Need help diagnosing significant MPG drop

Christo

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Oct 4, 2019
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Weymouth, MA
Okay, I'm stumped; maybe the community can help with this one. Over the past year my MPG has dropped from the high 7's (5 measurements, and yeah it's not great) to the high 5's (3 measurements); total miles driven is about 2000. I use 87 octane. I heard that the oil refineries used "winter gas" all summer due to excess inventory from Covid and that a lot of people experienced reduced mileage, but this seems like an excessive drop. Also, I do have some mild "whup-whup-whup" noise sometimes at the left front and suspect a worn bearing or CV -- this will be attended to over the winter but I think it would make a lot more noise if it were affecting MPG as much as I'm seeing. The engine runs great under all conditions: Cold start, warm start, accelerating/decelerating, idling, steady state operation. I did make some changes and inspections during this period as follows:

- Adjusted timing to 10 BTDC from 8. Operation of vacuum and centrifugal advance confirmed good. Using Paterson distributor and wires. No pinging.
- Vacuum lines seem good; vacuum gauge shows around 20 inches at idle and during steady-state operation on level roads.
- Choke opens fully when engine warms up.
- Quadrajet secondaries weren't able to open. Problem was determined to be the wrong accelerator pump piston which was preventing the throttle from opening more than halfway. After fixing this, the secondaries can open but they only do so when I really put my foot in it so they're rarely activated; in general I take it easy on the accelerator. I actually made a video today to confirm when and how much they open. During this service I also re-plugged the holes on the bottom of the carb -- the old epoxy was seeping and the fuel drained out of the bowl if the coach wasn't started for more than a week.
- Quadrajet primaries were dribbling at idle. Float level confirmed correct, needle/seat good. Problem was determined to be idle speed a bit too high; the dribbling went away as soon as I got the idle down to ~650. Idle mixture screws at 3.25 turns.
- Tire PSI and wheel temperatures are monitored constantly and are normal/unchanged (no dragging brakes).
- Oil pressure is solid, about 60 PSI when driving. Using 15W40 oil.
- Thermostat changed from 180 degree to 195. No overheating.
- I have a fresh air duct routed from behind the grill to the air cleaner snorkel. It's always active; thermostatic baffle in air cleaner is inoperative.
- No fuel leaks.
- Minimal generator usage.
- Transmission shifting normally.
- Spark plugs cleaned and gapped to .042.
- PCV valve rattles normally.

Any ideas? If I had a leaking fuel pump diaphragm I presume I'd have some symptoms like thin oil, smoke out the tailpipe, or hesitation/stalling. I haven't done a compression test yet.
 
Last edited:

Matt Colie

Active member
Oct 25, 2019
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South East Michigan near DTW
Christo,

You have listed about everything I would have checked.
If it was a normal performance loss, you would have noted that at the time.
Assuming that includes coming back to Weymouth, so you weren't going uphill a lot of the way, that eliminates climbing. We have seen this when mountain climbing at different times.
What the refiners add to make winter gas is butane and that is expensive so that is not your issue and actually increases the fuel heat.
That leaves about a hundred gallons of fuel that went somewhere.
If a ruptured fuel pump put that in the crankcase, I think you would know.
I didn't take that big a hit with a broken piston.

I did see a big loss with a cracked fuel line down by the selector valve. Just a drip and I never tried to quantify the fuel loss. I found it only when parked on concrete overnight.

You engine has HEI, and the gap is a little wide. While the gap should not be an issue at 0.042, I have seen modules start to fail at that value. It doesn't much to make HEI damage a module or coil to less than a complete failure. I don't know how to detect that failure until it is total.

Please let us know when you find it.

Matt
 
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Christo

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Oct 4, 2019
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Weymouth, MA
Undo all the changes one at a time until you find the one that made the difference!

Or maybe its a restricted exhaust?
Not likely to undo much, but restricted exhaust is certainly something to check. I suspect I'd see performance problems if that were the case, though, and as I said it's running better than ever.
 

JSanford

Active member
Oct 4, 2019
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Sacramento, CA
If your MPG dropped after your "fixed" the accelerator pump on the carb that could be it. If your secondaries never opened before, and now they are, that would make a difference.

Pretty common for old mufflers to rust and clog internally, back pressure is enough to effect MPG.

I have owned my GMC for a year and put 4592 miles on it with a average of 8.2 MPG.
 

tmsnyder

Active member
Oct 7, 2019
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Buffalo NY
Over the course of 2000 miles you noticed it dropped from high 7s to 5s? I'm not sure that's enough miles to see more than just the average mileage for that 2000 miles. Maybe the first fillups you weren't getting quite as much in, so it looked like you were getting more mpg. Then later maybe you got more fuel in the tanks, making it look like your mpg was lower?

Even if you're just noticing the noise b/c of small, inconsistent fill-ups, it does seem like your mpg over all is low if you're getting less than 7mpg.

Maybe you have a leak, check near the fuel filter, sometimes that housing or the flared fitting will have a leak and it's a little tough to see it. It's hot there usually too, so it evaporates quickly.

Also is your tire with the bad bearing getting warm? Maybe it's pushing that wheel down the road sideways and scuffing.
 

Christo

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Oct 4, 2019
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Weymouth, MA
Over the course of 2000 miles you noticed it dropped from high 7s to 5s? I'm not sure that's enough miles to see more than just the average mileage for that 2000 miles. Maybe the first fillups you weren't getting quite as much in, so it looked like you were getting more mpg. Then later maybe you got more fuel in the tanks, making it look like your mpg was lower?

Even if you're just noticing the noise b/c of small, inconsistent fill-ups, it does seem like your mpg over all is low if you're getting less than 7mpg.

Maybe you have a leak, check near the fuel filter, sometimes that housing or the flared fitting will have a leak and it's a little tough to see it. It's hot there usually too, so it evaporates quickly.

Also is your tire with the bad bearing getting warm? Maybe it's pushing that wheel down the road sideways and scuffing.
I always fill to the top and I typically don't have any problems with getting fuel into the tanks as some do. Also have double checked for leaks at the carb fuel filter. Also, temperatures normal at the left front wheel that has the bearing noise.
 
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tmsnyder

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Oct 7, 2019
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Buffalo NY
I always fill to the top and I typically don't have any problems with getting fuel into the tanks as some do. Also have double checked for leaks at the carb fuel filter. Also, temperatures normal at the left front wheel that has the bearing noise.

How do your plugs look?

Most of your driving is just using the primaries, and that mix is determined by the power piston spring, the rods and the jets.

Maybe someone enriched your carb mix with a stronger than OEM power spring (or whatever they call that spring under the primary metering rods), or a pair of smaller primary rods, or a bigger pair of primary jets.

It could also be that it's sticking in the up position and running rich. Can you open your carb and polish that power piston? Maybe you have some crud in there causing it to stick with the piston up, running rich.

Do you have an O2 sensor to see at what kind of mixture you're running down the road?

Based on the accelerator pump being wrong, I bet 'someone has been there', in your carb and has mucked things up. They probably set it up to run rich.
 

Christo

Administrator
Staff member
Oct 4, 2019
307
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Weymouth, MA
How do your plugs look?

Most of your driving is just using the primaries, and that mix is determined by the power piston spring, the rods and the jets.

Maybe someone enriched your carb mix with a stronger than OEM power spring (or whatever they call that spring under the primary metering rods), or a pair of smaller primary rods, or a bigger pair of primary jets.

It could also be that it's sticking in the up position and running rich. Can you open your carb and polish that power piston? Maybe you have some crud in there causing it to stick with the piston up, running rich.

Do you have an O2 sensor to see at what kind of mixture you're running down the road?

Based on the accelerator pump being wrong, I bet 'someone has been there', in your carb and has mucked things up. They probably set it up to run rich.
Plugs look good but they probably don't have too many miles on them. Given the MPG drop during my 1.5 year ownership, a sticking power piston seems like a good thing to check out. As for the other stuff, it might explain my overall mileage being in the 7's but I'm not going to worry about that until I can explain the MPG drop. Unfortunately I don't have an O2 sensor (yet?).
 

s trout

New member
Feb 17, 2020
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Some times the choke will stay on and cause a really rich condition. Might check compression/leak down to rule it out and for any major slack in the timing chain. As a last resort check the charcoal cannister and the emission valve in the left rear wheel well for operation.
 

pvfjr

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Oct 3, 2019
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Aumsville OR
I'd recommend you weld an O2 sensor bung into your exhaust pipe somewhere just past the exhaust manifold. I like to do this even on carbureted vehicles. I've had a vehicle with and exhaust restriction before, and it was very puzzling to diagnose as it always ran smooth. What eventually solved the mystery was a backpressure reading. I removed the O2 sensor from the exhaust and attached a gauge. Seconds after starting the vehicle, the gauge maxed out at 15 psi, then blew the hose off. Again, this vehicle ran smoothly if you can believe it. The exhaust system had already passed a full inspection with a borescope, so I was really pulling my hair out with this one. I don't allow for any assumptions when it comes to exhaust backpressure anymore.

I made as easy adapter by taking an old O2 sensor, breaking it, and punching out the internals. It left a 1/4" hole where I pressed in a piece of copper tube and flared it slightly in the hole for good measure. I keep this in my toolbox now and frequently make checks on vehicles of questionable health.

Having an O2 bung would also come in handy if you ever want to rent/borrow an AFR to give your carb a proper tuneup.
 
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