Need Help Deciphering a Vacuum Reading
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Need Help Deciphering a Vacuum Reading
I’ve got an 86 f150 4.9 w/ feedback carb. I hooked up a vacuum gauge to manifold vacuum with the engine warmed up.
Vacuum was steady at 19 inHg on idle. When I hit the gas it dropped to 14 or 15 then went back up to 23 then settled into around 18.5. I think that's normal, but someone should correct me if that's wrong.
BUT then a weird thing happens...11 seconds after I take my foot off the gas, the vacuum gauge goes up from 18.5 to 19 then down to 18 then settles back in at 19. That fluctuation happens in about 3 seconds. This corresponds to a drop in the idle speed. Is it normal to have the vacuum fluctuate so long after taking my foot off the gas? What causes this?
Here's a video of the reading: https://streamable.com/tp09er. I take my foot off the gas pedal at the 5 second mark, and don't touch it again.
Some additional context. My truck is running a little rough when it warms up. Also, it shifts hard into second and third. My first thought was the vacuum modulator was bad on the C6, but it doesn't have tranny fluid coming from it, and it holds vacuum, so I'm hesitant to replace it. I'm also getting an OBD1 41 when I scan it...which means the system is running lean. I replaced the o2 sensor, computer, ignition control module...without any noticeable improvement. Also, the engine was rebuilt recently. Plus it got a new (old) intake manifold.
Vacuum was steady at 19 inHg on idle. When I hit the gas it dropped to 14 or 15 then went back up to 23 then settled into around 18.5. I think that's normal, but someone should correct me if that's wrong.
BUT then a weird thing happens...11 seconds after I take my foot off the gas, the vacuum gauge goes up from 18.5 to 19 then down to 18 then settles back in at 19. That fluctuation happens in about 3 seconds. This corresponds to a drop in the idle speed. Is it normal to have the vacuum fluctuate so long after taking my foot off the gas? What causes this?
Here's a video of the reading: https://streamable.com/tp09er. I take my foot off the gas pedal at the 5 second mark, and don't touch it again.
Some additional context. My truck is running a little rough when it warms up. Also, it shifts hard into second and third. My first thought was the vacuum modulator was bad on the C6, but it doesn't have tranny fluid coming from it, and it holds vacuum, so I'm hesitant to replace it. I'm also getting an OBD1 41 when I scan it...which means the system is running lean. I replaced the o2 sensor, computer, ignition control module...without any noticeable improvement. Also, the engine was rebuilt recently. Plus it got a new (old) intake manifold.
FTE Legend
Joined: Nov 2015
Posts: 30,952
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From: Angier, NC
I think you said all you needed to in the first line
If the computer is still hooked up and "trying" to control the fuel mixture & timing then the vacuum reading will change as the computer it trying to bring everything back into factory spec for that run condition.
I would say that is all normal for a feed back system.
As for the running ruff when warm, lean will run ruff but you may not be able to change this on a feed back system?
If at idle do you have a idle mix screw and if so can it be adjusted? Back in the day they were capped so they could not be adjusted and yes they did run ruff at idle to it was richen up a bit.
Dave ----
I’ve got an 86 f150 4.9 w/ feedback carb.
I would say that is all normal for a feed back system.
As for the running ruff when warm, lean will run ruff but you may not be able to change this on a feed back system?
If at idle do you have a idle mix screw and if so can it be adjusted? Back in the day they were capped so they could not be adjusted and yes they did run ruff at idle to it was richen up a bit.
Dave ----
Post Fiend
Joined: Feb 2008
Posts: 5,831
Likes: 121
From: St Albert, Alberta
I’ve got an 86 f150 4.9 w/ feedback carb. I hooked up a vacuum gauge to manifold vacuum with the engine warmed up.
Vacuum was steady at 19 inHg on idle. When I hit the gas it dropped to 14 or 15 then went back up to 23 then settled into around 18.5. I think that's normal, but someone should correct me if that's wrong.
BUT then a weird thing happens...11 seconds after I take my foot off the gas, the vacuum gauge goes up from 18.5 to 19 then down to 18 then settles back in at 19. That fluctuation happens in about 3 seconds. This corresponds to a drop in the idle speed. Is it normal to have the vacuum fluctuate so long after taking my foot off the gas? What causes this?
Here's a video of the reading: https://streamable.com/tp09er. I take my foot off the gas pedal at the 5 second mark, and don't touch it again.
Some additional context. My truck is running a little rough when it warms up. Also, it shifts hard into second and third. My first thought was the vacuum modulator was bad on the C6, but it doesn't have tranny fluid coming from it, and it holds vacuum, so I'm hesitant to replace it. I'm also getting an OBD1 41 when I scan it...which means the system is running lean. I replaced the o2 sensor, computer, ignition control module...without any noticeable improvement. Also, the engine was rebuilt recently. Plus it got a new (old) intake manifold.
Vacuum was steady at 19 inHg on idle. When I hit the gas it dropped to 14 or 15 then went back up to 23 then settled into around 18.5. I think that's normal, but someone should correct me if that's wrong.
BUT then a weird thing happens...11 seconds after I take my foot off the gas, the vacuum gauge goes up from 18.5 to 19 then down to 18 then settles back in at 19. That fluctuation happens in about 3 seconds. This corresponds to a drop in the idle speed. Is it normal to have the vacuum fluctuate so long after taking my foot off the gas? What causes this?
Here's a video of the reading: https://streamable.com/tp09er. I take my foot off the gas pedal at the 5 second mark, and don't touch it again.
Some additional context. My truck is running a little rough when it warms up. Also, it shifts hard into second and third. My first thought was the vacuum modulator was bad on the C6, but it doesn't have tranny fluid coming from it, and it holds vacuum, so I'm hesitant to replace it. I'm also getting an OBD1 41 when I scan it...which means the system is running lean. I replaced the o2 sensor, computer, ignition control module...without any noticeable improvement. Also, the engine was rebuilt recently. Plus it got a new (old) intake manifold.
That vacuum fluctuation is normal on a carb feed back system. As for your Code 41 Make sure the enigne was up to operating temp The 41 actually indicates continuously lean and the EGO is not switching in closed loop The EGO should be seeing 8 switches (rich lean) within 4 mins. If it does not this will trigger the code 41.
So make sure you have no intake manifold leaks, If the exhaust has been modified and with today's fuels this may throw a code 41 also since it si rebuild if id does not have the factory cam this may also throw a code 41. So if you have no one of those issues come back and we can go through a comprehensive testing procedure to track down the issue. if you do have one of those issues fix the manifold leak and the rest can be resolved by up jetting the carb.
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