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Does this make sense. RPM Drop..........*UPDATE pg 3..

badmix

badmix

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I dont drive my K5 that often, usually just the weekends . Anyways. was driving on the freeway and noticed my RPMs were about 2-300rpm lower than normal at speed. Normally at 65mph. Im at 2100-2200rpm. but today it was 1800-1900rpm. The tach wasnt hung up or anything. But there have been some changes since I drove it on the freeway last, so you guys tell me if this all adds up or makes sense.


1) change oil - the old oil wasnt that bad but still. Put 10w40 w/ a bottle of Lucas oil stuff

2) Temps were cooler 65 degrees

3) Put air in the tires. Were reading 31psi. bump em up to 35psi

4) Replaced the 25 year old OEM starter with a napa one.


So is it possible that the old starter was putting some drag on the motor?

could cooler temps drop RPM like that?

Oil change with fresh oil make a change like that?

Tire pressure that big a diff. wouldnt think 4psi would?

Or could the tach be off? Ive never had any issues with it b4. Its an autometer superlight silver series.

Thanks
 
The only thing you listed that would affect the rpm's is the tires. How much taller did they get when you put air in them?
 
When the torque convertor goes into lock-up the RPM will drop about 300-400 RPM. Maybe you're having a problem with the lock-up? You might want to have that checked out because if it doesn't work you will eventually kill the trans.
 
Do you have a 700r4? Sounds to me like the torque convertor is locking up.
Maybe it wasn't locking up before because of a bad connection or something.
The work you did may have made it start locking again
 
Hmm. I have a 700r. Ive never had any issues with it in the past. It always goes into the proper gears it needs to. When Im running around town (below 45mph) I put it in Drive. But when im on the freeway, I put it in OD. I dunno, seems odd. i do have a tranny temp gauge, it barely gets to 150degrees. When Im really off roading, it wont go above 175degrees.

I do plan to change the fluid and filter in the tranny this spring.

I cant see the tires making the diff. They didng "grow" when air was added (they are 33 x 12.5 x 15 M/Ts).

One of my best guesses is the starter. But who knows.
 
The starter has nothing to do with how the engine runs, its only job is to crank the engine over until it starts then it is disengaged and does nothing.
 
My thought on the starter was that there could be some kind of drag on the flywheel. My started was getting "heat soak" when id do alot of stop and go errand running and wouldnt start, reason I replaced it.

I dunno, just weird that there would be an RPM drop. Im pretty "intune" with my truck that is why im perplexed at this. It doesnt make sense for it to drop. The air in the tires could play a part in that, more air, means taller fuller tire. But would it drop 200rpm?

I guess the oil change would help things spin easier. But ive put the same oil in there in the past and nothing changed then.
 
badmix said:
My thought on the starter was that there could be some kind of drag on the flywheel. My started was getting "heat soak" when id do alot of stop and go errand running and wouldnt start, reason I replaced it.

I dunno, just weird that there would be an RPM drop. Im pretty "intune" with my truck that is why im perplexed at this. It doesnt make sense for it to drop. The air in the tires could play a part in that, more air, means taller fuller tire. But would it drop 200rpm?

I guess the oil change would help things spin easier. But ive put the same oil in there in the past and nothing changed then.
Click to expand...

The only time the bendix gear even touches the flywheel is when the starter is cranking, once the engine starts the rotational force of the flywheel throws the bendix gear back into the starter.
 
My first guess is the tranny (more specifically, the torque converter locking/unlocking.)

It may be that the tach is wearing out or the ignition module is doing something different... but the difference you're talking about just screams "torque converter."

I think -- I've never had one of them fancy newfangled trannies. All I've ever had are Turbo 350's and 400's.

And if the starter was dragging, you would hear the most hellacious noise -- like if you crank the thing too long and the starter grinds? It wouldn't do it for too long as something would break or fry after a coupla minutes, prolly, but, umm, no, it's not the starter dragging you down at highway speeds :D

-- A
 
Jus curious why it would be the torque converter. My tranny shifts fine. It goes into Overdrive and dont have issues pulling off the line. I dont know much about the trannies and converters so im definently listening, I just want to understand it all and how if its going bad or what not, the RPMs would/could drop.
 
If the torque converter lockup was malfunctioning before, it would still accelerate fine and shift through all of the gears. The lockup is an efficiency feature and the trans will otherwise operate fine even if it is malfunctioning. The lockup function is applied when the trans has reached OD and the load is relatively light. When it locks, the torque converter stops slipping and the RPMs drop a little and mileage increases a little.

If this is what your problem is, it doesn't necessarily indicate a mechanical problem with the torque converter, it could be as simple as a loose harness.
 
a good way to check the lock up is while your running down the freeway just barely touch your brake pedal,you should hear the motor RPM change then return to normal when you release the brake
 
The torque converter does its job slipping when needed basically,then at highway speed it locks to eliminate heat and increase fuel economy in theory. In some vehicles its hard to tell when it locks up but if you feel it its like shifting into a 5th gear. Im not sure about the 700R4e`s but my jeep cherokee when crusing along at say 65 and you even touched the brake pedal it sends a signal to the computer and unlocks the converter and it could be easily felt,you`d gain about 300-400 rpm. I have some vehicles that sit weeks on end,If you said it gained 300-400 rpm Id say start looking at wiring because mice moved in and ate something up.
 
Wonder if anything got moved or tweaked when the starter got changed? Ill have to check things out tomorrow.
 
None of the items listed on the OP have anything to do with the engine running at a slightly lower rpm for a given speed, and the TV cable adjustment mentioned above only affects how the trans shifts. There is basically a direct correlation between engine rpm and vehicle speed and the only things that can change this is the gearing or tire size (and I don't think a few pounds of air in the tires would be noticable). There is no way you can magically make the engine run at a lower rpm at the same speed unless one of those two items is changed.

As mentioned previously this just screams "torque converter lock-up". The lock-up function is seperate from the normal torque converter function (take-off, gear shifts, etc...) and therefore would not notice anything. The only time the lock-up feature does anything is once you reach a steady state cruising speed it will lock and eliminate the small amount of slippage between the engine and trans, attributing so the 300 or some rpm change.
 
I agree with the torque converter locking up...

However, it seems as if it's working correctly, not the other way around, which is odd. If it wasn't working before, you wouldn't experience the lower RPMs at cruise on the highway. :confused:

It's like it suddenly started doing its job, which is odd. :thinking:

Like the others said, the stock lockup feature on the 700R4 is controlled electronically. If a connector was loose before, it may have not been working correctly. You may have hit a bump that jarred it back into position and suddenly caused it to work properly.

:dunno:
 
From the specs you posted on your truck, 700r4 33" tires. It is set up similar to mine.
At 65 on the freeway I run 2100 rpm with the convertor unlocked. When it lockes up RPM drop to 1900. Very similar to what you are describing.
There is a electrical plug on the trans that controls the TC lock up. Check the connection. It may be loose or corroded. As i said the work you did may have bumped the connection and got the TC lockup working again. I dont think there is a problem with the trans. It is just working properly now.
 
heh..thats cool. Maybe this will explain something. If I drive my drive in the D position and then get up to 60mph or above, I put the selector into OD and then the truck will shift and RPMs drop. Previously they would be 2100-2200rpm or so and stay there. But now, if I do the same exact thing, it goes to 1900-2000rpm.

So was it working b4?

I understand what you guys are saying about the TC, but wonder what it was doing prior to this. The only thing I can think that changed to effect things, would be the starter that got changed, not the fact that the starter was swapped out, but that someone was messing around under the truck, etc.
 
badmix said:
heh..thats cool. Maybe this will explain something. If I drive my drive in the D position and then get up to 60mph or above, I put the selector into OD and then the truck will shift and RPMs drop. Previously they would be 2100-2200rpm or so and stay there. But now, if I do the same exact thing, it goes to 1900-2000rpm.

So was it working b4?

I understand what you guys are saying about the TC, but wonder what it was doing prior to this. The only thing I can think that changed to effect things, would be the starter that got changed, not the fact that the starter was swapped out, but that someone was messing around under the truck, etc.
Click to expand...

If you were under there, it's much more likely that you nudged the wiring or connector going to the torque converter.

As others said, take it for a spin. Let the RPMs settle down at 65mph. Then GENTLY press the brake pedal. You should see/hear the RPMs go UP. The electronic lockup is tied into the switch under the brake pedal. When the brakes are applied, the torque convertor should unlock, allowing slippage, and therefore causing the RPMs to go up.
 
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