Infrequent Rough Idle & Peculiar Acceleration Issue


#1

At first I thought the issues were related, now I’m slightly worried they might be two entirely separate things.

I have a 97 Lumina sedan. When moderately accelerating between 50-60 on a slight incline, the car will sometimes start to “jolt” a little repeatedly. It continues to do it if I hold the pedal steady, but if the incline goes away or I press the gas pedal to accelerate harder the jolting will stop as abruptly as it started. The magic number seems to be around 55mph, never outside that range. I can reproduce it on certain interchange ramps or hills, but only in overdrive. Regular drive gear doesn’t do it but the engine is at a much higher RPM. If I accelerate harder than I usually do it also doesn’t seem to occur.

At first I thought it was just the same RPM loss issue at speed in a low cruising RPM, something that happens infrequently at idle. But now I’m worried the specificity might indicate a transmission problem. Car only has 87K miles on it for its age though. Any insight or advice would be greatly appreciated! Thanks for reading!


#2

Automatic transmission? Have you ever changed the fluid? Can you check the level and also note the color and smell?


#3

Sounds like you have an intermittent engine “Miss” I would do a basic tuneup…plugs, wires etc… It will probably solve this issue. Pretty basic.

Blackbird


#4

Thanks for the posts guys!

Yes it’s an automatic. I checked the fluid, looked clean, didn’t smell burnt, and it was still topped off from the fluid change <15K miles ago. It had a tune up around the last oil change.

If it is “missing” wouldn’t that create an engine check code I would see? It doesn’t happen at speed except under that specific scenario I gave, any speed outside 50-60 I can’t notice anything. (except sometimes at idle)

Could it be a sign of a spark plug starting to go bad? It had platinum plugs put in it ~52K miles ago.


#5

Platinum plugs should be good for well over 52K, so unlikely. Wires, maybe, ignition module, also maybe. An intermittent miss might not set a code.

To me, it sounds like the lockup torque convertor is acting up. It sounds like its trying to engage and dis-engage at the load point you describe. Old transmission fluid could be causing this. A fluid and filter change might fix this.


#6

Reset the computer. Sometimes things get fishy. :slight_smile:

Believe it or not, a simple solution like that actually helped me once. It forces the computer into relearn mode and clears out any corrupt data that might be causing what you’re experiencing. IF that is the problem, you would notice the change right away.

Don’t get too excited… Though, I figured it was worth mentioning since it is FREE, but it will only solve the problem if it is computer related. If that doesn’t work, you can start spending money. It probably is indeed advisable to service the transmission as suggested above. Topping it off isn’t really going to do anything for any sediment build-up. Lots of tiny parts inside the transmission… Things wear… metal shavings build up from friction…


#7

The next time the vehicle starts to jolt, and keeping up speed, slightly step on the brake pedal.

If doing this stops the vehicle from jolting, the problem is with the torque converter clutch shuddering.

Go to a local parts store, and purchase this product. http://www.lubegard.com/~/C-230/Dr.+Tranny+Instant+Shudder+Fixx

Add it to the transmission fluid and it will eliminate the clutch shudder.

Tester


#8

It is amazing to me what does NOT set codes on this car, oh well! I hope modern cars are getting much better about noting odd things in their systems even if it doesn’t trigger codes.

Thanks all of you for the advice! The transmission fluid was changed around 15k miles ago, my sincere apologies for not being clear on that point. A year ago the wires were replaced along with one ignition coil that had some rust on one of the connection points. I do not know if anything has been done to the ignition module.

Come to think of it this problem was occurring on the old battery that gave out. I just recently had to replace it and that certainly did reset the computer so I guess that rules that idea out.

I’ll do some research on this torque converter thing and try lightly tapping the brake as suggested next I’m at a spot in the road I can reproduce the issue. I’ll look into that shudder fix stuff as well, thank you!!


#9

I think it’s related to the torque converter lockup.
Did it do this before the trans fluid change?
Did they use the specified fluid? Not some universal stuff.
Is there a “kickdown” cable from the throttle to the trans? Make sure it’s adjusted properly.


#10

It sounds like torque converter shudder to me too. If so, the rough idle problem must be something else. Here’s a link to a short tutorial on converter shudder.

http://www.agcoauto.com/content/news/p2_articleid/203


#11

Alright, was able to borrow an interchange ramp in the wee hours to play with. I confirmed it is TC shudder as it would go away when tapping the brake.

The shudder will only occur when starting in a low overdrive gear with the engine at 1850rpm (cruising speed), then trying to accelerate past 55mph while taking an incline. Accelerating hard enough to kick the gear up (>2.6k rpm) prevents the problem. Only happens on inclines.

Going to talk with the mechanic before I put some of that Lubeguard shudder fix in the thing to see what he says about it. Thanks for all the help everyone, wouldn’t have ever figured it out without you guys! :smile:


#12

A fouling plug does not always set a code…since the coil is doing its job and firing…the plug is often times unable to be monitored tho some models can monitor the plug via resistance Im sure…its not out of the realm of possibility… Ive seen things that SHOULD light the Cel but dont for whatever reason.

Could be a simple plug…or similar…Many times the tune up you get is only as reliable as the mechanic doing the work. Take for instance transverse V engines…a tuneup requires all the plugs to be changed…guess which plugs get changed most often? Yup the ones in the front…as you need to do the others from under the vehicle many times. So they leave half of them in there to duke it out for another interval… I see this Constantly…its terrible. Of course you dont have a transverse engine…but the theory still holds water.

Blackbird


#13
Infrequent Rough Idle...

For this, try cleaning the throttle body and idle air control valve (if equipped).