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Warm Start and Engine Shuddering

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Hi all,

So, blackboxster and I appear to have different problems, so I decided to start a new thread concerning my car. My 98 2.5 non egas 5 speed under 10k miles Boxster seems to be having a idle problem on every warm start and frequently cold starts IF the outside temperature is below 60-65 F. What's weird (and good) I suppose, is that the engine kinda returns to normal after I've driven it for a few minutes. There also is no Check Engine Light. This isn't a chronic engine shuddering at idle problem, but pretty short term. And when the engine shudders, it is for like a second before the DME I think recovers it. It is however, really disconcerting because the engine shudder causes the whole car to vibrate.

There are 3 distinct categories that this engine shuddering can happen on my car. And each time before I start, I always wait for the 5 second countdown and before going anywhere, let the engine hunt for a stable idle.

1) Engine Cold, Outside Temperature also Cool <60-65 F. In starting, the tach shoots up to like 2000-3000, but then drops real low, causing the associated shuddering/vibration that is VERY noticeable. Then it overshoots the normal 900 or so idle by just a little before settling into normal idle.

2) Engine Warm can be off for like 2 minutes or 2 hours (but still kinda warm). Return to car to start it, starts fine unlike ex #1 and wait for it to idle stabley. Shuddering occurs later though. Shift into first gear, and start pulling into Houston's traffic. At stop-and-go (this is terrible stop and go, can't even get off the clutch completely in the few feet you can move forward), the engine shudders EACH time the engine is back into neutral to idle with the car stopped. However, this goes away once we finally get on our way with a green light and I can stay in 1st or 2nd gear for some duration. Then there's no more shuddering for the rest of the trip (the coolant doesn't even have to be completely up to temp yet either).

3) Same engine warm scenario BUT with no traffic and no upcoming stops, if I can allow the engine to run for a short while in 1st or 2nd gear at any speed, no shuddering.

Loren recommended looking for carbon build up on the throttle body and cleaning the Idle Air Control Valve attached to it. And while I was looking at PCA's tech Q&A section, Scott Slausen suggests in other posts concerning idle issues to look also for any intake leaks or a bad MAF.


Idle Air Control Valve/ Throttle Body and Butterfly

What kind of carb (carbon/carburator?) cleaner do you guys recommend?

Is the Idle Air Control Valve also cleaned with the lint free towel and carb cleaner or is it a flap of sorts that I just push to ensure it's moving well?

Is it reasonable for carbon to have built up on such a low mileage but older car?

Intake Leaks

Can anyone tell me what components are part of the intake tubes, so I can also check them for leaks?

I am also really baffled at the theory of this. I'm trying to understand why this idle issue is occuring but not all the time?!? How is it that a warm idle problem can resolve itself after driving for just a while? Am I helping calibrate my engine or some sensors by being able to run the warm engine for a little bit? Please assist me with this. I'm worried everytime the engine shudders the whole car.


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  • 1 month later...

Hey All,

So, I cleaned the carbon out of the throttle body and the IACV as suggested in another Renntech post that I posted this situation in (How hard to change engine temp sensor?). There really wasn't very much carbon. A thin film on portions of the IACV and a ring in the throttle body around the butterfly valve in the closed throttle position. I also pushed the little metal "swing-door" around in the IACV and it moved freely under pressure, but wanted to swing back in place to its original resting position when I let go of it. So, from what I can tell, it appears my IACV is functioning and not corroded. However, I still have the original problem of the engine shuddering sometimes at idle. I really don't know what it could be. Are there any other suggestions?

Everything in the intake/throttle body/IACV appears quite spotless, there is no check engine light, but the engine doesn't idle like it should! Truman at the PPBB board suggested a DME software update that corrects this problem and also makes the speedometer more accurate, but my dealer in Houston totally denies that such a update has ever been made available. I have posted Truman's post to me below. Please help, can anyone advise me?


More on the Software update (from archives)

The DME software update was the subject of a great deal of discussion in 1999, and dealt with exactly the problem you describe. The update also improved the speedometer accuracy. You might want to check your speedometer reading when you are in top gear at excatly 3000 RPM. An unmodified DME will show 75 - 76 mph. With the update, it will show 73 - 74 mph.

Here is some excellent input from Bob C., October 1999: "My service guy told me all he did was execute "Special Functions" or "Special Options" from the PST2. That took care of everything. On my receipt, it's specifically written that the fix to the speedometer was accomplished with the same fix as the one for the idle."

More from Bob C.: "The DME update is a software update for the Boxster's computer(s). Like the majority of cars today, most systems in the Boxster are controlled by computer. Each dealer service department has a computer called a PST2 (Porsche System Tester 2 ...I believe it's a glorified laptop computer) that can plug straight into the Boxster's on-board computer. The PST2 can be used either to diagnose problems or to download software updates. As I understand it, Porsche occasionally releases software updates on CD.

My service guy told me that he hooked up the PST2, executed "Special Functions" or "Special Options" or something like that, and then it was done.

IMHO, other dealers can deny it or ignore it or call it something different than "DME update" or whatever makes them happy. All I know is that my spedometer is now more accurate by 2.5mph at 70mph, my idle is now steady with the AC on, and my transition from clutch to gas from a stop is now silky smooth."

I would check for the DME software update as a first step.


Edited by Jameslovesporsches
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