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93 Volvo 850 engine revs high by itself

Help, Advice and DIY Tutorials on Volvo's extremely popular car line -- Volvo's 1990s "bread and butter" cars -- powered by the ubiquitous and durable Volvo inline 5-cylinder engine.
1992 - 1997 850, 850 R, 850 T5-R, 850 T5, 850 GLT
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93 Volvo 850 engine revs high by itself

Postby jreidko » 15 Jul 2010, 02:58

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 > viewtopic.php?f=2&t=30337&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?
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Re: 93 Volvo 850 engine revs high by itself

Postby jblackburn » 15 Jul 2010, 03:30

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.
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Re: 93 Volvo 850 engine revs high by itself

Postby asg1986 » 15 Jul 2010, 05:16

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:

viewtopic.php?f=1&t=22747
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Re: 93 Volvo 850 engine revs high by itself

Postby jreidko » 15 Jul 2010, 22:23

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!
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Re: 93 Volvo 850 engine revs high by itself

Postby jreidko » 25 Jul 2010, 13:15

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:
viewtopic.php?f=1&t=12325&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.
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Re: 93 Volvo 850 engine revs high by itself

Postby jreidko » 08 Aug 2010, 15:58

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!
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Re: 93 Volvo 850 engine revs high by itself

Postby Ozark Lee » 09 Aug 2010, 14:26

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.

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Re: 93 Volvo 850 engine revs high by itself

Postby jreidko » 10 Aug 2010, 04:17

Thanks for the suggestion Ozark, i'll give that a try.
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Re: 93 Volvo 850 engine revs high by itself

Postby jreidko » 09 Oct 2010, 22:49

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
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Re: 93 Volvo 850 engine revs high by itself

Postby jblackburn » 09 Oct 2010, 23:01

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?
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Re: 93 Volvo 850 engine revs high by itself

Postby jreidko » 10 Oct 2010, 02:24

Good suggestion, i actually pulled the entire cruise control control circuit when it first started happening thinking maybe it was related, but it didn't have any effect.

Not sure about the alternator, but the idle speeds at start up and at warm up and when in neutral have all been really consistent with the idle before the problem started occurring.

Does anyone know if there is there any way to eliminate the vacuum system as a potential culprit? Is there any specific things that happens when the brake or clutch are slightly depressed that would point to the system/s that might be malfunctioning?

- jreidko
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Re: 93 Volvo 850 engine revs high by itself

Postby Marcobrick » 10 Oct 2010, 07:06

Quite a tricky problem. The only thing I can think of that the clutch pedal has anything to do with is the cruise control.
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Re: 93 Volvo 850 engine revs high by itself

Postby Retired MVS Contributor » 10 Oct 2010, 12:50

The throttle regulates the amount of air into the intake manifold, thereby controlling speed...Any air entering the intake AFTER the throttle will cause a high rev...The throttle body-to-intake manifold gasket would be most suspect as well as the intake manifold gasket itself...Also the "O" ring on the vacuum distributer mounted on the top of the throttle body...Your problem is almost certainly vacuum related, just where is the problem...Unmetered air is getting in somewhere...
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Re: 93 Volvo 850 engine revs high by itself

Postby jreidko » 11 Oct 2010, 14:48

Thanks Jerrymcc, your comment gives me a new place to look. I've had the intake and some of the throttle related components on and off enough that it seems very plausible that i could have introduced a weak point somewhere. I'll post what i find.

Thanks again everyone, - jreidko
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