Cam Timing Off After Timing Belt Replacement?

[jb02]

I just finished replacing the timing belt, tensioner, and reset valve clearances. I was careful to double check that both the crank damper wheel and cam sprocket were aligned with their respective block and head markings. The engine starts but doesn't run well and will stall when put into gear. I put a vacuum gauge on the manifold port and the engine is only pulling 15" vacuum which suggests possible bad valve timing. Does this make sense given poor running symptoms?

I am assuming that to correct this I need to drain down and remove the radiator and hoses again in order to completely disassemble the up and lower covers.

[zaq123]

vacuum leak somewhere?

[jeffnhiscars]

I think your heading may answer your question. Did you spin the engine 2x by hand and then tighten the tensioner ?

Are you sure the valves are not too tight ?

[zaq123]

I think your heading may answer your question. Did you spin the engine 2x by hand and then tighten the tensioner ?

what is the point of that? If marks aligned, tensioner is tightened and it's as good as it gets. Spinning it with the tensioner loose can compress the tensioner spring delaying the cam with the belt slack and risking P to V contact.

[ForcedFirebird]

engine is only pulling 15" vacuum

The m20 does not pull a lot o vacuum in my experience. I came from the domestic car market before working on classic BMW's primarily. They really don't pull the typical 20" as most guides suggest (even stock cams have a lot of overlap compared to pushrod motors). I wouldn't use vacuum as a diagnostic on a BMW motor, in most cases anyways (~230° vs ~250 stock cams - hence m20's not needing a PVC valve, or EGR to pass emissions). You can line up the crank using the reluctor wheel, and pull off the upper timing cover with just a few bolts and visually check timing (small mirror makes the job easier).

what is the point of that? If marks aligned, tensioner is tightened and it's as good as it gets. Spinning it with the tensioner loose can compress the tensioner spring delaying the cam with the belt slack and risking P to V contact.

It's actually easy to have the belt off one tooth (cam retarded) due to the belt design and oil pump drive. Cam timing can be off several teeth, car will still run and not have V to P contact - EDIT: on a stock cam/pistons design.



To remedy this, I actually start by bolting the tensioner in the full "loose belt" position (spring completely compressed). Then back the crank up one tooth, wrap the belt starting at the crank, then cam, then pull the tension towards the tensioner and rotate the crank as I slip the belt on the tensioner. Then leave the tensioner spring fully compressed, rotate the engine two full times to be sure the marks repeat. Then loosen the tensioner bolt, let the spring do then tensioning, then re-tighten the tensioner bolt.

[digger]

what is the point of that? If marks aligned, tensioner is tightened and it's as good as it gets. Spinning it with the tensioner loose can compress the tensioner spring delaying the cam with the belt slack and risking P to V contact.

As you wind the engine over slowly the resistance of valvetrain ensures there's always tension so no harm is done by doing it loose. The purpose of doing it tensioner loose is it ensures the tension side is under some tension when the tensioner is locked off. If you had a bit of slack on the tension side and locked of the tensioner as soon as the crank rotates you'd get some slack.

[zaq123]

As you wind the engine over slowly the resistance of valvetrain ensures there's always tension so no harm is done by doing it loose. The purpose of doing it tensioner loose is it ensures the tension side is under some tension when the tensioner is locked off. If you had a bit of slack on the tension side and locked of the tensioner as soon as the crank rotates you'd get some slack.

Isn't that what that spring is doing, picking up any belt slack? I just trying to imagine how the tensioner will be steel loose with the spring applying all that pressure on the belt.

[zaq123]

To remedy this, I actually start by bolting the tensioner in the full "loose belt" position (spring completely compressed). Then back the crank up one tooth, wrap the belt starting at the crank, then cam, then pull the tension towards the tensioner and rotate the crank as I slip the belt on the tensioner. Then leave the tensioner spring fully compressed, rotate the engine two full times to be sure the marks repeat. Then loosen the tensioner bolt, let the spring do then tensioning, then re-tighten the tensioner bolt.

I do something similar as well, install the belt with marks aligned, spring fully compressed and its bolt locked; Release the spring and lock it, recheck marks; Rotate a couple of time a check them marks again; Put the dial gauge on the valve, gear on the crank and find the exact cam timing.

[digger]

Isn't that what that spring is doing, picking up any belt slack? I just trying to imagine how the tensioner will be steel loose with the spring applying all that pressure on the belt.

Only on the slack side. On a frictionless system it does both sides. there's enough friction in the crank and cam that means you can get some trapped on the other side.
You also need to rotate to 100% confirm you got the right tooth.

[jeffnhiscars]

what is the point of that? If marks aligned, tensioner is tightened and it's as good as it gets. Spinning it with the tensioner loose can compress the tensioner spring delaying the cam with the belt slack and risking P to V contact.

When first wrapped around the gears the belt is typically tighter on one side than the other which can effect valve timing so Its SOP to rotate the engine and then tighten the tensioner. I'm surprised you questioned this.

[zaq123]

Cam Timing Off After Timing Belt Replacement?

When first wrapped around the gears the belt is typically tighter on one side than the other which can effect valve timing so Its SOP to rotate the engine and then tighten the tensioner. I'm surprised you questioned this.

I’m not really questioning this...IMO if the belt is installed correctly, the tensioner spring will take all the needed slack from the get go. Belt teeth are going into sprockets and stay there. Rotate the engine a few times, that unbolt and rebolt the tensioner bolt to equalize the belt. Basically the same concept but different approach. I’m concern with P to V contact when cranking the engine with loose tensioner. The resistance of the aggressive cam can compress that tensioner spring IMO.
As it is, belt already has a normal stretch that nets 2 degrees





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[zaq123]

Cam Timing Off After Timing Belt Replacement?

Sorry. Double post

[digger]

I’m not really questioning this...IMO if the belt is installed correctly, the tensioner spring will take all the needed slack from the get go. Belt teeth are going into sprockets and stay there. Rotate the engine a few times, that unbolt and rebolt the tensioner bolt to equalize the belt. Basically the same concept but different approach. I’m concern with P to V contact when cranking the engine with loose tensioner. The resistance of the aggressive cam can compress that tensioner spring IMO.
As it is, belt already has a normal stretch that nets 2 degrees


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It's fine if you unbolt after rotating a couple times and relighted but If you do it that way you need to tighten the tensioner loosen it then tighten it again. Why not tighten it once only....

You don't crank the engine with tensioner loose you hand rotate it. If you rotate the crank the correct way the belt is skways under tension so the only p to v issue is if you put on wrong tooth and then it doesn't matter what you do.

[zaq123]

It's fine if you unbolt after rotating a couple times and relighted but If you do it that way you need to tighten the tensioner loosen it then tighten it again. Why not tighten it once only....



You don't crank the engine with tensioner loose you hand rotate it. If you rotate the crank the correct way the belt is skways under tension so the only p to v issue is if you put on wrong tooth and then it doesn't matter what you do.

I agree that the belt is always under tension but with loose tensioner, it’s just the tensioner spring agains all that cam resistance. I’m sure that the spring will hold since that’s how you guys are doing it....probably just paranoia on my part..haha


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[digger]

The belt has teeth you don't even really need much tension to rotate it slowly provided its sitting on the gears correctly it'll stay in phase. Just don't go backwards