A bit of a shudder

[ford40]

Hi All,
I need some advice on a shuddering noise I'm getting before the engine reaches idle. The motor reaches idle fine but sometimes it gives a shudder from about 1,100 rpm to the idle range. This does not happen every time it idles down but on occasion. The engine is warm when this occurs. I don't recall that I had been driving hard prior to the occurances. I just purchased the car a few weeks ago it is a 2004 boxster with 5,500 miles. I am reasonably certain that the shuddering is engine related as I have been rolling and I have been stopped when I noticed it.
I have a couple of street rods and have been a shade tree mechanic for years but this has got me scratching my head. I hope I'm not becoming a hypo-porsche-i-ack....I just love this car and don't want to overlook something that could be serious. It is a Porsche CPO with a 100k warranty.
Thanks in advance for your help!
Ford

Ford40:

Congrats on the new car. I found this info on the Porsche Tech Web-site. Possibly this can help to steer you:


"Usually the problem would be with a sluggish Idle stabilizer or also called an IACV. These valves can get corrosion in them and slow down until they get stuck and then cease to work. This valve controls the amount of controlled bypass air required to keep the car idling. These conditions change with load and the valve opens or closes to accommodate this. The valve is easy to access from its location on the side of the throttle valve housing. This condition will usually be exacerbated with the A.C. on.


Scott Slauson - PCA WebSite - 7/26/2004


A small drop in rpm's is normal when the A.C. operates. This is due to the load of the compressor turning on and off. Now there have been a couple of problems creating a drastic drop in rpm's with the A.C. switching on and off in the Boxster and 996.
The first is very simple, they were overcharged with 134a freon. This was done at the factory and often during an A.C. repair. An over charge of freon will cause an excessive load on the engine. This is simply taken care of by recovering all the freon and recharging it to the label under the hood. Depending on the year the charge rates are different.
The second is the Idle Air Control Valve (IACV). These valves tend to get corroded and they stick. Under normal running conditions they move very little. When the A.C. is turned on they are required to move the most. They will give you an erratic idle and sometimes even stall the car when completely bad.

Scott Slauson - PCA WebSite - 5/5/2004"

[ford40]

Boy you can tell I've been working on small block chevys too long...now that I think about your info it sure look like it could be the problem...Again thanks for your help!

Hey Ford40, I'm glad this helped you out! Good luck with your DIY project.