1991 Bonneville Rough idle & run when cold

[shilohin]

My Bonne recently started idling & running rough when the engine is first started. More like cutting out than an individual miss, like it's flooding, but no stalling at all, like it runs smooth for a second and then stumbles. The exhaust also smells funny when it's doing this, not like gasoline smell, but more like a chemical smell. When the engine warms up, it runs fine. No Service Engine Soon light and no codes besides code 12 (not running) when you check the ECM. I replaced the oxygen sensor, didn't make any difference. Has had the timing chain/gears replaced last year, new alternator last year, new spark plugs and new fuel filter 2 months ago. Checked for a vacuum leak but couldn't find one. I had an 87 Bonneville that had the MAF sensor go bad and it ran rough like this only it would die too (this one does not stall) and someone told me to peck on the MAF sensor and if it was weak or going bad it would cause the engine to stall. Tried it on this Bonne and it didn't stall or make any difference so I don't think it's the MAF sensor. The MPG is normal and hasn't dropped any. I've done all my own work on my Bonnevilles and normally anything like this would throw a diag code. Suggestions on what to check next, anyone? Thanks!

[jdq]

How bout catalytic converter, does exhaust smell like rotten eggs.

[shilohin]

Hi,

No, the exhaust doesn't smell of rotten eggs. I had a new catalytic converter put on about 18 months ago. The "funny" smell is more like paint or lacquer thinner. I was wondering if the EGR valve was stuck open would it make it run like this, rough when cold and normal when warmed up? When the EGR went bad on my 87 Bonneville, it threw a code and made the SES light come on but the 91 Bonne shows no codes and no SES light. Thanks!

[jdq]

Check vacuum leaks and clean IAC valve under throttle body.

[shilohin]

I checked THOROUGHLY for vacuum leaks, replaced one elbow and one line that were spongy but I don't think they were leaking. Still idles rough and surging. Cleaned the IAC and seat area. It was dirty. No change. I removed and blocked with gasket material the EGR valve so that it was still connected to the system but no exhaust gas was allowed into the intake. No change, still idled rough and surging.

The engine starts up immediately. Seems like it runs rough quite a bit when the engine is cold, and as it warms up, it cuts out and surges less and less as the engine gets warmer until it runs fine when the engine is at normal operating temperature. I will be checking back for more suggestions. Your help is greatly appreciated! Thank you!

[HotZ28]

You need to check the coolant temperature with a scanner. The ECT sensor is a thermosistor, and as the coolant temp gets higher the resistance drops. The ECM supplies the sensor 5v and measures the voltage drop through the thermosistor to determine temperature. Some resistance vs. temperature values shown below:

*F--------Ohms
210----------177
158----------467
104----------1459
68------------3520
32------------9420
-4-------------28680
-40-----------100,700

The sensor is used for open/closed loop operation and is used in the calculation for fuel and ignition. It acts like a choke on a cold engine. The colder the engine, the more fuel required to operate smoothly. If the ECT is not providing the correct information to the ECM, the ECM will try to compensate, hence the surging back & forth until it warms up.

[shilohin]

Hi again,

Since the ECT sensor is rather difficult to get to under the throttle mechanism and since it's a relatively inexpensive part, I decided to just replace it, which I did. No change, still stumbles and surges when cold which gradually smooths out as engine warms up and when at normal operating temperature, the engine runs smooth like normal at any rpm range. This engine has 135,000 miles on it, but when I replaced the timing chain, gears & oil pump last year, the parts did not look worn much at all. The engine uses about 1/3 quart of oil between 3000 mile oil changes (regular using Castrol GTX) and gets decent mpg. Even when the engine is running rough when cold, it doesn't seem to have loss of power. I use injector cleaner fairly regular, probably every 3rd or 4th tankful of gas. Any other suggestions? Thanks for your help, everyone.

[shilohin]

Well, I replaced the computer (Electronic Control Module) behind the glove box and it did not make any difference, the car still runs rough when first started, gradually smoothing out until the engine is at normal operating temperature, at which time it runs smooth.

I'm wondering if the coil would act this way? I have never had a coil go bad on a Bonneville but others probably have. Would it misfire when first started and then operate normally as the engine warmed up? Thanks for any help!

[auto trainy]

Just one quick question, are you losing any amount of anti freeze?

[Jrs3800]

Thats a very good question.. With a car of this age it may be due for intake gaskets.. But I don't think he's using any coolant but will await an answer on that one..

Does your 91 Bonneville have the original Magnavox DIS system? I have at times seen these go bad... And if its damp of very cold they can have a hard time starting and or idling until they warm up or dry out, but that does not mean its the problem you are having..

You need to look at what the car is doing via a scan tool... Not always easy to get to a scan tool tho unless you know someone that has one..

Are there any codes at all stored in the ECM?

[shilohin]

Hi,

Not losing any antifreeze and no excessive exhaust steam or water in the oil. My Bonneville does not have the Magnavox DIS system. I do not have access to a scan tool.

As I have stated before, the car starts right up, but idles rough. When I start to move the car, seems like it runs rough, then will run smooth for a second or two and then rough and then smooth for another second or two, almost like it's surging only when it surges it is running smoothly. When the engine is warmed up to normal, it runs fine. In fact, I think I'm getting better mpg since I put the oxy sensor in.

Since the car has started acting up, it has not thrown any codes at all. Thanks for your comments and suggestions! Appreciate it!

[Mickey#1]

Coils are a possibility. You can check the resistance of the primary & secondary side of all three coils. All three readings should be close.

[Jrs3800]

This is the Delco Type 2 Ignition system..



Lower left hand corner is the Magnavox Ignition System


Just being sure..

If I was near you I'd say bring it over and we'd scan it to see what it was doing.. Wish I had more thoughts to offer

[shilohin]

Hi,

Hmmm. My coil pack does have the "three square" coil like the middle photo with the spark plug wire terminals on both sides. However, it does not have any type of display on the left side of the temperature gauge on the instrument panel. It does not have the "Delco type 2" coil pack. I actually have one of those (Delco type 2 coil) here that a friend gave me several years ago.

Also just to clarify, when I say that the engine runs rough when I start it cold, I mean when the engine and components are the ambient air temperature which may be 50 or 60 degrees. If I drive the car and it sits for 30 minutes and cools down (but not COLD), when I restart it it will run a little rough until it warms back up to normal operating temperature.

Also, I have a digital VOM and know how to use it but I have never tested a coil like this before so I don't know which are primary / secondary terminals.

In any case, thank you for continuing to offer help & suggestions! Appreciated muchly!

[Jrs3800]

It almost does sound like there is an unhappy sensor... Almost as if the ECM thinks its in Closed loop when its really needing to be in open loop.. That would be one reason I'd love to Throw a Scan Tool at the car, I would love to see what the ECM is seeing as opposed to the dash gage..

The interesting thin is that on the later years GM went to a single sensor for both the dash gage and the PCM..

When you get to testing the coolant sensor let us know what you find..