Hi.
I'll try to be brief but concise with this trouble I am experiencing....
'98 C5 6 speed 130k miles stock except for the typical bolt-on stuff- B&B PRT mufflers, variaram intake, speedline z06 wheels, lowered, bilstein shocks, hurst shifter.
I've been fortunate (sarcasm) to have the usual trouble-oil pressure sensor failure, rocking sport seats, puking differential and the misc. plastic interior items shaking/rattling/falling apart.
Car is 99% street only w/ 4 HPDE events at Mid-Ohio road course.
Here is my trouble:
2 situations that yield same result-
1. Set cruise control at ~60mph (1200~1400rpm). Now crest over the top of a hill. Cruise incrementally reduces throttle to maintain speed. At this point, the car wants to violently "buck" like severe driveline lash. Accelerate and no shaking. Apply brake from cruise set condition and the "bucking" occurs.
2. Hold steady throttle application (w/o cruise). Now very gradually reduce throttle. At this point, the "bucking" once again occurs.
No perception of this condition under acceleration. Haven't pinpointed if temperature dependant. However, seems like maybe more severe in warm weather. I've considered removing the hood to watch for any rocking of the motor on trailing throttle. However, my preference would be to resolve w/o requirement of removing hood just to confirm the motor movement.
Rear suspension mounting is tight as well as trans mount, etc. Axles, CV joints have no perceptable play or slop.
My guess right now is possibly a motor mount causing this. I believe this condition to be a gradual occurance over time and not a single, catastrophic event. However, now it is rather violent. If motor mount, no problem, I will fix. But, quite a bit of effort if problem isn't fixed.
Please help me out. I've searched forums and haven't found quite the same scenario as my situation.
Thanks!

 

First off, I doubt you have a motor mount issue. If there was something jumping around the first place you would notice it would be in the shifter, no need to remove the hood.

1200 to 1400 is pretty low rpm for 6th gear. Dirty MAF sensor, iffy injector (dripping at low throttle settings), bad plug, or a number of other items could affect low throttle settings.

Are you getting any codes?

 

Could be a lot of things. I once thought I had excessive driveline backlash in my F-body. Turned out that the rubber bushing in the axle trailing arm had moved axially, and one side of the trailing arm was almost metal to metal with the bracket. Under the right conditions, it would hit and make a "clank". Never did fix it, but I slept a lot better knowing it was just the trailing arm hitting the bracket.

Not saying this is your problem. Just that hunting down noises and rattles can be a real pain.

 

Hi Eric D.
Thanks for the response.
Currently no codes and runs strong up to redline.
If I use my C5 for commuting for work, it's good bragging rights to get 30mpg (because of running 1200~1400rpms) and then turn around and track it and get 6mpg. Commuting to work is my reason for running low rpms.
My basis for thinking motor mount/something w/ a worn rubber/urethane bushing etc. is/was that I can hold a steady throttle at 2k~3k rpms, then gently reduce throttle and not have this situation. My guess from this is that a load is being applied and then some compression braking, etc. that mutes the condition? At very low rpm (1200~1400) there is minimal load but considerable amount of torque.
Plugs are relatively fresh (about 5k miles) but easy to pull-so this will be one of my checks.
Your idea of a leaking injector peaks my interest. Any thoughts on how to confirm other than pulling them and checking spray pattern?
Thanks again in advance for any suggestions/help.

 

Seeing the plugs are fairly new I would start with more simple things first. I would check or replace the air filter, run some Techron through the tank and maybe spray the MAF sensor down with some cleaner to see if this helps at all. The Techron can help the injectors. If none of that works, then I would check the fuel pressure to make sure the regulator is doing its job. Seeing you have no problem at wide open throttle the fuel filter most likely is fine.

I too like to brag about the MPG on the highway. I normally will get between 30 and 32 mpg if I keep the speed around 70 to 75.

Keep us posted on what you find and wish you luck on getting it figured out.