| Re: [GMCnet] losing power [message #220376] |
Thu, 29 August 2013 17:30  |
KB
Messages: 1262
Registered: September 2009
Karma: 0
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Senior Member |
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> Ok. Double check the throttle body hold down bolts, and all your vacuum
> lines. Temporarily block off the cruise control at the back of the
> manifold. Steady idle at 750 rpm on a new engine should yield 18" of
> mercury with a rock steady needle. If it is not steady, perform a
> compression test or a cylinder balance test. RV camshafts usually pull high
> vacuum at low speeds. Did you check your intake manifold for cracks? Enough
> for now.
Turns out it's really hard to do the usual vacuum leak tests with EFI
(eg carb cleaner, propane, etc) because the computer compensates before
you can tell there's a change. I've gone over everything carefully
though and can't find a vac leak.
manifold looked really good when we had it off. I'll do a compression
test as a baseline, but the needle is steady.
I think the problem is likely the fuel tables in the EFI,
specifically too rich. After a few wideband learns at idle
which brought the fuel table values down, we're pushing 17
vacuum at 750rpm. Hopefully some more tuning will improve
it from there. The clue was that when we deliberately pulled
a small vac cap off the manifold to try and see what an obvious
leak would do, the vac gauge immediately jumped to 20.
Think we're just going to have to get our new fan shroud
and fan clutch installed and drive the thing.
thanks
Karen
1973 23'
1975 26'
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Karen
1975 26'
San Jose, CA
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| Re: [GMCnet] losing power [message #220381 is a reply to message #220376] |
Thu, 29 August 2013 17:49  |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Senior Member |
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Karen, don't you just love it when the computer tweaks stuff before you can
even figure out what it did? If you have a steady needle on the vacuum
gage, that is a good sign. Sounds like you are making the right moves. I
had a Howell Throttle body with EBL that drove me nuts for several days.
Problem turned out to be a failed fuel selector valve on the gas line
before the electric fuel pump. Those things do not like ANY restriction.
They will cavitate like mad.
Drive the coach and let it learn, then see what you get.
Jim Hupy
Salem, Or
78 Gmc Royale 403
On Aug 29, 2013 3:30 PM, "KB" <kab7@sonic.net> wrote:
> > Ok. Double check the throttle body hold down bolts, and all your vacuum
> > lines. Temporarily block off the cruise control at the back of the
> > manifold. Steady idle at 750 rpm on a new engine should yield 18" of
> > mercury with a rock steady needle. If it is not steady, perform a
> > compression test or a cylinder balance test. RV camshafts usually pull
> high
> > vacuum at low speeds. Did you check your intake manifold for cracks?
> Enough
> > for now.
>
> Turns out it's really hard to do the usual vacuum leak tests with EFI
> (eg carb cleaner, propane, etc) because the computer compensates before
> you can tell there's a change. I've gone over everything carefully
> though and can't find a vac leak.
>
> manifold looked really good when we had it off. I'll do a compression
> test as a baseline, but the needle is steady.
>
> I think the problem is likely the fuel tables in the EFI,
> specifically too rich. After a few wideband learns at idle
> which brought the fuel table values down, we're pushing 17
> vacuum at 750rpm. Hopefully some more tuning will improve
> it from there. The clue was that when we deliberately pulled
> a small vac cap off the manifold to try and see what an obvious
> leak would do, the vac gauge immediately jumped to 20.
>
> Think we're just going to have to get our new fan shroud
> and fan clutch installed and drive the thing.
>
> thanks
> Karen
> 1973 23'
> 1975 26'
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
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