Just bought an '87 D100 (auto) 2 weeks ago with only 60,000 miles on it for $500. Trucks' in awesome shape, but I do have a performance related question. If I drive the truck around town, (under fifty-five), its incredibly smooth and rather quiet. Once it gets on the highway, the engine sounds like its screaming, (over 55mph), and when I take my foot off the pedal, it feels like I've dropped a gear, (like a manual trans). The tranny is shifting up all three speeds, and I've changed the tranny filter & fluid, (no surprise pieces or smells from the pan). Is this normal for slant 6 in a full sized pickup? I understand that it wouldn't be a "speed demon", but am I asking too much for a smooth/quiet highway ride from a 20 yr old, low powered truck? Do you have any advice on a possible underlying problem that I should be looking for? The truck has sat for some time and the u-joints may need replacing, (small clunk!~ from P/N to D/R), could the noise and performance issue be due to a driveline vibration at a specific speed?

That's easy to answer...if you detect any performance to speak of, you're not in an '87 D100 with a slant-6!
It is not. It might be interesting to learn what rear axle ratio is installed and what tire size you're running; severe engine braking effect upon lifting off the throttle and "screamin' on the highway" are indicative of very low rear-end gears (3.91...?), but such ratios were generally not installed in trucks like this, so unless some previous owner swapped in a non-stock ratio or the truck was specially ordered, this leaves transmission problems as a possible cause. Are you quite sure you're seeing all three gears? When you're on the highway, does anything change if you shift from D to 2?
No, you're not asking too much, and — depending on what kind of emission test you need to pass, if any — there's actually a fair amount of improvement that can rather easily be made to improve power and efficiency. But first we've got to find and fix the cause of your present symptoms.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thats the second time I've heard that performance wasn't a strong point... No worries. The truck shifts from 1st to 2nd around the 20-35 kms range, 2nd-3rd between 35km - 60km, then just gets increasing louder after 80-85 kms, (120km sounds like a wind tunnel under my feet, I don't push it much past). tire size is 205/75/15, (I've been trying to find 236/75/15's for camping as well as getting rid of the "car tire" look)...are you saying that its proobably not the greatest idea to go bigger? I'm pretty sure the gear ratio has not been altered. I live in Calgary, emmision testing only happens if your killing mosquitos and small birds as you drive by them in flight...

Bigger tires will slow down the engine for any given road speed. I run 235/75R15s on my 1989 D100 (sadly, no slant-6; it's got a 318 and I have to pass a yearly smog check). Your shift speeds sound about right, but you're still describing symptoms that suggest a problem with the transmission (or, now I think about it, with the torque converter).

What level of do-it-yourselfer are you?

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I'm an apprentice Agro mechanic for John Deere, I can follow a shop manual on regular vehicles, but if it ain't green and yellow and covered in cow crap, it takes me some time to work it out. What would you suggest? I'll tell you if my skill sets match.

Put a tachometer on the engine, set the parking brake, hold your left foot firmly on the service brake, shift into "Drive", and rev the engine. Quickly note the RPMs the engine achieves under this kind of load, then take your foot off the accelerator, shift to "Neutral" and let the engine idle a few minutes. This tests the stall speed of the torque converter and can point up certain problems therewithin.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I'll give that a whirl. Just took the truck out for a spin, took 'er to 85kms and dropped it into 2....glad I wasn't going faster. She is getting up to 3rd gear, and dropped to second as soon as my hand left the stick. Although...no traffic on a new road and the noise wasn't too bad from the engine. The truck is probably geared more for pulling than racing as a 3 speed, but the downshift feeling wasn't as apparant. (like I said, the truck has been sitting a while). As to "easy"power and efficiency improvements, what would you suggest? Other than changing all the fluids and filters, I haven't done anything to the stock truck.

The gear changing symptom is caused by the lock up torque converter disengaging when you let up on the gas pedal.

Mmmm...no, a strong engine-braking effect when letting up on the throttle could be caused by the torque converter clutch failing to unlock.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

If you can get a tach on the motor, and as long as the torque convertor is locked up and with a 28" tire, 2800 rpm should be about 72mph/120km/h with a 3.23 rear. 3.55 would be a bit over 3000.

BUT,
The computer controlled torque converter still disengages when you let of the gas.

That is how it is supposed to work. If it's not disengaging properly, the severe-engine-braking sensation reported by garagamus will result.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.