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Old 12-26-2019, 06:26 PM   #1
Mini-Skoolie
 
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Engine: 350 V8
Not another fuel economy thread (Thomas Chevy P30 / 350)

Just got my bus smogged and took it on a little road test in the SF area. Mostly hills, the hwy 29 pass between Santa Rosa and Lake County and back.


I seem to be averaging about 3.7Mpg. I've got a 1100 mile road trip coming up and need to get this thing more fuel efficient.


My bus is a 1986 Chevy P30 Thomas.


Just got it smogged - has thermac/heat stove, egr, efe, smog pumps in workin order. new Air box vacuum sensor, EFE TVS, and new EFE valve


TH400 3 speed auto trans


JB9 braking system - hydroboost, dual rear axle, front and rear disc


80k on a Chevy 350 with a rochestor quadrajet, just rebuilt with electronic choke


Air bin label says 89 octane which I have been using...no pinging though occasionally the fuel smells unburned. I have pinhole leaks in the exhaust headers.


No oil or trans leaks and a clean bill of health. The master cylinder does leak a little which sounds like a common problem on P30s. It's a new master cylinder but it gives me terrible belt squeak which I am hoping is also contributing to the poor mileage


I have not looked at the evap system closely. I think it needs a new breather filter or canister but all the hoses seem fine.


The bus was OHV in oregon for about 20 years putting about 100 miles a year working on a tree farm. It was missing its external smog pump filter in a can in the front of the bus for a long time so I'm not sure of the condition of the pumps or deceleration valves. The pumps had some grind to them when I had the belt off but make no sound otherwise.


New hydrobooster, water pump, and power steering pump.


New throttle springs, but I'm not sure if I have them in the right place. I feel like I have to put the pedal all the way down to get to 55 and when I hit hills I'm struggling to stay at 45 with a virtually empty vehicle. I'm wondering if this is a limitation of the transmission since it sounds like most other folks have an overdrive gear on their P30s.



Vacuum advance is working properly and timing appears to be fine, but I haven't measured it.



Some recommendations I've found reading here have been to install a vacuum guage as a driver assistance tool but I'm curious what other pointers folks can give here. I'm operating on a tight budget.

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Old 12-26-2019, 09:12 PM   #2
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Yeah, after reading all that you had done to the bus I was thinking that it just might not like to be run at those speeds. I hope you figure something out, those mpg's suck!
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Old 12-27-2019, 02:20 AM   #3
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The first thing I was looking for when reading about your problem was "Smell of unburned fuel". Something is making your engine run rich, which could be a reason it's not generating as much power as you'd like it to. Something's not right with the carb, maybe you have a vacuum leak? Check all the vacuum lines. Is the choke staying closed? Oh, and how's the exhaust? Catalytic converter restricted?


I'm rather surprised it passed a smog check if it's smelling of unburned fuel and getting such poor fuel economy.
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Old 12-27-2019, 06:50 AM   #4
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You can buy an air/fuel meter and oxygen sensor to go with it. Now that you have passed smog take the belt off the smog pump so you can get a correct air/ fuel mixture reading. With this feedback you can change the jets in the carb to correct your fuel mixture if running too rich or lean. Unburnt fuel smell and poor mileage all point to a very rich condition, and or not enough spark advance.
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Old 12-27-2019, 09:09 AM   #5
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How old is the rochestor quadrajet ?

I would start with buying a new carb, quadrajets are know to drain gas tanks when they get old. Also check all vacuum lines.
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Old 12-27-2019, 04:56 PM   #6
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I would be looking at a TBI setup for improve better fuel mileage. Holly makes one that will bolt in place of a Holly four barrel.
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Old 12-30-2019, 12:56 PM   #7
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Since no-one mentioned it -- look at a spark plug color chart -- either online or better yet at a parts store.

Then pull your own spark plugs -- what color are they?
Have you verified proper timing?
What is the compression of this motor?
What is your tire pressure?

You have a lot of truck for a small engine. (that means the small engine gets worn out faster than the same eng used in a smaller car)
You have a heavy duty auto with the th400 but the price for that is high parasitic drag -- it will never deliver good mpg like a 4L60 would... but the price for a tranny swap is high...
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Old 12-30-2019, 01:32 PM   #8
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also the TH400 is a 3 speed non OD, non Lockup transmission.. its the equivalent of an AT545 in a big bus... granted the 400 is a really good beefy transmission, its not the most efficient out there..
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Old 12-30-2019, 01:40 PM   #9
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Carbureted Smog engines work by having a rich mixture to lower combustion temps then burning off the excess gas in the catalytic converter with air added by the smog pump, so an add on TBI setup with an oxygen sensor will get you better economy
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Old 12-30-2019, 05:25 PM   #10
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Tbi would be a great improvement, but will not pass smog most likely. Problem is it will be less emissions, but will not have the factory correct emissions equipment, like the smog pump. The smog pump will cause an incorrect o2 sensor reading and cause the TBI to run rich to compensate for the extra air in the exhaust.

I know in Virginia haveing the factory correct is more important then less emissions.
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Old 12-30-2019, 06:17 PM   #11
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So most of the emissions stuff that was just put on comes back off to do a good running fuel injection setup. I would ask the emissions test stations what is legal to do other then original equipment.
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Old 12-31-2019, 01:48 AM   #12
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Would a 700R4 be a worthwhile swap?

I haven't played with gassers in many years. IIRC: the 700R4 is a four speed overdrive with a lock up torque converter..
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Old 12-31-2019, 07:09 AM   #13
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700R4 is a decent trans if you get one thats been built up a bit... I broke more than one of them till I did a few things.


1. upgraded forward Sprag
2. B&M TransPAK kit
3. Corvette Servo
4. Big cooler
5. Heavy duty lockup converter


match the converter to your engine.. high stall are good for street.. low stall or good for engines that make torque in the lower end of their RPM bands..



then they were bulletproof even in my heavy Built-350 Box-caprice police car..


the TransPak has 2 "modes" you can install.. 1 being a heavy tow and the other a street performance.. I had the performance mode.. but for a bus you'd use the heavy tow. and the shifts arent as hard but still solid enough to not burn up the band or clutches.



the 700R4 uses an electric lockup.. you need to hook this up or you will burn up the transmission in short order..



I used these on my hotrods.. they work really well.. they are speed controlled lockup controllers that are adjustable ..


this one if your spoeedometer on your vehicle is already electronic.


https://www.jegs.com/i/B-M/130/70248/10002/-1


this one includes the speed transducer for cable speedos


https://www.jegs.com/i/B-M/130/70244/10002/-1


-Christopher
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Old 12-31-2019, 10:32 AM   #14
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Quote:
Originally Posted by PNW_Steve View Post
Would a 700R4 be a worthwhile swap?

I haven't played with gassers in many years. IIRC: the 700R4 is a four speed overdrive with a lock up torque converter..
The 700R4 is the earlier designation of the 4L60 -- same banana, just a GM nomenclature change. The 4L60E is the later electronic evolution of the same xmsn.

Everything Christopher said -- only thing to add is make sure you get a case with a K embossed on it. This designates the "truck" version -- it is a heavier duty casting and is important in this application.
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Old 12-31-2019, 02:23 PM   #15
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I just rebuilt a Rochester for a buddy. 1985 vintage, actually on a Dodge. Anyway, they are a real good carburetor when new. However, with grandpa driving it for the last thirty years, the secondary throttle plates probably never opened once. So, you end up with the primary shaft wearing out, and the secondaries stuck shut. Or worse, secondaries stuck partially open.

Trying to get the idle mixture screws correct with a vacuum leak from a worn shaft and the secondaries stuck partially open is hard. Getting any power without the secondaries opening is also hard. You could push the secondaries closed with a screwdriver and give that a try.

Only rebuild the carburetor if you are going to get the primary throttle shaft drilled and bushed. Otherwise you're just wasting your time.

I have two older big trucks with Holley 600's on them. On of them starts on fire a couple times a year. We call it fire starter. LOL. Anyway, when I get some mad money I am going to put an aftermarket EFI and HEI on it to see how much better life can be. It has a 427 tall block in it and can use any help it can get.

Also, you can buy a rebuilt Rochester for 3-400 with yours as a core. That would be the fastest way to see if you can to better in the millage department.

BaconFarms
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Old 01-04-2020, 04:09 PM   #16
Mini-Skoolie
 
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That year looks like it takes an A/C Delco R44T spark plug. Do you have a new set of these plugs in there? I would also replace the coil too. Look at the distributor cap and see if there are any carbon trails indicating a cracked distributor.

You mentioned fuel smell. I always look at spark plugs when I smell fuel.

The smog pump system needs to be checked as does the catalytic if it has one. Anything wrong there and you are going to have bad fuel economy and no power.

Make sure that you have an OEM style kickdown cable for the transmission and that it is adjusted properly.

Rochester carburetors are good if they are rebuilt properly. Many people adjust the floats wrong which results in raw gas going down the intake.

I had to learn to do all my own repairs on my 1972 G30 with a 350. No one else had a clue or gave a damn about setting it up correctly.
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Old 01-04-2020, 08:47 PM   #17
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Those Rochester carbs are bad about the plugs underneath the baseplate leaking fuel. We used to in weld em up, but I’m not sure I’d recommend that but it might be worth checking out.
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Old 01-05-2020, 12:16 AM   #18
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Quote:
Originally Posted by yuletree View Post
Just got my bus smogged and took it on a little road test in the SF area. Mostly hills, the hwy 29 pass between Santa Rosa and Lake County and back.


I seem to be averaging about 3.7Mpg. I've got a 1100 mile road trip coming up and need to get this thing more fuel efficient.


My bus is a 1986 Chevy P30 Thomas.


Just got it smogged - has thermac/heat stove, egr, efe, smog pumps in workin order. new Air box vacuum sensor, EFE TVS, and new EFE valve


TH400 3 speed auto trans


JB9 braking system - hydroboost, dual rear axle, front and rear disc


80k on a Chevy 350 with a rochestor quadrajet, just rebuilt with electronic choke


Air bin label says 89 octane which I have been using...no pinging though occasionally the fuel smells unburned. I have pinhole leaks in the exhaust headers.


No oil or trans leaks and a clean bill of health. The master cylinder does leak a little which sounds like a common problem on P30s. It's a new master cylinder but it gives me terrible belt squeak which I am hoping is also contributing to the poor mileage


I have not looked at the evap system closely. I think it needs a new breather filter or canister but all the hoses seem fine.


The bus was OHV in oregon for about 20 years putting about 100 miles a year working on a tree farm. It was missing its external smog pump filter in a can in the front of the bus for a long time so I'm not sure of the condition of the pumps or deceleration valves. The pumps had some grind to them when I had the belt off but make no sound otherwise.


New hydrobooster, water pump, and power steering pump.


New throttle springs, but I'm not sure if I have them in the right place. I feel like I have to put the pedal all the way down to get to 55 and when I hit hills I'm struggling to stay at 45 with a virtually empty vehicle. I'm wondering if this is a limitation of the transmission since it sounds like most other folks have an overdrive gear on their P30s.



Vacuum advance is working properly and timing appears to be fine, but I haven't measured it.



Some recommendations I've found reading here have been to install a vacuum guage as a driver assistance tool but I'm curious what other pointers folks can give here. I'm operating on a tight budget.

unburned fuel? .....fuel leak? fuel smell confirmed at the tailpipe? assuming good plugs, coil, wires, and timing - are the exhaust leaks before or after the o2 sensor? does it even have an o2 sensor? I think 86 was near the beginning of computers in cars, but maybe not on trucks... does it have a computer? odbI port? am I asking stupid questions?

Is the 80k on the speedo correct? not 180k? does the analog spedo go over 100k (have 6 digits plus a decimal point)?

What is the rear gear ratio? tire size? Do you have good acceleration on take off? what is rpm @ 55mph? Are your tires fully aired up? brake shoe draging - seriously?

I have an 89 P30 with a Fed Ex type aluminum step van body, cargo area is18' long behind the drivers seat and 7' ceiling, and 454 with auto trans, 19.5 tire, 11000 pounds empty. Gets 5-6 mpg. Rear end is around 5.10 (counting wheel and driveshaft rotations.) Was my daily driver for 2 years. Stupid, I know.

Talked to several other owners and the only one that had better mileage had a 4bt in it, claimed 15 mpg with a 3.07 rear gear, smaller truck, but, hey, that really sounded good...but he was trying too hard to sell it to me, so I did not believe him.

I am content to drive it at 50-55 mph (it will do at least 60 but I don't like the way that big box handles in cross winds) and I don't care if it slows down going up hill as long as long as it gets me there. Seems to be a lot of surprise cross winds in the mountains around here.

I'm thinking that you need more motor and/or taller gears (bus made for city driving, lots of stop and go, low gears)... or more patience. I'm not much of a mechanic, (your more than likely better off to ignore me) but I have put a lot of miles on old tired crap (and a lot of time on the side of the road cussing) and found out that my maintenance cost and fuel cost go down when I go a little slower. Also, low tires and speed combined really eat up the mpg.

Speed costs money. Cheapest way out is just quit thinking that you need more speed and leave a little earlier on your trips, at least until after that motor burns up. Back off the pedal and see if your mileage goes up, that might point you in the right direction. Buy less expensive gas LOL Relax and enjoy the drive.
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Old 01-05-2020, 01:24 PM   #19
Mini-Skoolie
 
Join Date: Oct 2017
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Get a Haynes manual on Rochester carbs. It will tell how to use epoxy to reseal the metering wells.

I did not have time to work on my old van once. I took it to a supposedly reputable mechanic. He used chewing gum to seal the metering wells!
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Old 01-05-2020, 01:37 PM   #20
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Isn't 4mpg about average for one of those gas engine buses from that era?
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