1995 Riviera frustration with repair

I Am new to this forum  I have a 95 Riv  3.8 Supercharged with 125,000 miles that has a lower rpm miss that I cant seem to locate  I have done all 3 coil paks, plugs, wires, ignition module, all 6 injectors, mass air flow, throttle position sensor, AIC, I thought it was a vibration at first and replaced the harmonic balancer, motor mounts, trans mounts, I have looked at as many grounds as I can find, If I unhook the battery and reconnect and let the computer do its thing it seems to be better at first then resort back to the missing issue , not showing any codes by the way,   Any helpful knowledge would be much appreciated I've done enough research to figure out these 95's can be hard to diagnose  Thanks MichaelB

If u are not getting any codes it may still be just a vibration.
I have a 96 series II and have chased what may be just what u are experiencing for yrs and some $. There is a lot of info on here about mounts and subframe bushings and such. Many threads I have looked at seem to just end without resolution.
I have actually begun to accept that it may never be resolved but I keep trying.
Good luck.
Jbird

MichaelB wrote:

I Am new to this forum  I have a 95 Riv  3.8 Supercharged with 125,000 miles that has a lower rpm miss that I cant seem to locate  I have done all 3 coil paks, plugs, wires, ignition module, all 6 injectors, mass air flow, throttle position sensor, AIC, I thought it was a vibration at first and replaced the harmonic balancer, motor mounts, trans mounts, I have looked at as many grounds as I can find, If I unhook the battery and reconnect and let the computer do its thing it seems to be better at first then resort back to the missing issue , not showing any codes by the way,   Any helpful knowledge would be much appreciated I've done enough research to figure out these 95's can be hard to diagnose  Thanks MichaelB

A lower RPM miss *could* be you got the "wrong" coils or that something else is wrong. This has comeup before. See the following topics/posts (I suggest you read thru the whole thing):

https://rivperformance.editboard.com/t6709-ignition-coil-spark-voltage?highlight=hot+coil.

Using RockAuto as a reference guide, I see that the coils shown for the SC and NA versions of the 3800 are the same. This is wrong, sort of. Originally the SC coils were different. A good way to get the right coils, at this point in time, might be to get them from a junkyard car. You're likely to see good coils on a ParkAve Ultra or Olds with the SC 3800 engine. The right coil is the Delco D576, for the SC engine, not the D555 nor the D555 equivalents listed on the RockAuto site for the SC engine. I am not sure where you even get a D576 except from junkyard, or Amazon, nowadays. On Amazon they are about $50.- + each. Reading through the thread I referenced above, you will see the explanation of why the D576 coils should be used with the SC engine.

Short explanation: The Delco D555 coils will work and they will misfire when you're in enough boost. BUT read the thread and decide for yourself what to do -- there's other issues (MAF, wires) covered.

A reason you don't see more of this might be that the original coils don't fail unless something else goes wrong. In 330,000+ miles the only coil failures I have had were two.

On one, a plug wire's internal conductor broke. The shop (a Firestone in Pleasant Hills section of Pittsburgh - diagnosis and repair on a Sunday morning) found the problem, got me the coil they could get (a standard Standard coil), put a used wire on N/C (their techs are old-school and keep some discarded-but-good items around for just this kind of problem) and told me to get new wires when I could, preferably factory wires. Because the other wires would eventually go as this is a 'high energy system.' Repair was not crazy $ and the car ran w/o setting a code. However -- it was on this incident I discovered that the SC coils made a difference -- if you were being peckish you could hear the engine "loped" a little at idle. I replaced the wires the week I got home -- still a little lope. I was reading this site about other's problems, planning my maintenance for the year, and it occurred to me that I ought to measure those coils. It was *several* years after the install. ANYWAY -- using an ohmmeter I found the 'new' coil varied from the others. Hmm... it worked... I wondered if it would work better if they were all the same. SO - , I took the meter to a junkyard and found some good coils on a '97 Park Avenue with the SC engine. Bought them (for a song) and swapped the "new" one out with one of the JY pulls. Lope GONE. I mean, I checked smoothness of idle by balancing a nickel -on its edge- on the engine cover.

The other coil failure I had was due to a wire terminal that had loosened (they can do that) and its metal ablating (vaporizing) over time. I got a lope at idle that time apparently because that cylinder was firing hit-and-miss and that action basically fried the coil. Did not take me so long to figure it out this time ha ha. I replaced it with one I bought off Amazon (in 2015) b/c my other 'spare' coils went to another enthusiast-in-need.

Bottom line is my coils are working great, and at least one of the set has been in service for more than 20 years and 330,000+ miles.

MichaelB wrote:

I Am new to this forum  I have a 95 Riv  3.8 Supercharged with 125,000 miles that has a lower rpm miss that I cant seem to locate  I have done all 3 coil paks, plugs, wires, ignition module, all 6 injectors, mass air flow, throttle position sensor, AIC, I thought it was a vibration at first and replaced the harmonic balancer, motor mounts, trans mounts, I have looked at as many grounds as I can find, If I unhook the battery and reconnect and let the computer do its thing it seems to be better at first then resort back to the missing issue , not showing any codes by the way,   Any helpful knowledge would be much appreciated I've done enough research to figure out these 95's can be hard to diagnose  Thanks MichaelB

Is the CEL on? Is it actually reporting a misfire?

No the light is not on and it shows no codes showing a misfire

you may have a weak cylinder causing uneven power flow
which is not noticeable at higher rpms.

perform a compression test.
the compression gauge is not expensive. maybe $20 ?
it's important to carefully follow test instructions:
warm engine
all plugs removed
same number of "bumps" for each cyl
(it's good to start with a well-charged battery)

don't try to remember anything.
write the numbers on the valve covers with felt-tip pen.
put motor oil on the gauge threads & o-ring.


get back to us with results.
best.

Hi Abaddon

In my case the mismatched coils caused a *very faint* low rpm miss or chuggle. I admit that I could only hear it when I was being, well "peckish" and the service manager at my local GM dealership could not hear it at all (it's that close to normal).

It *never* set a code.

It didn't make a real difference in my driving (I drive *a lot* but I do not thrash the car with dragracing, autocrossing, etc.).

My butt-dyno tells me it made a little difference in performance (e.g., passing on the highway), however I never actually measured that.

The big "aha" for me was it took a very long time (years) to figure out what caused it.

I think all that's really happening in causing this "slight miss at idle" is there is a slight difference in rise time between the NA and SC coils, and I bet that one can only hear it is one is musically or mechanically inclined - because it's not a random chuggle like you get with say an idle air problem but it's a regular arrithmyia - every third detonation will be a little off time if you have a coil mismatch. If your engine is OK otherwise you can hear it at low RPMs. Raise the RPMs, then not so much. Get into boost, it's too quick to pick out but it's keeping you away from optimizing power. I kinda wonder if you could see it on a scope, you know, one of the old Sun machines. Remember the "cylinder balance test?" On the Sun 720 you could look at the oscilloscope. This might show up on the scope as a difference in the peaks when each cylinder fires. Every third one would be higher or lower than the others depending on if you had one or 2 NA coils on the ig module.

Or you could just pull the coils and measure the primaries. However, I do not have those specs.

Bottom line is... I don't really *know* for sure but other than looking for iffy wiring at the ig module or excessively dirty spark plug wires, I'd measure those coils.

Albertj

It could quite literally be anything. It might even be something that's normal that you don't like. The 3800 isn't "smooth" by any means. The damn thing misfires at idle by design

You could have a leaky Injector. You could have (as mentioned already) slightly low compression on a cylinder. Could be a leaky/burnt valve. Could have a slightly collapsed lifter, etc etc etc.

I think you need to get this thing on a scope and see if it's actually misfiring before you waste any more money/time.

My $.02