Decel fuel cut vaccum switch

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Output Shaft said:
The EGR valve can still "test to spec" yet continue to create a stumble every time it opens. Just to rule it out, pull the vac hose leading to the EGR valve & plug the hose. Drive the truck around the block. See if that fixed the stumble.

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Just back from a good test run with the indicated hose plugged. No noticeable change.

Here is a bit more data: I just bought a vacuum gauge and it reads about 10 inches at idle coming off of the gas filter thing. Will pop up to 13-14 after revving and letting off.

Also, I think something may be funky with ignition timing. The last time I set the timing, my timing light crapped out so I did the old school approach of getting it in range, going for a drive and listening for a ping. I advanced timing until I got a ping under load, then backed off until the ping went away. Well, I just got a working timing light and it is WAY advanced. I'd guess at least 15-20 BTDC because the mark isn't in view in the window. I tried setting it back to 7 BTDC and it ran like crap, stumble was worse, no power.
 
Don't test the vacuum off the gas filter. It has three pipes & there are bleed lines connected to it slowly leaking vacuum (though 10 inHg is too much)

Test the vacuum off the brake booster fitting.

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If you've still got low vacuum, there's a big leak somewhere as @CaptClose mentioned
 
Output Shaft said:
Don't test the vacuum off the gas filter. It has three pipes & there are bleed lines connected to it slowly leaking vacuum (though 10 inHg is too much)

Test the vacuum off the brake booster fitting.

View attachment 1433896

If you've still got low vacuum, there's a big leak somewhere as @CaptClose mentioned
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Ok. It reads 13-14 inches at idle on the brake booster. Goes up to about 20 after decelleration.

So, if spraying carb cleaner everywhere doesn't ID the leak, where do I go next?
 
Connect your vacuum gauge to that brake booster fitting with engine idling and prop it up so that it's in clear view.

Then rule out each vacuum line by pinching off each one, waiting a bit & seeing if the vacuum changes on the gauge. Needle nose pliers work well.

If you've pinched off every vacuum line (one at a time, or two at a time) & no change in the vacuum was detected on the gauge, then it can be reasonably assumed that the vacuum leak isn't related to any of the vacuum hose connections & may be caused by a leaking manifold gasket or carburetor base.
 
Output Shaft said:
Connect your vacuum gauge to that brake booster fitting with engine idling and prop it up so that it's in clear view.

Then rule out each vacuum line by pinching off each one, waiting a bit & seeing if the vacuum changes on the gauge. Needle nose pliers work well.

If you've pinched off every vacuum line (one at a time, or two at a time) & no change in the vacuum was detected on the gauge, then it can be reasonably assumed that the vacuum leak isn't related to any of the vacuum hose connections & may be caused by a leaking manifold gasket or carburetor base.
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Ok. Just did that with no joy. I guess I pull off the air filter and check the torque on carb base and manifold?
 
I just pulled the air cleaner. Carb nuts seem to be ok...at least they were all similiarly tight. You can't get a torque wrench in there but I think they're ok.

The front most intake manifold bolt though, was only finger tight. I could spin with without a ratchet just with bare fingers on the extension .

I'm assuming that they are all loose. Is there a trick to getting to the others?
 
You can get to all the manifold bolts. You'll need to move some of the junk that's in the way temporarily. Bigguns are 17mm hex socket size. Youll need a few different lengths of extensions and possibly a swivel joint and wobble tipped extension. Look at the diagram in the FSM as to where they are. You'll have to go by feel for a few of them to find them.
 
Hey Curt... I'll give you a gold star if you can get to all the manifold bolts without removing the spaghetti. Two gold stars if you can do it without cussing hahaha..

I think it's safe to assume the leak is at the manifold base. You may want to consider putting a new gasket on as well, but may not be necessary.

Try tightening all the bolts you can reach first. See if that helps your vacuum. If you're lucky, you'll be able to seal the leak with a few turns of the wrench. If not, you've got a bigger job ahead of you.
 
Got to the front and rear bolts. Clawed back to 17 inches of vacuum. Better, but not done.
 
Great work... keep at it. One thing that really helped me was a bit of advice from @Output Shaft about Zen Mechanics. Just concentrate on one bolt at a time. Sometimes, it can be tightened only a quarter turn at a time. Some days you might only get one bolt finished. Just relax and breath. Enjoy the struggle.
 
Got to all of them but one. The all took at least a half turn. No further improvement on vacuum reading, tho.
 
Great job Curt! Which one didn't you get to? The one by the firewall, or the one tucked under the EGR between the intake and exhaust manifolds?

Can you make a video of the vacuum gauge for us? Preferably with a warm engine around 950-1000 RPM's.
 
Output Shaft said:
& what elevation are you at? The higher the elevation, the lower the vacuum because the air is thinner.
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100 ft above sea level.

I'm not sure where to go next. Compression indicates a slightly tired engine...would be a pain to replace the manifold gaskets and not see any change.
 
CaptClose said:
Great job Curt! Which one didn't you get to? The one by the firewall, or the one tucked under the EGR between the intake and exhaust manifolds?

Can you make a video of the vacuum gauge for us? Preferably with a warm engine around 950-1000 RPM's.
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It was the third one from the back that I couldn't reach.

I'll do a video.
 
Here are couple of videos. Did one cold start , and one at idle after warming up It looks like youtube dithered the resolution but the gauge is right on 15.
 
Thanks for the videos Curt... I think you're okay on the manifold gasket. If you're losing vacuum there, it's not much. A valve adjustment and a carb rebuild is what I think you're looking at. 15 in hg is a little low for 100 feet of elevation, but probably due to a worn out engine, than a leak. Regardless, I think the carb is the source of your sputtering.
 
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