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Freshly Rebuilt rb25det no compression!!

Driftieboy
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Driftieboy Apprentice

Driftieboy

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Freshly rebuild rb25det s2 has no compression! Was sitting in my shed for 6months (nice dry shed and every hole in motor was plugged).

I brought it from a well renowned motorsport guy who had it built for me (bit of a deal for a Neo head i had) i had also brought a Jerico wc4 dogbox off him 2 years ago...

Timing was good checked it over a few times, compression test came up 0-5psi...

Any help would be great! 

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Ben C34 Grand Master

Ben C34

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You would confirm with another gauge, and confirm cam timing is correct, then pull the head off for a look.

 There is no trick or shortcut.

 

For all cylinders to be super low I would be thinking it is something silly.

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Driftieboy Apprentice

Driftieboy

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Is it possible the camshafts are 180° out with the crank? Timing marks all line up correctly but as the crank rotates double the amount as the cams then could they be 180° out?

I've just purchased a leak down test kit so will wait for that to arrive then do that.

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GTSBoy Grand Master

GTSBoy

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4 hours ago, Driftieboy said:

Is it possible the camshafts are 180° out with the crank? Timing marks all line up correctly but as the crank rotates double the amount as the cams then could they be 180° out?

I've just purchased a leak down test kit so will wait for that to arrive then do that.

Wouldn't cause no compression. It would work exactly like it should, except it would be trying to spark at TDC on the exhaust stroke.

Although that depends on what exactly you meant by "180° out". 180 crank degrees or 180 cam degrees? 180 crank would be weird, but timing marks wouldn't line up. would need to be 360 crank for the timing marks to line up.

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Driftieboy Apprentice

Driftieboy

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1 hour ago, GTSBoy said:

Wouldn't cause no compression. It would work exactly like it should, except it would be trying to spark at TDC on the exhaust stroke.

Although that depends on what exactly you meant by "180° out". 180 crank degrees or 180 cam degrees? 180 crank would be weird, but timing marks wouldn't line up. would need to be 360 crank for the timing marks to line up.

I mean when checking the timing i noticed that when i put the crank on it's timing mark, both camshafts timing marks were on the opposite sides of what they should have been so then i spun the crank a full 360° untill the crank was back on it's mark again and now both of the camshafts were on there correct mark, so when assembling the eninge how do you know when your not 180° out? They guy i brough the motor from done the timing so i wonder if he has got it wrong?

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GTSBoy Grand Master

GTSBoy

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1 minute ago, Driftieboy said:

how do you know when your not 180° out

The same as on any engine. TDC compression on cylinder #1 is when the piston is at the top (easily found by timing mark on crank if correct, or by feeler rod through spark plug hole if suspicious of markings) and the #1 cam lobes pointing at the sky.

TDC on exhaust, the cam lobes are pointing downish. The exhaust cam will be close to closing and the inlet cam will be close to opening.

We don't talk in cam degrees. There is no "180 out". You are talking about 360° out, engine degrees. And as I said, if it was 360° out, you would still get compression. All the mechanical things are still working in concert. TDC is still TDC. If the inlet valve opens just before TDC, then the engine will still ingest air. If the exhaust cam is closing at that same point, then the air will still be trapped inside the cylinder and get compressed on the next upstroke. Etc.

The question of whether the engine will run or not will depend on how the ignition is timed wrt to the engine. If it was a crank driven distributor then it would attempt to fire on the wrong stroke, while the valves are on overlap. Not going to run. If the ignition timing is driven from the cam (like it is on an RB) then the engine should actually run. It's just that all the marks on the timing chain won't line up when they're supposed to so you'll have to have it 360° out to get them to line up.

But, regardless, your lack of compression is far more likely to be either all teh valves are bent/stuck open, or you have another severe cam timing error (other than 360° out), if it is even possible to get it that bad.

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Driftieboy Apprentice

Driftieboy

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1 hour ago, GTSBoy said:

The same as on any engine. TDC compression on cylinder #1 is when the piston is at the top (easily found by timing mark on crank if correct, or by feeler rod through spark plug hole if suspicious of markings) and the #1 cam lobes pointing at the sky.

TDC on exhaust, the cam lobes are pointing downish. The exhaust cam will be close to closing and the inlet cam will be close to opening.

We don't talk in cam degrees. There is no "180 out". You are talking about 360° out, engine degrees. And as I said, if it was 360° out, you would still get compression. All the mechanical things are still working in concert. TDC is still TDC. If the inlet valve opens just before TDC, then the engine will still ingest air. If the exhaust cam is closing at that same point, then the air will still be trapped inside the cylinder and get compressed on the next upstroke. Etc.

The question of whether the engine will run or not will depend on how the ignition is timed wrt to the engine. If it was a crank driven distributor then it would attempt to fire on the wrong stroke, while the valves are on overlap. Not going to run. If the ignition timing is driven from the cam (like it is on an RB) then the engine should actually run. It's just that all the marks on the timing chain won't line up when they're supposed to so you'll have to have it 360° out to get them to line up.

But, regardless, your lack of compression is far more likely to be either all teh valves are bent/stuck open, or you have another severe cam timing error (other than 360° out), if it is even possible to get it that bad.

Ok cheers thanks for the clarification!

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Driftieboy Apprentice

Driftieboy

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Thanks for that picture, yes all i checked timing correctly as per picture with the harmonic balancer removed aswell to be sure.

Have just done the leakdown test (had to rush back to work) i mananged to quickly test cyl 1&3 each at TDC and the air was rushing out of the exhaust side and out the exhaust, couldn't hear anything from intake but next time i test I'll remove the intercooler to listen properly

 

 

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admS15 Grand Master

admS15

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You could stick an inspection camera/borescope down a plug hole and check if you can see anything but if you've got air escaping in your leak down test, I feel you're going to have to pull the head anyway.

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