engine peaks around 4000 rpm. vibration at idle

stantheman

Original poster
Member
Dec 26, 2012
4
Hello all,
i have a 2002 envoy 4x4.
let me start by saying i have been on your site all day looking at threads trying to diagnose my vibrating envoy. i have learned a lot so far and have checked mutiple suggestions and have gotten to a point where i am stuck.
bought truck used in 06 with 68000 miles, now bumping 250000. this has been a great reliable truck and hope to get many more miles down the road.
history. Have maintained vehicle very well, all service has been done close to on schedule. The trans was replaced about 40000 miles ago but all has been well with that. about a month ago i started feeling the vibration at start up and when in gear but would go away when truck was in motion. did notice some power loss but have had no lights come on at all. after reading your forums i decided to replace motor mounts. put mounts in today and vibration is still there. i read some more and decided to do as suggested and remove belt and run. vib went away. at this i am elated because at least i know its something on the front that can be change easily. checked all pulleys and belt. all seem to spin free and tight, not loose in any way side to side ,up or down. water pumpo and clutch fan were replaced about 20000 ago , couple years on alt, never on ac or p/s pump(that i know of). so i put belt back on to see if i could hear anything to try and n arrow it down. Vibe comes back , hear no funny noise up front, all looks like spinning well, nothing looks out of shape. i had my son press the gas to see if that made a difference. nothing. I then remember passing a car on the road the other day and at about 3500rpm i heard a ratling noise from the motor just before it changed gears. so i decided to gas it in park. floored it peaks at 4000 rpm or there abouts. no more no matter how long u hold it there. can definitely hear something but not sure. yes, tb has been cleaned. Last thing i did before posting was was pull coil wires one at the time and all 6 made a noticeable difference in idle, for the worse. so i think all 6 are firing. last note i drove this to my brothers house for xmas on interstate and cruised at 75 as always . no problem for 200 miles, except vibe at idle.
Want to thank all of you for the wealth of info i have gotten so far from your site. Thanks in advance for any help. Maybe cats? thanks Brian
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
There is a rev limiter in neutral and park to reduce transmission abuse from kids who would rev up, then dump it into drive. That's entirely normal.

Motor mounts are a specific fix for a 10 Hz rotational vibration that starts out being worde in DRIVE than neutral.

You probably have a bad ac compressor or PS pump. Ever changed the PS fluid? Does the nature of the vibration change if you turn off the AC using the snowflake button? How old is the belt? Have you checked compression?
 

stantheman

Original poster
Member
Dec 26, 2012
4
Replaced belt when w/p and c/f were changed 20000 ago. last changed p/s fluid at 200000 mark. turning snowflake on does cause idle to drop for a sec and vibe continues. thanks for the quick reply.
 

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin

blwn31

Member
Jan 26, 2012
12
I too have this same issue. I haven't looked into motor mounts yet, hard to believe with only 136K on the clock they would be bad. But, I have suspension parts that are shot or have already been replaced, so... I will be following along as to your fix.

Keith
 

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