running rough all of a sudden and stuck for anwers

[KenD]

Car was running smooth seems like forever and then one afternoon, i had the jerkies when flooring it to get to a little higher hwy speed. Now the car idles rough, but will still drive nicely in cruise control and when held at a steady RPM. So, i had some extra plugs lying around and with the limited amount of time that I had, just replaced the front 4 plugs. The old plugs looked fine, but I replaced them anyways. Took off the boots on the coils to check contacts and there were 3 suspect contact but i cleaned them with a wire brush.

Starting the car is no problem. Driving it is really no problem other than when start off from a start or i stomp on it, it will hesitate all the way up to hwy speeds then once in cruise control, it's fine and dandy. Mechanic buddy says to start with plugs and wires, then coils and finally ICU since it's under the coil. Other than that he says they can put on a fuel pressure gauge and check it out, but he would start with basics first.

(Where is the fuel pressure regulator located?)

Thoughts on this? It's getting colder up north here and the more work i can get done before it freezes over, the better. Car has 252000kms on it now and i have been running regular fuel in it for the longest time, but on Friday put $20 of premium in to see if there was a difference.

Oh, no codes are showing up on DIC.

thx

[growe3]

Hi Ken,

Just a few thoughts.

Did the roughness start after a recent fill-up?

. Might be water or other contaminants in the fuel.

Suggest:

Add fuel "dryer" to fuel tank.

Run the tank low, and then refill from a different station.

Your fuel filter may need to be changed.

. There may be an accumulation of debris in the fuel filter, reducing the fuel supply.

Suggest:

Change fuel filter.

Have you been losing any coolant?

. Might be a head gasket starting to fail.

Checking fuel pressure is easy, and will help the diagnosis.

Need to check pressure at idle, then at about 2000 RPM. Shut off engine; does it hold pressure?

I am not sure of the pressure you require, I think between 38 and 43 PSI. Someone else should have the specs

If the wires are original, they may be bad. Look at them with the engine running at night, or in a dark place.

Check at idle, and with a little bit of steady throttle. Mist wires with water, and check again. If the engine is rough, and you see arcing, replace the wires.

PCM is pricey, and pretty durable; replace as last choice.

-George

[KenD]

roughness started at about 1/2 tank left after fill up. Mileage still seems to be good even with the rough idle and take offs.

i will maybe replace the fuel filter (under drivers side door correct)

i had replaced my alternator and of course spilled a bunch of coolant getting the upper rad hose off. I had driven it a bit after the alternator replacement and then the low coolant light came on after a week ( i guess i didn't fill it up enough) and filled it up to the top and have had no problems since and that was for sure a few months ago.

i think i'll start wires and then with the fuel pressure and see.

[growe3]

I believe that the fuel filter is on the frame rail, below the driver's side rear door.

-George

[lothos]

I believe that the fuel filter is on the frame rail, below the driver's side rear door.

-George

yeah, that's where it was on my 95. Real easy to change.

[jackc]

My 93 would occasionally start running rough out of the clear blue. Each time, it was due to a bad coil.

You can check the coils using the on-board power balance test. This test lets you kill power to the fuel injectors one-at-a-time. If a cylinder is working correctly, the idle will drop when you kill an injector. If the idle does not drop, there is a problem with that cylinder. Note that the RPM drop is very subtle - you may have to run through the test a few times to notice the difference.

If you find 2 cylinders with problems, look to see if they're on the same coil. If they are, swap the coils around and see if the problem moves with the coil.

If you find one cylinder with a problem, suspect the wire or plug.

The link below will walk you through running the test.

http://caddyinfo.ipbhost.com/index.php?sho...r+balance\

[Ranger]

Ken, that sounds just like the problem I had after the fuel rail recall on my '97. Turned out to be the wires. I think the thread was entitled "Stutter, Engine or Trans".

[lothos]

My 93 would occasionally start running rough out of the clear blue. Each time, it was due to a bad coil.

You can check the coils using the on-board power balance test. This test lets you kill power to the fuel injectors one-at-a-time. If a cylinder is working correctly, the idle will drop when you kill an injector. If the idle does not drop, there is a problem with that cylinder. Note that the RPM drop is very subtle - you may have to run through the test a few times to notice the difference.

If you find 2 cylinders with problems, look to see if they're on the same coil. If they are, swap the coils around and see if the problem moves with the coil.

If you find one cylinder with a problem, suspect the wire or plug.

The link below will walk you through running the test.

http://caddyinfo.ipbhost.com/index.php?sho...r+balance\

I was looking for that link, thanks :-)

[PAUL T]

I believe that the fuel filter is on the frame rail, below the driver's side rear door.

-George

yeah, that's where it was on my 95. Real easy to change.

It is real easy to change if you can get the threaded end off. My 97 Deville was a nightmare to change because I could not get the threaded end to budge. I ended up having to cut the gas line and get a kit with a compression fitting to put it back on. The repair kit comes with a short piece of gas line, a pipe fitting, and a compression fitting.

I agree with the power balance test, if it is missing as you describe it should show up good. It can be very suttle though, so doing it more than once is recommended until you get some consistancy. If it is a coil you should have 2 cylinders bad. There is another step on this test that will let you display the rpm's if you don't have a tach. After you get into the override "PS04" press "off" to maintain the override. then select "PCM Data PD11". This bring up the rpm's in the info center. Press "cooler" button and note rpm change. Press "warmer" button to go between cylinders shown on the climate control panel. When the injector is killed the rpms should drop. If they do not then that is a bad cylinder.

[growe3]

I believe that the fuel filter is on the frame rail, below the driver's side rear door.

-George

yeah, that's where it was on my 95. Real easy to change.

It is real easy to change if you can get the threaded end off. My 97 Deville was a nightmare to change because I could not get the threaded end to budge. I ended up having to cut the gas line and get a kit with a compression fitting to put it back on. The repair kit comes with a short piece of gas line, a pipe fitting, and a compression fitting.

The 93-94 STS do not have threaded fittings on the fuel filter, they are quick disconnect clips.

-George

[lothos]

The 93-94 STS do not have threaded fittings on the fuel filter, they are quick disconnect clips.

-George

That's what my 1995 has as well.

[PAUL T]

The 93-94 STS do not have threaded fittings on the fuel filter, they are quick disconnect clips.

-George

That's what my 1995 has as well.

My 94 Deville and the 97 Deville have a quick disconnect clip on one end and a threaded fitting on the other.

[bobbnsueb@sbcglobal.net]

Hi.

If/when you replace the fuel filter, shake it out and if water is a problem, you should see water come out of the filter inlet end.

Bob B

[BodybyFisher]

What was the result of your ignition wire check?, look in each end for a burned core