another engine
    .as SR heads can crack around spark plug holes if they get super heated from detonation/or low coolant overheat,
    with the head off now you can check most of the above

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    Quote Originally Posted by ANDY black s13 View Post
    I see in work done to engine you mentioned relined cylinders, a big drop in compression on all four,not equal but all for have something
    suspect's could be, piston to bore size is incorret,all four pistons have broken rings, if head was dismantled all the shims and rocker arms are mixed up
    and at least one valve on each cylinder is slightly open, is the cylinder head known to be good ?,or is it from another engine
    .as SR heads can crack around spark plug holes if they get super heated from detonation/or low coolant overheat,
    with the head off now you can check most of the above
    I have actually noticed slight cracks around the spark plug holes! But they arnt deep cracks

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    was the cam angle sensor installed properly >? i didnt read up when i rebuilt my engine had starting issues and finally find out you have to line the gear up on cam angle sensor haha

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    Quote Originally Posted by cooltiger View Post
    was the cam angle sensor installed properly >? i didnt read up when i rebuilt my engine had starting issues and finally find out you have to line the gear up on cam angle sensor haha
    Yeah at first I didn’t know about this, but once I googled around I made sure it was lined up
    A

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    Quote Originally Posted by lewi565 View Post
    I have actually noticed slight cracks around the spark plug holes! But they arnt deep cracks
    check for cracks in the exhaust valves, if it has got very hot/near detonation previously the exhaust valves may have tiny hairline cracks
    or the valve seats are scortched and very pitted/not sealing properly
    ,cracks in the combustion chamber around the plug holes might not leak or bleed all of the compression loss,depends how deep they go
    if valves and the head seem ok,piston to bore clearance and any signs when wound over of score marks
    in the bores from broken piston rings needs checking closely

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    It just seems to be little cracks in the spark plug holes, not the valves themselves. Over ordered some new valves just incase and all lap them in. Also the piston rings are fine, I’ve had oil sat on the pistons since I’ve removed the head to test how well the rings are seated. It’s been a week and the oil has only seeped through a tiny bit. No scoring. The pistons and piston rings are all new

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    that leaves valve clearance as suspect compression loss, the rockers are sort of coded and numbered
    ,I think it relates to how much was machined of the 'finger pads' and the machined cam lobe wipe area
    if rockers and the shims were mixed up chance a valve was held open a touch on each pot
    new valves when lapped in will need clearance carefully checked to get rocker pad flat to cam lobe
    as in one rocker with two fingers and no two valves will be exact same height ,the correct thickness shim levels rocker pad
    there are about 10 shim sizes in thickness,you may need some to get clearnce good on all 16
    order sizes you need from nissan main stealer or try find a cheap scrap head to rob its shims, do a shims wanted thread on here?

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    Or valves can be machined to same height, then you can use the original shims that are all the same thickness. (At least mine were)

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    should have done a leak down check with the cams removed before whipping the head off.

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    Quote Originally Posted by bahis View Post
    Or valves can be machined to same height, then you can use the original shims that are all the same thickness. (At least mine were)
    This, can't just swap them over for new ones, you should also replace the guides at the same time and have them honed to accept the new valves.

    Quote Originally Posted by green_rs13 View Post
    should have done a leak down check with the cams removed before whipping the head off.
    Also this, you seem to be just swapping bits out without actually working out the problem.

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