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Off Idle cruising causing a vibrations/shaking

1063 Views 12 Replies 3 Participants Last post by  mccviii
Greetings astro family

I've been nursing my 03 astro for the last number of years. and I have hit a wall with what I think I can diagnose on my own. I suspect that a mechanic with a real time data analyzer could help pinpoint where the issue is, but I don't have a trusted mechanic with that equipment and don't want to go $earching for that guy/gal just yet...

This issue has been around for at least a year or so, seems to be getting worse over time (deterioration of a component). When I initially felt it, I thought it was from changes in the pavement on the highway making the van feel/sound slightly different. Now, the shaking/sound is more pronounced, and has a direct relation to gas pedal input. It can happen at various speeds depending on transmission gear, but for example, if I'm cruising at 35mph or so, and looking to sustain speed with "maintenance throttle" level of gas, the van will sound/feel like it's going over mild rumble strips. To get it out of the vibration zone, I can give it more gas (if traffic allows) or remove my foot from the gas pedal altogether. with no throttle input, the van idles smoothly.

I thought initially that the issue was linked to a specific RPM, but as the issue has grown more pronounced, I think it's closer linked to throttle input primarily. Plugs and wires were replaced about 2.5 years ago. Fuel pump was replaced a month ago, which seems to have slightly changed the character of the throttle response. The fuel injectors were replaced at a dealership before I purchased the van in 2016.

No engine error codes except for an occasional "open reference circuit B" code that will appear and clear on it's own. previously had a sporadic "cylinder 2 misfire" code but that hasn't appeared in months.

I fear that the problem lies in the injectors, being unable to supply proper atomized fuel at small throttle openings, causing stumbling, but theres an argument to be made for electrical/ignition components as well...

any thoughts or relevant experience to share?

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What you describe "might" be the very common worn distributor issue. The worn gear causes erratic timing "at part throttle" only... idles fine and full throttle fine. It can happen in any gear, and virtually any speed, when vacuum is highest at part-throttle. The erratic jerking operation is often misdiagnosed as a transmission issue. It can be subtle or obvious. It only gets worse with time.

Can't say for sure this is your issue.. but it was mine on several vans now, as well as many others who've dealt with the same trouble.

I'm less inclined to believe it's an injector issue, since it was replaced already.
But it's always a possibility too.

Here's an excellent link on the distributor:
Don't over-think this. The distributor gear wear is quite common. I've personally found the distributors to be good for about 150K.. but experiences vary. It may or may not be your problem, but what you describe sure sounds like it... it was for me on more than one occasion (especially the part-throttle thing).

Fortunately distributors are fairly cheap (less than $100) and easy to install. Carefully mark old one before pulling, and be sure to drop new one in lined up identical. They spiral in.. so I make two marks (direction rotor points), one at top of gear, and one when fully seated. If done accurately, there will be no further action needed. Don't turn over engine. If it doesn't fully seat, you can tweak oil pump shaft with large long screw driver.

Even if it turns out not to be the trouble.. it very well could be soon enough anyway. Changing the distributor on these engines is almost like changing the plugs. Even if not needed, it will eventually need to be done most cases.
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Here's my own video of my worn distributor that caused issues for me. The amount of play in old unit doesn't seem overly excessive to me.. but apparently the computer had serious issues with it. My guess is that it physically jumps around while the motor is running.. and I discovered the problem most pronounced at part-throttle high vacuum when timing is over 35-40 degrees.

the vibration issue somehow is the worst it's ever been. found the culprit. god damn exhaust heat shield that sits above the muffler...
Where I come from.. something "rattling" is different than a vibration.
Good job.. glad to see you resolved the trouble.
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