FAQ: Hesitation, Stutter(Studder), Shudder, Stumbling, on Acceleration/Load

yeah its all good no more codes. and no i mean i got one of the vac harnesses to compare and as parts. mine does not use that setup at all. i took pics of mine will post shortly

where a "t" is sposed to be i got this

the front side


my vac diagram

like i said mine does not have the t style vac

got it thanks. On the later sc3800s the connection at the LIM is a tee.

I've been having some stutter issues once I get up to and above 40 mph and slowly accelerate.  It seems to start once I get in the higher gears with low throttle input and higher load.  It stops as soon as I let off the gas.  Once I get up to speed and am on a flat road it doesn't seem to do it anymore as long as I don't have to accelerate much to maintain speed.  But if I get to a hill or need to slowly accelerate it will start stuttering again.  The RPM doesn't really look to be fluctuating much during the times it is stuttering, maybe 100 rpm plus or minus.   If I give it more gas to accelerate it runs fine and doesn't experience any symptoms.  No noticeable issues at WOT, seems to idle ok, no harsh shifts or issues taking off from a stoplight and no check engine light it on.

So far I have replaced the spark plugs and wires.  Autolite 605 and AC Delco wires.  The old plugs were blackened/fouled? and I did smell a little fuel on one or two of them but nothing extreme.  I have the torque app and don't see any misfires on any of the cylinders.  Still have the stock coils and ignition coil module.
Sprayed cleaner on the MAF
I tested the TPS with a multimeter and the volts slowly move up and down when the throttle is opened and closed.  Does this prove the TPS is good?
Both LTFT and STFT are around zero during the stuttering.  No KR is showing up either. Vacuum looks to be good at idle.
Transmission fluid and filter were replaced about a 1,000 miles or so ago.  Approx. 105k miles on the car.

Not really sure what is going on so I appreciate any help!

Check for exhaust restriction.  Vacuum gauge connected to manifold vacuum.  Should be 20" at idle fully warm in Park.  Raise the RPM to 1500, and watch the gauge.  It should remain steady at 20" or higher.  If it starts to drop off, you have an exhaust restriction, converter or muffler.

LARRY70GS wrote:

Check for exhaust restriction.  Vacuum gauge connected to manifold vacuum.  Should be 20" at idle fully warm in Park.  Raise the RPM to 1500, and watch the gauge.  It should remain steady at 20" or higher.  If it starts to drop off, you have an exhaust restriction, converter or muffler.

Is it better to hook up a real vacuum gauge instead of using the computer reading? I just checked with the torque app (I don't have a vacuum gauge) and it was at 19.6 to 20 in park and increased to 21 to 22 at 1500 RPM.

No, that should be accurate. The spark plugs being blackened does not sound right at all. That doesn't make any sense to me at all. Either the engine is running rich and your APP is lying to you, or the heat range is wrong and they are fouling. I have never used anything but AC Delco 41-101 iridium plugs. See what the sensor values are for the MAF.

Is the air screen for the throttle body in place and (relatively) undamaged?

The reason I ask is, the air screen is used to create "straight line" laminar air flow past the mass air flow sensor. This makes the fuel trim readings from the MAF sensor to the PCM more reliable. If this is not done, then the MAF readings fluctuate enough that the car will experience rich and lean spikes in air/fuel mix as the PCM tries to compensate for what it thinks are changes in throttle. The rich and lean spikes cause inconsistent throttle response (throttle response doesn't match physical throttle/gas pedal input), and cause inconsistent idling too. When the sensor over-reports air flow, the car will run rich. When it under-reports, it will run lean.

Other question, how old is your MAF? I recently replaced mine because it was showing a similar pattern of behavior to yours, essentially it was lying to the PCM. I did not have a MAF screen problem, basically I had a 'chuggle' I could not otherwise explain. Torque Lite readings were fine except the fuel trims were BS. I replaced it with a junkyard unit out of a '96 Camaro. The 96's unit looked like it had been rebuilt or modified, the cover over the electronics had been opened and neatly epoxied back shut. Installed it - Problem solved. YMMV.

The 'hint' for me is your fuel trims should not be just sitting at 0. Here is an article which give you background and details on managing fuel trims: https://us.autologic.com/news/fuel-trim-fuel-control-diagnosing-faults-part-1

Bottom line for me is I'd swap the MAF for a known good one, and I'd also make sure the air screen is undamaged and installed. This isn't a 'slam dunk' fix, especially since you're not setting codes (yet) - the variability isn't helping you operate will but it is apparently within 'normal' operating range.

If you buy a replacement MAF go with a good one like Hitachi -- there is a difference. If you have a damaged air straightener, you can get a replacement from performancemrp.com although you will have to work it with a dremel to chamfer or notch around the outside edge so it will fit behind the snap ring retainer. You can also make a straightener from 3 or 4 layers of aluminium window screen - user ugotbit02 on the SVTPerformance site said he did that and it worked fine, see https://www.svtperformance.com/threads/straightening-air-flow-before-maf-with-screen-or-honeycomb.873609/.

Also check your air inlet forward of your air filter. I had some critters make a nest in there that caused me a very similar problem.
Once they were relocated and the inlet duct cleaned out all was good.
I always try the easy stuff first.

The plugs I replaced were kind of old but I agree they were a little darker than what I have seen in the past when I pulled them.  I had just figured it was due to age but maybe something else caused them to foul.

I do still have the screen in place at the throttle body but will check it to make sure nothing hasn't changed since the last time I had the intake off.  The Maf is original to the car.  A few years ago I did get a quick stumble and it threw a code for the Maf.  I cleaned it up and it seemed to fix it.  It wasn't the same feeling like the car is now experiencing. The LTFT aren't exactly zero but were hovering around -2 to -1 or so.  Is there a way to monitor the MAF to see if it is doing something it shouldn't be doing?  I think I can pull it up on the scanner but not sure what normal would look like.

I did order a new TPS to test it out to see if it was intermittently failing due to vibration while cruising but the new one didn't work at all and actually triggered a service engine soon light so I will see how the replacement is later this week.

I think I will pull the intake off and make sure everything looks good and look into the MAF some more.

I just checked the screen and air filter area and they both are clean. I did go ahead and order a Hitachi MAF to test out to see if it fixes my issue since it was like $60. Definitely hard to track down the cause.

BTW are you burning regular or premium gas?

albertj wrote:

BTW are you burning regular or premium gas?

I'm using Premium, I tried filling up at another station to see if it would help but no change.

I'm now wondering if it could be torque converter shudder.  I changed the transmission fluid earlier this year and used Mobil 1 Synthetic again.  I've used it before but I'm wondering if it has something to do with it.  I found this Facebook group https://www.facebook.com/GM3800Transmissions/posts/346453239158209?__tn__=K-R that mentions you can get torque converter shudder if use Dexron VI instead of Dexron III.  Although I have used Dexron VI for 7 years now, maybe the new fluid has something different in it.  If the Maf doesn't fix it I'm thinking about switching the fluid out for Dexron III and then adding some Lucas or Shudder Fix to it if it still has the problem.  Any recommendations on good Dexron III fluid?  

Are any of these good choices?
https://www.rockauto.com/en/moreinfo.php?pk=8701976&cc=1303322&jsn=963
https://www.amazon.com/Valvoline-DEXRON-MERCON-Automatic-Transmission/dp/B002ZSM1RU?th=1 Dex/Merc Version

Thoughts?

98riv wrote:

albertj wrote:

BTW are you burning regular or premium gas?

I'm using Premium, I tried filling up at another station to see if it would help but no change.

I'm now wondering if it could be torque converter shudder.  I changed the transmission fluid earlier this year and used Mobil 1 Synthetic again.  I've used it before but I'm wondering if it has something to do with it.  I found this Facebook group https://www.facebook.com/GM3800Transmissions/posts/346453239158209?__tn__=K-R that mentions you can get torque converter shudder if use Dexron VI instead of Dexron III.  Although I have used Dexron VI for 7 years now, maybe the new fluid has something different in it.  If the Maf doesn't fix it I'm thinking about switching the fluid out for Dexron III and then adding some Lucas or Shudder Fix to it if it still has the problem.  Any recommendations on good Dexron III fluid?  

Are any of these good choices?
https://www.rockauto.com/en/moreinfo.php?pk=8701976&cc=1303322&jsn=963
https://www.amazon.com/Valvoline-DEXRON-MERCON-Automatic-Transmission/dp/B002ZSM1RU?th=1 Dex/Merc Version

Thoughts?

Hmmmm.... Dexron VI is lighter viscosity than III but maintains stable viscosity for about 3x as long I am told.  I very much like Dex VI.  Dex II is not the 'evil elixir' BUT the problem with Dex III is that for the Riviera you have to be sure to get Dex III(F) and not the earlier Dex III fluids.  Dex III(F) was released in 1993 and was standard fill for the 4T65E starting in 1997.  I dunno if GM still polices the Dex III(F) standard. There is a nice Wikipedia article about this.  Excerpt from the article:

1993 – DEXRON-III(F)

In 1993, GM released the new Dexron-III (F) fluid (GM Spec GM6417M and later GMN10055).[17] Dexron-III(F) was composed of Group 2+ base oil plus an additive package. According to GM TSB 57-02-01 issued Oct-2-1992. The improvements in Dexron-III(F) include better friction stability, more high-temperature oxidation stability, and better material compatibility. Dexron-III(F) has the same low-temperature fluidity as Dexron-II(E), for better transmission performance in cold weather. This specification failed to address a number of issues concerning long term durabilities such as shear stability and fluid oxidation.

Dexron-III(F) underwent a number of iterations in an attempt to address various shortcomings but was eventually replaced by new thinking i.e. DEXRON-VI(J). GM Dexron-III(F) licensed products have a license number on the can that begins with the letter F. Example: F-30001. This fluid is backward compatible with all previous Dexron fluids as well as the Type "A" Suffix "A", and the Type "A" fluids produced from 1949–1966.

In 1994–1995, some early OBD-II phase-in vehicles experienced a P0300 DTC (Random Misfire). Engineers determined that road forces being transferred through the TCC were affecting the normal rotational fluctuations of the crankshaft and tricked the ECM into thinking there was a cylinder misfire.

The solution was to create a new kind of TCC that would normally slip around 35 rpm. GM called it the Variable Capacity Converter Clutch (VCCC), other manufacturers had their own names. Some VCCC systems had a shudder or vibration during normal operation. Engineers tried several computer calibration changes, but a revised fluid was also needed to address the issue.

Ford released the new Mercon V Fluid Specification in 1996, GM released the Dexron-III (G) Fluid Specification (GM6417M) in 1998, and Chrysler released the MS-9602 Change C Fluid Specification in 1999.

This fluid was first used in the following transmissions:

1997 GM Hydra-Matic 4T65-E 4-Speed transaxle with Variable Capacity Converter Clutch (VCCC).
----------end excerpt----------


My PERSONAL experience:

- I strongly prefer Dex VI.  50-60K miles between fluid changes, unlike 20K/2 years for Dex III(F), for my driving (pretty much all expressway). In my driving basically I burn up Dex III. It'll start turning brownish around 40K miles.  

- With a Dex III fluid change it can take months and miles for the fluid to behave right.  Just after the change there's always an annoying but not fatal shudder problem. I didn't experience that with Dex VI changes.  With Dex III I got used to putting up with bullsh-t for about 3000 miles/2 months after a change.  Just at city speeds.  Didn't make a difference cruising on the highway.    No such issues with Dex VI for me, it always worked fine right outta the bottle.

I read the specs for Mobil Synthetic ATF...  I think your problem is it's targeted for Dex II/III specs more or less. Seems to be a synthetic fluid designed for backward compatibility for cars you can't really get design spec fluid for anymore -- which would not be the Riv by the way.  To get the lack of shudder you'd need to use Mobil DEXRON VI ATF.  Read the spec sheets and you'll see that the two fluids are indeed different. The viscosities actually are way different (hint - if you go to a GM dealer, this DEXRON VI is the fluid you'll most likely get for your car, from the bulk tanks or the little GM bottles) and I think that difference is what's killing you.  See the specs pdfs  at links from the following page:

https://www.mobil.com/en/lubricants/for-personal-vehicles/our-products/automatic-transmission-fluids

If you do it soon, a drain and fill with Dex VI ought to fix this, since you apparently can't get Dex III(F).  

Albertj

I have been using Valvoline ATF since buying my Riv in 2004 with 86,000 miles. At first, I was using their "High Mileage" formulation. Not sure what it was back then. For the last 3 changes (20,000 miles), I have used their Max Life Full Synthetic ATF,

https://www.valvoline.com/our-products/automatic-transmission-products/maxlife-multi-vehicle-atf

I am now approaching 200,000 miles on the original transmission. It shifts flawlessly.

LARRY70GS wrote:

I have been using Valvoline ATF since buying my Riv in 2004 with 86,000 miles.  At first, I was using their "High Mileage" formulation.  Not sure what it was back then.  For the last 3 changes (20,000 miles), I have used their Max Life Full Synthetic ATF,

https://www.valvoline.com/our-products/automatic-transmission-products/maxlife-multi-vehicle-atf

I am now approaching 200,000 miles on the original transmission.  It shifts flawlessly.

Sounds great!  They claim Dex II, III and VI... Hmmm... also JWS 3309 (Aisin Warner) ...just curious. Wonder what's in that bottle...

I think we got it fixed. I ended up changing the transmission fluid out twice with the blue gallon jug of Valvoline and it seems to have cleared up the issue. The first change cleared it up most of the way but it took two changes to get it to go away.

98riv wrote:

I think we got it fixed.  I ended up changing the transmission fluid out twice with the blue gallon jug of Valvoline and it seems to have cleared up the issue.  The first change cleared it up most of the way but it  took two changes to get it to go away.

Larry FTW!

Well it looks like I will be needing a new transmission.  Last month I was driving my car and when parked I noticed a noise coming from the transmission area.  I decided to drop the pan again to change the filter and to see how things looked.  I found several pieces of clutch in the pan and the magnet was covered in fine clutch material.  I then opened up the filter and it was full of clutch pieces so things don't look good for the transmission.  The weird thing is that it worked ok when I parked it.  I'm guessing it is the 4th gear clutch.  

I messaged Dave at Triple Edge Performance and he thought it looked like water had gotten into the transmission at some point.  I opened up the radiator cap and noticed some drops of red oil floating on the surface.  I believe coolant had leaked from the radiator into the transmission cooler and ended up destroying the transmission.  I had replaced the radiator around July/August 2020 due to the old one cracking.  I never noticed a large drop in coolant so it never occurred to me to pressure check the system.  It probably helped when I changed the fluid a couple of times for the stuttering but ultimately the damage was done.

I'm not sure what I will do now with the car.  For the most part the car is in good shape only with some surface rust in the typical places under the car.  I've had the car forever so I hate to see it go but also hate to put a lot of money and work into fixing a 23 year old car.  Parts alone for a rebuild look like it will be around $1000 to $1500 which is probably more than what the car is worth.   Dave suggested that I would also need to get a new torque converter.

I was thinking of finding a lower mileage junk yard transmission and putting that in but not sure where to find a good quality one.

Try car-part.com. Then call around and talk to them about condition and fluid appearance. When you search, you can search within your area.

98riv wrote:

Well it looks like I will be needing a new transmission.  Last month I was driving my car and when parked I noticed a noise coming from the transmission area.  I decided to drop the pan again to change the filter and to see how things looked.  I found several pieces of clutch in the pan and the magnet was covered in fine clutch material.  I then opened up the filter and it was full of clutch pieces so things don't look good for the transmission.  The weird thing is that it worked ok when I parked it.  I'm guessing it is the 4th gear clutch.  

I messaged Dave at Triple Edge Performance and he thought it looked like water had gotten into the transmission at some point.  I opened up the radiator cap and noticed some drops of red oil floating on the surface.  I believe coolant had leaked from the radiator into the transmission cooler and ended up destroying the transmission.  I had replaced the radiator around July/August 2020 due to the old one cracking.  I never noticed a large drop in coolant so it never occurred to me to pressure check the system.  It probably helped when I changed the fluid a couple of times for the stuttering but ultimately the damage was done.

I'm not sure what I will do now with the car.  For the most part the car is in good shape only with some surface rust in the typical places under the car.  I've had the car forever so I hate to see it go but also hate to put a lot of money and work into fixing a 23 year old car.  Parts alone for a rebuild look like it will be around $1000 to $1500 which is probably more than what the car is worth.   Dave suggested that I would also need to get a new torque converter.

I was thinking of finding a lower mileage junk yard transmission and putting that in but not sure where to find a good quality one.

The win for you is there were a lot of GM cars that used the same transmission and I am told it is not that tough to configure it for the Riv. I'd look for an old-fashioned junkyard.

It looks like I found a local one from a 98 Riv with 112k miles on it for a around $300.  I'll probably go look at it and see how it looks.  If I bought a used one, is it worth doing a few preventative maintenance items on it like the TCC solenoid, 4th Gear clutch shaft before putting it on the car?

98riv wrote:

It looks like I found a local one from a 98 Riv with 112k miles on it for a around $300.  I'll probably go look at it and see how it looks.  If I bought a used one, is it worth doing a few preventative maintenance items on it like the TCC solenoid, 4th Gear clutch shaft before putting it on the car?

$300 is a Nice Price for that core with 112K on it and the way I am told most Rivs have been driven (coffin car for nonenthusiasts) simply dropping the pan, replacing the filter and topping off with Dex VI sounds like Plan A. However since you are asking... I would ask a local builder (not a local installer). My transmission was rebuilt by a man who builds transmissions for dirt track cars, and maintains fleet vehicles for villages, companies and towns in rural NY. When my TCC quit, while he had the transmission open to fix that he replaced the frictions and steels (raybestos blue clutch pack), put it a Transgo shift kit, and a Sonnax 'sure cure' kit for the valve body. Superior makes a more pricey shift kit intended for taxi use. I do not remember whether he replaced the torque converter. The Transgo and Superior kits both have the TCC fix.

While you are getting the transmission pulled off that 98 Riv you might want to get the parts that have become Unobtainium. Depends on what you have.

Any suggestions on some hard to find parts that I should look for?

98riv wrote:

Any suggestions on some hard to find parts that I should look for?

Just look at your car and tally up what's not working. Maybe the lights in the door pulls or remote lever wells. Maybe missing a Riviera script. Maybe the door weatherstripping is in better shape than yours. Maybe you have an issue with the horn or the steering wheel switches. I dunno...

Wheel hub covers can usually be resold on ebay for a neat profit, depending on the savvy and desire of the junkyard.

Rear package shelf. Headliner bits. Speakers. Gas cap.

The self - resetting thermal breakers under the rear seat.

...think.

Albertj