Rough Idle: possible fuel sending unit issue?

[kameraguy]

How many people have this issue:

From a stop on level ground, lightly apply throttle until you barely start rolling. Once you start moving, let go of the throttle.

Let the car start coasting. As the car starts to slow, do you notice your revs start to bounce between 500 and 1000 RPM? Or something along these lines? It feels like the car can't stablize idle.

Or how about this. At a stop my car idles at approx 500 rpm. But on occasion it sporatically dips to say 250/300 RPM and seems like it's going to stall. But this happens intermittently and when it does happen, it seems to happen only once. Then it takes an unpredictable amount of time until it happens again.

I have had this issue for some time now. And it only happens when my car is stopped, or coasting to a stop.

I would like to fix this issue if it is a problem.

I've already changed my fuel filter due to a major tune-up. Is there anything else that might contribute to this?

Such as the distributor or rotor?

How about the fuel sending units? Would these be the same guys responsible for my erratic fuel gauge? Maybe they can cause this issue too?

Aside from this, the car seems to run great.

Anyone else have this issue?

[Rocky]

Kamer, I spend about 10 + hours reading and tring to figure out this kind of problem.

Keep in mind the throttle actuator is a 3 in one deal. It controls the idle, the cruise control and the ASR (even if you don't have ASR your thottle actuator still has this piece.)

The TA is one expensive Mofo. Avg $1200 USD just for the part.

My problems seem to have gone away. It might have been due to bad gas but I think I have a warped head again or a gasket meltdown. After I put in a bottle of Bar's leak the car is good, not great.

I have a crapload of theories:

1) Fuel pump, our cars have two. expensive each.
2) OVP (shouldn't be a problem since ours is a 1997)
3) Loose vacuum line somewhere
4) Dirty Thottle plate (i cleaned mine out)
5) Clogged EGR tube

and many more theories.

How does it start and how does it idle?

Any CEL lights?

[kameraguy]

Regarding starting the car, more often than not it starts like a champ on the first try. The car settles into a consistent idle.

But sometimes the car starts with the idle spiking to 2000 RPM but settles down quickly. This doesn't happen very often.

I have a VERY intermittent CEL issue. It has only happenned three times in 20K miles. Each time was a cylinder misfire code. But a restart corrects the limp mode. The CEL goes out a day or so latter.

I've dropped my car off to the dealership in this state and, as luck would have it, they could not determine the root cause. Even with the CEL on, they had no idea why it happenned.

For me, it is an intermittent issue. All the frustrating ones are intermittent!!

If I had a bad or faulty TA, would the ASR light sometimes go on? Because I also have an intermittent issue where i lose ASR completely. It happens when driving. But once again, a restart corrects the issue.

This is great info. Thanks Rocky!

[Rocky]

What codes did you get?

I have probably the exact same problem as you but I have no clue whats going on.

In Idle and at a stop light with the brake applies..the car sometimes sputters.

It use to start up goign vrnvrnvrnvrnrvn for a few secs the goto normal.

My CEL light is intermittant ans hasn't been on in awhile.

I got a code for a potemieter in the TA and a Injector #3 circuit malfunction.

Lucky for me I don;t have ASR hahaha so I never see the ASR light.

[kameraguy]

The last time I had the CEL on, it was when the car misfired. The only codes I had confirmed which cylinders misfired.

I was able to get the codes from a previous time I had a misfire, and unfortunately there was no consistency. I was hoping maybe the same cylinders would misfire to help isolate the issue, but it was different cylinders both times.

But yup, your description of what happens when at a stop with the brakes is exactly what my car does. Not all the time, but sometimes.

It's problems like these where root cause is nowhere in sight that drive you mad!

[Rocky]

kamer, if you have a misfire problem (which cylinders?), most likely it's the boots underneath the coils. There are 3 boots and they are known to go bad. It's like $10 bucks a piece. If you never changed them..do it. It's preventive maint and it's only $30 bucks for the parts. You can do the labour yourself.

I think they are called Coil Ends. They go bad easily on M104 motors and usually that is the misfires you get. Most people change the coil ends each time they change the plugs.