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Engine Vibration


cpk

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First, some background...

1989 Cadillac Allante with 4.5. I rebuilt the engine last summer, painted the car last fall, and drove it somewhat regularly through the winter till today.

It runs well. Since the rebuild, I ended up replacing the oil pump with the newer gerotor style, and replaced the fuel injectors due to a few leakers. All the engine mounts were replaced, has new bearings and rings, waterpump, plugs, wires, cap, radiator. Put in a used transaxle over the winter as well.

Currently, I have a slight exhaust leak.

Anyhow, I have a slight, but annoying vibration that is a little difficult to describe. It seems worse when cold and idling, but it sticks around at all RPMs. Varies with RPM. I thought it was just the exhaust leak, but I noticed that sometimes when it is loudest, if I take my foot off the brake at idel, the car will creap forward in a with a slight jerkiness that mimicks the vibration.

So, my first thoughts are to try and find ways to isolate the problem, assuming I can reproduce it regularly enough. I can't hear it under the hood, only inside the passenger compartment. More importantly, I can feel it through the steering wheel and brake pedal.

I'm thinking a good first step would be to run the car without the serpentine belt. This would eliminate the power steering, water pump, A/C compressor (I turned the climate control to econ to eliminate this). Also, if this is the notorious bearing thump (which I believe is unlikely, but stranger things have happened) that should also be eliminated/mitigated by taking that stress off the balancer pulley.

If that doesn't work, then the next step would seem to be to disconnect the torque converter from the flywheel, push the torque converter out of the way, and run the engine. That would eliminate the TC and transaxle as sources.

This isn't a huge thing, and I'm probably the only one who'd notice, but it annoys me. Its a difficult noise to describe, almost like a severely muffled rattling, but it just feels like something is out of balance somehow. Maybe I'm interpreting it wrong and its some sort of imbalance between the cylinders, either fuel or ignition.

I've got some exhaust parts coming in, so hopefully I can address the noise while I have the car in the garage for repairs.

Any other suggestions I should consider?

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it is not clear from your description what is it vibration or noise or both. are your brakes OK? rotors, calipers etc. check out the rotors first. I have got a rattling noise after an accident (skidding) i cannot identify. I have made some reserch through the internet and foun tonns of similar cases when people cannot find the source of the noise/vibration. try to give us more specifics. i Know it may be difficult, but just try.

The saddest thing in life is wasted talent

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You note that you rebuilt the engine.

When you separate the engine from the transmission, you are supposed to mark the torque converter and flexplate so you can return it to its original orientation. If this was not done, the only way I know to realign them is trial and error.

If the engine runs well and the vibration feels like it is from the engine, a possible source could be the torque converter balance. This type of vibration is best felt by gently accelerating the engine, and holding it at 1500 RPM. Then try a few higher speeds, holding for at least 10 -15 seconds each time noting the difference. Do not maintain an engine speed of over 3000 RPM while in park; no need to drive it.

Mark the flexplate and torque converter with chalk or other marker. Remove all attaching bolts and rotate one bolt hole. Reattach the bolts and run the engine.

You may have to do this two or three times. Keep track of which position is the smoothest position.

-George

Drive'em like you own 'em. - ....................04 DTS............................

DTS_Signature.jpg

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Thanks guys. The vibration occurrs at idle, without the vehicle moving, so I'm pretty sure its powertrain-related.

I used the TC from the replacement trans, so rotating it for better balance sounds like it would be a good idea.

Seems like I could mark it from below (to record where it is now), then disconnect the converter. If the vibration goes away, I could try rotating the TC to one of the other two settings when reassembling. While I did a pretty good job of rebuilding the engine (so far) , I could never get the transmission to work after I rebuilt it. A known-good, lower-mileage used one became available at a great price, so I just replaced it and put off troubleshooting the original trans for sometime in the future.

I have replacement exhaust parts, so I'm hoping to replace those this weekend. That should quiet the overall noise down so I can hear if the vibration is audible too. I think it is, but its hard to distinguish from the exhaust.

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How about defective or loose engine mounts or the engine grounding out at some point? Lift the hood, hold the brake hard and in drive and reverse see if the engine lifts when you press the accelerator, gently in case you do have a broken mount, the engine will rise quickly if you are not careful. Also have a look at the tranny mounts. That it is only at idle makes me think its a harmonic. Who rebuilt the engine and what was done? I think I heard where the harmonic balancer can be defective, maybe it was damaged during the rebuild. Guru would zero in on this type of problem! The idea below about the torque convertor and flywheel is good, but the parts would really have to be out of balance to cause that. I may be wrong but I doubt that they consider that issue when the car is built as all parts should be balanced. My dad always marked things however, and I do understand the reasoning. I dont know how bad your muffler system is, but you may be surprised that is the cause of your vibration (uneven backpressure issues on certain cylinders), and once you replace your muffler system you may find the engine is smooth. Good Luck and let us know how it turns out, Mike

Early on sailors navigated by the stars at night and the North star became the symbol for finding ones way home. Once you know where the Northstar is you can point your ship in the right direction to get home. So the star became a symbol for finding ones way home or more symbolically even finding ones path in life.

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This may sound silly compared to the all the other complicated fixes, but my advice would be, replace the gas cap.

Don't crinkle your brow yet!

I replaced mine after a discussion here on the board about such a "proceedure", and I was supriesed at the difference.

Turns out the gas cap is a sophisticated part of the fuel system, for it acts as a valve to control pressures and vapour flow in the fuel system. They are said to need replacement every 2 to 3 years, for its ability to "regulate"wears out. This can cause a mis pressured fuel system and all sorts of gremlins could apear...poorly operating fuel injection, bad fuel economy, ROUGH RUNNING ENGINE, and so on...

That little massage on it that tells you to turn it tillit clicks a couple of times is important too. That gives it the proper tightness to do its job.

Like I said, I immediatly noticed the difference in my Northstar - ran much smoother, better fuel economy, and didn't seem to have to work as hard, for it shifted at lower RMPs than I was used too!

You can search the archives to find the discussion where I got al this info.

For a mere 20 bucks, try it!

and if you havnt't replaced it for a few years, you'll know you made a sound investment, even if you problem is more complicated!

Hope this works!

Scott

" ...'took my Cobra down t' the track, hitched to the back o' my Cadillac..."

- Jan & Dean, 'hey little cobra'

Scott

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Great post Scott! Thanks I will replace mine!

Early on sailors navigated by the stars at night and the North star became the symbol for finding ones way home. Once you know where the Northstar is you can point your ship in the right direction to get home. So the star became a symbol for finding ones way home or more symbolically even finding ones path in life.

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Thanks again for everyone's replies.

I did replace all the mounts when I rebuilt the engine.

I apologize that I haven't been able to describe this better, but its a little hard for me to describe. The vibration or roughness is most noticable at idle, although I can feel it throughout the RPM range.

The sound is consistent, in the way that a stuck lifter ticks with RPM, but the sound/vibration isn't distinct like a tick, it's more along the lines of a muffled shudder, if that makes any sense. There's no stumbling as with a rough idle and no hesitation when giving a bit of gas.

I just came back from lunch. I held the car with the brake in gear and rev'd the engine to about 1500. There was still a discernable, but less evident, vibration. When it was back down to idle, I let off the brake without any gas and the car "chugged" forward in sync with the sound/vibration. RPM at idle in gear is about 625.

Oil pressure is great all the time (the gerotor oil pumps are amazing). No coolant issues. Fluids are all relatively fresh.

I don't expect to solve it here and now. I'm just looking for good ways to diagnose the source so I can keep the time spent on it to a minimum. Seems like weekends go by so fast now that the weather's improving. As much as I enjoy working on the car, driving it is far more pleasurable!

Removing the serpentine belt is a quick and easy operation, so I'll head there first. If that has no effect, pushing the TC out of the way is also pretty easy. I'll see where I get from there.

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Now that you have said the vibration occurs throughout the range it seems to be ignition to me.

Do me a favor, when you are under the hood, pull each spark plug wire and look into the boot and see if you can see if the center core is burned out. Check both ends of the ignition wires for burned cores, including the end that goes into the distributor or coil pack depending on what you have.

If you can not see the cores, check them with an ohm meter and record the readings. It does not matter for now what the readings are, you are looking for one wire that is defective and sticks out from the rest, a wire with a burned out core will run rough as the spark arcs, to complete the circuit, but will show up bad in an ohm meter test. Mike

Early on sailors navigated by the stars at night and the North star became the symbol for finding ones way home. Once you know where the Northstar is you can point your ship in the right direction to get home. So the star became a symbol for finding ones way home or more symbolically even finding ones path in life.

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Scotty,

The 4.5 has a distributor. I'm a little reluctant to remove the plug wires. Seems like one of the connectors always comes off still attached to the plug.

However, if I do narrow it down to the engine, I guess it is worth checking. The wires, plugs, cap, rotor are all new ACDelco (well, new as of last summer, but they have maybe 10k miles on them).

Will the resistance vary with the length of the wires? Some are quite a bit longer than the others.

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THATS EXACTLY WHAT I AM TALKING ABOUT!!! YEAA!!! If any of your plug wires is NOT making 100% connection like THAT one! You will get a roughness, that will tend to lessen as you run up in the RPM range (sometimes)..... How old are your ignition wires, it may be time to replace them, do a careful inspection! I would replace THAT wire IMMEDIATELY!! Let us know, Mike

Early on sailors navigated by the stars at night and the North star became the symbol for finding ones way home. Once you know where the Northstar is you can point your ship in the right direction to get home. So the star became a symbol for finding ones way home or more symbolically even finding ones path in life.

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Will the resistance vary with the length of the wires? Some are quite a bit longer than the others.

The resistance will vary but thats not too important right now, you are looking for ONE or MORE wires that are WAY out of range!! That will cause your roughness.. If the carbon core is burned out or making a poor connection the ohm meter should isolate it, Mike

Early on sailors navigated by the stars at night and the North star became the symbol for finding ones way home. Once you know where the Northstar is you can point your ship in the right direction to get home. So the star became a symbol for finding ones way home or more symbolically even finding ones path in life.

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THATS EXACTLY WHAT I AM TALKING ABOUT!!! YEAA!!!  If any of your plug wires is NOT making 100% connection like THAT one!  You will get a roughness, that will tend to lessen as you run up in the RPM range (sometimes)..... How old are your ignition wires, it may be time to replace them, do a careful inspection!  I would replace THAT wire IMMEDIATELY!!    Let us know, Mike

Sorry for not being clear.

I was just speaking generally; whenever I remove plug wires it seems like one of the connectors always pulls out and stays attached to the plug.

The wires on there now were new last summer and I have never had a reason to remove them. Hopefully none will come apart.

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You were very clear, if you do not get the connector back on the wire correctly, you could experience the problem you are describing. Have any of your current wires separated like that? If they did, did you replace the wire? Or how did you repair it. You have standard plugs, you can purchase a plug removal tool that will grasp the boot firmly and eliminate the problem you are having. Also make sure you give each boot a shot of dialectic grease to help future removal and to help prevent arching from moisture. Mike

Early on sailors navigated by the stars at night and the North star became the symbol for finding ones way home. Once you know where the Northstar is you can point your ship in the right direction to get home. So the star became a symbol for finding ones way home or more symbolically even finding ones path in life.

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Here is a picture of what I am talking about from the SNAP ON site, Mike

post-3-1082756317.jpg

Early on sailors navigated by the stars at night and the North star became the symbol for finding ones way home. Once you know where the Northstar is you can point your ship in the right direction to get home. So the star became a symbol for finding ones way home or more symbolically even finding ones path in life.

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Sorry for the mistake.... I missed that one badly... (but I know what I meant!) LOL

Early on sailors navigated by the stars at night and the North star became the symbol for finding ones way home. Once you know where the Northstar is you can point your ship in the right direction to get home. So the star became a symbol for finding ones way home or more symbolically even finding ones path in life.

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