93 Volvo 850 engine revs high by itself

jreidko · Post by **jreidko** » 14 Jul 2010, 20:58

Hello, i'm a first time poster here, but i've been relying on this site's hugely helpful forums for a couple years.

My beloved 93 850 has started to do something that i, for the life of me, have no idea where to start a diagnosis. The engine is reving up by itself all the time, not even intermittently just constantly. If i lightly depress the clutch or the brake it will disengage this behavior, but no matter if it's just been started and idling or if i'm driving at any speed, once i take my foot off clutch (and brake) it revs (or attempts to rev if i'm in gear) to above 3000 RPM.

If i'm on the highway it performs pretty good as a cruise control as it basically keeps the car at 70mph when in 5th gear. In fact, I thought maybe the cruise control had freaked out or something but i pulled the cruise control control module and that had no effect.

I'm totally stumped on what this could be, i saw an s90 thread here at > https://www.matthewsvolvosite.com/forums ... 7&p=137203 but it doesn't seem like a good match to my problem.

Does anyone have any thoughts? Or suggestions on where i might start looking?

Post by **jblackburn** » 14 Jul 2010, 21:30

Look at the throttle cable or throttle body itself for something sticking. My Saab's cable kinked one time, and it would redline itself as soon as the engine started.

Post by **asg1986** » 14 Jul 2010, 23:16

Have you messed with your vacuum hoses? When I replaced the transmission on my '97 850 I connected one of the hoses to the wrong plug, and it did this in park and while I was driving, but as soon as the brake was depressed it stopped the revs. There is a hose that runs from the brake pedal (and in your case the clutch pedal) along the L side of the car. I believe it is a blue plastic hose. My guess is it is vacuum related if not the throttle cable is sticking. Check this out:

https://www.matthewsvolvosite.com/forums ... =1&t=22747

jreidko · Post by **jreidko** » 15 Jul 2010, 16:23

Thanks for the thoughts guys,

I was hoping it was something simple like what you mention jablack, but all seemed fine in the throttle cable.

I did messed with the vacuum hoses several weeks ago and was wondering about the vacuum lines. The car has been running well since (until this thing started happening two days ago) but it certainly seems possible that something either fell off, got disconnected or was, indeed, patched wrong.

Thanks for the cross link and description I have never had a good grasp of how the vacuum lines all relate and where the heck they all go, so that link and your description are my next task.

I'll post what what i find!

jreidko · Post by **jreidko** » 25 Jul 2010, 07:15

Well, it's a week later and i've probably spent several hours staring at all the vacuum connections under the dashboard and in the engine compartment. Everything looked all right.

The closest i got to what might have been the issue was a vacuum hose under the battery platform that goes to the cruise control device/s that are mounted to the underside of it. Looked like this vacuum line might have been at least partially compressed under the edge of the platform. Not really convinced this was it, but after putting everything back together (battery platform, air box) the problem went away.

Hoping it stays like that! But we'll see. Thanks for the suggestions!

If anyone's looking, the following link was also helpful:
https://www.matthewsvolvosite.com/forums ... 25&p=98764
This vacuum routing diagram was closer to my 93' 850 (non turbo) than the basic routing diagram in the 95' 850 shop manual PDFs that are floating around here. But none of the vacuum routing diagrams i found showed how the EGR hoses and a few others also route through the cruise control components under the battery platform.

jreidko · Post by **jreidko** » 08 Aug 2010, 09:58

The engine revving up by itself has returned. It's actually been *sort of* intermittent. What i've discovered is that it seems to only do this when it's pretty hot outside (like 90º or so).

I don't think it's the vacuum hoses. I've inspected them everywhere i can see any connections, and everything appears to be in good working order and properly connected. I'm starting to wonder if it's a sensor somewhere. When it's happening the engine will rev up very quickly (as if you stomped the gas pedal to the floor), but like before if you slightly depress the clutch or brake it will disengage the revving.

When it's happening, It happens 100% of the time from the moment the car has been started till you turn it off. It's never been intermittent while actually driving.

If anyone has any thoughts, any suggestions would be hugely appreciated!

Post by **Ozark Lee** » 09 Aug 2010, 08:26

I think the next place I would go is to the IAC valve. Remove it and clean it. It may or may not fix the problem but it can't hurt anything. I normally use carb cleaner but you can use about anything.

...Lee

jreidko · Post by **jreidko** » 09 Aug 2010, 22:17

Thanks for the suggestion Ozark, i'll give that a try.

jreidko · Post by **jreidko** » 09 Oct 2010, 16:49

Well, its been a while and i'm no closer to finding the culprit on this issue.

The IAC valve is good working order, removed it and it was pretty clean, but sprayed it out with carb cleaner and also experimented with it disconnected entirely. It ultimately had no effect on this intermittent behavior where the engine will just rev up to very high RPM (over 5000 if you let it) all by itself.

The only clue i have to this thing is that any slight pressure on the brake or clutch pedal immediately relieves the revving. But since this is happening intermittently i can't figure out how it could be vacuum related.

Could it be a relay somewhere? Or a sensor with an occasional faulty reading? This revving problem doesn't happen often more like once a week when i'm driving every day.

Totally stumped... - Jreidko

Post by **jblackburn** » 09 Oct 2010, 17:01

2 more suggestions:

What's your alternator voltage? When I had one die on my Accord, it would idle around 3000.

The only clue i have to this thing is that any slight pressure on the brake or clutch pedal immediately relieves the revving. But since this is happening intermittently i can't figure out how it could be vacuum related.

Pull the fuse for the cruise control pump. Any difference?