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Thread: OT Need Opinion on Removing Emissions from 1985 GMC S15 4WD Pickup

  1. #1
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    Default OT Need Opinion on Removing Emissions from 1985 GMC S15 4WD Pickup

    I've had this S15 2.8L V6 standard cab long bed since the early 90's. Recently it has become my daily driver so I'm wondering if removing the emissions crap from the engine will improve my mpg. I don't know where this S15 came from but it has a ton of emissions stuff on it. Currently I get 13.3 mpg in 4wd where it stays because of the slick roads. It runs quite well although lacks a bit of power in 5th gear.

    In my younger days I removed emissions stuff from engines in hopes of getting better performance but can't really remember if I got any significant gains in performance or mpg. So I figured I'd ask here where there is more traffic than on the GMC truck forum.

    This 2.8L V6 has an air pump, vapor canister with 6 hoses going into it, a carb with 7 hoses connected to it, 2 temperature controlled vacuum valves in the air cleaner, 2 temperature controlled vacuum valves in the water jacket, several electrical plug connectors, 2 vacuum manifolds with vacuum hoses going every which way and a bunch of other stuff.

    Have any of you guys been successful removing emissions and improving performance and/or mpg? If so, what kind of stuff did you remove and what did you leave in place? Clearly the EGR, PCV and vacuum advance system for the distributor will have to stay. Can you think of any other systems that I should look at more closely before removing? Also, the choke is electric so that will have to be traced out so I don't remove anything critical to it's operation.

    Those with first hand experience and/or very knowledgeable of 1985 vintage emissions please respond.

    Thanks in advance.

  2. #2
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    EGR is semi-commonly deleted to aid with engine longevity, in that soot in the exhaust can cause some damage as it goes back through the engine again. But it will be unlikely to have a significant effect on mpg.

    Same with choke, as unless it is faulty, it should only come on when you start the engine (and then it really should only go on when it's cold out).

    Only thing I can think of that could help is perhaps the air intake part of it. Manufacturers seemed to put in restrictive air intakes, and larger intakes with better filters, and even better getting air from outside the engine compartment.

    But, I don't have first hand experience and/or knowledge of '85 (or thereabouts) emissions stuff. I jumped from the '70s (74 rabbit 1.6L, 78 impala 305?) to a 97 F150 5.3? and now an 04 sierra 6.0. Oh, there was a '87 Jaguar xj6 in there, but I did nothing to it other than maintenance to keep it running.

  3. #3
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    A truck with a lot of emission controls sounds like it's from California. The only ones I know of who actually got better mileage without messing up some other aspect of the performance did it by buying an aftermarket "chip" to replace the ECM.

    My 92 F150 4WD got around 13 MPG on a good day. I blamed it on the gearing and my heavy foot.

    Dan
    At the end of the project, there is a profound difference between spare parts and extra parts.

  4. #4
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    My 1989 Toyota 4WD pickup gets about 18 MPG, but it's a 4 cylinder, and probably not as big as an S-15.

    Maybe convert to electric?

    https://www.diyelectriccar.com/forum...on-197762.html


  5. #5
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    Based on the thread title, I was going to recommend a shovel.

  6. #6

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    That truck is from the bad old days of emissions control. Just a couple years later, they got fuel injection sorted out, and performance and MPG rose considerably. You would probably have to go old school and build an engine from the early 70's. Your engine probably has funky heads and other built-in issues that de-smogging it aren't going to correct. I had an '85 Ford, and it was a dog, got terrible mileage, and flunked the emissions test every year, requiring an expen$ive trip to the shop to get it to pass. Bear in mind that messing with emissions equipment likely violates federal law, even though your state may not care. Just had to say that; I've never heard of anyone getting busted by the feds for emissions violations.

  7. #7
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    My opinion is don't do it. I don't want myself or my family to have to breathe your excess emissions. I'd be OK if you piped the excess into your house. Where do you think emissions go?

  8. #8
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    Quote Originally Posted by rklopp View Post
    My opinion is don't do it. I don't want myself or my family to have to breathe your excess emissions. I'd be OK if you piped the excess into your house. Where do you think emissions go?
    Agreed. Reminds me of Cousin Eddie. Also, fireplaces are inherently inefficient due to all of the heat loss up the flu/chimney. Maybe he should try an open fire in the center of his home to prevent all of that heat loss.
    Work hard play hard

  9. #9
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    13mpg really doesn't sound too bad in 4 wheel. with lower power in 5th, I'd guess poor tune or large tires.Check timing etc, including the chain make sure any vacuum lines are good.

    Air pump is for the cat, if the exhaust has been changed and doesn't have a cat, you can safely remove that, if you have a cat, it's better to make sure it's working correctly. Canister is for evap, it catches fumes from the fuel tank and stores them till the engine is running, it keeps raw fuel vapors out of the air, won't effect mileage or power at all unless it leaks.
    EGR delete can add a slight bit of power but reduce mileage

    Since it's a carb, you are pretty wide open on what you can do with it as far as tuning, if I remember right, the only real electronics should be the HEI distributor which just needs switched 12v power.

    Should be able to find a diagram of the emissions system, then determine what can be removed.

  10. #10
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    The main problem with pre fuel-injection engines was head, camshaft and carbs designed to minimize emissions at the expense of mileage and power. The heated inlet air to the carb is best left alone as it prevents carb icing in cold damp weather. The rest of the stuff is mainly gas vapor capture and cleaner exhaust gas related which doesn't hurt power or mileage.

    RWO

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