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Thread: OPS relay mod

  1. #1
    Registered User drocker's Avatar
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    Default OPS relay mod

    New here and this is my first post. I currently have a 1994 GMC Yukon that I recently picked up.

    I have searched and read many post about the OPS, LP and the relay mod. I am pretty sure how to begin this task but I do have a few questions.

    If you look at the 2nd picture you will see how I plan to wire up the relay to the lift pump. Where I get confused is in the 3rd picture. What is this LP relay shown here for (had to borrow the pict) and will my relay mod hook up before or after this relay. I was planning on cutting the wires to OPS about 3 to 6 inches back from the plug to the OPS.

    One other question I have is with the thermostat. Some say 180 is better than the 195. Some say the fuel mileage will be hurt using the 180. I am reading so many different stories. I am running the 180 stat and before I switched it from the 195 my truck ran just fine before. Now I changed it about a week after I got the SUV and my current mileage is around 14.5 mpg. This seams pretty low since I don't tow but some say it could be cause of the winter diesel. Any ideas why it could be so low?

    Thanks.

    Thanks.
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  2. #2
    MONROE COUNTY SHERIFF Chevylover's Avatar
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    Default

    I'm happy with my 180°F thermostat. Never I'll go back to the standard 195°F. With this my engine will get to hot while towing 5 tons. With the 180, the temp stays at 185° on the temp gauge.

    But that's just my 2 cents.


    Cu,
    Sven

    Germany ? No thank's ! Anybody here, who offers a job incl. greencard/work permit ? - USA or Canada, it would be a pleasure to immigrate !

    SOLD : 1995 Chevrolet K2500 Ext.Cab SB, FSD-Cooler, 3" Rancho Suspension Lift, Rancho Add-A-Leaf, Warp Speed Performance 4" Exhaust, Custom K&N Intake, 285/75/R16 BFG All-Terrain with 8x16 Alcoa's, 4x4 Posi-Lok, Deezee Bullbar, Stepbars, Lunarvisor, Roll-N-Lock Bedcover, Softtopper Bed Extender, Truck Nuts, H.I.D. Headlights, ShokSafety LED Lightbar and much more ...

  3. #3
    OBD 1 Master Pruittx2's Avatar
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    well first off Welcome to the site! Next when you get a minute,, go up and look for the USECP link in the site hearder, to fill out a Signature, to show us all the options your your nice Yukon, RPO codes are on a sticker on the glove box, you could look up to see what gears you got, and other usefull stuff. A few diff factors will get your mileage down, But do tell if you have done any Mod's like air box mod, downpipe, and such.

    some else will chime in on the relay info,, as I have been thinking of doing this to mine also, just ain't got the time right now.

    Again,,,, and


    1995 GMC SLT Suburban 153,000 miles
    Buddy Super chip, 4" exhaust, FTB Mod, Bosch Injectors, GU6-G80, Killer PMD Cooler FS PMD #9
    Duraterm Glows, TurboMaster, K-47 airbox, D-max fan New Mod'd clutch






  4. #4
    Registered User drocker's Avatar
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    Updated signature. Not sure what gears yet. I know with the 285/75/16 tires I am running right at 2000 RPMs at 65 mph. Photos below show my custom CAI and ground mod to stop the gauges from bouncing.
    Attached Images Attached Images
    1994 GMC Yukon 2dr 4wd 6.5 TD
    8th VIN#: S
    Mods: CAI, custom ground, 180 T-Stat, Hi Beam Mod, 285/75/16 BF Goodrich T/A, EGR block off and Hogged out upper intake. OPS mod. #9 resistor.

  5. #5
    Registered User ak diesel driver's Avatar
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    Can't help with the relay. FYI your cai is not a cai just free flowing. It's sucking hot under hood air. I think your mileage is low because of your tire size and #1 fuel doesn't help either.
    96 gmc k2500 4" exhaust, marine inj, #9 resistor, GL4, turbomaster, duraterms,-1.94 TDCO,FTB, remote pmd, isspro pillar gauges, lift pump relay added ,141 block, 2 1000 w block heaters, 2 150w glue on oilpan heaters, 2 150w glue on transpan heaters, 97 K47 air filter , IP replaced at 229k ,gear drive set awaiting install, Parts for intercooler waiting to be installed

    94 k1500 excab sb 6.5 plow truck. Fisher plow w/engine driven pump, F intake, h/m turbomaster, plowing chip by Buddy, remote PMD w#9 resistor, Duraterms

    96 k3500 CC 6.5 dually
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  6. #6
    Registered User drocker's Avatar
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    ^ Sorry but I use the CAI pretty loosely in terms when talking about this type of setup. Also I dont see how my tire size can effect fuel mileage that much. I know they are one size above stock but running a smaller tire would mean more RPMs when doing the the same speed. Of course running to tall of a tire will mean more fuel for get up and go. I guess I will wait until spring or summer to see what my mileage turns out to be.
    1994 GMC Yukon 2dr 4wd 6.5 TD
    8th VIN#: S
    Mods: CAI, custom ground, 180 T-Stat, Hi Beam Mod, 285/75/16 BF Goodrich T/A, EGR block off and Hogged out upper intake. OPS mod. #9 resistor.

  7. #7
    Registered User vail426's Avatar
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    The third picture is the factory relay I believe. I'm not familiar with the 94 set up. If i am correct, leave that relay be and fashion your own relay in to reduce the load on the OPS according to your diagram.
    1995 Silverado K2500 / 174k miles / stock / vin F / ext cab / long bed / boost gauge /60G glow plug override mod / Gray Stanadyne PMD on sink in bumper nostril / lift pump relay mod / stainless steel crossover / fuel pressure gauge / early model lift pump / Kennedy 20" fan / Hayden 2839 fan clutch / Opti-Lube XPD at every fill up

    1965 Dodge Coronet 500 426 wedge head / 4 speed / 2dr hdt coupe

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  8. #8
    Registered User drocker's Avatar
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    ^ thanks.
    1994 GMC Yukon 2dr 4wd 6.5 TD
    8th VIN#: S
    Mods: CAI, custom ground, 180 T-Stat, Hi Beam Mod, 285/75/16 BF Goodrich T/A, EGR block off and Hogged out upper intake. OPS mod. #9 resistor.

  9. #9
    Tropical 6.5er matuva's Avatar
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    1994 full size BLAZER
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    Completely rebuilt 506 + 2nd gapless rings

    and more ...

  10. #10
    Registered User drocker's Avatar
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    ^ Thanks. I think I read the thread a thousand times before I felt comfortable about doing the mod. Then I saw the relay picture for LP on the firewall which is like my Yukon. So then I got confused again.
    1994 GMC Yukon 2dr 4wd 6.5 TD
    8th VIN#: S
    Mods: CAI, custom ground, 180 T-Stat, Hi Beam Mod, 285/75/16 BF Goodrich T/A, EGR block off and Hogged out upper intake. OPS mod. #9 resistor.

  11. #11
    Lieutenant JiFaire's Avatar
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    Welcome to the site, drocker.

    My opinion on your thermostat is a bit different than Sven's... actually, a LOT different than Sven's. Make of it what you wish.

    Putting in a 180* thermostat won't help keep you cool when towing, other than starting the floor a bit lower. The thermostat sets the bottom temperature, not the top one.

    Not sure what you know, so bear with me:

    FIRST OFF: if you replaced your thermostat with anything but a genuine ACDelco thermostat, the PROPER one for your truck, there's your problem. You have a 1994 1Tstat system, and you need the right thermostat... Stant won't work right. Neither will NAPA, Autozone, Harbour Freight, Walmart, or one from a gasser. (The proper Stewart thermostat will work, also. Just don't expect to get a cheap one and be happy) These things like to run hot, and will get your best fuel economy around 195-200*.. 180* is too cool, IMHO.

    - when the engine is coming up to temp (below 180*), your thermostat stays closed. Once things hit 180*, it opens to allow coolant flow and circulation through the rad, where cooling happens.

    - A 195* thermostat does the same thing, only at 195*. Now... when you're towing, your engine is probably at operating temp, meaning the thermostat is wideopen. From this point forward, the thermostat might as well not be there... it's wide open, right?

    If your rad cannot eject heat fast enough, and your engine continues to add heat, you will overheat. Period. With either thermostat. Every time.

    That means if you're pulling 9800# high-front 5th wheel, into a hot wind, in traffic, in Southern Utah, and your IATs keep climbing, and your EGTs keep climbing, and your tranny unlocks and you end up running in 2nd or 3rd, you are screwed. Ask me how I know this.

    Your truck has only a single thermostat, so there are some critical things you need to do:

    - pull rad and clean it. Many times. Takes 3-4 hrs to do it right.
    - replace thermostat with genuine ACDelco thermostat. Make sure it's the right one for your truck. Use no substitutes!
    - check your fan clutch to make sure it kicks in properly and at the right temp. If not, either replace it or modify it to work right.
    - do the TCC lock mod.
    - Minimum3" exhaust, definitely replace your 1994 frankenstein downpipe with a 3" mandrel-bent one. I mean that.
    - Turbomaster and GL4 chip.

    Voila... no overheating with big load.

    Now.. about your mileage. You are in Kansas. It's December. In your ambients, I bet the truck isn't warming up with that 180* thermostat; between that and #1 winter fuel, yeah, it'll cost you 5-7 mpg..

    I suggest going back to the proper 195* thermostat, but at the very least, Put cardboard over your grille for a tankful and see what happens to your mileage.


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    1994 Silverado SLT c2500, 3.73 gears, "F" engine, 300 000 km, Snorkle-ectomy and Airbox mod, TCC mod, Mandrel-bent DP and crossover, 3" Duramax resonator, 4" constant-radius out the back.

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  12. #12
    OBD 1 Master Pruittx2's Avatar
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    Yeah.. What he said"


    1995 GMC SLT Suburban 153,000 miles
    Buddy Super chip, 4" exhaust, FTB Mod, Bosch Injectors, GU6-G80, Killer PMD Cooler FS PMD #9
    Duraterm Glows, TurboMaster, K-47 airbox, D-max fan New Mod'd clutch






  13. #13
    Registered User drocker's Avatar
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    JiFaire, current t stat is one from autozone. I know for sure it is a stant brand. I will purchasing a ACdelco from one of the online dealers that is a 195 stat to replace the current. It wasn't overheating before but the 195 stat that I replaced looked just like the stant one. I will never probably tow anything with this vehicle. I have seen the ACdelco and they do look a little different. The link below shows the one I will probably get but it list it as a 190 t stat. Part # listed is ACDELCO Part # 13191. So is this right?

    http://www.rockauto.com/catalog/moreinfo.php?pk=79801
    Last edited by drocker; 12-27-2009 at 12:48 AM. Reason: Additional text
    1994 GMC Yukon 2dr 4wd 6.5 TD
    8th VIN#: S
    Mods: CAI, custom ground, 180 T-Stat, Hi Beam Mod, 285/75/16 BF Goodrich T/A, EGR block off and Hogged out upper intake. OPS mod. #9 resistor.

  14. #14
    Registered User Woody35's Avatar
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    btw 285/75/16 are not 1 tire size above stock its two, and on some trucks its almost 3 that came with 235s.
    1997 Chevy K1500 4X4 EC/SB 6.5TD/4L80E
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    Walbro FRC-10 LP, Bosch Injectors, Bosch Duratherm Glowplugs, Relocated PMD,
    Leroy's Steel Braided Oil Cooler Lines, 4 inch Diamond Eye Exhaust, 177K miles SOLD!
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  15. #15
    Registered User drocker's Avatar
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    ^ sticker on driver side door panel has tire size as 265/75/16. My 1994 fullsize chevy blazer (350 gas) that I sold had 265/75/16 which was shown on door panel as well.
    1994 GMC Yukon 2dr 4wd 6.5 TD
    8th VIN#: S
    Mods: CAI, custom ground, 180 T-Stat, Hi Beam Mod, 285/75/16 BF Goodrich T/A, EGR block off and Hogged out upper intake. OPS mod. #9 resistor.

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