Rapidly moving vacuum gauge at idle - smooth above 1k
Posted: Sun May 13, 2018 4:59 pm
360ci LA Mopar (iron headed street engine) 11.3:1cr. Has developed a rapidly fluctuating vacuum gauge needle between 19 and 21", at idle speeds upto around 1000 rpm. Above 1k rpm it smoothes out nice and steady.
When the transmission is put in Drive the fluctuations increase upto a 4" sweep. Engine runs rough too and usually needs some throttle to keep up the rpm. The vacuum needle fluctuates right from the start on a cold engine and doesn't improve when hot.
Engine vacuum is normally 'high' around 17-21" during regular driving.
I understand this kind of needle behaviour could usually be pointing to a bad valve guide(s).
My question, can this rapid needle movement also occur when a lash is too tight on a valve and holding it open ever so slightly?
Some more info:
I've had this engine broken-in on an engine stand were the needle was a bit more steady.
Engine currently runs on a MegaSquirt digital ignition in the car where only ignition is controlled for now.
Did a cranking compression test with an old MotoMeter push-on tester, which didn't show much other then the readings went off its 12 bar (174psi) scale. Cam is CompCams XE256H.
The heads have had new bronze valve guides installed and all valves received positive valve seals, so I would think valve-guide related vacuum issues would be closed off by the seals. But I haven't been able to rule that out yet. Valve seats have been leak-tested with brakecleaner while watching for leakage.
Now for the curve ball in the mix;
The new hydraulic lifters have been converted to solids with a spacer inplace of the spring and check valve, because a few of them started collapsing on one side of the engine. I suspect too much/deep inner lifter bore camfer, and/or lifters with a too high oil band.
Valves were lashed (a number of times) at 0.003 to 0.006", and are currently at 0.004" cold (have not checked them hot yet). Iron block and heads. 1.6 rockers.
So my concerns about the lifters is I think they are still able to build some pressure inside and perhaps manage to close up the tight lash at idle and cause vacuum issues.
This seems not something I can easily test with a compression tester, unless someone else will prime the engine's oil system at the same time perhaps.
When the transmission is put in Drive the fluctuations increase upto a 4" sweep. Engine runs rough too and usually needs some throttle to keep up the rpm. The vacuum needle fluctuates right from the start on a cold engine and doesn't improve when hot.
Engine vacuum is normally 'high' around 17-21" during regular driving.
I understand this kind of needle behaviour could usually be pointing to a bad valve guide(s).
My question, can this rapid needle movement also occur when a lash is too tight on a valve and holding it open ever so slightly?
Some more info:
I've had this engine broken-in on an engine stand were the needle was a bit more steady.
Engine currently runs on a MegaSquirt digital ignition in the car where only ignition is controlled for now.
Did a cranking compression test with an old MotoMeter push-on tester, which didn't show much other then the readings went off its 12 bar (174psi) scale. Cam is CompCams XE256H.
The heads have had new bronze valve guides installed and all valves received positive valve seals, so I would think valve-guide related vacuum issues would be closed off by the seals. But I haven't been able to rule that out yet. Valve seats have been leak-tested with brakecleaner while watching for leakage.
Now for the curve ball in the mix;
The new hydraulic lifters have been converted to solids with a spacer inplace of the spring and check valve, because a few of them started collapsing on one side of the engine. I suspect too much/deep inner lifter bore camfer, and/or lifters with a too high oil band.
Valves were lashed (a number of times) at 0.003 to 0.006", and are currently at 0.004" cold (have not checked them hot yet). Iron block and heads. 1.6 rockers.
So my concerns about the lifters is I think they are still able to build some pressure inside and perhaps manage to close up the tight lash at idle and cause vacuum issues.
This seems not something I can easily test with a compression tester, unless someone else will prime the engine's oil system at the same time perhaps.