6.0 Low vacuum readings
Thread Starter
Teching In
Joined: Apr 2023
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From: Missouri
6.0 Low vacuum readings Hello all, recently I built an lq4 (6.0). I put all new bearings, stage 2 sloppy cam, timing gear and chain, pistons, rings, valve seals, lifters, push rods, and valve springs. I spent a lot of time making sure all of these components would work with each other and they did... for a few hours. The engine started up perfectly after being rebuilt, ran, and idled very smoothly. After starting up about a week after it ran perfectly it started to act funky. Ran like it had a miss and the Holley terminator x was getting thrown all over the place because of the miss fire. So far I've done just about everything I can think of. Tested compression (good and close numbers all the way around), checked for spark, changed the o2 sensor, and messed around with the tune. After testing for vacuum I realized that was my problem. The gauge goes back and forth from 0 to 5 in/hg. I replaced the intake gaskets and made sure there were no leaks in the lines yet I still have no vacuum. If anyone has any ideas or thoughts it would be greatly appreciated, Thanks.
TECH Fanatic
Joined: Jan 2009
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From: Bostwick, GA.
Use the data logging to help get a handle on the problem....
KOEO... What does the MAP sensor read?
Have you tried running in open loop?
Post the tune and a data log. Has to be in a zip file.
KOEO... What does the MAP sensor read?
Have you tried running in open loop?
Post the tune and a data log. Has to be in a zip file.
Thread Starter
Teching In
Joined: Apr 2023
Posts: 19
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From: Missouri
KOEO the MAP sensor reads 90 kPa. I sucked on the line going to the sensor and it moved as I thought it would. But when the engine is running it basically doesn't move because there is no vacuum. When I get around 3k RPM it starts to move down into the 97kPa range but still not where it should be. Also, intake gets very hot indicating a backfire? I haven't tried running it open loop but I can... I'll run it for enough time to read the data log, get that and the tune zipped, and post it. Won't be at the car for a couple of weeks so bare with me...
TECH Fanatic
Joined: Jan 2009
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From: Bostwick, GA.
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Thread Starter
Teching In
Joined: Apr 2023
Posts: 19
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From: Missouri
I did seem like the compression should have been higher.
Although the car ran a lot better when I fist started it. I let it run around a total of 4 hours before I drove it the first time. The fist time I drove it is when it started to run funky. I’m starting to think I have a component that failed internally. What do you think?
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Joined: Jan 2009
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From: Bostwick, GA.
I would start with pulling the plugs and taking a look.
Next would be the valve covers and take a look.
A leak down with the valves backed off might provide some information.
What intake and t/b is that?
Next would be the valve covers and take a look.
A leak down with the valves backed off might provide some information.
What intake and t/b is that?
Thread Starter
Teching In
Joined: Apr 2023
Posts: 19
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From: Missouri
pulled valve covers and cranked the engine. All rockers and valves are moving but that does not mean they are sealing. I think a leak down will be a telltale if I have an internal problem like bad rings or valves. Pulled the plugs and saw spark on all plugs. Can't really test injectors but since they were brand new id have to assume they are good.
This is my intake and throttle body (mine is in black)-
https://jackspaniaracing.shop/produc...aa729374&_ss=r
This is my intake and throttle body (mine is in black)-
LS Low Profile Metal Fabricated Intake Manifold DBW LS1 LS2 LS6 102mm Cathedral
https://jackspaniaracing.shop/produc...aa729374&_ss=r
Thread Starter
Teching In
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From: Missouri
Yes, I made sure to plug EVERY port on the intake except the map. The one I'm using is directly on the bottom of the intake right in the middle.