S70 stumbles on start

jimmyboy · Post by **jimmyboy** » 02 Aug 2011, 09:08

Occasionally when I start my car after driving it, parking it, and coming back 10-15 min later, it will feel like it stumbles a little bit after the engine catches.

Otherwise, it runs great - but I notice the RPMs dip below 700 sometimes and it gets very rough. With a prod of the gas, it chokes a little bit and then recovers and runs fine the rest of the day.

This doesn't happen often; just once a week or so. I feel like it's been increasing in frequency, though, so I'm beginning to worry.

Car in question is a 1998 S70 GLT with ~150K.

I think I covered everything. Let me know if you need anything else and thanks in advance!

Post by **Ozark Lee** » 02 Aug 2011, 10:04

I think I would start by cleaning the IAC valve. You can get fairly aggressive with it and not hurt anything but keep whatever solvent you use off of the hoses.

...Lee

jimmyboy · Post by **jimmyboy** » 02 Aug 2011, 10:08

Thanks for the response Lee!

Where is this valve and how do I go about cleaning it?

Post by **jblackburn** » 02 Aug 2011, 11:21

Here's the valve. You can remove the hoses and such and it will pop right out of place. Then get a can of carburetor cleaner and just spray it straight through.

I'd also have a look at the vacuum lines to your fuel pressure regulator. One comes off the fresh air hose down to your turbo, and another comes out of the front of the engine near the black throttle plate cover from the front of the intake manifold. The fuel pressure regulator on your S70 *should* be behind the front passenger side wheel near the steering rack. Follow the fuel lines up from the tank and you'll find it.

That it seems like it only happens when the engine is hot after sitting sounds to me like you're experiencing vapor lock in the fuel rail. It should vent that excess fuel pressure back into the tank as the car is sitting.

DrewsT5 · Post by **DrewsT5** » 02 Aug 2011, 13:51

I think I might have this issue. Seems quite similar when I start my car. It just has this quick, 1/2 second stumble when I turn the key, whether hot or cold. Idle is normal after and the engine is fine, no misfires.

Question: I have a can of the MAF sensor cleaner. Will that work in lieu of the carb. cleaner?

Post by **Ozark Lee** » 02 Aug 2011, 14:09

I'm not sure how well MAF cleaner will work - you can't hurt anything trying it but I would get a bit more aggressive with carb cleaner or I have even used brake cleaner depending on what I had laying around in the garage.

...Lee

jimmyboy · Post by **jimmyboy** » 08 Aug 2011, 10:06

Hmm...cleaned the idle control valve as suggested. I did notice that when I come to a quick stop, it doesn't almost stall out anymore like it used to.

I looked at my vacuum lines and I didn't see any cracking or missing lines - I recently changed out a lot of them.

But the "stumble" was still there on startup after running to the store this weekend. Outside temp was around 93 at the time - again, it only seems to happen when hot.

Any other ideas guys?

Post by **jblackburn** » 08 Aug 2011, 10:20

I know it sounds obvious...but when was the last time things like the distributor cap/rotor and spark plugs were changed?

jimmyboy · Post by **jimmyboy** » 08 Aug 2011, 10:47

All of that is less than 1 year old. I wound up replacing all ignition stuff on this car when I first got it because it kept showing misfire codes. I later found out that it was from random unmatching injectors the kid that owned it before me had put into it.

I've finally got the car more or less back into stock form (removing the stupid loud exhaust, getting matching injectors, and removing the stupid blow-off valve from the turbo). I'm hoping this one is the last hurdle.

Post by **jblackburn** » 09 Aug 2011, 09:59

Hmmm...ok, one last idea.

The EVAP purge valve should remain shut at idle. If it's not, and it's a hot day, the charcoal canister can flood the intake manifold with fuel vapors and cause the engine to choke out.

Try to recreate the situation several times. I think you've got an idea of when it is most likely to happen now. Disconnect the hose highlighted in red here from the intake manifold (at the white one-way valve will do just fine) and plug off the end of it. It may or may not throw an EVAP emissions code, but that can be easily cleared with a scanner.

If it doesn't stumble at all anymore, you may well have found your culprit there. The square purge valves (if you have one) seem to be a common failure point on 1998's for some reason or another.