Help Please! Bogging down on highway!

Hi everyone. So I just purchased my first Riv, a 98 Sc with 135k. Driving to work yesterday, (a short 8 miles down the highway, no city) was fine, but today while cruising at 70 he engine started bogging down. I gave it gas, but the whole car started to shudder, and it was an effort to keep speed until I got off the highway. After that it was fine. On my way home just now the same thing happened, but when I slowed down to about 40 the problem stopped, and I was able to get home no problem. This is a huge concern for me because this car is a daily driver and I have had it less than a week. Any thoughts or similar experiences would be greatly appreciated

If you can, try shifting into 3rd next time it happens. Any change?

First thing I would do is a SeaFoam treatment

(post by jax95riv)

https://rivperformance.editboard.com/t123-write-up-seafoam-to-restore-lost-horsepower

Any engine lights on? I'd check the fuel pressure, and change the fuel filter if you haven't done so.

Shuddering under gas? First check for me would be plug wires. Then the things others have suggested.

i did try shifting down on my way home, and it shifted fine, but the problem continued. I was definitely leaning towards a fuel sending problem, but the fact that this only seems to happen at cruising speeds bothers me.
What is Seafoam, I've read about it in forums like these. I added 4oz of Marvel oil, when I did my first fill up, as the owner td me it it had been sitting a while. Perhaps one of the causes of my problem?

I checked the plug wires and they are shiny new, no corrosion at all,

I would change the ignition wires like karma suggested. Also plugs. Mine looked like new and were bad at 109k miles. These need changed at your mileage, even if they turn out not to be the problem. Best of luck!

In addition Pull the Fuel Sending Relay and check the posts for corrosion. Also, While driving leave the back seat out, shut the radio off and wait for it to happen again and listen for any relays clicking in the black fuse blocks. Along with all that, my most famous answer, Check The Grounds! All of them. You can get a schematic printed out at any GM parts Counter. Best to you and let us know what it turns out to be.

Ok, I just spent the day doing various errands, and The problem occured three times. It seems to be when slowing down, or letting my foot up off the throttle. When i go to press the gas again, the whole car will start to vibrate. when on the highway, I have to slow down below 40mph for it to stop. When in town, began to lag again, but instead of letting off the accelrator i kept it pressed and the car eventually worked through it. Another thing, the car started to shift rough, like it was jamming into gears. Ive only just bought the car, and I have no vehicle history on it. I'm concerned this might be a transmission

It is sounding transmission-y to me from your description. Is the fluid level okay? What color is the fluid?

Where in NH are you?

mechanic said it looks great. levels fine, no leaks, good color. i was starting to think this might have something to do with the torque converter?

I live in Hampton

Seriously. Replace the plugs and wires. I've had just what you describe in the past on the wife's 98. It doesn't matter how new the wires look. Its a new car to you, get new plugs and wires. Then you KNOW they are good. This should be the first thing you do with any "new" used car. A failing plug wire is notorious for seeming like a fuel or transmission issue just as you describe. Its a pretty cheap thing to try and can save you a bunch of headache later, and if it doesn't solve it then at least you are running around with new wires and plugs.

Alright, I'll do plugs and wires tomorrow, and let you guys know.

Be sure to take note of how the plugs look. (Burnt, black, white, oily, or gas smelly.) Which can assist in further troubleshooting.

also.. how many tanks of gas have you run through it? You said it has sat for awhile, could be slugs of water in the gas from sitting too. Could fire in some water remover stuff just in case too.

If it is a misfire, it will eventually cause the SES (service engine soon) light to illuminate. If you connect a scan tool to the car, you can look at the misfire history for each cylinder. I can't imagine why any mechanic wouldn't do this. Autozone will do it for free.

No lights as of yet, and when the mechanic checked the codes it came back clean. The problem occured again today, and i popped it into neutral, while keeping the rpms constant, and the shuddering immediately stopped. when i shifted back, it began again. As I said, the key seems to be slowing down, or relieving pressure from the throttle. The mechanic mentioned something about the clutch having trouble engaging, and said he knew of an additive that would take care of the problem. Does this seem likely?

im on my first tank of gas, at 1/4 after putting in 10 gallons.

You might have a problem with the torque converter clutch. Very difficult to tell over the internet. What I am saying is that even with no SES light, the computer will keep count on the misfire history of each cylinder. All you need to do is hook up a scan tool and look at the misfire history. Takes 5 minutes max. Each coil fires 2 cylinders, 1-4, 2-5, and 6-3. If you see misfires on 2 companion cylinders, it could be a coil pack. If it is one cylinder, a wire/plug issue. If it misfires, it does it under load. Putting it in neutral removes the load. Your mechanic needs to diagnose this if you can't.

The suggestion to shift into 3rd was meant to rule out the TC clutch. If you push the gas at all in 3rd while at highway speeds, the clutch will release (no pulsation). If you are hard on the throttle in 3rd and it shudders, it's more likely ignition wires, plugs, or coil.

I'm guessing either of the pressure control solenoids is sticking. One is for general fluid pressure, the other is for the torque converter.

Another way to force the torque converter to release is to tap the brake pedal while keeping your foot steady on the gas. The RPMs should jump up a bit, then lower again when you release the brake pedal. Don't slow down, just enough on the brake pedal to make the lights blink and release the torque converter.

I did try to tap the brakes while keeping my foot on the throttle, didnt change anything.

Green-irwinman wrote:

I did try to tap the brakes while keeping my foot on the throttle, didnt change anything.

If you feel like driving down to MA I can hook up a scanner and see if the torque converter is engaging or not, and record some operating parameters. We should be able to figure out what's going on.

-Derek

I read a little about these trannies on a couple other forums, about othe people having problems with the pcs. How difficult a part is this to replace?