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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: 1979C20 on October 05, 2011, 05:58:15 am
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After I firat put my new tires and wheels on my truck, according to my GPS unit my truck was running 1450RPMs at 45mph for a few days. And the past couple days its been running 1900RPMs at 45. What would cause a change all the sudden? Its the truck in my sig.
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I would say your GPS was wrong the first couple of days. GPS units are computers and it's possible that the data it was gathering and prcessing wasn't accurate. It seems unlikely that no change to your vehicle could result in a lost of MPH at RPM. If it were an Auto I suppose the converter could be slipping but since you have a manual, I'd can't see a way.
The only way RPM could increase when your drivetrain configuration stays constant would be if there was some new drag on the drivetrain that caused the motor to have to work harder to maintain the same speed. A gain of 450 RPM is significant if you ask me. Even being in 4 wheel drive by mistake wouldn't likely result in such an increase.
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Well, today my RPM to MPH changed again. It went to 1700RPM at 45MPH. BUT, last night I cleared some mud off of my headers, control arms, trans xmember, and it rained yesterday and cleaned off my wheels, tires, and stuff. So I guess it was the mud. Maybe I had mud or sand in my drums or caked on my rotors. Ill take a power washer to everything and see if it helps. And I need to check my rear dif fluid level, its starting to moan while im slowing down.
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Also, my stock speedometer is off by 10MPH, 50 on the speedo is 40MPH. And that matched the GPS unit, 55MPH when the GPS said 45MPH. So, my RPMs did infact change.
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Would have been good to know about the mud. Mud can add a significant amount of weight to a truck which can make yor motor work harder...
Just wash your truck and drive it dude. :o
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i dont understand how RPMs would change without the MPH changing in a vehicle with a manual transmission. Wouldnt that mean the clutch is slipping?
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i dont understand how RPMs would change without the MPH changing in a vehicle with a manual transmission. Wouldnt that mean the clutch is slipping?
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sounds like the GPS unit isn't accurate enough for this kind of testing. whats the margin of error for the GPS unit?
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And I need to check my rear dif fluid level, its starting to moan while im slowing down.
Might want to look into this soon.
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yeah mud or not you could pull a trailer loaded with 20k lbs with a manual rpm to mph is going to stay the same as long as the clutch is not slipping and if it was slipping by 500 rpm you would smell it and be on the side of the road before to long
only thing that would change would be the thottle position with the change in load
if you are running a 33 tire 45mph = 1879 rpm / 35 tire 45 mph = 1771 rpm
which one are you running
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Im running 31's, and its at 1450RPM.... And thats accurate witb my GPS speedo on my phone, my GPS on my dash, and my stock speedo. And its also accurate with speed signs around town.
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Alrighty, for the past few days, my truck has been running 1600RPMS AT 45MPH and 1450RPMS at 40Mph consistantly, and accurate with my GPS unit, phone GPS, and speed sensor/signs around town.
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with running 4.10s and 31s you should be close to 1800 rpms so something is off. with what info your giving your running a 3.27 or around there
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No, its 4.11s. Unless my tach reads wrong.
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with 4.10s and 31s you should be at 2k at 45mph
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i thought ford had 4.11?
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the .01 is going to make that the difference
if you are trusting the gps units the I would say the tach is going
does the motor sound different when you are having the big swings in rpm on the tach 500 rpm should be audible difference
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Okay! I think I discovered the weak link tonight. I almost never drive over 50 anymore. But, last night my dad and a bunch of family friends wanted to go camping and its mostly highway to get there. My tach didnt go over 2000RPM past 50MPH in 3rd gear. I was cruising 70MPH and my tach said 2000RPM. But, in first and second it has full swing. I was "race shifting" from a stop to see if the tach would go past 2000RPM and it worked like normal. I hit 5500RPM and shifted to second, rode that to 4000RPM and then in 3rd it wouldnt go any farther than 2000RPM when it should have been at like 3000RPM at those speeds. The tach would stay at 2000RPM until my speed went under 50MPH. What would cause this behavior with the tach? Is it the tach itself, or something in my distributor?
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since your distributor has no ideal what gear your in i dont think its the signal there. and your tach seams to be the problem. buy another one since you will probably buy one anyway for your "other" truck and check what it says
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ok i have the same tranny and gears but i have 33's. i run 1750-1800rpm at 45mph and 2krpm at 50mph, its sounding like your tach is going south... sad to say i've tached mine out at 7k before on a slick logging raod in 2wd and it still works fine and accurately. good luck
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The changing of the tires and wheels could have started the whole problem. There could be a vibration coming from the new tires. And, at a certain speed the vibration is affecting a wire, marginally burnt coil, or something else, causing an interference with the signal inside the distributor (harmonics coming from vibrations can do strange things.) Also, Don't leave out the weather, rain can cause problems too.
Moving on, I'm assuming your truck is a 1979 and still has the HEI distributor. If that is not the case, make sure your tach is connected to the negative side of the coil. It's been awhile sense I've worked of this year truck and, so I don't know if Is this an OEM tach? If the tach is correctly hooked-up, and the tach seems to be the only device malfunctioning. What I would do is, see if you can find a cheap tune-up tach (from a friend, garage sale, or buy one with a money back guarantee :-) hook it up and see how it responds. If it responds normal, you have your answer. Your tach is a boat anchor.
Hope this helps...
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Its HEI, and the tach is hooked up to the designated "Tach" plug on the cap. There was originally a green wire plugged into, but I have no idea where that wire went. My truck has developed a decent vibration. The tires and wheels are all balanced. I think its from mud, or maybe my driveline. My motor mounts are going out again and my tranny mount is missing 2 bolts now. And my speedo cable broke last night. Along with an oil leak, coolant leak, header leak, power ateering leak, high temps, low oil pressure, and high idle. My heater core started leaking at the end of last winter so I bypassed it and now its getting cold. And since this is Arizona, it has random 100° days, and I havent put refrigerant in my AC yet. This truck is really getting on my nerves lately. I need to get my K20 soon so I can do a hit of an overhaul on my C20.
ANYWAYS, so vibrations could be throwing my tach off?
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It is somewhat of a stretch but, it's certainly possible. The replacing of the tires and wheels shortly before the problem started may not be a coincidence. Glass breaks at a certain frequency, right? Well, vibrations are a frequency! As I said, you could have a problem in the HEI unit or a wiring problem. And a vibration coming from mud on the wheels, a bent drive shaft, or incorrectly balanced tire, could be exacerbating the problem. If I were troubleshooting this, I would splice another tach in to that " TACH" wire and see what happens. Or you could remove the radiator cap and drive another truck under it :-)
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My drive shaft isnt bent, but it might be out of time. Ill try another tach soon. Theres one on both of my parents jeeps, so ill see if one works.