noob needs help with mileage - only 10 MPG!

[dusterny]

My slant never got good mileage but it was always a carb issue. Even with rebuild it wasn't great but still ran too rich at idle which I think was a bad float. I was tired of messing with it due too going with a 360 in a year. I run one summer with 8 3/4 with 3:23 and that killed mileage to the piont 360 gets better overall even with your foot in it a little bit. If your slant is in pretty good shape I would be suspect of the carb.

 

[Woods74]

My slant never got good mileage but it was always a carb issue. Even with rebuild it wasn't great but still ran too rich at idle which I think was a bad float. I was tired of messing with it due too going with a 360 in a year. I run one summer with 8 3/4 with 3:23 and that killed mileage to the point 360 gets better overall even with your foot in it a little bit. If your slant is in pretty good shape I would be suspect of the carb.

X2 same issues. Same outcome.

 

[Captain Easy]

A narrow A Body that vintage in decent mechanical condition and an auto tranny in normal driving should get 14-16 city and 16-18 hwy mpg easily. My 66 Dart w/a tired 225 and 3 spd manual gets 21 on the hwy if I drive easy.

If the fuel pump isn't leaking into the crankcase (already mentioned) and the timing is ok, and it runs well, you should do much better. I'm sure you have already checked out the auto choke when it is hot, to make sure the butterfly is open all the way. Those choke thermostat springs die with old age. Also, some of the 80s era had an electric heating element that was supposed to kick in and pull the choke off (stupid, eh?) so worth a check. A partially closed choke will sure burn more gas.

Does it have the original Carter carb, or did they bring the Holley over with the engine transplant? How about the rest of the smog stuff from 1980? Slants in the post-smog era were much worse; one of the nails in their coffin. The 74 Swinger with 225 and auto, 1 bbl Holley, PS,PB,AC I had never got much better than 15 to 16 on the hwy. If the smog set-up was carried over in the transplant, you have de-tuning and surgery to do. I think I recall the cam/valve timing on the smog models was a little different, and as they came from the factory, would never perform or get mileage like the 60s versions.

The Carter BBs were better for mileage. I found if you can locate a Carter BB made for the 170 Cu In, it will absolutely get better mileage with little noticeable performance difference in a non-smogged 60s A Body. It had a slightly smaller air horn and venturi. Whatever carb, they were simple. Get a kit and a new float and some carb soak and rebuild it; just pay attention to the drawing in the kit as you take it apart and lay out the pieces; the kit will have parts in it for other variations of the carb.

If that does no good, I'd look to the tranny. If it's slipping, if the torgue converter seals are leaking, if the pump is weak, if there is a crack in the shift body galleries or not locking up in high gear, you'll never get decent mileage. I've seen this happen in the older cars. Especially if the car is new to you, it may seem ok. See if you can find a 'vintage' mechanic who is still in business and isn't senile yet and have him check it out. It is my guess that is your problem. Sometime a shot of tranny tune-up will help the seals, at least for awhile..maybe...

It's hard to get a Slant to get 10 mpg even if it's running poorly - unless you like to race. In that case, the 'swap it for a big V-8' is good advice. If mileage is you main reason for having an A Body, keep your eyes open for one with a manual. They were not the best manual tranny ever, and hard to find parts for, but they were always good for a couple more mpg.

Good luck!

Captain Easy
:coffee2:

 

[slantsixdan]

You've probably got a problem with your carburetor. See here for what I've seen cause ridiculously bad mileage despite the engine running well. Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download. Tune-up parts and technique suggestions in this thread. And don't forget to keep on top of the valve adjustment.

 

[memike]

Low tire presure to worn wheel bearings and rear axle bearing will rob hp and gas milage.. along with old u-joints and a bad alignment and all above post
I just had to throw this in here because it was not mentioned.
Don't give up on the /6.. Take the potato out of the exhaust and let it breath, I removed my intake and exhaust manifold and seen my heat riser was stuck half closed and the pea shooter ran fine but it struggled to mix well.. do a new exhaust system and remove the very small pipe and go with a 2 1/4'' down pipe to a muffler and exit with a 2''.
loose the points and go electronic and enjoy :cheers:

To the na sayers , I get over 20 mpg...in my 2.700 lb standard trans car.

 

[Renato]

First, thanks to all who responded with tips and suggestions.

I just got the car back from the shop and it is running much better.
Actually it ran well before, but it had idled rough since I got it. The idle is still a bit rough, but a lot better than before.

Here's what was done to it:

Pertronix electronic ignition installed
carburetor overhauled (it was missing the throttle return spring!)

the mechanic also ran a leak down test, here are the results:
cyl %
1 35%
2 30%
3 25%
4 25%
5 34%
6 25%

So I'm guessing (and hoping) that the electronic ignition and overhaul of the carburetor will improve mileage considerably.
I'll make another post once I refill the tank and can calculate the mileage.

What does the board think of those leak down percentages?
On a new engine they'd be terrible, but a 1980 engine with no telling how many miles are it on it, they don't seem that bad (or do they?).
Should I consider having this engine rebuilt? or looking for a rebuilt slant six to replace the one I have?

 

[67Dart273]

More than 20% is considered suspect, and certainly 25 and greater is not that great.

BUT does it use oil? Did the shop say WHERE it is leaking, IE valves, rings, what?
What did they say about the "rough idle?" Did anyone ever set the valves? They should have been able to get a clue with the leakdown test.

In my opinion, this is probably NOT bad enough to explain the very poor mileage, and it seems to me you are asking this question before you have found out IF you've managed to improve it.

I think the answer to an overhaul is based on you: Your expectations, just how much you intend to drive it, and how long you may be keeping the vehicle.

 

[Outkast]

I agree...I've gotten acurate part #'s from slantsixdan that the Chrysler dealerhip said didn't exist...very sage advice from slantsixdan.

x3 for slantsixDan he knows his sixes,he is also more often found on the SlantSix forum. It is great for info on these motors.

 

[memike]

I had a 225 with no visible gas leaks, turned out the fuel pump diaphragm was bad and leaking into the oil pan. A lot. Might want to smell inside the oil filler cap. 10 mpg is pretty bad unless you're mud bogging

This just happened to me, glad I caught it right away :coffee2:
Artex pumps was all I could find locally and I was determined not to
put another one on, I am home awaiting my new carter to show up on the mail.

 

[Renato]

67Dart273,

I wanted to update the board with the work that was done on the car.
Obviously I won't know if the mileage has gotten any better until I fill up the tank next.

The mechanic said the leakage is due to worn rings and/or cylinders.

The oil level is stable.

 

[derrangedgadgeteer]

The carburetor overhaul should do the trick. The hallmark of a carb running over (Bad/clogged needle and seat, or sunk float) is the engine running well when cold, (when it needs a richer mixture) and having trouble idling once warm. Also, pulling your plugs you'll find they're dark or even black with soot.

I tell people who have trouble like this, to take off the air cleaner, start the engine, open the choke, and look straight down the carb with a flashlight, if anything is wet, that is the problem. If everything is still dry, have someone cut the car off while you're looking down the carb. if it's running over, fuel will drip from the boosters onto the throttle plates.

In either case your problem is either a bad needle and seat, trash in the needle and seat, Your float may have gone bad, may not be adjusted properly, or it may be excessive fuel pressure (No carb should ever be fed over 5psi of fuel pressure, and hi-po and electric fuel pumps are good at doing that. you'll want a regulator if this is the case.) Regardless, a carb rebuild is in order, and that should fix the problem.

 

[rmchrgr]

If it runs rough and has excessive leak down readings (#1 35%!), you have mechanical issues. Valves are probably toast, could be rings. The shop should have told you where the air was leaking from. A new carburetor and a tune up ain't gonna fix your problems. General spec for a healthy engine is 10% between cylinders. Like said above, anything over 20% leakage usually warrants a tear down.

Before leaping to any conclusions though, perhaps you could try some combustion chamber cleaner treatment type thing to see if that might free things up a little. Could be a lot of carbon in there causing problems. Mopar makes good stuff available at the parts counter. I think it's like $7-$8 a can. Worth a shot. Least it will be cleaner when you take it apart.

I was going to ask if you had an electronic ignition and it looks like you took care of that with the Pertronix. However, I hope you/they looked carefully at the distributor before putting it back in and verified the rest of the components were good. Might be excessive play in the shaft which could cause your timing to be erratic/unstable. Slants have a plastic gear that can get worn/chipped. Just a thought.

BTW, new electronic ignition packages are relatively cheap and easy to install. I put one in my old '65 Valiant (Slant) and it was like a new car. Seriously. Better mileage and all around better performance - start up, idle accel. Great bang for the buck.

 

[stewy22]

I have a 76 dart and everything is stock at the moment. Complete tune up and carb cleaning and adjustment and the best I can get is 15-17 mpg.And thats driving like I have an egg between my foot and the accelerator.Wonder whats gonna happen when I get the Hi-Po rebuilt in with headers and 4bbl.

 

[Renato]

I finally burned thru a whole tank of gas and calculated the mileage:
13mpg (mix of city and hwy)

I was hoping for 15mpg or better, but 13 is sure beats 10.

 

[slantsixdan]

13 is still lower than it ought to be. Something's still the matter.

One question: have you checked whether your speedometer and (more importantly) your odometer are reading correctly? If you've installed larger tires or a numerically lower rear axle without changing the speedo drive pinion, you'll record fewer miles than you've actually travelled. And have you checked carefully for fuel leaks?

 

[rmchrgr]

I finally burned thru a whole tank of gas and calculated the mileage:
13mpg (mix of city and hwy)

I was hoping for 15mpg or better, but 13 is sure beats 10.

Does it still run rough like you said initially? Mechanical problems usually don't fix themselves.

 

[Renato]

slantsixdan,

I checked the odometer when I first got the car, and again earlier today. Today I did 15 miles on the interstate (according to the mile markers), and the odometer showed 15.3 miles. If anything that should slightly overestimate mileage, but not significantly - if the interstate mile markers are accurate, mileage is overstated by 0.2 miles per 10 actual miles.

There are no obvious fuel leaks...

I posted these above in reply #31, but here are the leak down percentages again:
cyl %
1 35%
2 30%
3 25%
4 25%
5 34%
6 25%

I know these are not good, but do they explain the poor mileage?
Are there specific things I should be asking my mechanic to look at?


rmchrgr,

The car doesn't run rough, but idles rough once it warms up. It idles fine from a cold start.

 

[/6 Matt]

Nobody else has stated one blatantly obvious thing here, check your drum brakes and make sure they're in good working order. Tire pressure too.

BTW If your carb is reman, drop it like it's hot and get a NOS carb.

 

[Renato]

The carb is Holley 1920 and I just had it overhauled.
Would you recommend replacing it with a NOS carb? if so, which one?

The drum brakes are good, says my mechanic.

 

[rmchrgr]

cyl %
1 35%
2 30%
3 25%
4 25%
5 34%
6 25%

I know these are not good, but do they explain the poor mileage?
Are there specific things I should be asking my mechanic to look at?

rmchrgr,

The car doesn't run rough, but idles rough once it warms up. It idles fine from a cold start.

Yes!

When there is electricity, I am a tech at a dealership. I do powertrain exclusively. When a customer comes in and complains their car is running rough at any point, it usually means a misfire. We have the benefit of a scan tool to tell us which cylinder is missing but on your car, your guy will have to rely on older technology and/or experience for diagnosis.

Anyway, The first step we usually take with this scenario is to swap coil/plug to another cylinder to see if the miss moves. Since you have one coil, he might do a plug. If the miss does not move with the plug, it can be ruled out as faulty.

So if the plug is OK, the next step is usually a leak down test. In our shop, again, anything over 20% will warrant an engine tear down. At least the head would be coming off to check the valves/seats/springs etc. 35% leak down represents a very large combustion leak. Something, whether it be a valve, head gasket or whatever is not sealing during the combustion process and is allowing excess fuel/air mixture to not be properly burned.

Does he have a chemical type combustion leak tester that goes over the radiator opening? Probably. If it has a head gasket leak, have him show you the color change with the car running.

Could be a simple vacuum leak, Does the condition go away or get worse when you put it in gear?

It is odd that the car runs OK when cold and idles rough when warm, usually its the opposite as parts warm up. Usually that type of situation gets worse over time if the condition is not corrected.

Regardless, the drivability does not explain the large leak down numbers. I'd say that your mechanic probably does not want to deal with taking the head off a car that he might be stuck with for a while before he can find whatever parts he needs. Taking the head off would be no problem but if he finds bad valves, bad seats, springs or whatever, he will need to find parts somewhere. Then it's at least a head rebuild/reman which ain't gonna be cheap.

If I had to guess, your guy is gonna need to get pretty far into it to solve your issues. Unfortunately, he really can't give you a definitive answer until the head comes off though. I guess I'd speak to him about what's involved with that and whether he wants to do it. If he does take the job on, I'd get a written estimate and make sure he either shows you what he finds and/or leaves the worn parts for you to inspect.

We often run in to this with people at work. They have an older, high mileage car that's been neglected for a while and exhibits some mechanical problem. Unless they have a warranty, we usually ask for several hours labor time up front to take it apart and make sure we diagnose it right. It's not always possible to know exactly what the problem is until you see it. Often they decline because the cost to repair is more than what the car is worth.

I'd think about doing this yourself. Do you have the space/tools/time? Most gear head types are gonna want to do this at some point, sort of a right of passage. Is that you? Would be a good way to learn. Or maybe just find a better running Slant and drop it in for now while you rebuild the one in your car. There's probably someone near you yanking one out for a V8.

Bottom line, you need to figure out why you have 35% leak down and where it's coming from whether it be exhaust/intake/head gasket/rings. If you have to pay your guy an hour to tell you, then that's what you have to do.

I could be way off base here though, hard to diagnose a car over the internet. Maybe post a video of how it runs?

 

[BillGrissom]

Before you pull the head, find out where the leak is coming from. Apply air thru the spark hole to each cylinder at TDC-compression stroke. Listen at exhaust pipe, carb throat, and valve cover. Since a solid-lifter engine, it could be as simple as the valves need adjusting since being held open by the cam.

 

[Captain Easy]

As all the others have said, there's lotsa possibilities. The smooth-when-cold and rough-when-hot scenario w/poor mileage sure seems to suggest a too-rich mixture. As loose as the engine is, you may bever get 20+ mpg until you do an engine overhaul. Bad rings won't generally cause rough running, but if you have some cooked valves, along with uneven carbon build-up in the head, you may not be able to get adequate compression in the cylinders for good mileage and a smooth hot idle.

An NOS Carter BB carb is not cheap, but lots cheaper than a rebuild. So if you've tried all the suggestions, it's the only one left I'd put money in. The good up side is, even if it does not cure the problem, you will have a quality, high mpg carb to use after your engine re-build. A quality factory rebuilt long block Slant 6 from a good rebuilder is not the end of the world price-wise; like, way less than a year payments on a new car. Since your car is pre-smog, you can do the engine swap sans the smog stuff you may have inherited on yur current engine. 20 years ago, I'd have suggested cruising the junkyards for a good salvage engine (probably what the previous owner did) but the chances of finding a tight, low miles used stock engine at this point is pretty tiny.

Good luck

 

[Renato]

rmchrgr,

What my mechanic had to say was very similar to what you wrote above.
He said that the high leak down percentages are probably due to worn rings and/or valves, but that he can't be sure without taking the head off. He said that worst case I'd be looking at about $2,000 for an engine rebuild/reman.
Does that sound like a reasonable figure?

And the idling rough when warm thing, maybe "idling rough" is not the best way to describe what it is doing. From a cold start the engine sounds fine when idling or revving. When the engine warms up it makes a sound like engine knock when indling, but not when revving. When I picked it up from the shop where I had the work done I started the car and left it idling in park while I talked to the mechanic. Sure enough, once the engine warmed up it started making that engine knock sound. The mechanic had just explained to me that the high leak down percentages were probably due to worn rings or valves, and he also said that worn rings, once the engine warmed up, could explain the knocking-like sound.

To those of you work on engines, does that make sense?

Captain Easy,

Why is it that you recommend the Carter carb over the Holley 1920 that I have now?

As a serious noob, I'm grateful for all the input here.
Thanks, guys.

 

[slantsixdan]

he also said that worn rings, once the engine warmed up, could explain the knocking-like sound.

No. Worn rings do not cause a knocking-like sound.

Why is it that you recommend the Carter carb over the Holley 1920 that I have now?

See this post for detailed discussion on that matter.

And did we ever get confirmation that your valve adjustment is proper? If the valves are too tight it'll cause all the problems you list except the mysterious "knocking-like sound"; it'll cause rougher running when hot than cold, it'll cause high leakdown, it'll cause cruddy gas mileage. I don't think your "knocking-like sound" is related to your poor mileage.

 

[rmchrgr]

rmchrgr,

What my mechanic had to say was very similar to what you wrote above.

Hey, sometimes I guess right. :thumrigh:

He said that the high leak down percentages are probably due to worn rings and/or valves, but that he can't be sure without taking the head off. He said that worst case I'd be looking at about $2,000 for an engine rebuild/reman.
Does that sound like a reasonable figure?

I guess it's reasonable, I have never priced a Slant 6 rebuild myself. Stuff adds up quick - gaskets, freeze plugs, rings, valves, timing chain etc. All kinds of stuff that you don't think about until you have to rebuild. Plus, he has a car that has to get pushed around the shop/yard or stays on the lift until it's done. A head rebuild could be pretty quick but doing rings is definitely time consuming. There's a lot of clean up involved too, use of special tools, fluids, etc.

Cool that your guy is willing to do it, lot of guys would say no way for whatever reason - not worth it, can't get parts etc.

And the idling rough when warm thing, maybe "idling rough" is not the best way to describe what it is doing. From a cold start the engine sounds fine when idling or revving. When the engine warms up it makes a sound like engine knock when indling, but not when revving. When I picked it up from the shop where I had the work done I started the car and left it idling in park while I talked to the mechanic. Sure enough, once the engine warmed up it started making that engine knock sound. The mechanic had just explained to me that the high leak down percentages were probably due to worn rings or valves, and he also said that worn rings, once the engine warmed up, could explain the knocking-like sound. To those of you work on engines, does that make sense?

No, that does not really make sense. A knock is usually something like a bad wrist pin, rod or crank bearing or some other mechanical malady. Worn rings do not usually make noise themselves. A tell tale sign of bad rings is blue exhaust smoke.

I'm gonna go out on a limb here and suggest that it's possible the timing chain jumped a tooth or has a lot of slack. That might explain your leak down numbers, knocking noise and poor mileage because the engine is not timed right. It would still run but pretty crappy. It's not real likely given that the car runs OK at first and usually there is a hard start condition associated with that scenario. Regardless, a new timing chain is part of any real engine build. A quality chain is good investment.

Can you tell where the knocking noise is coming from? You could check into it yourself. If you have a long extension, you can put one end of it at various points where there are moving parts, then cup your ear around the other end of the extension and listen for anything that sounds wrong. Can you hear it under the front cover or is it more evident under the valve cover/engine block? Just watch the fan!