Intermittent rough idle

1997 SC bone stock, 71K miles, pampered.  I have read lots of posts - not sure where to start.  Intermittent rough idle in all running conditions and engine temperatures, cannot determine any pattern.  freeway driving to a stop, sometimes smooth as silk, sometimes rough.  Not a miss or a stumble, just rough enough to be annoying.  Cold or warm, street driving at a stop smooth, next stop rough - and so it goes.  Lots of sensors, egr, pcv, iav - where do I begin
Richard

Put an inexpensive vacuum gauge on the engine to rule out a vacuum leak. I would also look at the IAC and clean the throttle body. Can someone scan the computer for you?

also check for bad pulleys and worn belts. 71k would be about the mileage and way past the age for thee to get bad enough to cause vibration. Assuming they are original.

Thanks for the input. Throttle body cleaned, new belts and pulleys, motor mounts, supercharger rebuilt, no discernable vacuum leaks, injector o rings, lower intake manifold gasket set all done. No codes on scan. What else have you got?

Did it start after one of those was replaced?

How about balancer pulley? look for cracks in the rubber part of the pulley.

Richouser wrote:

Thanks for the input.  Throttle body cleaned, new belts and pulleys, motor mounts, supercharger rebuilt, no discernable vacuum leaks, injector o rings, lower intake manifold gasket set all done.  No codes on scan.  What else have you got?

The IAC contols idle. Maybe it is sticking occasionally.

Larry,

Is that part serviceable or just replaced?

I think you may be able to clean it, but it is replaceable. This is what it looks like,

http://www.rockauto.com/info/154/AC66Tphoto%20primary__ra_p.jpg

It gets carboned up like the throttle body. Maybe it is sticking. Try cleaning it. New ones are 20-30.00 on www.rockauto.com

might also want to give cleaning the MAFF a try. buy MAF cleaner, dont try other cleaners on it.
Also, are the plugs and wires original?

As far as I know, original plugs and wires - both look great.

I don't see any mention of ignition wires. Worn ones can cause intermittent issues. Could also be leaking injectors and maybes on many many other things. We can all take guesses, but start with the basics. make sure everything is clean.

If you do go after new ignition wires, don't get cheap ones. There are a lot of posts here about them being the cause of headaches. The supercharged motor is picky about what wires it will like.

Ok, thanks everyone.
I went back in and cleaned the throttle body again, checked the MAF visually, did not clean it, and this time pulled and cleaned the IAC. It was carboned up on the tip of the pintle, so I hit it several times with carb/throttle body cleaner and carefully burnished it with 0000 steel wool. Reassembled and put electrolytic grease in all connectors. It now idles with good manners.

However, in the process of smoothing it out I now believe that I am getting a vibration from the idling engine into the frame, and up into the steering column that I could not detect when the idle was modulating. I believe it is subtle enough that motor mounts should be insulating it. I have researched the motor/transmission mounts in the factory repair manual, and there isn't too much there except replace them if they are bad. I am not able to find either factory or aftermarket replacements in an initial search. It appears I will have to remove them to inspect them. How deep should I dive into this, or am I heading in the wrong direction?

Richouser wrote:

Ok, thanks everyone.
I went back in and cleaned the throttle body again, checked the MAF visually, did not clean it, and this time pulled and cleaned the IAC.  It was carboned up on the tip of the pintle, so I hit it several times with carb/throttle body cleaner and carefully burnished it with 0000 steel wool.  Reassembled and put electrolytic grease in all connectors.  It now idles with good manners.

However, in the process of smoothing it out I now believe that I am getting a vibration from the idling engine into the frame, and up into the steering column that I could not detect when the idle was modulating.  I believe it is subtle enough that motor mounts should be insulating it.  I have researched the motor/transmission mounts in the factory repair manual, and there isn't too much there except replace them if they are bad.  I am not able to find either factory or aftermarket replacements in an initial search.  It appears I will have to remove them to inspect them. How deep should I dive into this, or am I heading in the wrong direction?

the motor mounts on this car are a PITA. You can see if they are collapsed on visual inspection, the supported bolt goes thru a thin rubber "X" in each undercar mount. If the X is sagging the mount needs to be replaced. Depending on how the car is driven they don't last all that long. IIRC there are inserts for the transmission mount. I don't have details.

Hi gang,

Not sure if this is the right thread, but here goes.....

I have a 95' Riv that typically runs great, but recently when I start the engine when hot (like after stopping to fuel up), the engine idles very rough, best way to describe it is "chugging", after a few seconds it will smooth out to a high idle then settle into a normal idle, when this happens the check engine light comes on and stays on till next restart, engine starts fine every time when cold.
I have a very nice Bosch code reader, but as most of you know, the 95' is a crossover year (OBD-l computer with an OBD-ll interface) so I can't download the codes, and the only place locally that can read the codes charges $89.00 to do it.
I have recently replaced the TPS, fuel filter, and plugs & wires, any thoughts would be appreciated....thanks!

SpiralArchitect wrote:

Hi gang,

Not sure if this is the right thread, but here goes.....

I have a 95' Riv that typically runs great, but recently when I start the engine when hot (like after stopping to fuel up), the engine idles very rough, best way to describe it is "chugging", after a few seconds it will smooth out to a high idle then settle into a normal idle, when this happens the check engine light comes on and stays on till next restart, engine starts fine every time when cold.
I have a very nice Bosch code reader, but as most of you know, the 95' is a crossover year (OBD-l computer with an OBD-ll interface) so I can't download the codes, and the only place locally that can read the codes charges $89.00 to do it.
I have recently replaced the TPS, fuel filter, and plugs & wires, any thoughts would be appreciated....thanks!

Throttle Position Sensor - usually not bad, one major cause of "bad" readings is voltage drop on the circuit it gets its power from, for some other stupid reason... however they do wear out over time and are not too pricey.

Fuel Filter - with the gasoline in the US the past 20-30 years or so and plastic filler tubes/tanks, fuel filters are rarely dirty. They do tend to trap water from in-tank condensation and what not so they really do need to be replaced on schedule. Whatever you do, Don't Use a Crap Fuel Filter.

Plugs and wires - if these were the problem you'd chug cold or hot, or misfire under load/at speed, or lope.

If you have a problem idling, problem is likely to be in the idle circuit: an "overkill" fix that is not costly is to get a replacement Idle Air Control (these have a pintle that can be cleaned but do wear out, I think it's thermal cycling but I don't really know) and a throttle body gasket. PULL THE THROTTLE BODY and clean it (and its passages) with an appropriate cleaner. GM dealers sell a cleaner that works well. Do Not Use Carburetor Cleaner, you will remove the coating the the TB and then have to clean it daily (it will feel like daily) thereafter. Be sure you have your air screen in the throttle body, if not get a replacement they are cheap on rockauto.com, it is held in by a ring clip. Air screen helps the MAF work right. Clean the air passages on the intake manifold at the throttle body, with that TB cleaner also.

On a hunch I would check the wiring to the ignition module for an intermittent, while I was under the hood.

[Hey Abaddon, if you're reading this, do you think it might be the MAP sensor? He'll need a tech II to read it but I bet it is misreading or not reading at all. What do you think about the following? ]

What you have to do is get a scanner that will read the values of the sensors and read the MAP sensor while the engine is OFF and COLD, and at the same time find out what the true local barometric pressure readings. The two figures should reasonably match. If not, replace the MAP. Next -- After you start the car when it is idling MAP goes down under vacuum. Does the MAP sensor reading change? I had one that just sat at 90, what a pain. If the MAP does not change, replace it. It won't set a code at that time due to the GM engine management strategy, they don't read the MAP all the time.

I suspect that what may be happening, maybe, is the ECU's getting the BS reading from the MAP on the warm start, mis-setting the engine controls, when it reads it again the MAP reading's not making sense so it ignores it and sets some default values and sets on the CEL, your idle/running straightens out and you wonder why the car is running so well with the CEL on. So again if the MAP reading is off from the true barometric pressure reading by more than 1 unit when the engine is off and cold, just replace it. If it is not changing readings appropriately when it should (blip the throttle up to 3000-3500 RPM, the MAP should change reading appropriately) just replace it. A MAP failing from age will do all sorts of dumb stuff including drag down the voltage feed to other sensors and cause them to misread; your ECU then says whiskey-tango-foxtrot and sets a code, ignores the MAF and the car then runs OK with very slightly worse gas mileage and leaves you wondering what all the fuss is about. Problem is that since the sensors are misreading because the MAF is semi-shorting or whatever, the code won't necessarily be a MAF code. Which leads to people replacing parts that don't have a problem -- the telltale is the sensors that are yelling are the other sensors that are on the same power feed circuit, not the MAF. Especially if your MAF is just dying from old age.

Thank you for the very informative response!
I was leaning towards IAC and MAF based on other posts I have read here, and I guess I should have mentioned I replaced the TPS, fuel filter, and plugs & wires to resolve other issues prior to the idle problem manifesting itself, I just wanted save readers from suggesting those things as causing the current problem.
IAC's are cheap, and based on the age and mileage of the car I'm sure it wouldn't hurt to just go ahead and replace it whether it helps with my current issue or not.
As for the MAF sensor, I appreciate the very detailed suggestions and would love to follow through with the testing, however I just can't afford to spend hundreds of dollars on a compatible scan tool.
I really love the look, ride, and performance of this "era" of Rivieras, but I really regret buying a 95' due to the "OBD" issue, this is a beautiful car but it's been a real pain in the ass to keep up with.....lol, I guess I will just have to take it to a shop and pour more money into it.

Thanks again for the suggestions !

Boy did I luck out !!!......My whole "hot start" problem was simply the fact that after the mechanic changed my TPS he did not get the plug in all the way, I pushed it in till it "clicked" and the car is fine now.....who'd of thunk it, eh?....lol.
Just wanted to post it so anyone else experiencing this problem can be sure to check the TPS and it's plug.

Thanks again albertj for your suggestions, they were very informative, but I'm happy it turned out to be so simple.

As long as I'm here, can anyone suggest an affordable scan tool that overcomes the whole 1995 OBD-l computer/OBD-ll interface issue?