Shaking: Could it be harmonic vibrations?

[Chad91Q45]

Ok, on a search for vibrations, I read some of Q45Techs stuff... but kinda got lost in the verbage and math of it.

Basically what happens is when I am doing a slight acceleration uphill at 42 or 48mph (no other ranges really) i get a shaking kinda like when you have a manual in 4th when it should be in first. So I thought maybe the transmission was trying to down shift, but I didnt see any rpm spikes like I was expecting. I was figuring ~40 would be a speed that it could shift so it didnt seem too out of the question, but with rpm staying constant... I dont know how feasible that is. It only happens when I am at those 2 speeds, though, and then disapears, so it only lasts a moment and not very often at all (came acrossed it by chance the first time then took some hills to see what speeds and how to replicate the event). The other possibility I was thinking was that it was harmonic vibrations since it disapears at higher speeds and doesnt come back (in my tests i took it up to 90 and it never came back).

Any ideas? Would it be possible for the transmission to upshift and down shift fast enough I wouldnt be able to see spikes? I mean, its not like I drive at that speed all day long or on hills much at all for that matter, just wanted to know if this was a correctible thing and if so, how.

[maxnix]

Gear, speed, altitude, temperature and inclination of road are what?

[Q45tech]

32 valve Engines are not really designed to run under 1700 rpm [50 mph] in 4th witnh 26' tires and 3.538/0.694 gearing..........they can, but are constantly an inch away from lugging ..........every thing must be perfect.

[Chad91Q45]

max, 3rd or 4th i assume, whatever the at puts it at at those speeds.Speed- 42 and 48Alt- umm, probably 150' above sea levelTemp-70ishInclination- first time I noticed it 6% grade, when ive been testing it, probably 2 or 3% grade

[maxnix]

Lock up in 4th would be extra hard on the transmission. Best to be in third.

Please report back rpm, gear and speeds.

My advice, don't shift into 4th below 55-60 mph on the level. I do almost all climbing in 3rd gear or less. Better for engine, transmission, and feeling good about maintenance.

[VimyJ]

I constantly manually manage the transmission in my J. Expressway jams under 30 mph I'll leave it in 2nd and not race to keep on the bumper of the car in front jamming on the brakes to slow down, etc. Surface streets I'll leave it in 3rd 'til about ~45 mph. Since I've started doing this my mileage has actually increased substanitally. Lugging is kept to a minimum and shifts are considerably reduced.

[Q451990]

Chad,

What you're describing is definately lugging. I noticed the symptom more when I had back knock sensors, but anything can cause it including dirty injectors, dirty air passages, etc. Even when the car is in perfect mechanical condition, it's still right on the edge of lugging the engine. The ECU has the car in 4th at 40MPH for EPA fuel ratings, but overiding it into 3rd unless you're at 50+MPH is probably a good idea for engine and transmission health.

Heath

[Chad91Q45]

yeah, forcing it into 3rd got rid of it completely. My maxima never lugged and the basic feel of the cars was similar so I hadnt even really thought it was a possibility.

How long could you run on bad knock sensors if that is the case? I wouldnt have time to do it myself until like... January of next year. Completely swamped until then and I dont want to pay someone else to do it.

[maxnix]

Let me ask how long you want your catalytic converters to last?

Price them (OEM, not aftermarket short-tun cheapies) first before answering, please.