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Discussion Starter #1
My son in law has a nice 90's stepside with a 305 & 5 speed, with throttle body injection, that gets absolutely horrible mileage - like 8.

No engine codes, new tune-up, nothing apparently wrong, engine runs OK.

I'm not familiar with that type injection, where to start?

Thanks for any suggestions.
 

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Examine the plugs under a 8X-10X magnifier and see what they look like, sounds like you have multiple problems including dirty injectors, worn engine, old O2 sensor, worn out cat, partially blocked EGR, sticky EGR valve...etc. etc. etc. To start I would check to see how much blowby you have, if its a little under hood chimney I would start with a rebuild first.

Old vehicles, don't you love em.:)
 

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Come Home Safe Soldier
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I would pull those plugs and look at them to make sure they are gapped correctely.Ensure all plugs are firing,Make sure that the cap and rotor are good.I know you stated new tune up.Was it a "Major tune-up?New everything,not just plugs?If the engine seems to be running fine than it has to be with the tune up procedure.Install new fuel and air filters.Another thing that is often overlooked in fuel mileage problems are the tires,An underinflated set of tires can destroy mileage and you will not even know it.
 

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and to add to the above, how is the alignment? does he have huge tires on the truck???
 

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Discussion Starter #6
More Info

Yes, should have put in more detail on the tune-up. All new plugs, wires, cap, rotor. He does have oversize tires on it that are softer than normal, I've mentioned that to him, but would it take 5 or more mpg off? He should be getting mid teens or close on the hiway with the manual OD, 75 is barely over 2000. Long time ago I had a carb'd 305 in a 3/4 ton van and, with an advance curve tuneup, got it up to 15 on the highway with an auto trans without OD and carrying a good load.

And, no, he hardly ever gets the engine
over 2500 even when upshifting. Drives it like a baby. So does his wife (my daughter).

I'm going to recheck his timing, maybe throw a timing tape on the damper and see what he's got. Probly can't change the curve other than to advance initial, if it will stay out of detonation. I was thinking maybe the computer was keeping it in closed (?) loop, like if it
was full throttle. I even asked if his engine was running at the right temp - warm enuf for the computer to go into regular mode. It's got quite a few miles, maybe the 02 sensor could stand replacing. I guess we could check out the temp sensors, too. Anybody know what volts or resistance they should show? If the computer temp sensor is telling the computer the engine isn't warm enuf to go out of cold start "choke" mode, that might do it, too.
 

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Come Home Safe Soldier
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but would it take 5 or more mpg off?


Easily,if they are not inflated properly.Plus oversized tires are pulling more and will throw off the settings you were accostomed to when the stock tires were on it. Try airing the tires first and see if that helps a little. Weird how even the smallest mod to a vehicle will do isnt it.Also like Old School stated,the alignment,If it is toed in or out too much you have drag,your engine is working to overcome the degree of the tire in or out.With the tires,are they newly put on?If so you should expect a drop,plus have an alignment checked if just installed.They can get wore out real quick if misaligned.
 

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Old School Nut said:
and to add to the above, how is the alignment? does he have huge tires on the truck???
and does he drive around in 4 wheel drive?

also if he has large(tall) tires on the thing that will throw the odometer off, giving false mileage readings.
 

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You would probably be shocked what a set of tires can do to gas mileage.

I went from wee lil 29 inch tires that achieved about 15-16 mpg,
Went with a set of 42 inch tires about 5-6 mpg but was then informed they were illegal so pulled them off and put a set of 37 inch tires on and i'm getting about 8-9 mpg. Might I say this is on a Mopar 318.

How big are his tires and how big were the stock tires, don't forget the width either.
 

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Discussion Starter #10
Just 2wd

It's just a 2 wheel drive and by oversize
tires I don't mean the big mudders, just
like 255's instead of 225's on the stock rims . They've been on awhile and are not wearing crooked, so isn't likely out of alignment.
 

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Discussion Starter #12
Big Ties

Back when I had muscle cars, I always put the biggest
tires on them I could fit under the fenders, esp on the
back. Being street tires, they did increase diameter some
but mainly the width = up to 10-1/2" wide in a couple cases,
and usually at least 2 sizes wider on the fronts on wider rims.

This was on things like a 426 Hemi 4 speed, 375hp 396 Camaro SS 4 speed, Challenger Rt/SE 440 6 pack 4 speed, SS El Camino 275hp 327 4 speed, and a warmed up 440 in a full time 4wd Ramcharger with BFG All Terrain radials all around -
the bigger of the sizes available (9" or wider), on 8" rims.

Never noticed more than a mile per gallon difference on any
of them. Of course, kept them inflated properly and the
alignment correct.
 

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My tires went from 29/8.75/16.5 to 42/15/16.5 to a 37/15/16.5 this type of change I could understand horrible gas mileage with but with the little done it shouldn't effect it what-so-ever IMO.

Eric
 

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I remember reading a technical article (maybe from Bosch) that O2 sensors can loose performance reading abilities (not necessarily a bad sensor), and not set off a code. They are like spark plugs to a degree and should be replaced every so often. If memory serves correct, a one wire about 50k miles and three wire about 70k miles. This article said that a poorly performing O2 sensor will affect mileage in varying degrees, but it could be substantial.

Also, look for a cracked vacuum hose to the MAP sensor. I once left the hose loose and because the MAP sensor could no longer read vacuum (i.e. load), it thought I was under WOT all the time! It was very obvious that something was wrong so I searched and fixed the problem. Check this hose carefully because mine was plastic and brittle. You may want to replace the MAP sensor anyway.

Good luck, Ed www.edgesz28.com
 

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Although, 8 is pretty bad.............try to remember, you are pushing around a "brick" here, not much areodynamics.

Is it a high milage truck?

Have the injectors been cleaned?

By, tune up, was everything replaced?

I gained a little mileage on my 305 van, by advancing the timing a couple of degrees.
 

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Discussion Starter #17
Now We're Getting Somewhere!

Hey - appreciate all the input -
thoughtful and helpful!

I had thought about 02 sensors, too,
I replaced them on my 200k mile 89 Lincoln LSC 302HO and things did improve, course also changed out the intake air and engine temp sensors at the same time that gives info to the computer, but had never thought about that MAP sensor - and of course vac leaks.

He said he had pulled the codes and found none, but you're entirely correct on "marginal" sensors.

He's starting a body repair business, so any major expense right now just won't happen. But I'll get with him and do some more checking on some of those things -

Thanks again!
 

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also if he has large(tall) tires on the thing that will throw the odometer off, giving false mileage readings. [/B]


yeah what he said...
 
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