8.1L vortec - Low vacuum at idle after rebuild - Need expert help for this puzzle
06-24-2018, 09:04 PM
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8.1L vortec - Low vacuum at idle after rebuild - Need expert help for this puzzle Hey everybody,
I know I'm not a regular poster, and my application isn't *exactly* right for this forum, but if my years of reading internet information has taught me anything, LS1Tech is more likely to help me hone in on my problem the best. I'll apologize up front if this tends to get long - I'm working on being less wordy...
So, the application is a marine engine - a Volvo Penta 8.1Gxi - which is the 8100 Vortec motor without any EVAP and using a MEFI-4B controller in speed density mode. I completely rebuilt the engine over the winter and converted it from an 8.1Gi to the Gxi - the only difference being the cam - the Gi used the GM HP1 cam (375 hp) and the Gxi uses the HP2 cam (420hp).
Here's a quick breakdown of the rebuild:
Due to external corrosion, I replaced the heads with used heads that were in really good shape.
Due to some slight pitting on the cylinders, I had the block bored +.030. Crank was balanced and polished. All GM gaskets were used.
On the intake, the 8.1l intake is known for a severe restriction right after the throttle body that can be enlarged by cutting off the bottom of the lifter valley side of the intake to gain access for milling the restriction clear and then the bottom is welded back on (the importance of this will be evident in a moment).
I replaced the cam with a brand new HP2 cam - not adjustable - and I used a factory timing chain - also not adjustable. As nothing in the cam setup was adjustable, I didn't bother to degree the cam during installation.. .a choice I am now regretting.
Here are the cam specs according to one online source:
224º/233º @ .050” (222º/232º)
.300”/.300” Lobe
.510”/.510” Valve
114º/115.5º
The factory process for this engine's rocker arm nut 'adjustment' is to simply torque it to 18ft pounds. There is no procedure to ensure that the valves being torqued are on the base circle or otherwise - just "torque to 18 ft-lbs"
Due to the cam and intake changes, I wanted to be certain that I was not running too lean, so I installed a wideband O2 sensor in the port riser.
I believe I followed all of the assembly steps in the Volvo(GM) engine manual to the 'T'. I really did believe I was putting this back together text-book.
I purchased 'MEFIburn' from OBDDiagnostics so I could reprogram the MEFI-4b controller to the Gxi tune. Bob at OBD Diagnostics sent me a .bin that says it's the "gxi" tune, but it's not clear if it's actually a copy of the OEM tune, or just something that's supposed to be close.
Upon first fire, the motor started *immediately* and seemed to idle fine. It sounded a little lumpy, but I assumed that was the new camshaft. It would want to stall out after coming off throttle and it has this issue where when you first feed it throttle it revs up and then pulls back the revs a little and this continues with additional gradual feeding of throttle. Above a certain throttle percentage, though, and it just will rev up quick. The O2 sensor is pretty much always reading low to mid 13's.
At this point, I assumed that the tune was either not a replica of the factory tune or that my change to the intake manifold was throwing something about the tune off and needed to be tweaked, but I was having some connectivity issues with the scanning software, so I sent Bob a copy of the log to show him how the datastream was dropping out. He clarified that sometimes the connection to the ECU needs to be disconnected and reconnected if the signal is dropping out but then also commented how there was very little vacuum at idle and he thought I installed the wrong cam sensor. Prior to this, I hadn't done the conversion of what the MAP values meant to manifold vacuum.
After adjusting the TPS settings so that 0-100% are read correctly, and opening the idle stop on the butterfly to get the IAC value at idle down (now is at around 55), the engine starts immediately and will idle, albeit not super smoothly. The engine continues to have the rev up-then pull back phenomena when gradually giving throttle. At higher RPMs the engine doesn't miss at all and sounds pretty nice.
At idle, the MAP is reading 60-64 kPa (99.2 with engine off). With a vacuum gauge, the idle reading is ~9 inHg at pretty steady (maybe oscillates a total of 0.5inHg). All of my sources (online and my old books) suggest that this is too low and would be likely caused by valve timing or an intake vacuum leak.
-I snugged up the intake manifold bolts and sprayed carb cleaner all around the intake manifold seams and saw no change in idle vacuum. (no obvious external leak?)
-I changed the cam sensor and MAP sensors back to the old ones and there was no change. (cam sensor OK and MAP sensor not leaking)
-When the engine is running at idle, according to Scannerpro, the Ignition advance moves between 0 and 4 degrees
Some focused questions for the pros:
- If I was off a tooth in either direction, would the engine run this 'well' - no misses, no knocks and revs well?
- If the valves were overlashed, that would result in low idle vacuum, correct?
- Does the factory installation process for the rocker arm nuts seem poor to anybody?
- Are there any ways of determining whether or not I have an intake leak into the lifter valley without replacing the intake manifold? I'm worried that perhaps the intake manifold porting job, because of how it is performed, resulted in a leak straight into the lifter valley. I did look for obvious inclusions/porosities in the weld joint when I got it back and it looked OK, but obviously now I'm questioning everything.
- Would 0-4 degrees of ignition timing at idle account for such a low vacuum? What advance would you recommend?
- Does anybody know how to tweak this in Tunerpro? Adjusting the main spark table doesn't seem to be changing anything, but I may not be adjusting it enough..
- Are there any other sensor data that I should be looking at to hone in on the root cause?
Here's a video of the engine idling:
https://photos.app.goo.gl/MpeRyyJ8ZqQmi1QZA
Here's a link to the ScannerPro Log of it running:
https://drive.google.com/open?id=1SM...dEgJ6SXo7zNTRK
I know this was long.. I sure appreciate anybody who's willing to jump in and help me solve this puzzle. Thanks!
I know I'm not a regular poster, and my application isn't *exactly* right for this forum, but if my years of reading internet information has taught me anything, LS1Tech is more likely to help me hone in on my problem the best. I'll apologize up front if this tends to get long - I'm working on being less wordy...
So, the application is a marine engine - a Volvo Penta 8.1Gxi - which is the 8100 Vortec motor without any EVAP and using a MEFI-4B controller in speed density mode. I completely rebuilt the engine over the winter and converted it from an 8.1Gi to the Gxi - the only difference being the cam - the Gi used the GM HP1 cam (375 hp) and the Gxi uses the HP2 cam (420hp).
Here's a quick breakdown of the rebuild:
Due to external corrosion, I replaced the heads with used heads that were in really good shape.
Due to some slight pitting on the cylinders, I had the block bored +.030. Crank was balanced and polished. All GM gaskets were used.
On the intake, the 8.1l intake is known for a severe restriction right after the throttle body that can be enlarged by cutting off the bottom of the lifter valley side of the intake to gain access for milling the restriction clear and then the bottom is welded back on (the importance of this will be evident in a moment).
I replaced the cam with a brand new HP2 cam - not adjustable - and I used a factory timing chain - also not adjustable. As nothing in the cam setup was adjustable, I didn't bother to degree the cam during installation.. .a choice I am now regretting.
Here are the cam specs according to one online source:
224º/233º @ .050” (222º/232º)
.300”/.300” Lobe
.510”/.510” Valve
114º/115.5º
The factory process for this engine's rocker arm nut 'adjustment' is to simply torque it to 18ft pounds. There is no procedure to ensure that the valves being torqued are on the base circle or otherwise - just "torque to 18 ft-lbs"
Due to the cam and intake changes, I wanted to be certain that I was not running too lean, so I installed a wideband O2 sensor in the port riser.
I believe I followed all of the assembly steps in the Volvo(GM) engine manual to the 'T'. I really did believe I was putting this back together text-book.
I purchased 'MEFIburn' from OBDDiagnostics so I could reprogram the MEFI-4b controller to the Gxi tune. Bob at OBD Diagnostics sent me a .bin that says it's the "gxi" tune, but it's not clear if it's actually a copy of the OEM tune, or just something that's supposed to be close.
Upon first fire, the motor started *immediately* and seemed to idle fine. It sounded a little lumpy, but I assumed that was the new camshaft. It would want to stall out after coming off throttle and it has this issue where when you first feed it throttle it revs up and then pulls back the revs a little and this continues with additional gradual feeding of throttle. Above a certain throttle percentage, though, and it just will rev up quick. The O2 sensor is pretty much always reading low to mid 13's.
At this point, I assumed that the tune was either not a replica of the factory tune or that my change to the intake manifold was throwing something about the tune off and needed to be tweaked, but I was having some connectivity issues with the scanning software, so I sent Bob a copy of the log to show him how the datastream was dropping out. He clarified that sometimes the connection to the ECU needs to be disconnected and reconnected if the signal is dropping out but then also commented how there was very little vacuum at idle and he thought I installed the wrong cam sensor. Prior to this, I hadn't done the conversion of what the MAP values meant to manifold vacuum.
After adjusting the TPS settings so that 0-100% are read correctly, and opening the idle stop on the butterfly to get the IAC value at idle down (now is at around 55), the engine starts immediately and will idle, albeit not super smoothly. The engine continues to have the rev up-then pull back phenomena when gradually giving throttle. At higher RPMs the engine doesn't miss at all and sounds pretty nice.
At idle, the MAP is reading 60-64 kPa (99.2 with engine off). With a vacuum gauge, the idle reading is ~9 inHg at pretty steady (maybe oscillates a total of 0.5inHg). All of my sources (online and my old books) suggest that this is too low and would be likely caused by valve timing or an intake vacuum leak.
-I snugged up the intake manifold bolts and sprayed carb cleaner all around the intake manifold seams and saw no change in idle vacuum. (no obvious external leak?)
-I changed the cam sensor and MAP sensors back to the old ones and there was no change. (cam sensor OK and MAP sensor not leaking)
-When the engine is running at idle, according to Scannerpro, the Ignition advance moves between 0 and 4 degrees
Some focused questions for the pros:
- If I was off a tooth in either direction, would the engine run this 'well' - no misses, no knocks and revs well?
- If the valves were overlashed, that would result in low idle vacuum, correct?
- Does the factory installation process for the rocker arm nuts seem poor to anybody?
- Are there any ways of determining whether or not I have an intake leak into the lifter valley without replacing the intake manifold? I'm worried that perhaps the intake manifold porting job, because of how it is performed, resulted in a leak straight into the lifter valley. I did look for obvious inclusions/porosities in the weld joint when I got it back and it looked OK, but obviously now I'm questioning everything.
- Would 0-4 degrees of ignition timing at idle account for such a low vacuum? What advance would you recommend?
- Does anybody know how to tweak this in Tunerpro? Adjusting the main spark table doesn't seem to be changing anything, but I may not be adjusting it enough..
- Are there any other sensor data that I should be looking at to hone in on the root cause?
Here's a video of the engine idling:
https://photos.app.goo.gl/MpeRyyJ8ZqQmi1QZA
Here's a link to the ScannerPro Log of it running:
https://drive.google.com/open?id=1SM...dEgJ6SXo7zNTRK
I know this was long.. I sure appreciate anybody who's willing to jump in and help me solve this puzzle. Thanks!
06-27-2018, 01:55 AM
#2
The main things that jump out are that you should have used an adjustable pushrod to measure the proper pushrod length for the new cam, and your idle timing seems quite low. As for the cam timing, a tooth one way or another would still fire up, but be down on power. Does a compression test show unusually higher or lower than normal readings?