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vacuum guage reading

scott_duerring

Regular Contributor
what would one expect for a vacuum gauge reading at idle for a 8.2 MPI Crusader engine.
Both of mine read 12 at idle, which seems low. I have read that low consistent idle may indicate retarded ignition timing. Should one use a vacuum gauge in conjunction with a timing light when setting the initial timing? Needle vibrates very little which I understand is ok with these type engines and their valve overlap.
 
12 is pretty low. A healthy engine should read at least 16 inHg and as much as 21 inHg at sea level (depending on engine). Yes, timing can effect the engine's vacuum reading. The lower the manifold vacuum, the harder your engine is working to stay at idle.

Very little fluctuation on the vacuum guage should be okay. A lot of fluctuation may indicate a stuck/broken lifter, bent pushrod, uneven valve lash adjustments, etc...

I would recommend setting your base timing, and your total timing advance, THEN re-checking your vacuum. If it is still really low, I would do a compression check.
 
Not sure of what you mean by setting base timing and total timing advance in two steps? My engines are 8.2 EFI, Muti Port injection... to time them you turn a plug that goes to the ECM to "timing" mode and then shoot your light on the timing marks and turn the distributor to line up the marks... once the marks are aligned you just turn the plug to "run" mode and presto, your done. I am pretty sure that by switching the plug to timing mode you are disabling the timing advance which is controlled by the ECM and by returning the engine to run mode the ECM takes over the timing advance function which will work off of the base timing that is set in the "timing mode"... or sumtun like that...
did a compression check on stb engine last year, all cylinders between 150 and 165, so no issue there.
 
12 seems very low to me also. You're correct, the ECM controls the timing advance. Are you sure you have an acurate gauge? Why are you asking, is there a runability issue?
 
Sorry, you are not setting base timing AND total advance (TA). You set your base timing at idle, then verify you are getting the proper amount of TA (~3000+ rpm). Timing tape comes in handy for this and is very cheap. Would have nothing to do with your vacuum issue, but it is always a good idea to check TA after messing with your timing. Not sure the specs on your motors, but my 454's (model 350's) should get 35° total advance, I believe. The wrong time to be too far advanced is when you are under load.

You have any vacuum leaks going on? If you have a stethoscope, take the tip off so it is just the hose, and listen around the engine. Pay close attention around your injectors, throttle body gaskets... just about everywhere on the intake side.

If your timing is good, and no vacuum leaks, I would re-run a compression test.
 
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Yep I have twins, I am pulling the vacuum from the fuel pressure regulator connection to the engine. Fits the hose on my gauge perfectly. Both are pulling 12 and maybe I should try a different gauge as the one I am using I bought when I was 16, I am now 53... soooo.
My issue is with stb engine been trying to figure out low, rough idle issue, also when I bump rpms up to @ 11-12 hundred (in neutral) the engine "hunts" up and down... so thought I had a vacuum leak... but I have no issue with port engine and my old gauge reads the same on that engine. I have done the spraying around of carb cleaner to see if I can locate a leak... I KNOW the safety issues... but old habits blow up hard.. I mean die hard... and could detect no leak.. the compression test was run last year, twice with the same readings, the readings for 1 and 3 were high and it was thought that some oil was blowing by causing the high readings. the thought of the mechanic that did the tests was that perhaps the compression rings were ok just the oil ring may be "clogged" with sludge... I am burning oil in that engine at a faster rate than port engine and get oil on my plugs in the 1 and 3 cylinders, I only have 500 hours on the engines, this year I ran 50 weight Royal Purple synth. oil in the stb engine as it was suggested that my not only help the oil consumption issue but it may also "clean up" the oil rings on those cylinders... not sure if that was sound advice but could not see how it could hurt... specs call for 40w oil.
 
I would try to find a better spot to grab vacuum. I am not familiar with your engines, but do you have any vacuum ports on the throttle bodies? I would make sure wherever you plug the guage into, it is as close as possible to the intake manifold. If the fuel pressure regulator is your only option, hook the guage up to the manifold side of the hose; not the regulator side of the hose. If you already are... disregard and maybe couldn't hurt to try a different guage. They are fairly inexpensive.

For future reference, a method to check for leaks that is safer than making puddles of flammable liquid is to use propane. Same idea, same general procedure, but less likely to start a major fire. Camping-size propane tank, valve, and small length of hose. Or the stethoscope idea to listen for hissing...

Oil build-up on the oil rings should not affect your compression readings unless the compression rings themselves are worn out; which it does not sound like this is the case. The oil rings are just there to basically scrape the cylinders clean of oil. The compression rings do the compressing. If you have oil on the compression rings, though, it could bump up your compression readings. Unfortunately, there is no real test to see if your valve stem seals are leaking, but if these seals leak, they would cause oil to enter the cylinders and could increase the compression. The higher compression in two cylinders, the oil on those plugs, and the oil consumption you are having would lead me to suspect the valve stem seals.
 
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