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Old 09-19-2019, 02:40 PM   #1
FFG
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Increasing MPG - 1994 P30 454

Getting around 6 mpg in a 25 foot Itasca Passage (1995) and want to improve this as others in the Uk are getting double this. I'd be happy with 10 so what re the most obvious things tho change/upgrade. I'm looking at Tri-y from Doug Thorley to increase torque but to help with engine and cab temps and I have a manifold gasket leak so its an obvious upgrade.
Look pretty stock as it is and I've changed a fair amount of consumable but I need to look at other options.
Its a 7.4 Chevy big block, HEI. Not really had any experience so any help much appreciated. Seems to run well but ticks under load (which I suspect is the gasket) but is quiet when cruising.
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Old 09-19-2019, 02:59 PM   #2
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I would think it should do better without doing anything except possibly a tune up and timing.

We just sold our 454 with TBI powered 26' Flair which averaged 7.5 mpg on several OR to WV and back trips at 70 MPH. Ours had OD and ran at about 2500 RPM at 70 MPH. Gearing can make a difference.

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Old 09-19-2019, 03:04 PM   #3
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6 mpg sounds about right for that vintage in a MH.
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Old 09-19-2019, 03:12 PM   #4
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There should be a cold air snorkel bringing air to the air cleaner. If the engine is breathing hot engine compartment air, your power and mileage will decrease. I have the same motor in my 95 Suburban and usually get just under 10mpg with normal around town and highway driving. I have about 275,000 miles on it and it's always been about there. Anyone getting the same 10 mpg or more in a 25' motorhome is a bit unbelievable to me. I'd have to see it to believe it.
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Old 09-19-2019, 04:07 PM   #5
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Quote:
Originally Posted by FFG View Post
Getting around 6 mpg in a 25 foot Itasca Passage (1995) and want to improve this as others in the Uk are getting double this. I'd be happy with 10 so what re the most obvious things tho change/upgrade. I'm looking at Tri-y from Doug Thorley to increase torque but to help with engine and cab temps and I have a manifold gasket leak so its an obvious upgrade.
Look pretty stock as it is and I've changed a fair amount of consumable but I need to look at other options.
Its a 7.4 Chevy big block, HEI. Not really had any experience so any help much appreciated. Seems to run well but ticks under load (which I suspect is the gasket) but is quiet when cruising.
FFG
Generally I'm getting 8 mpg on the back roads with some stop and go in the towns and villages. On the parkways and expressways closer to 9 mpg at 60-63 mph. On the Hotter days with air conditioner on 'Max" and accompanying alternator drag from the fans cuts that mileage by 1 mpg.

How is your bus running? is it smooth? Black smoke from exhaust? ECM temp sensor is problematic on your engine and tend to make the engine run rich as they fail. I had to change mine. part#15326386 <$25 USD.
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Old 09-19-2019, 10:23 PM   #6
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I should have also said that the 7.5 average I've gotten over about 35k miles also includes having a tracker in tow, at 70 MPH most of the time.

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Old 09-20-2019, 12:50 AM   #7
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I don't drive it hard and its not towing anything, so weight is just over 5,000kg. Speed around 45-50 so not driven hard (it felt like a jelly hence keeping it down but changed shocks, airbags etc since). It runs OK as far as I can see and doesn't shake so pretty smooth. It does smoke on start up but white smoke so may just be burning moisture (always damp here) as its OK when hot. Just need to find someone/somewhere locally who has any idea about these engines as they aren't common in the UK.
People are saying they are getting 14-15 but I don't buy it. I'm sure I can do better but nearer 10 I'd be happy, and when we retire and do some european trips I'll go lpg.
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Old 09-20-2019, 01:42 AM   #8
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Seems like a common part number:
https://www.ebay.co.uk/itm/New-Genui...UAAOSwmSdaaed1
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Old 09-20-2019, 04:16 AM   #9
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People are saying they are getting 14-15 but I don't buy it. I'm sure I can do better but nearer 10 I'd be happy, and when we retire and do some european trips I'll go lpg.
FFG
They might be getting that on some of the diesels. But not in gasoline motorhomes. My 4.3 liter 2500kg van would barely get more than 15mpg
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Old 09-20-2019, 08:38 AM   #10
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I had a 1988 Itasca 454 towing a Tracker that was averaging about the same as yours. Had the carb rebuilt, tuned it up, new tires, etc. Changed very little.
I installed a vacuum gauge in the dash hooked to the intake manifold and if I watched it and tried very hard to maintain the highest vacuum the mileage could go as high as 9 mpg while towing.
The gauge helped me also identify at what speed the mileage was best. Two mph could make a difference in the fuel mileage.
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Old 09-20-2019, 08:45 AM   #11
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I think that consistently getting 10 miles per U.S. gallon with that engine isn't likely to happen. 6 to 8 seems to be the range I hear the most for those, and what I used to get on mine. WHERE it's being driven makes a big difference too. If much of your driving is in town the the mileage will go down, if mostly cruising on the highway then it should be towards the high end of the range.
If it hasn't had a tune-up recently, do that. But if it's going to take 10mpg to make you happy, I'm afraid you're destined to be unhappy...
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Old 09-20-2019, 10:57 AM   #12
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I didn't say if I don't get 10mpg or I'll give up, just wanted nearer 10. The UK gallon is 25% more than a US gallon so 6-8 US should be 7.5-10 UK. Its mainly been motorway and taking it easy as its a new driving experience being so long, heavy, wide, LHD, and the wife screaming constantly that I'm on or over the line. Combined with piss poor handling, especially when lorries (big rigs) overtake, its been pretty scary. Lots of things been changed now and spending some money and giving TLC so looking at options to make it better. If 6mpg is it and its running sweet with no issue so be it, but I don't believe that is the case and close to 10 is achievable and anything spent will be worthwhile in future lower fuel spend. I may then look at LPG but want it running sweet first.
I've just changed the leaf spring U bolts as one had broken and it failed its annual test and the rear brakes were imbalance so its going in for a re-test and a full check over underneath. Then got some new tyres to fit for the new season and hopefully reap the rewards of all my recent efforts.
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Old 09-21-2019, 06:53 AM   #13
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I didn't say if I don't get 10mpg or I'll give up, just wanted nearer 10. The UK gallon is 25% more than a US gallon so 6-8 US should be 7.5-10 UK. Its mainly been motorway and taking it easy as its a new driving experience being so long, heavy, wide, LHD, and the wife screaming constantly that I'm on or over the line. Combined with piss poor handling, especially when lorries (big rigs) overtake, its been pretty scary. Lots of things been changed now and spending some money and giving TLC so looking at options to make it better. If 6mpg is it and its running sweet with no issue so be it, but I don't believe that is the case and close to 10 is achievable and anything spent will be worthwhile in future lower fuel spend. I may then look at LPG but want it running sweet first.
I've just changed the leaf spring U bolts as one had broken and it failed its annual test and the rear brakes were imbalance so its going in for a re-test and a full check over underneath. Then got some new tyres to fit for the new season and hopefully reap the rewards of all my recent efforts.
FFG
Get the front end re-aligned to Super Steer Specs and it should handle much better. Check your anti-sway bar bushings to make sure they are still pliable not cracked or missing. Koni FSD front shocks (PN # 8805 1006) made a significant improvement in the ride and handling on mine.
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Old 09-21-2019, 08:54 AM   #14
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Rick, I,ve changed the front shock (Bilstein 24-xxx898). The sway bar bushes have been changed to poly before I bought it. airbags new (50 psi) and just changed rear shocks (Bilstein 24-015875 - know that one off the top of my head) so will ask the garage to look at the alignment while it is in. Many thanks.
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