Rough Idle Only When Cold

[IQT45]

I just purchased a 97 Q45, and have a question about cold idle. When the engine is cold the idle is rough, rough in that it seems to have a miss. Not dramatically severe, feels as though it is only one cylinder?? The vehicle was inside the dealership and was about 70 degrees. The rough idle is not noticeable while the vehicle is in park or nuetral only when in gear and the rpm drops. When the engine warms to normal operating temp. it is perfectly smooth, in or out of gear. I read a post by Q45 tech that states.

"Gasoline has a vaporization point and the blend changes at least 4 times per year in a single location to attempt to match the average outside temperature. The ideal range is pretty narrow unfortunately the injectors start flowing at some pretty cold temps and the gasoline won't evaporate until the intake valves reach a certain temperature [it reliquifies and coalesces into drops that don't burn] so every thing is double rich to try and make sure something [some part of it] burns. When the idle smoothes out you know it is getting warm enough. Remember the old choke well the injectors just double/triple up the fuel [cold enrichment as a function of coolant temperature and high idle to make sure it stumbles the least amount].

I thought this may have been the issue however the vehicle was inside and was about 70. I have read about possible dirty MAF, IAC, plenum, injectors etc. But what I don't understand is why it idles perfectly when the engine is at normal operating temp.

Any ideas??

Thanks
Modified by IQT45 at 12:56 PM 1/9/2006

[Q45tech]

At 60-70k the first thing you do is install fresh spark plugs and use BG44k to clean injectors and valve stems and piston tops. Abusive SLOW city driving can create problems reducing the viable spark plug life.

Most don't drive these cars in an rpm aggressive way for long enough to clean up combustion chambers.......you can't without going to jail so you are stuck with chemical cleaning.

Anyway a rough idle is little concern until the coolant warms up and the vehicle is ready to drive.

OBD2 is very sensitive so as long as you don't have a MIL light.

[IQT45]

Thanks for your reply Q45tech.

The vehicle has 153,000 miles, so it may be overdue for it's second set of plugs, I will look in to that. Good point about the unaggressive driving. The vehicle was owned by an elderly couple since new and it's a safe bet it has not been driven aggressively it's entire life. I will also add a can of BG44k, the good thing is there is no MIL.

Thanks again, I really appreciate your response.

[IQT45]

Just thought I would update you all on my rough idle only when cold issue. The dealership called and said their diagnosis revealed a faulty coil on #7 cylinder. The dealership had pressure washed the engine prior to replacing the valve cover gaskets, they claim water had gotten into the coil pack and caused it to fail.

[CrimsonQ]

I hope they are replacing it free?

[IQT45]

I hope they are replacing it free?

Absolutely!

The day I test drove it I noticed the miss, they stated they thought it was just some wet components under the hood from pressure washing and when it dried out everything would be fine. They were not completely finished with detailing it so I was to take delivery the following day. We completed the paperwork and I agreed to purchase the car with one contingency, the car needed to idle smooth, engine cold, the next day when I picked it up. Well guess what the next day engine cold rough idle. So they agreed to fix the problem. Kudos to them