Bad idling on '93 850

Dunoyer · Post by **Dunoyer** » 29 Mar 2006, 20:30

My '93 850 idles terribly, sometimes it stalls, but mostly it does something I've heard called idle hunting, where when idling it cycles from 1500 to 500 rpm every 2 seconds. Also it sometimes surges where you're in gear at 3000 RPM, then you put in the clutch and stays idling at 3000 RPM until you coast to a stop. These problems occured under warranty and I paid numerous visits to the dealer, and was corrected, until 2002 when it reared its head even worse. Mechanic is puzzled. Cam sensor was replaced as a diagnostic replacement. Didn't fix it, numerous other failed attempts to dignose and repair, like cleaning the leads on the harness.

Any wisdon out there? Something tells me it's a design failure in the circuits. Mechanic thinks replacing the CPU might do it, but it's a big expense (how much?) for a 13 year old car which has 100k on it. Plus it might not work, or make the car sicker!!!

Post by **MadeInJapan** » 29 Mar 2006, 20:34

Don't go for an ECU just yet...though can probably be found on eBay for a reasonable price.

How long has it been since a tune up? Plugs, ignition wires, rotors? How about a throttle body cleaning? Fuel filter? Fuel pump and/or fuel pump relay going out? Last of all, how about the MAF sensor? These are all things that can contribute to the symptoms you describe.

Any engine lights or CEL on?

Dunoyer · Post by **Dunoyer** » 29 Mar 2006, 21:00

Tune up was the first thing I did when this problem came up, it's been almost 3 years, 25K miles, so yes, it is time for another tune up, though "tune" is too generous a word. I had plugs, dist cap and rotor, air filter, fuel filter, none of those things improved the idle.

No throttle body cleaning has been done. Is that easy?

So I guess I'll replace the fuel pump as a diagnostic effort, then the fuel pump relay going out, then the MAF sensor?

Sadly CEL is not on, there are no codes.

I think in 1993, they were really still trying to figure out how to use computers to control internal combustion, fuul injected engines...

They say Volvos last forever, and mine is in great shape so I despair when it runs so rough.

Thanks for your help.

Jean

Post by **MadeInJapan** » 29 Mar 2006, 21:32

I'm not saying to replace all of those parts...just check them out to make sure they aren't the reason for your troubles.

A throttle body cleaning isn't hard...just a pain. Take off the hose from the air box to the throttle body and see what kind of condition everything is in. There is a butterfly flap in there (lets the air in for correct air/fuel mixture)...if it's got gunk all around it and the flap isn't closing good, this could be the reason for your issues. Use a good carb cleaner, and spary, let it sit, take a tooth brush and scrub away, spray again....try to get all the gunk out. Some people even take the throttle body off and clean as deep as they can get into the intake manifold area. This is good, but if you do this, which is a very thorough cleaning, you'll need a new TB gasket (anly a few bucks from www.fcpgroton.com). Let everything dry, then make sure when you put the hose back on after the cleaning, you get the clamp good and tight...you don't want unfiltered air entering the throttle body (sets codes). Hope this helps.

Post by **matthew1** » 29 Mar 2006, 23:44

Check vacuum lines and elbows also.

Post by **akitawagon** » 30 Mar 2006, 04:21

My '93 850 started doing a similar "idle hunt" like yours except it cycled between 800 and 1500 rpms when at idle and the ambient and engine temperature didn't seem to make a difference. It never stalled, but it always sounded like it was about to. My stage 0 / basic maintenance items had been done recently so I was puzzled.

I finally got a check engine light and one of the codes pulled was
A6 | 4-1-1: Throttle Position (TPS) signal from fuel injection system missing or faulty.

This lead me to do a search on the throttle position sensor on the volvospeed forums. People with similar symptoms reported that replacing or cleaning the sensor helped them, so I ended up replacing my throttle position sensor with a used one from a local Volvo repair shop (Volvo Part #1336385; $49 used; $89 new). No more "throttle cycling": now.

You might want to try cleaning the connections for your throttle position sensor first and see if that helps before buying a new sensor. Also check the wiring from the sensor leading to the ECU/TCU.

Post by **MadeInJapan** » 30 Mar 2006, 08:47

You're right...check the TBS (don't know how this slipped my mind)....some have reported looking for it and finding it loosely connected or unplugged. Cleaning it would be my first step. If you don't have contact cleaner (cheap at radio shack), then plug and unplug it 20 or so times to clean the terminals. Good luck!

Dunoyer · Post by **Dunoyer** » 31 Mar 2006, 07:12

How do I locate the throttle position sensor. This is really helpful, guys. Thanks a million.

Jean

JavaJiver · Post by **JavaJiver** » 31 Mar 2006, 07:19

Will take two orders of how do you find that sensor...My car started doing this on the way to work...Patience so thin...

Post by **MadeInJapan** » 31 Mar 2006, 17:58

It is right in front of the throttle body...where the short hose connects to...a very large in diameter hose on the side of the intake manifold. When I say in front....it is between you and the throttle body when you stand in front of the car. The sensor appears to be turned sideways with a plug in the left side. You might not be able to see it unless you take the throttle cable cover off with its torx screw. Good luck!