Motor Home Mileage

[Pick]

Ok, when are one of you guys going to bite the bullet and do a re-power with a Duramax/Allison? They are starting to become plentiful on Ebay.

[outofcontrol]

I love the idea of transplanting a Duramax/Allison set-up into the MoHo. I figure at some point we'll have no alternative than to fit diesel motors and use alternative fuels. Biodiesel or whatever. Once petroleum based products are more expensive than gold, the entire US will figure out how to get around the problem. American's love their cars - no getting around that. At some point you'll just go to the GM dealer and get the Tribium drive motor that needs fuel every 10,000 miles, and it's a $4,000 conversion.

Until then, I made up a little Fuel Log sheet if anyone wants it. I'm putting together a 3 ring binder with all my notes, articles, manuals, etc.

-KM

[Chuckles]

Hi All,
I have just joined this thread, in desperation more than anything!

My 1982/3 310 with the 454/4bbl was I thought (stupidly!), running well, but after about 500 miles of reasonably easy driving, no towing, no Aircon, no speeds over 55mph and only ½ a tank of water, I fell through the floor when I calculated about 5.5 to 6.5 per UK Gallon!

She has done about 34K; I say about because I was told that in her early years she was rebuilt. (What is it with rebuilt engines under 100k!!)?

The engine sounds sweet enough except after a good run it will miss and chug after a short idle, say at a stop sign etc. A quick blip on the throttle and it clears after running up to about 1500 revs.

I suspect something’s going on with either a crank oil breather (oil traces in the air filter) or the Auto Choke - if there is one. I say the latter because during the recent cold weather the temp gauge never got over 80 and it never idled less than 1100 - 1300 revs; now warmer weather is here the temp goes to 120+. I was assured by the PO that the higher 80 stat was fitted, and I recently fitted a new radiator cap (14.7/1.1bar).

Any thoughts as to where to start would be welcomed, as I was hoping for around 10MPG (UK Gallon) before I convert her to LPG.

Chuck

[swebster]

Chuck,
I would start by establishing a baseline on your engine. Begin with a cold compression check of each cylinder (should be around 150 - 160 psi but more importantly they should be close to each other in all eight cylinders). If one is low then re-check a few times. If it it still low then get a leak down test to figure out if it's rings or valves. Check for a gasoline smell in the oil, also look for coolant drops in the oil, or oil drops in the coolant.

While the plugs are out for the compression tests, check each one for color. Too much soot = rich mixture. Oily and gummed up = oil in the cylinder.

Once you know the internals are good, you can then start looking into each system; fuel, air, spark system individually knowing that your internals are good to go.

Your mention of the choke makes me want to zero in on that right away. If you choke is not fully opening then you're engine will run very rich and burn a lot of fuel. Once it’s warmed up, just pull the air cleaner cover and look. If it’s open and your running 1300 rpms then it’s time to take a good look at the idle speed adjustments, clean the linkages, clean the carb, etc.

You also mentioned missing at idle. Chances are good if it’s missing at idle, it’s also missing at speed, but you can’t feel it since the mass of the engine is spinning through the missed fire. Time to hook up a timing light and check each plug for consistent firing. If one is intermittent, or dead, start isolating this and chasing its cause. It could be fouled plugs (too rich, burning oil, etc.) or you could have a bad plug or boot. Many of these engines burn up the spark plug wires and can short or fail from time to time.

Double check your timing and make sure it’s set to specifications.

When you say 80 on temps do you mean 180? Most of these engine run between 180 and 220. 195 is the specification from Chevy for the P30. If you’re indicating 80 then I think your gauge is not working.

Oil in the air filter is a concern. Replace the breather and double check the oil level to make sure it’s not over filled and you're seeing excess blow-by.

Once you’re sure of the baseline, good compression, choke is working properly, no blow-by, the plugs look correct (light brown with no soot) and all eight are firing then you should be OK to look into the carb, the distributor, exhaust etc.

I chased many of these same issues on my own engine a while back. By focusing on each system and repairing or replacing components one at time (carb rebuild, ignition replacement, high temp racing wires, replaced stock manifolds with header and free flow exhaust) we now get 8.3 when towing a car and 9.4 when empty in our 345.

Keep us posted on your work. Many of us have chase out each of these gremlins and can lend advice.

[GlenCoombe]

Yup, what he said. Steven has the right ideas. I changed my wires, exhaust, headers, THEN I had the carb rebuilt by a professional. The difference was dramatic.
I'm getting 8's and 9's from a US gal. not an imperial gallon.
Oh, one thing that also helped was to give it as much air as possible. The old air cleaner just does not provide enough air to the carb in my opinion. Check out my photos for the custom DIY air intake you can make in no time.
BTW what RPM range are you operating in at 55mph?

[Chuckles]

What a marvellous response. Thank you very much. I will do exactly the order you suggest and yes I will keep you all posted. I hope to do some thing this weekend, well at least make a start.

We are off again the following weekend to a local C&W weekend. (Its one of our holiday weekends here, then). Therefore, it would be nice – and help with the sleeping – if I can ‘bottom-out’ a couple of causes.

The Airstream certainly attracts the gazes and admirations at these shows so a sweet engine will be even better!

Yes I did mean 180 degrees (F). I keep going back to Centigrade!!!

At 55mph she is showing about 2250 rpm. I think the Speedo is a bit slow judging by the trucks here. Nearly all trucks run at 56/57mph (90k/hr) with governed engines, and we kept catching them up! It is also my favourite speed, I have tucked enough ‘miles under my belt’ over the years (60-70k for the past 43 odd years) to not be a boy racer any more and take my time. At least running with the HGV's (truckers) it saves a lot of overtaking, and hard breaking when an idiot argues with another idiot!

Thanks again Chaps
Chuck

[GlenCoombe]

Chuck, that sounds like you've got a very "short" rear end... sorry if that sounds a little too personal. Short is good actually as "tall" rear ends drive the tach up.
If you're only turning that speed at 55mph it's got to be choke or carb that needs looking at ... and likely both.
You should be able to get it sorted and find yourself close to double digits after some time and money is invested in getting it running as it should. Lots of luck and we'll be looking for new and improved numbers as you work out the bugs.

[Chuckles]

Glen, I must admit that the carb area was my first instinct, particularly as it "chugs" at times. It just sounded like a fuel problem, but I will go through the list given by Steven, as there were some very valid checks.

The PO had not necessarily neglected it for about 18months, but perhaps didn’t have the enthusiasm/knowledge to keep on top of things. It has stood most of that time.

Perhaps as a short term "quick-fix" I may have a look at the carb first, closely followed by new ignition leads and plugs. I did find two leads, and some odd plugs in the slide out tray, so perhaps that has reared its head in the past.

Regarding the ‘short or very short rear end’, that’s a cute term for what I assume is the final drive or back axle differential. I am going to hunt through the books from GM describing the “unfinished vehicle” and see if there is any reference to ratios. (Very short rear–end…. indeed…… perhaps the diets working?……)

Thank you Steven and Glen for such quick and interesting replies.

Chuck

[Chuckles]

Just found some rust!!

When the engine cover is raised, you are looking at the rear of the engine..... So looking at the carburettor on the right hand side, (Passenger side) there is a rubber pipe coming from a plate in the inlet manifold to what looks like a pressure/hydraulic damper fixed to the side of the carb.

This rubber pipe has fractured due to heat but the metal part of the pipe connected to the plate fixed to the inlet manifold is rusted through and broke off. The hole left is blocked with rust.

The question is what is the plate for/called and can it be replaced. Also is it essential. I need to use the beast this weekend, and there will be no way to get this part before then!

Chuck

[swebster]

Are you talking about this one?

[Chuckles]

The shape of the rubber is similar but the carb looks different, well apart from shiny and clean that is. The hose part goes to a plate in the top of the mainifold. I did take a picture but it was on the the brothers phone....and I cant get it off!

I am going down in the morning again so I will look a little closer.
Thanks
Chuck

[Druupy]

Looks like an E.G.R. (Exhaust Gas Recirculator) setup.
Was designed to allow burned fuel/air (therfore inert) into the manifold.
It was to lower the combustion temperature to GREEN-up the exhaust emmisions. It does work if the components are all new, like when the original test was done for the EPA. But if some of the parts get old, bummer.
The effect of worn out EGR stuff, and tuning to get the motor to run at all,
is 3 cylinders running very rich, 3 cylinders running very lean, but you still get 2-out-of-the-8 that are running just fine.
More money, or a very knowlegable mechanic to plug and bypass the offending bits.

[swebster]

Chuck,
That is a newer carb than yours (mine too) but it's still a Rochester QuadraJet. My first thought after reading your post was EGR valve, but when you mentioned that the diaphram was on the carb on the passenger side I thought about the pull offs on that side of the carb.

The EGR valve is mounted on the manifold, just behind the upper radiator hose and connects to the carb with a short hose (kind of opposite from what you described).

If it is the EGR than can be replaced (so can the carb or it's components). One quick check to make sure you still have the original QuadraJet carb on there and not an aftermarket replacement - look at the casting of the carb body - it should say Rochester QuadraJet on gthe driver's side.

Obviously a picture would be worth a thousand posts. If you can post one I think we'll have more to go on.

Also, a vacuum leak like that bad hose could be playing havock with your carb performance/fuel consumption as well. You should check your other vacuum lines while you're in there....just replace them one at a time so you don't mess up the routing.

[overlander63]

Originally Posted by Chuckles

At 55mph she is showing about 2250 rpm. I think the Speedo is a bit slow judging by the trucks here. Nearly all trucks run at 56/57mph (90k/hr) with governed engines, and we kept catching them up!

Chuck, go through a measured mile with your coach. You may find that your odometer is a little slow as well. Our Chevrolet van has a speedo/odo that are off by 10%. For every mile traveled by the odometer, we really are going 1.1 miles. If this is the case, you can pick up another 10% of gas mileage, without doing anything...

[Chuckles]

[ATTACH]

[/ATTACH]So... Here goes with some photos...

By the way thanks for the advice to get the extra 10%.... interesting to say the least...!

Back to the photos.
The arrows indicate the two pipes. The little curved arrow in the top picture shows the location of the plate and where the rubber hose attaches to it (Metal pipe rusted out and missing). One is black rubber from what I am now told is the choke bi-metal spring. The other is the longer metal pipe with a rubber sock to the top of the carb.

The air is sucked from the top of the carb (metal pipe) down to the plate in the intake manifold. This pipe is rusted through with many pinholes. The air then runs through what I assume is some sort of coil/heat exchanger inside the manifold, picks up the heat, that then passes up the rubber tube to the choke bi-metal coil.

Rightly or wrongly I was further advised that this circuit can be "taken out" and the choke coil, after taking off the black cap that is riveted to the body, can be wound back to be permanently open. Starting would then be a case of a few pumps on the gas pedal (getting good not calling it petrol or throttle!!) and wait for a while before reliably starting off.

How does that all sound to you chaps?

There has been a second train of thought .... To buy an adaptor and fit the Holly 650cfm carb.
Chuck

[swebster]

Chuck,
I found this resource for QuadraJet diagrams online and thought it might be helpful. (Use the next, back, home links to navigate around this information http://www.tocmp.com/manuals/Carbs/R...age050_jpg.htm) Looks like you have a hot air choke.

Hope you don't mind but I doctored up your original images a bit so they are easier to view.

Seems like it's plumbing all related to the choke. I doubt that any of this is opening a vacuum leak in the manifold but I've never actually seen a hot air chock on the 454 before. I suspect it's just a plenum not open to the intake plenum which allows hot air from the intake to open the choke. You may consider converting to an electric choke (there are a number of kits available).

This may also explain your rich running condition, if the hot air isn't getting to the choke then it's never fully opening.

Other than your choke not fully opening I don't think it will cause you any harm. Anyone else (who's actually seen on of these) have additional opinions?

[Chuckles]

Thank you Steven, good job on the photos.

The link was great, A little more info gladly gleened for the beast.

I am going to try and remove the plate, after copious amounts of WD40 or Plus-Gas to free off the bolts, don't want them snapping off!

As it seems unlikely to replace this part I will attempt either a repair or a modified adaptation. I will let you all know if I am successfull

Chuck

[GlenCoombe]

Chuck, you might also check your speedo/odo with a hand held GPS. Hope you find the solution for your carb woes. If you can find a carb technician who knows his stuff the difference, as in my case, is worth more (much more) than the cost of an overhaul of the carb.

[Chuckles]

Glen, I am still in a bit of a quandry about the carb, does it stay or does it go (Fuel Injection or Holly replacement).

but I take your point and I think once I get this repair sorted, and this next weekend out of the way, then I will have a week or two 'breather' to have the carb properly sorted.
Chuck

[GlenCoombe]

Chuck, many have pointed out in the past, on this board and on RV.net that the original Rochester Quadrajet, in good original shape, is the best carb for the job. Fuel injection is great...at a price. The Holley family is a great performance add on but the cost vs. a simple rebuild of your existing carb may not be the most benefit for the $$...or Pound as is your case.
In my case the rebuild on my carb was the first. It had never been touched since new and had over 40,000 on it when I had it rebuilt.
I vote for a rebuild and a new electric choke assembly to replace the hot air choke. That should serve you well for the next five or so years.