96 Riv S/C Stuttering/Misfiring

Just recently purchased a 1996 Riviera S/C.
Car Idles fine I can rev it up to 4-5k in park with no rpm fluctuations.
But when I start rolling if I give it too much acceleration it starts to stutter/misfire badly that I have to let off the gas for a second and gently reapply throttle. Its not bad when the engines cold but once it warms up its alot easier to make it stutter.
It has new spark plugs, wires, and 1 new coil pack.
The previous owner is unsure if the Gaskets have been changed and I've heard the stock ones are plastic.
Somebody mentioned the possiblity of the snorkel air tube pipe having cracks and under acceleration the cracks let in air and make the MAF go all out of whack.
Ive also heard it could be another coil, but it revs fine in park?
Possible O2 Sensor?
Also a vacuum leak? Whats the best way to check for a leak, someone suggested spraying something in the engine bay and looking for rpm changes. Ive tried looking and listening with the motor running with no luck.
Or the worst I can think of the trannys giving out :/

Spark related. Swap the old coil back in, replace one of the other coils with the new one.

Hrm I was thinking that but wasnt sure cause the fact I can rev it up in park with no big misfires/rpm fluctuations

there are something like 5 or 7 members with this allmost identical problems. so far a bunch of parts are being thrown at the Riv but these problems still persist...Im stumped

Ill prolly get a 2nd new coil pack and swap out the 2 older ones see if that helps anything, the coupler on my s/c is bad for sure I can hear it when i turn the car off, could that produce misfires and or hesitation?

did you clean all the obvious parts? egr aic pcv maf etc?

K i know what EGR and MAF are but enlighten me on AIC and PCV.

EGR and MAF are on my to do list, I just need a new EGR Gasket if im not mistaken when I clean it? and whats the best thing to use on the MAF to clean it?

air idle control valve pcv is the pcv.
only clean electrical connections with electrical partss cleaner....

I read up on the PCV looks like a quick easy cheap maintenance thats probably been neglected in the cars15 years..

yup. another thing not mentioned very often is the maintenance of the injectors. additives wont do anything. you have to do a search online for "fuel injector filter kits" when my Riv started acting up,I changed the filters in the inj. and she ran a whole lot better. still havent solved my issues but one less thing to worry about.

For me, it was plugged injectors, I have a silt problem in my tank. Probably from a year of sitting with the gas going all varhishy. I got them cleaned, and it was smooth sailing again (until it happened again)

trumbo71 wrote:

Hrm I was thinking that but wasnt sure cause the fact I can rev it up in park with no big misfires/rpm fluctuations

This is how you know it's a spark issue. There's no load (no boost) in neutral, so no demand on the ignition system. Only in boost are your coils and wires actually tested. If it were a clogged cat, the rev test in park would show the problem, but spark will only show up under load.

AA wrote:

trumbo71 wrote:

Hrm I was thinking that but wasnt sure cause the fact I can rev it up in park with no big misfires/rpm fluctuations

This is how you know it's a spark issue. There's no load (no boost) in neutral, so no demand on the ignition system. Only in boost are your coils and wires actually tested. If it were a clogged cat, the rev test in park would show the problem, but spark will only show up under load.

Plugged injectors will do the same. Rev it up in park and it sounds pretty good, punch it is drive and the bottom falls out.

Could be. More reading about this issue:
https://rivperformance.editboard.com/t4285-stuttering-sometimes-when-driving

Seems like wires solved his problem i bought new ones not too long ago as well as plugs must be the coil packs cause what he describes sounds just like my issue.

Swapping your old ones for the new should fix it. You can also remove the coils and have them tested to see if they're bad.

If you don't find a problem with the coils, you can try buying a set of new/reman injectors, but hope you don't need to!

K I bought new coil packs swapped em out with the old and no improvement.
Failed to Mention in 1st gear I can get on it with no stuttering it doesnt start til the car hits 2nd, it almost feels like the car overshifts but would that cause misfiring?
Somebody suggested the transmission shift modulator, right below the snorkel for the intake connected to the front of the transmission.
I know the car is misfiring its throwing the codes P0300 and P0420 I think the 420 is from the previous owner removing cat.

I also just replaced the PCV noticeable improvement in idle the old one didnt look likes its been changed in the cars lifetime.

Still seems like a spark issue to me. You have new wires, so I'm stumped. I had the same exact symptoms at 109k miles; I thought it was the transmission, but new wires solved the problem.

One thing you can try is carefully removing the SC belt from the blower and pulleys, tie it off to the side. If you can't duplicate the stutter without boost, you know it's load induced (probably spark).

IF you are not getting improvement from replacing coils, you may need to remove *all* the coils and clean the mounting. You will need conductive heatsink grease or dielectric grease for this. NoAlOx will work too. There are other posts on this site detailing that R&R.

Do you think you might have a bum ignition module? Or a bad wire to/from it?

Albertj

I pulled off the ignition module it is very dirty but autozones tester gave it the all clear i didnt use conductive heatsink grease and there was none when i pulled it off could that be a problem?

Im unsure about their tester aswell cause it got warmer with a current running through it but not nearly as warm as sitting in an engine bay with a motor running..

trumbo71 wrote:

Ill prolly get a 2nd new coil pack and swap out the 2 older ones see if that helps anything, the coupler on my s/c is bad for sure I can hear it when i turn the car off, could that produce misfires and or hesitation?

One issue: the right thing to do when replacing a coilpack is to R&R the ignition module, clean it and refresh teh conductive heat sink grease. Not everyone does that... until they do, they are likely to keep chasing the same problem.

Another issue: there is not just one reason for this symptom - if you fix this problem and another still persists (air leak at the throttle body for instance) the problem does not go away. Have to make some guesses as to what's wrong by direct observation. The best we can do is share experience but each person has to do some wrenching and troubleshooting of their own to get things right, especially when buying a older car of any marque that has no (or incomplete) maintenance records.

So please whether you rotate or replace coil packs clean up the ig module and other coils. As for your SC coupler, slip the belt off the tensioner and turn the sc pulley. if it has play you'll tell immediatly. If not, that ain't your problem (hint - they go bad slow and will run for *years* bad without causing other problems besides noise). If you have other modifications (a Northstar TB for instance) now is a good time to re-check them.

Albertj

Hi all....

I am having a misfire problem as well as of late...let me give brief history.

This fall I had a leaky injector o-ring which was repaired with a fresh set, and two junkyard injectors (to replace two OE injectors that were damaged...). During the repair, one of the fuel lines was ruptured and required repair with an intermediate brass-barb fitting kit and the visible leak was solved. The car ran fine and on my daily drive to work has had no problems for the past 3 months (daily drive does not typically involve much spirited driving, may get into slight boost but RPM rarely crosses 3k). Anyhow, I noticed one of my spark plugs was loose yesterday so I tightened it back up and then took it for a drive getting into WOT to make sure things were in working order.

Sure enough I encountered a misfire that seemed to be RPM dependent at about 4000 RPM. No matter the throttle position or gear (locked into first and misfire appeared under very light throttle at 4000). I changed out the old crusty plug wires today to no avail. The problem persists....Coils are OEM AFAIK....I have AL104's that are 2 years old with a max of 10,000 miles....and I know they are cheap and I plan on swapping them ASAP but the fact that this misfire doesn't seem to be load induced is leading me to believe it may be fuel (O2, ghetto fuel line fix?) not spark. Anyhow, I took a quick scan tonight and figured I would host it for you all to see. Scandepot seems to have disappeared so I apologize for the viewing inconvenience on google. Let me know if there are any other conditions you would like to see, I didnt have much time to play around with it since I have to get to work tomorrow morning. View misfires in highlighted areas. I appreciate any insight into this problem. I understand I could change out the coils and plugs but before I do that I just want to see if the scans help tell a different story. At the point where I am getting some significant KR my LTFTs are maxed at 16.4....O2 jumps from 733 to 30 to 594 all at constant throttle...WTF. Is my O2 going crazy? TIA.

https://spreadsheets.google.com/ccc?authkey=CNPwwdUI&hl=en&key=tXe9aTH68r7dVO42Vb8cjSw&hl=en&authkey=CNPwwdUI#gid=0

EDIT: Header is now frozen for easier viewing....

Mine seems to always happen at about 2000 RPM when warmed up, chances of it happening in first are very low and rarely happen but do occur sometimes..

Forgot to mention the fuel pumps been recently changed maybe 1-2k miles ago the last went completely out..

Where could I go to get such in depth scans? Autozone just has the OBD reader which tells me P0300 and P0420 not which cylinder is misfiring :/

trumbo71 wrote:

Mine seems to always happen at about 2000 RPM when warmed up, chances of it happening in first are very low and rarely happen but do occur sometimes..

Forgot to mention the fuel pumps been recently changed maybe 1-2k miles ago the last went completely out..

Where could I go to get such in depth scans? Autozone just has the OBD reader which tells me P0300 and P0420 not which cylinder is misfiring :/

in depth scans = get your own scanner (not code reader) such as www.obd-2.com or go to an independent mechanic or GM dealer that has the equipment.

Quote :

One thing you can try is carefully removing the SC belt from the blower and pulleys, tie it off to the side. If you can't duplicate the stutter without boost, you know it's load induced (probably spark).

See if you can replicate the problem without boost.