Home » Public Forums » GMCnet » New here but not to the GMC (Fuel milage thoughts)
| New here but not to the GMC [message #79729] |
Thu, 08 April 2010 10:15  |
Hot wheels
Messages: 9
Registered: April 2010
Location: Holiday City Ohio
Karma: 0
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Junior Member |
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Good Morning,
I have been working latley on the old 77 palm beach that has been in our fammily since about 1978. I'll give you the long and short versions of the story.
I'm 42, grew up seeing the states in the 77 PB dad owned, we put 250k on the old thing over the years and have upgraded just about everything. Sway bars, bilstiens, 3 bearing hubs, 4 bag air, alcoas and rear disk brakes, new rear frame and the lower 3.50 drive gears has a custom pain job as well. I have put 2 rebuilds on the old 455 and the most recent one a year ago with a new throttle body fuel injection system. I sold my low mile (69k) '76 elegonza when Dad decided to move into an 86 prevost and spend more time living in one winters in FLA. I have been working on these, swapping motors and repairs since i was 16 helping the rv seating company we worked for keep a small fleet of them running as seating show vehicles. I have spent way more time in and around these things than most can imagine.
Anyway I picked up dads 77 and decided it was time to freshen the engine and trans and add a fuel injection system to try and get the most out of it both in power and fuel milage. I have spent the last year fighting overheating problems that it never had before and very poor milage. I think i finally found the source of the heating problem after installing a new griffen radiator and flow cooler pump I still had the problem, i found one of the chambered mufflers was broken and plugging the outles partially and making it build heat the harder i ran it. A good pair of glass pack mufflers really brought the engine back around and seems to have gotten rid of the heating problem. I usually pull a 14x7 enclosed trailer around with me so im used to poor milage but on the last shake down run I got 4.8 mpg., up from 3.5 the previous run, set at a constant 65mph. I think I have the injection tuned pretty well now but the lower gear and faster highway speeds may be taking a toll on me. We used to get around 7 pulling the trailer @ 65 or a little less. at 2700 rpm down the road I think i need to go back to the higher gear and get the rpm back down a little.
Is anyone running a simular setup, have any thoughts on the milage? I also have been digging around a little here and see a couple referances to a 6.5 diesel conversion that has been done. If i could find out a little more about who and how it works I would be reateful. I really love this old gmc but im having a hard time accepting that i cant get to 10mpg pulling a pretty small trailer around. i know its alot to ask for, we used to get 9.8 as best ever running 55 mph with taller tires and no trailer. the speed increase alone probably costs me 1-2 mpg then add the trailer gets me down to 7 but 5? Im a little frustrated i guess and looking for options to keep this a usable viable unit for my family. we are running 100-150 miles each way to karting events and such and I get really frustrated at haveing to fuel at both ends of the trip.
I have a picture of it floating around here somewhere i'll see if i can post it up.
Sean Rupp
Holiday City Ohio
[Updated on: Thu, 08 April 2010 10:52] Report message to a moderator |
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| Re: New here but not to the GMC [message #79738 is a reply to message #79729] |
Thu, 08 April 2010 11:04  |
Rick Denney
Messages: 430
Registered: January 2004
Karma: 0
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Senior Member |
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| Hot wheels wrote on Thu, 08 April 2010 11:15 |
Good Morning,
I have been working latley on the old 77 palm beach that has been in our fammily since about 1978. ...
.....any thoughts on the milage? I also have been digging around a little here and see a couple referances to a 6.5 diesel conversion that has been done. If i could find out a little more about who and how it works I would be reateful. I really love this old gmc but im having a hard time accepting that i cant get to 10mpg pulling a pretty small trailer around. i know its alot to ask for, we used to get 9.8 as best ever running 55 mph with taller tires and no trailer. the speed increase alone probably costs me 1-2 mpg then add the trailer gets me down to 7 but 5? Im a little frustrated i guess and looking for options to keep this a usable viable unit for my family. we are running 100-150 miles each way to karting events and such and I get really frustrated at haveing to fuel at both ends of the trip
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Sean, it is axiomatic that GMC owners with engines and coaches in good working order get 8-10 mpg depending on whether they tow. Doing better than that will require extensive re-engineering with a cost in excess of potential savings. But you should be able to do better than the 5 mpg you've been getting, even pulling your (not that small) trailer.
The usual cause of poor mileage is running too rich. Since you have installed fuel injection, you should install a wide-band oxygen sensor and measure the mixture. In highway cruise, lean operation to as high as 16 or 17:1 will work, but when you need power, it should drop down to 13:1 or so. Remember that with throttle-body injection, we are still subject to the vagaries of differential flow for each cylinder, so the mixture has to be rich enough to use all the air when you need power, and lean to use all the fuel when you don't. The richness when you need power should be enough to overcome the variation in mixture to each cylinder. Otherwise, the lean cylinder might be subject to lean detonation.
There are a number of guys into the whole fuel-injection thing, and there is much there to consider. With all that considered, though, you'll still get 8-10 mpg, depending on whether you tow.
You might also check for any vacuum leaks, which will confuse the closed-loop operation of a fuel injection system. These vacuum leaks could come from a variety of sources, one of which can be cracks in the intake manifold. Given that you found a plugged muffler, the heat from that bank will have been forced thorugh the exhaust crossover in the intake, and that can cause severe overheating and cracking.
Rick "welcome to GMCnet" Denney
'73 Glacier 230 "Jaws"
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| Re: New here but not to the GMC [message #79743 is a reply to message #79729] |
Thu, 08 April 2010 11:28 |
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I had the same problem as you was getting only 5-6 mpg, and engine running hotter. I found that the vacuum advance was not holding any vaccum! I installed a crane adjustable vac advance and my milage instantly went up and engine temps went down. I'm running total timing around 45* olds motors love timing, so advance it as far as you can without detonation. Attach a vac pump or even just suck on the vac hose to the advance. My money is on the advance not holding vac. In which case you will only have centerflugal and static advance leaving as much as 25 degrees on the table. Als chec for stuck advance weights, that can cost you even more timing if those are binding or stuck.
Let us know what you find. On my last trip I averaged 10.5 mpg over 870 miles with an average of 13 at one point! The only thing different I have on my coach is a holley street avenger I dialed in to curise nice and lean, other then that its a stock 455...
73 Canyon Lands, (a.k.a. The Yellow Submarine) West Los Angeles CA
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| Re: [GMCnet] New here but not to the GMC [message #79864 is a reply to message #79729] |
Fri, 09 April 2010 08:52 |
GMCWiperMan
Messages: 1248
Registered: December 2007
Karma: 1
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Senior Member |
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Sean,
Welcome aboard; we look forward to enjoying hearing about your GMC
experiences and learning from it.
I've taken the liberty of enrolling you in GMCMH-EFI, a mailing list
exclusively dedicated to EFI on the GMC, a sort of sub-set of this group.
I'll bet you can solve your problems with help from that group.
Ken H.
On Thu, Apr 8, 2010 at 11:15 AM, Sean Rupp <sean@ruppenterprises.com> wrote:
>
>
> Good Morning,
>
> I have been working latley on the old 77 palm beach that has been in our
> fammily since about 1978. I'll give you the long and short versions of the
> story.
>
> I'm 42, grew up seeing the states in the 77 PB dad owned, we put 250k on
> the old thing over the years and have upgraded just about everything. Sway
> bars, bilstiens, 3 bearing hubs, 4 bag air, alcoas and rear disk brakes, new
> rear frame and the lower 3.50 drive gears has a custom pain job as well. I
> have put 2 rebuilds on the old 455 and the most recent one a year ago with a
> new throttle body fuel injection system. I sold my low mile (69k) '76
> elegonza when Dad decided to move into an 86 prevost and spend more time
> living in one winters in FLA. I have been working on these, swapping motors
> and repairs since i was 16 helping the rv seating company we worked for keep
> a small fleet of them running as seating show vehicles. I have spent way
> more time in and around these things than most can imagine.
>
> Anyway I picked up dads 77 and decided it was time to freshen the engine
> and trans and add a fuel injection system to try and get the most out of it
> both in power and fuel milage. I have spent the last year fighting
> overheating problems that it never had before and very poor milage. I think
> i finally found the source of the heating problem after installing a new
> griffen radiator and flow cooler pump I still had the problem, i found one
> of the chambered mufflers was broken and plugging the outles partially and
> making it build heat the harder i ran it. A good pair of glass pack
> mufflers really brought the engine back around and seems to have gotten rid
> of the heating problem. I usually pull a 14x7 enclosed trailer around with
> me so im used to poor milage but on the last shake down run I got 4.8 mpg.,
> up from 3.5 the previous run, set at a constant 65mph. I think I have the
> injection tuned pretty well now but the lower gear and faster highway speeds
> may be taking a toll on
> me. We used to get around 7 pulling the trailer @ 65 or a little less.
> at 2700 rpm down the road I think i need to go back to the higher gear and
> get the rpm back down a little.
>
> Is anyone running a simular setup, have any thoughts on the milage? I also
> have been digging around a little here and see a couple referances to a 6.5
> diesel conversion that has been done. If i could find out a little more
> about who and how it works I would be reateful. I really love this old gmc
> but im having a hard time accepting that i cant get to 10mpg pulling a
> pretty small trailer around. i know its alot to ask for, we used to get 9.8
> as best ever running 55 mph with taller tires and no trailer. the speed
> increase alone probably costs me 1-2 mpg then add the trailer gets me down
> to 7 but 5? Im a little frustrated i guess and looking for options to keep
> this a usable viable unit for my family. we are running 100-150 miles each
> way to karting events and such and I get really frustrated at haveing to
> fuel at both ends of the trip.
>
> I have a picture of it floating around here somewhere i'll see if i can
> post it up.
>
> Sean Rupp
> Holiday City Ohio
> _______________________________________________
> GMCnet mailing list
> List Information and Subscription Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
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| Re: New here but not to the GMC [message #80461 is a reply to message #79729] |
Tue, 13 April 2010 11:04 |
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just a related thought, has anyone ever tried building a MegaSquirt unit for a GMC? the Turbo ford guys swear by them and they are relatively inexpensive.
wasthinking about making a nice MPFI for the caddy motor I got with one of these...
73 Canyon Lands, (a.k.a. The Yellow Submarine) West Los Angeles CA
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