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need help with my Bonneville it seems to have a stutter at an idle and going down the highway at normal speed

Probably a mis-fire. How old are the plugs and wires?

Today my car just developed a similar issue. Ran great in the AM. Immediately when I started it after a couple of hours (still warm), the idle was very rough, and the engine misfires/shudders under acceleration. The lower the rpm, and the higher the load, the worse it feels. This is NOT a transmission issue, the misfire is obvious at idle whether in or out of gear.

I'm wondering what could have happened in the interim? I can't detect any vacuum leak. There is no MIL, and the two FLAPS which I frequent say that they can't do any diagnosis unless the light is or has been on. Daggone it. I don't have access to a proper scan tool, which will read output from all of the sensors.

Tune-up was done at the time of the car's resurrection, December 2012, perhaps 16K miles since.

Suggestions?

Can you tell if it's shuttering due to shifting issues or just engine running rough?

plugs are about 5 years old maybe more. I just recently replaced the plugs, because the car sat for 5 years before I got it and the some of the wiring harness was chewed due to rabbits. I replaced the wiring harness and the plug wires to get it to start. It runs. at an idle you can hear a slight miss, but going down the road it seams that the transmission seems to be slipping. I know that the transmission is not doing that. when you put in park and rev on the gas she stutters until you hold a certain rpm. I do not know what the problem is.

I do have a mil on but they can cant figure why.

Transmission slipping can be caused by bad fluid and old filter. Replace those for a cheap fix. The engine running rough is usually a combination of a few things: Fuel, Air, and Spark. Check the throttle body, the fuel system and injectors, and the spark plugs, wires, and coils. Also, find a shop that will read off engine codes for free if you have a check engine light. Could be a EGR, O2, MAP, if it's any of those things the codes will tell you.

if it is the 94 in your signature you may have to have a certain scanner to be able to pull that code.

OK.

if you have a 94 or 95 a lot of auto parts stores will not be able to read them. It has an obd2 connector but runs under obd1 parameters. Where are you located it's possible we may have a member near you with the correct scanner.

The issue with my car seems to have gone away for now. Never did set a "check engine" light, even though it was running so poorly.

I added a bottle of Techron to the tank, but I can't say that was the cure. When the problem started, the fuel level was under 1/4 tank, and I filled it during the rough running days. I was out of town for a couple of days, but my son drove the car a couple of times. Sunday AM it was ok again.

So, I'll be chalking it up to a fuel-related problem. The car is past due for a fuel filter, so I'll be putting one on as soon as I can get to it. I did not check the fuel pressure, but that was on a list of things to look at.

This had gotten me thinking that I need a good connection for access to an OBD I capable scan tool. Hmmm.

Sorry to hijack/share your thread, but it certainly seemed as though we were having the same type of problem.

It's BAAAACKK!

Dang.

Sons were driving the car in Canada over the weekend and it began acting up again.
I have put on a new fuel filter this afternoon, and added Techron again. Fingers crossed!

The car seems to idle better since the filter change, but when driving, it exhibits a very consistent problem, resulting in significantly reduced power:
Whenever the throttle is opened past about 20%, the engine stumbles badly and vibrates a lot. It will cruise down the highway, but passing is out of the question, as all acceleration is very gradual.

I'll be cleaning the MAF as soon as it cools down, and maybe swapping out the TPS with one off of the newly-aquired parts car in my back drive. Otherwise, I am stumped. It was run several hundred miles from Toronto to Michigan, and still did not set a MIL! How am I supposed to know where to look for a solution, if the system will not give me an indication regarding the source of the problem? Grrrrr.

Could be fuel pressure. Check it. When my pump was dying, as the car got warmer, the amount of gas I could give it was less and less. When it got to the point that I couldn't maintain 65 when warmed up, I swapped it. The progression was a couple weeks from no problem to "I can't stand this".

OK, the latest:

First I took out the MAF and did the spray clean thing. Did not expect any change, and that was the result.

#2 was to check the fuel pressure. It appears as though the pump is putting out enough. I picked up a nice "loan" unit from O'Reilly, and hooked it up to the valve on the back side of the fuel rail. Results: 35 psi immediately upon ignition cycle to the On position. With engine start and run, still 35 psi. A blip of the throttle will bring a rise to near 40 psi, followed by a dip to about 32 while the engine returns to idle. Then back to 35 again.

I am kind of thinking that fuel pressure is not the problem, (can't find the 2nd volume of my shop manual.... ) as I recall 35 being sufficient for this system? Time to hunt for some other issue.

BTW, the car ran fine until the second restart after buying fuel in Canada. I wonder what alcohol mix they have in fuel outside Toronto? Also, do they use more than 10% Ethanol, or 5% Methanol?

The fuel filter I replaced was original to the car, and probably was less capable of flowing gasoline than the new one, but it was not plugged up, and the swap did not solve the problem.

I have no way to diagnose the TPS, other than to move the rotating lever with my finger, without moving the throttle plate. It does raise the rpm (presumably by adding fuel via the injectors) until a certain point, and then it begins to decline again.

Alan,
My gut feeling would also be fuel pump (was the fuel allowed to get rancid while car was stored?)
When I had problems, fuel pressure checked out good but when it got warm it would act up. It was worst after idling in summer traffic.
In my '00+ cars the pumps failed quickly to the point where the motor would not run. In the older cars it seemed to linger on a long time just getting weaker (as said above).

It also could be ignition related though.
Since the pump is somewhat of a PITA to change I would start with the following.

Plug wires and coils are usually weak links (assuming you have good properly gapped plugs).
I usually run engine in pitch black darkness and lighly mist wires and coils with spray water bottle looking for blue corona.
Check for corrosion at coil plug wire terminals.
Coils commonly have intermittent failures and it's easiest to check by susbstitution (i.e. swap 1 at a time from you parts car).
Make sure you have good clean grounds on ICM (very very slim chance ICM itself could be going also but its not common).

Neither fuel pump nor ignition problems will show up directly on a scanner (you will only see fuel trim being out of whack).
MAF and TPS failure would probably not present these symptoms.
If ignition checks OK I would replace the pump & sender with new good quality ones.

Hey, RR!

Well, this car was never really stored.... Has been run consistently since you helped me get the subframe issue sorted out in December of '12.

The symptoms of rough idling are worst when the engine is cold. The "no power, misfire under more than a slight acceleration" problem I have not looked at without the engine being at normal temp.

The wires and plugs were changed at the time of the car's SF swap. I'm thinking those would not be the issue, since this same thing happened a couple of weeks ago and then went away, only to return Friday night in Canada. Fading fuel pump is looking more unlikely, see above ^

Intermittent coil, I might believe. Failing ICM, I could also see as a possibility.

There was junk in the filter when I pulled it out, and it drained backwards into a jar. Might something have gotten through and plugged up the screens in the injectors? Also possible, I think. Since the tank is FULL, I'm going to have to resist considering replacing the pump, at least for a couple of hundred miles, so that it will be emptier.

The majority of problems I've had over the years turned out to be ignition related in some way.
Didn't catch the rough cold idle symptom - vacuum leak in an accessory item itself?
I would still start with ignition though.

In my experience, when fuel pressure gets below high 30s, you get drivability problems.

Years ago when I was beating my head trying to track my problem I did this to get live readings:
Bought a Nordskog 100 psi fuel pressure sender and shrader adapter fittings.
Ran couple wires inside car and hooked up a multimeter to read resistance.
To calibrate pressure vs resistance I had a tee fitting off the fuel rail shrader for my pressure gauge.

The times when it would act up real bad (being barely drivable), pressure would drop to 25 psi with heavy throttle.
There were also times when it would test OK in the driveway with no engine load.

Roadrunner wrote:The majority of problems I've had over the years turned out to be ignition related in some way.
Didn't catch the rough cold idle symptom - vacuum leak in an accessory item itself?
I would still start with ignition though.

Roadrunner


Ding Ding Ding Ding!!! We have a Winner!

I began this afternoon's investigation with a simple test: Pull each plug wire individually at the coil, and see if the engine idle drops, or what else I can see.

For the top two wires, the idle got worse, and a spark was jumping all over the place, anywhere a ground might be found, even an inch or more away from the posts...

But for the second coil, there was no change in the rough idle at all. More significantly, there was no arcing. None. (I was hoping that it was just the coil, although the electronics panel (name?) that they mount to should also be good in the parts car.)

So, I pulled the one from the white car in the back drive, cleaned it up and installed it, and.... Success!

Smooth idle immediately apparent, and a quick drive confirmed that ALL of the power and great running characteristics have returned.

YAY!

The lesson here for me, is to not get hung up thinking in only one direction (fuel, this time), and consider ALL of the possibilities which may be causing an issue. Thanks for everyone's help, I hope this is it as far as problems for a while....