Need Help: 92 Riv Shudders When RPM's Backoff

[bluto00]

Hi all,

My son has a '92 Riv (mine is a '91) and is having some problems.

When the engine RPM's normally drop back while going into the "*coast down" mode on the highway, the car starts to shudder mildly. It is similar to when driving a stick and the gear is too high for the speed of the car. The steering wheel shakes and all that good stuff.

When stepping on the accelerator to get out of this mode it seems to stop shuddering.

Just replaced plugs and wires in his car a few months ago so that shouldn't be a problem. The tranny fluid is not low.

No Codes have been tripped and the Service Engine Soon light has not come on.

Can anyone give me some idea's what to check or where to start?

Thanks,

Clark

*When driving on the highway with a steady accelarator the RPM's normally drop like going into an overdrive mode.

[Guest Ranger]

Clark,

Since you have a 3.8 motor and drivetrain which is the same as the Reatta, you may have better luck posting this on the Reatta forum where there's dozens of 3.8 experts. It's just below the ROA forum.

Good Luck,

Jim

[Rivieraman]

Clark

Have you checked the EGR Valve? If it sticks open on coast done it could do this. Also you may wnat to check the computer behind the glove box. pop the mileage chip out of yours and put it in the new one. Give it a try

[tim63riv]

Clark,

How many miles are on the car? Is it worse hot or cold? Does brake application change the vibration at all. I have couple ideas, but they are somewhat "mileage sensitive."

Let me know!

Tim

[NTX5467]

A shudder on coast-down is unusual, whereas a shudder when the torque converter clutch tries to lock-up can mean the torque converter clutch is on its last miles. In coast-down, the torque converter clutch is normally disengaged with a vacuum switch in the torque converter wiring harness, just as it disengages under certain lower vacuum (i.e., acceleration or "deeper throttle setting" situations). The brake light switch is in that converter wiring circuit too, so touching the brake pedal slightly would break that circuit and take the converter out of lock-up mode, but it should be not be in lock-up on decelleration, I would not think.

Does this happen on a gradual transition from cruise to coast or only when the throttle is suddenly closed while at highway-speed cruise speeds? Is it from the onset of coast or build as coast-down time increases? How quickly does the shudder go away if the you throttle back into the engine while it's doing it?

What you might look at would be the IAC motor in the throttle body, though, to make sure it's not trying to put the engine rpm too low in these coast situations. If the car has a tach, it should probably "float" just above about 1000rpm in the coast-down situation, as I've observed newer vehicles to do.

Rather than chunking parts at the car, you might take it to the dealer and have them put their Tech __ diagnostic computer in the car and drive it, watching to see what sensor or whatever might be close to "out of parameter" readings when it does this shudder situation.

Might also be the negative torque is causing one or more of the motor or transmission mounts to be in an unusualy position, from age and such, that might be causing the vibration under those negative torque situations? A quick diagnosis of this might be done by putting the transmission in "D" and/or "R", with your foot FIRMLY on the foot brake and then gently applying throttle to see how much the engine might rotate on its mounts. Seems like there is a torque strut on the front of the engine that has rubber bushings that can be "age or use-worn", which is pretty each to change? The other engine and transaxle mounts can be more intense to change yourself.

Just some thoughts,

NTX5467

[bluto00]

Hi Rivieraman,

Thanks for the response.

In answer to your question I did not check the EGR Valve. I will have him check this as soon as I talk to him next (I live in Iowa and he is in Wisconsin) and replace it.

I am not sure I understand about the computer behind the glovebox. Is this safe pulling my chip and putting it in another year auto? Also what am I looking for by doing this?

Thanks,

Clark

[bluto00]

Hi Tim,

Thanks for the help.

As to the miles I will guess and say around 150K. The symtoms do not seem to be effected by the engine being warmed up or not.

I only test drove it once and I really can't tell you whether braking changed the vibration. Off the top of my head I will say it did not but I will check with my son on this.

Thanks,

Clark

[bluto00]

Hi NTX5467,

Again thanks for the help guys.

As I stated I only drove this once but it seemed to me that at about 55 mph I held the gas pedal steady to engage the coast down. The rpm's dropped normally like in my '91 and it seemed to me that the shudder started at that time. I will have to check with him to be positive on that though.

When I accelerated it seemed to stop the rough running/shudder. I also put the car in lower gears and I didn't "seem" to have any problems after that.

At the time I believe that the rpm's were around the levels on my '91 which is about 1200 rpm's when the problems started.

What would the test be for the throttle body IAC motor?

I will have to wait until I see him again to test the mounts as you stated. I will set my son up with this forum and maybe he can answer some of the questions.

Thanks,

Clark

[tim63riv]

Clark,

Some "pattern failures" that occur on this vintage Riviera could be the cause of your issues with the car. Please don't start throwing parts at the car without doing some diagnosis first. If you adopt that approach, you may find yourself frustrated and out a lot of money, too. If the steering wheel is shaking, I would look at inner and outer tierod ends for wear. Another cause that Willis mentioned is the motor mount becoming weak and letting he engine and transmission contact the subframe or the body and sending these driveline vibrations into the body of the car. One way to test this is to reproduce the vibrations a couple of times so that you know the speed at which the vibration occurs and the severity of the vibration. To diagnose this, you have to find a stretch of road were you can duplicate the speed without any other traffic around you. First off, drive the car to the speed that the vibration occurs and shift the transmission into neutral while you decelerate. If the vibration is gone, there is a good chance that one or more of the motor mounts has failed. Another method would be to bring the car up to the speed where the vibration would normally occur. Hold that speed and turn the ignition switch to the "OFF" position, NOT the lock position!!! If the vibration is still there it is not engine related. If the vibration is gone, then it has to be an issue with the engine or transmission or the motor mounts.

Just some more ideas!

Tim

[NTX5467]

On the issue of swapping the computer PROM chips (the smaller ones of the two in there), if the cars are the same year and same specs, this should not be a real issue, but if one is not the same model year, it COULD be an issue then as many of those chips changed from year to year. Each PROM has an identity (other than the number and letter codes on the chip), which I think can be read by many scan tools, but I don't recall the model year range where that is possible.

Just some thoughts,

NTX5467