NC280z Posted January 20, 2014 Report Share Posted January 20, 2014 Hello all, I'm still troubleshooting the low vacuum gauge readings on my engine and decided to do a compression test on all cylinders to see the results. With the throttle plate wide open I'm getting dry readings on all cylinders at around 125-130psi, a far cry from the 160+ the book states. This engine is a L28E and brand new in terms of a bore/hone, pistons/rings (Flattops with a .030 overbore) along with a valve job and new valve stem seals etc. I did the 20min/2000rpm break-in procedure, but other than a few startups it has zero road miles. Strange thing is that it starts up just fine, however it smells like it's running rich and even after verifying the valve lash as correct and finding no leaks in any of the vacuum hoses I can't get a vacuum gauge reading higher than 5inHg. I don't have any oil/fuel in the coolant and the oil is clean as well so I'm doubting it as a head gasket issue. What would cause such low compression readings on all cylinders, and such a low vacuum reading? I'm at a breaking point in terms of frustration with this car. I love Z cars but the trouble this thing is putting me through is unbearable. Just need some advice from you Z gurus. Quote Link to comment
Eagle_Adam Posted January 20, 2014 Report Share Posted January 20, 2014 you need to put some miles on that motor before you worry about those numbers IMO. set the rings!!!!!!! 3 Quote Link to comment
TENDRIL Posted January 20, 2014 Report Share Posted January 20, 2014 ^^^this did you test it hot? Quote Link to comment
Sealik Posted January 20, 2014 Report Share Posted January 20, 2014 Sometimes it takes a few miles to get the compression up.....depending on what rings you used Rich conditions can wash the cylinder walls which hinder the seating of the rings I would try a leak down test and see what's up Quote Link to comment
Sealik Posted January 20, 2014 Report Share Posted January 20, 2014 Even at 125 lbs.....vacuum readings should be higher Quote Link to comment
NC280z Posted January 20, 2014 Author Report Share Posted January 20, 2014 I figured that it might take a bit to seat the rings, I'll be doing a leak-down test next weekend when I get a chance. I'm just trying to think of what would cause the low vacuum readings given that I can't pinpoint any large leaks in any vacuum lines or gaskets. I also figured that even with the little bit of run time it's had that the compression readings would be higher than 125-130 dry. Also, I did the test with the engine cold. Intake and exhaust valve lash set to .010 and .008 respectively, per the FSM, leading to a MSA header with no muffler at the moment. Quote Link to comment
Just Joel Posted January 20, 2014 Report Share Posted January 20, 2014 Needs miles and warm to get a true reading. Also, where is the vacuum testing from? Quote Link to comment
ggzilla Posted January 20, 2014 Report Share Posted January 20, 2014 I've used compression gauges that read 125 normally. Has you gauge been calibrated? If not, even readings are good. 1 Quote Link to comment
NC280z Posted January 20, 2014 Author Report Share Posted January 20, 2014 I can't say the last time the (compression) gauge has been calibrated, but I'm hoping the even readings are a good sign. The vacuum gauge is hooked to the intake manifold via the port used for the brake booster. It's hooked using its own line, not T'd in with the b/booster. I checked the gauge on my '08 Frontier and it read normal at idle so it's a verified working gauge. I used a Smoke Pro tester and the only smoke I had coming from anywhere was the BCDD, at a pinhole right above the BCDD adjustment screw. Hooked the Smoke Pro up to the manifold and to the oil dipstick tube and no smoke from anywhere else. Quote Link to comment
620Turbo4X4 Posted January 21, 2014 Report Share Posted January 21, 2014 Double check your camshaft timing. Having the cam timing too far retarded would cause a loss in intake vac. Is the gauge always at 5hg or does it get better at higher RPM's? 1 Quote Link to comment
NC280z Posted January 21, 2014 Author Report Share Posted January 21, 2014 I thought about the cam timing, but the notch on the gear is where it should be at with #1 TDC and 0' on the marker ( notch just right of the line in the cam tower). I may remove the front cover and reset the bright links on the cam and crank gears just to be sure, but as far as I can tell the cam is slightly advanced, not retarded. Same with the dizzy, got it set to the FSM spec of 7' BTDC. I reused the old injector O-rings, I didn't see smoke coming from that area during the smoke test but I'm replacing them next weekend just to be sure they're not the source of the vacuum leak. I've been racking my brain for the past 2 days trying to think of where else I could have such a massive leak without a sure sign, any ideas? Also, it does move up from 5inHg when the rpms are held higher. Quote Link to comment
datzenmike Posted January 21, 2014 Report Share Posted January 21, 2014 Don't bother re setting the 'bright links'. The V just under or slightly right of the etched line governs. If this is correct.... the cam is timed. period! Lack of intake vacuum and low compression. Valve lash way too tight. Bent or broken valve or valve seat, possibly just worn out valves. Large duration cam with high overlap. Cam timing retarded Quote Link to comment
620Turbo4X4 Posted January 21, 2014 Report Share Posted January 21, 2014 Cam timing fine then... Do you have the EGR valve hooked up? The only other thing I can think of is the EGR is connected to manifold vac and allowing exhaust into the intake at idle. That would also cause poor vacuum and typically rough idle too. If it's hooked up, try disconnecting it just for curiosity.... Quote Link to comment
datzenmike Posted January 21, 2014 Report Share Posted January 21, 2014 Typically EGR isn't on at idle for that reason... it just won't idle. It could be crusty with deposits and not fully closing though. Remove and wire brush. Quote Link to comment
EricJB Posted January 21, 2014 Report Share Posted January 21, 2014 Close the hood. Run a few tanks of gas through it. Vary the rpm and load. After the first tank, start driving it harder. Readjust the valves . Than check it. Quote Link to comment
NC280z Posted January 26, 2014 Author Report Share Posted January 26, 2014 No EGR on this motor, it's the '75 Federal emissions L28E. The camshaft I'm running is a stock e30 reground to Schneider Racing specs, 260 intake duration/ 270 exhaust duration with 112' LSA and .460 lift. I ran the engine to operating temps and the hot valve lash is spot-on for the FSM specs. A friend of the family that is a service station tech and familiar with Bosch fuel injection came over and messed with the AFM bypass screw, along with the idle speed screw and another smaller screw on the throttle body next to the idle speed screw that isn't listed in our books (another idle mixture maybe?) All in all he was able to get the car at a steady idle of 800rpm with about 10inHg of vacuum, needle steady on the gauge. I was told (and read) that performance cams can shift the vacuum gauge readings at idle, anybody running a similar cam here? I did a warm leak-down test with a differential leak-down tester from Snap-On. All cylinders have less than 2% difference at 90psi, however the little bit of air that does bleed off almost sounds like it's exiting the exhaust, so I'm going to check that tomorrow. As for ignition timing, can someone post a pic of the adjustment tab on the side of their dizzy? Just need something to compare to. Unfortunately during the tuning process my brand-new Champion aluminum radiator sprang a leak from the top end tank, throwing a wrench in any further testing. Not too happy as the radiator has only gone under the break in process and a few idle sessions, no real road miles. Temp gauge stayed at the halfway mark the whole time so it may have been just a spotty weld or bad row. Once I get another radiator in and complete the exhaust I'm going to put some careful road miles on it and see where that brings me as far as compression numbers and vacuum readings. Quote Link to comment
620Turbo4X4 Posted January 26, 2014 Report Share Posted January 26, 2014 Well, it's not the EGR valve then. Your cam shouldn't be causing low vacuum. 260/270 is not big at all. I'm no expert on z cams, but the isky master catalog states that 280 degrees is stock duration for the l26 & l28's so your schnider would be actually smaller than stock. I wonder if your harmonic balancer hasn't slipped on the hub, shifting your timing marks? Still seems to me that your a tooth off. You should have much better numbers than 130 and 10 in/hg. I would pull #1 plug and turn motor over by hand to find TDC, then make sure timing marks are where they are supposed to be. Something is not right.... Quote Link to comment
datzenmike Posted January 26, 2014 Report Share Posted January 26, 2014 Throw vacuum gauge away and try a second opinion. I don't think a motor will rin on only 6 in. Hg.b Quote Link to comment
NC280z Posted January 26, 2014 Author Report Share Posted January 26, 2014 Stock duration for the E30 cam on a L28E is 248/248. Whenever I bring the motor up to TDC I always have the cam cover off and spark plugs out to make sure everything matches up correctly. Since I have to pull the radiator out anyways I may just end up removing the front cover and double-checking all the marks. I might try to post a video of it running as well once the new radiator is in so you guys can hear it run and maybe make better guesses at what's wrong. Quote Link to comment
NC280z Posted January 26, 2014 Author Report Share Posted January 26, 2014 Also, I "borrowed" this photo from another forum to show the dizzy drive position with my engine at TDC for the #1 cyl. I see posts online where the drive is more inline with the two bolt holes. Could this be a cause for the low vacuum readings? I've tried dropping the oil pump and changing the tang position but the only other position it will drop into is around 11:20, with the bottom portion of the tang pointing past the bottom bolt hole just slightly. I tried running it like this and it wouldn't start. Quote Link to comment
datzenmike Posted January 27, 2014 Report Share Posted January 27, 2014 This is the position of the distributor. So that everyone is (more or less) on the same page the factory 11:28 position has been universally agreed upon. It's looks slightly advanced, but should be adjustable on the scale. Distributor position can be anywhere you like 12:30, 1:35, 2:20 doesn't matter, as long as the rotor is under the correct #1 wire on the cap at TDC and is withing proper timing range on the adjustment. The above in no way affects the compression. Cam timing can though. A late closing intake valve can. A tight intake valve can. A broken intake valve can. Throw vacuum gauge away and try a second opinion. I don't think a motor will rin on only 6 in. Hg.b Still say the gauge is wack. Quote Link to comment
ggzilla Posted January 27, 2014 Report Share Posted January 27, 2014 No that won't cause low vacuum readings - it only controls spark timing. Verify spark timing with the degree marks on the timing cover & pulley, and set it correctly. If the distributor is rotated too far it won't run right. On the other hand, cam timing can cause low vacuum. Quote Link to comment
datzenmike Posted January 27, 2014 Report Share Posted January 27, 2014 Well what do you call normal. 15-20 and closer to 20 would be good. Set TDC and pull valve cover off. To check cam timing lok for the V notch on the back of the cam sprocket to be just below or slightly to the right of the etch line above it on the cam thriust plate. Like this... Quote Link to comment
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