Gentlemen,
I have a 1995 Bonneville 3.8 Series II.
Lately has developed rough idle and most noticibly under load at ~2200 rpm and up
Holding Brake down while reving in gear really shows this "stuttering"

Whats been done:
Harmonic balancer died ~3 months ago, replaced with no issues.
---I did NOT bang up the crankshaft sensors
Recently replaced water pump Xover "O" rings leaking.
Thought it was a water pump at first, but turned out to be the plastic elbow O-rings.
After burping, no issues with losing coolant.
This is a remanufactured engine with new manifold gaskets etc.
---After blowing 1st engine to that problem, I am very aware of the level of coolant!!!!!

Upon reinstall found alternator dropping out to battery voltage.
Figured that original alternator had a bad brush, and moving it around broke it loose.
Replaced Alternator, but eventually found that the wire to the Voltage Regulator was
pulled out of its plug and causing the intermittent drop out. $136 wasted.

In any case, after this routine repair, I noticed that the engine was running rough under load

I pulled the plugs and eyeballed them.
All appear OK for their age. White/Tan deposits, gap slightly eroded but normal.
Plug Wires all have good continuity under 10K
Removed coils from plate, cleaned spark tower, and base connections.
Replaced plugs w/new to take them out of the equation.
Cleaned MAF and PCV

Still stuttering and rough under load.

Began thinking about vacuum leak.
Had wife PowerBrake car while I sprayed carb solvent all around intake manifold.
No help there, still stuttering under load.

Began thinking that #2 fuel injector wires were mangled when I laid the alternator back on
that side of the manifold. Listening to it w/stethoscope sounds like the others.
--#2 plug looks perfectly normal, no fouling, wetness, etc., after the stuttering.

Reviewing the posts found by searching "rough idle" and "stuttering" I've followed most
if not all the advice so far, and no fixes yet.

Any Ideas??

Mike G.

I had the same issue as you with the stuttering under load....I replaced all 3 coil packs with some from oreillys and the problem went away....it would only stutter under load at the same rpm range as you....just a suggestion have you tried changing your coil packs

Have you cleaned your throttle body?

Have you tested your fuel pressure?

Do you have any engine codes? Have you seen the SES light flash when you get the stutter?

I checked each coil and juel injector with ohmmeter
-all OK

No codes are shown when I borrowed a scan tool.
--This is a 1995 and it has only OBD-1 which apparently does not have as many goodies as OBD-II
No SES light but I did see the ABS light come on once. Never repeated.

We own another Bonneville, a 1999 and I swapped the coils and ignition control module.
The stutter/dropout is still there.

Given that the 99 Bonnie runs perfectly, I do not think it is in the ignition side.

Any ideas on how to test fuel system beyond the ohmmeter??

Thanks

Mike


Fuel Pressure:
I do not have any adapters that can connect to the fuel lines
I've got gauges, just no way to connect. Is there an inexpensive way or tool??

Mike...I'd be looking at the wires closer. Plug wires can ohm out fantastically and yet they aren't laying on the engine with a ton of energy going through them when we test them. I've found the best test for plug wires is to test them at night with a hose or spray bottle of water... Mist them very well and if you see a pretty light show..you need plug wires.

Another idea is if the #4 or #6 wires are touching the O2 sensor/wiring that'll mess with things pretty quickly. Tie them back off the O2 wiring and it'* amazing how quick the engine runs nice.

In most, but not all cases, a stutter is ignition. That'* coils, plug wires, plugs, ICM. Fuel system is next in line, low fuel pressure can cause a stutter. After that it'* a mechanical problem, bent or burnt valve, broken valve spring, clogged cat.

Test coils both cold and hot. (freezer and oven) Chances are you will find a bad coil hot. Say about 150*

You can rent a fuel pressure tester from places like autozone.

Had a car here just the other day that ran rich, O2 looked funny, and didn't idle real smooth. Looked at the O2 wire and it was laying on the #4 plug wire. I moved it over and the scans changed. The O2 wire needs to be as far away from plug wires as you can.

Right.. Being that the engine was recently swapped. I've got my usual suspects. Plus I like to start with the easiest items. A quick visual check can get me smackin my forehead about the O2 etc at times.

Thanks for the ideas, unfortunately not the basis of the problem.

I had high hopes for the O2 sensor--the #6 wire was near it, but no effect.
Just to be sure about the wires, I swapped the wire sets from my wife'* 99 Bonneville
Again, no effect.

Just to clarify the problem, there is a roughness that happens cold or hot.
This roughness starts just off idle, and is most noticible about 1500-2000 rpm
most noticlble under load as I climb my driveway (~10 vertical feet over 50yds)
Above about 2100-2200 rpm'* it is not noticible.
--either from the shorter firing interval at elevated rpm smoothing it out so you don't feel it
or--a fuel problem that is reduced by injecting more fuel or higher pressure or something like that.

I'll give our Autozone and Schucks/O'Reilley a call to see if they have a fuel pressure tester I can use.
What should I look for? -specific pressures @ certain rpm?? idle pressure??

Any ideas that I can test with only general garage equipment??

Thanks,

Mike

PS The engine was swapped a couple years ago. It was the EGR Plastic manifold issue.
I'm just much more aware of the coolant level from that expensive learing experience.

Have you checked the PCV? That could cause a rough condition. Also, if it'* about the same RPM, you should check the TPS, throttle position sensor. Having another similar car you can swap the TPS and see if that helps.