Horrible fuel mileage, can't be right. Can it?

My 1988 5th gets about 20L/100km(11mpg) from a straight highway tank. It has 205/70/15 tires, 318/904/2.22 combo. It has been switched to electronic ign with vacuum advance, and the larger TQ 4bbl. Yeah, i know the primaries are way larger than the 2bbl it replaced. However your foot only need to rest on the pedal to have enough air/fuel come through to sustain the 2000rpm hwy cruise.

The timing is very advanced as well. It has been a while, but I am thinking I have it at 15* initial, mechanical pulls in like 14*, and vacuum like 12-13*. For a total of like 42-43*. Also the vacuum is manifold, not ported. It seems to like it quite well. It runs well and the plugs look good after many many miles.

Although, the mileage. I was wanting to up the HP and gearing! but can it possibly get worse than that?! lol

You should get better than 11 mpg. Provided the base engine is sound, my GUESS is the carb may be too rich. I just saw you said that was highway mpg with 2.22 gears so your mpg is WAY low. My Aspen with a 500" engine averages 11 mpg (if I keep my foot out of it). About the only way to really know if the carb is calibrated correctly is with a wide band air/fuel ratio gauge. Even if it seems fine and the plugs look fine as well, it can still be too rich. Mine seemed fine also but after installing the gauge and playing with jetting and idle air bleeds, along with ignition timing and curve, to get the numbers where they should be, the mpg's went from the previous 8 average to the current 11. Also, you need to check the ignition timing at the highway cruise rpm. If, for example, the total mechanical comes in at 2,800 and your only turning 2,000 it is basically running with retarded timing

Mhm, definitely need to get an A/F guage.. Just they are all like $250-300, and that is some good sized cheddar for a summer car. lol Got any leads to some that don't break the bank and are somewhat accurate?

I'd say the carb is definitely not tuned properly for every aspect, it is after all just a $10 junkyard carb with a rebuild kit! lol from a 440 truck.

I might try and borrow a friends setup small bore q-jet and see about what it does.

The thing with the timing is, it is at least sitting at ~30* from initial and base timing to begin with before mechanical ever comes in to play. I am fairly certain it was all in by the 2000-2200 area.

I would recommend the AEM brand. I've got the 30-4110 model. There's also a lower priced version. Not sure what the difference is other than it doesn't have different face plates and mounting rings. Plus you can select the mode from the front of the gauge. The 30-4110 one you have to use a tiny screwdriver through the back of the gauge but,,,,,,there's really no reason to switch modes from A/F to Lambda or the other modes for tuning purposes.

When you figure the cost of fuel and the potential savings from using less, and the cost of random jetting or metering rod experimentation plus the time it takes to know if you've made an improvement,,,,,,,,,,,worth every penny of the purchase price.






AEM Electronics Wideband Air/Fuel UEGO Gauge Kits 30-4110

AEM Electronics X-Series Wideband UEGO Air/Fuel Sensor Controller Gauges 30-0300

Wow 11mpg from a big block let alone a big big block.

Sitting at the Tim Horton's drive thru in high idle makes bad gas mileage.

Do you have a Canadian Y pipe?
All those cats and a clogged exhaust sucks gas/HP.

I run 93 non ethanol, which is the same price/cheaper than the big guys, and that brought up the gas mileage.

Ontario winter gas really sucks the gas mileage.

A properly running carb helps almost anything, but very few can work on them these days.

Aspen, I like the price point on the cheaper one you listed. Seems like it will fit the bill quite well.

OldIron, I wish it were a 500hp BB causing the mileage... lolol

The Dr., Yeah seriously. 6 years of my life has been spent in a tims drive-thru. No canadian y-pipe, but the main cat and muffler were new a few years ago when i did the 4bbl/ignition swap. It does run out of steam WAY early in the RPM's though, so perhaps it is getting bottle-necked by the pre-cats and single exhaust. Like asking for more than 3500 in 1st it just looses power and stops pulling. cam and heads limiting it that badly?

Also, this mileage was also on non-ethanol 89 and 91 too throughout the summer.

20L/100 is 14 mpg (imperial gallon). Why is the timing so advanced? Consensus seems to be that initial advance should be 8-10 deg. and total should be around 34-36 deg., all in at 2500-3000. You are running almost full advance at idle (15 initial and 13 vac, abd vac is being applied at idle because it it manifold vacuum, not ported). Why are you running manifold vacuum and not ported? Does the carb have a ported vacuum port? if so, is it blocked off, considering you aren't using it?

Hang on a sec ;go put the timing lite on and rev it up to to cruise rpm and THEN read the timing.Next, disconnect the Vcan, and repeat the test.Next put the Vcan back on and check the manifold vacuum at cruise rpm, still in park.
Next plumb a vacuum gauge on the manifold, that you can read from the drivers seat while driving.Then go for a ride and record the vacuum at your cruise speed. Then check your water temp with an IR gun. The hottest temp at the stat house
Then post that info.
Then go park that fully warmed up, including the diff oil, car on flat level, hard, pavement. Shut it off, put it in neutral, and try to push it ahead.If you can't move it, find out why. Maybe your calipers are stuck,lol. Maybe your tires are flat. Maybe your U-joints are seized. Maybe your front wheel alignment is way off.

Next let's do some math; 20l /100 km is 5 km per liter, and My calculator converts that to 5x4.55x.62=14.1 mpg imp or 11.28mpg USg
Granted 14.1 is still lousy.

Next, 2000 rpm should be 70 mph. And my dad's 84 got 25 mpgs at that speed, so.......there is hope for you.
Next IIRC,the TQ primaries are 1.5 and 1.375, about the same as most 318 2bbls.But it really doesn't matter, cuz you should be cruising at less than 50% throttle valve opening anyway.

Does your 88 have the 3 cat set up?
Well they suck big time, but you are in Ontario, so you can't get rid of them.

That's why I only buy 87 and older.
My 86 F/A, I went a Canadian Y pipe, into a straight pipe to a Flowmaster 40 and a tail pipe.
I put a MSD box on and that warmed up the spark.
Kept the 2bbl.

Those 2 things really woke the F/A up to the point I'd like to see that it would do side by side with my Dip cop.

Unless you changed out the stock exhaust manifolds, you should have an oxygen sensor already in place on top side of driver’s side exhaust manifold.
Being it is 30 years old – I would recommend replacing it first, though – to get accurate test results.

Direct replacement Oxygen sensor part numbers are (they may others out there):
Bosch 12919
Denso 234-1002
NTK 23004

Universal sensors (will fit the hole, but connector will not. In your case, this will work fine, sense sensor will not be communicating with your Lean Burn Computer anymore.
Bosch 11027
Denso 234-1000

Then get a digital multi-meter and hook one end (+) side to the output wire and (-) side, using a battery clip, clipped onto the hex nut part of sensor. Run your wires (lengthened) into cabin of car, and go drive.
Having a rider make notes would be advisable.
Note: readings will be all over the place, between 0.2 volts (lean) to maybe 0.8 volts (rich).

You want readings when under load and cruising. Idea reading will be 0.45 volts (450 millivolts) when cruising

A more permanent solution is to get an EGT (Exhaust Gas Temperature) sensor setup – which comes with its own (ie: different kind of) sensor and gauge that you can install on dash, somewhere) or what was mentioned in posts, above.

Before switching out carburetors, can you get me the number off of your TQ? It will be on drives side rear lower corner of carburetor and report back, please.
It may look something like:
9123
123 4

Note: the cats on my ’86 are presumed to be stopped up - for my fuel mileage is in same range as you.
Matter of fact my, um, muffler fell off a while back and car is still just as quiet as any car on the road is, with (except for the 3-cats) open exhaust – which again, I point a finger towards the cats being stopped up.
BudW

kkritsilas said:

20L/100 is 14 mpg (imperial gallon). Why is the timing so advanced? Consensus seems to be that initial advance should be 8-10 deg. and total should be around 34-36 deg., all in at 2500-3000. You are running almost full advance at idle (15 initial and 13 vac, abd vac is being applied at idle because it it manifold vacuum, not ported). Why are you running manifold vacuum and not ported? Does the carb have a ported vacuum port? if so, is it blocked off, considering you aren't using it?

Click to expand...

Who measures in IMP gal though, other than across the pond? lol I listed the "Canadian and American" values as most on here I think are in North America . Timing is set to where it pulls the best/steadiest vacuum and runs its absolute best. 15* was that magic number. I read a good manifold vs. ported article and thought i'd give it a go. It seemed to make it idle even better. Seeing as how it pulls vacuum in as soon as you touch the pedal it is really only affecting idle no? It does have a ported port, it is blocked off yes of course.

AJ/FormS said:

Hang on a sec ;go put the timing lite on and rev it up to to cruise rpm and THEN read the timing.Next, disconnect the Vcan, and repeat the test.Next put the Vcan back on and check the manifold vacuum at cruise rpm, still in park.
Next plumb a vacuum gauge on the manifold, that you can read from the drivers seat while driving.Then go for a ride and record the vacuum at your cruise speed. Then check your water temp with an IR gun. The hottest temp at the stat house
Then post that info.
Then go park that fully warmed up, including the diff oil, car on flat level, hard, pavement. Shut it off, put it in neutral, and try to push it ahead.If you can't move it, find out why. Maybe your calipers are stuck,lol. Maybe your tires are flat. Maybe your U-joints are seized. Maybe your front wheel alignment is way off.

Next let's do some math; 20l /100 km is 5 km per liter, and My calculator converts that to 5x4.55x.62=14.1 mpg imp or 11.28mpg USg
Granted 14.1 is still lousy.

Next, 2000 rpm should be 70 mph. And my dad's 84 got 25 mpgs at that speed, so.......there is hope for you.
Next IIRC,the TQ primaries are 1.5 and 1.375, about the same as most 318 2bbls.But it really doesn't matter, cuz you should be cruising at less than 50% throttle valve opening anyway.

Click to expand...

I do not have the car at my place for the winter. Its spending the winter at my grandmothers, under her carport. 1.5hr away. I can't do any of your tests! lol

However, not even fully warmed up on near flat ground with the car in neutral it will roll away by itself. No push needed. The tires aren't wearing weird, so I assume a pretty fair alignment. Brakes could be dragging up front a little, it has been a loooonnnnng time since they've been serviced.

Fully agree, highway cruising is barely resting your foot on the pedal. honest to god.

Dr Lebaron said:

Does your 88 have the 3 cat set up?
Well they suck big time, but you are in Ontario, so you can't get rid of them.

That's why I only buy 87 and older.
My 86 F/A, I went a Canadian Y pipe, into a straight pipe to a Flowmaster 40 and a tail pipe.
I put a MSD box on and that warmed up the spark.
Kept the 2bbl.

Those 2 things really woke the F/A up to the point I'd like to see that it would do side by side with my Dip cop.

Click to expand...

That it does, but the rear cat and muffler are brand new. I could still very easily remove the front two cats, as they'd never know to check for pre-cats ... as they don't even give two shits if there is a rear cat.. lol Diesels are a visual test of the exhaust opaqueness, my 88 jetta doesn't have an exhaust system. Straight off the turbo pointed at the ground in the tunnel basically. No smoke while idling? pass. The guys I go to are real solid guys.

When you say you kept the 2bbl, you went to the 2280 or w.e the other holley number was for the non-ELB stuff? It is definitely not the 4bbl aspect of this swap causing the mileage, as it never goes in to secondaries unless your on the floor and really hold 1st or 2nd out on the gear selector. ORR, 160km/h in 3rd will open the secondaries too. Seen it first hand. Original suspension is a little sketchy up there though lol.

Sadly I don't think this cars demeanour fits a loud exhaust, maybe a very very subtle burble.. at least, not a glass pack sound anyway.

BudW said:

Unless you changed out the stock exhaust manifolds, you should have an oxygen sensor already in place on top side of driver’s side exhaust manifold.
Being it is 30 years old – I would recommend replacing it first, though – to get accurate test results.

Direct replacement Oxygen sensor part numbers are (they may others out there):
Bosch 12919
Denso 234-1002
NTK 23004

Universal sensors (will fit the hole, but connector will not. In your case, this will work fine, sense sensor will not be communicating with your Lean Burn Computer anymore.
Bosch 11027
Denso 234-1000

Then get a digital multi-meter and hook one end (+) side to the output wire and (-) side, using a battery clip, clipped onto the hex nut part of sensor. Run your wires (lengthened) into cabin of car, and go drive.
Having a rider make notes would be advisable.
Note: readings will be all over the place, between 0.2 volts (lean) to maybe 0.8 volts (rich).

You want readings when under load and cruising. Idea reading will be 0.45 volts (450 millivolts) when cruising

A more permanent solution is to get an EGT (Exhaust Gas Temperature) sensor setup – which comes with its own (ie: different kind of) sensor and gauge that you can install on dash, somewhere) or what was mentioned in posts, above.

Before switching out carburetors, can you get me the number off of your TQ? It will be on drives side rear lower corner of carburetor and report back, please.
It may look something like:
9123
123 4

Note: the cats on my ’86 are presumed to be stopped up - for my fuel mileage is in same range as you.
Matter of fact my, um, muffler fell off a while back and car is still just as quiet as any car on the road is, with (except for the 3-cats) open exhaust – which again, I point a finger towards the cats being stopped up.
BudW

Click to expand...

I have a replacement 02 for it already! Bought it for the tune-up i was doing just before i decided a tune up wasn't gonna solve the dead lean burn system being the reason it was just plowing through fuel.. lol

Interesting, I never knew you could tune a gas engine with an EGT. Always just figured those were for the diesel world . I have a sweet one on my diesel jetta, it is a 4 digit LCD and the thermocouple is accurate to like 0.01%. Its a beauty. might have to buy another!

It will ABSOLUTELY run out of steam in the 3000-3500 area.. so I wonder if those pre-cats are just plugged like right solid.. don't think you can get those canadian y-pipes anymore though can you?

I would bet you have to much total timing

Its possible, however with the initial way down (5-8*) or vacuum on the dizzy not even hooked up on separate tests yielded no difference. It doesn't seem to affect anything but idle quality if anything.. lol

The carb is likely way too rich on them primaries sadly.. The bad part of the TQ is having to source a few or more old carbs to scavenge parts from to change the jets and rods. Where as a common 4160 holley you can buy a trick kit or the likes.

My guy gets parts for them and tunes a few a year.
I was just talking with him about setting up a pair for a tunnel ram.

Have you tried adjusting the metering rods down to lean the cruise afr?

You also need to make sure the air bleeds on top of the carb are open, if not it can cause a rich condition.

My guess is combo of carb and timing issue. You may have also jumped a tooth on the timing chain and are masking the symptoms with advancing the timing.

Float levels with also affect afr, too high will show rich, low will show lean.

The TQ you have was built on 351th day of 1974 (351 4).

It came off of an early 1975 B or C-body 400 Automatic 49-state (is not from a High Performance 400). It does have the larger primary’s and does have Idle Enrichment (or more accurately described as an Idle "In-lean-ment") valve.

The carburetor rates in the middle of “my ranking” of TQ’s. It is not the best one but is also far from the worse one out there. No need to look for another one – just might need to tune this one, as needed.


The Holley 2280 2-bbl is not computer controlled. The Holley 6280 is almost exact same carburetor - but is computer controlled. My Holley 6280 is gathering dust in my garage . . . somewhere.


EGT’s (Exhaust Gas Temp) is more known for Diesels and for turbocharged vehicles – but data from one can be used to tune a car with. The oxygen sensor method might be a more cost effective method (and is an older version).
All I was trying to say that your car already has a “test port” installed – so you might as well put it to use.


I started a post a while back: https://www.forfmjbodiesonly.com/classicmopar/threads/the-canadian-y-pipe.5866/ . If a person is not able to find a Walker Y-pipe (# 40430), a muffler shop can still make one for you.
I had one made (3rd picture, post #1) – which by the way, is still for sale.
The Y-pipe in the 2nd picture is going into my 5th Ave, after it warms up, outside.

I’ve “heard” that “some” muffler shops can build a Y-pipe (or tailpipes) without the car present, by using measurements from a book (or computer) – but no first-hand experience with that.
BudW