home register FAQ memberlist calendar

Go Back   Turbobricks Forums > Mechanical > maintenance & nonperformance

Thread Tools Display Modes
Old 02-06-2019, 09:37 PM   #1
Board Member
Kestrel's Avatar
Join Date: Sep 2010
Location: Albuquerque, NM
Question Volvo 940 NA High Idle / Surging

Hello all,

Recently picked up a $500 '92 940 NA (Bosch LH2.4 with EGR) and are trying to get it back on the road. Car was missing a few items at time of purchase - namely an exhaust system and catalytic converter. Amazingly somebody had ripped out the cat, and welded in a giant ugly 'muffler'. As this was an EGR car, when they yanked the cat, the O2 sensor went with it, leaving the wire hanging in bare space. Yikes. Anyway, getting it all put back together... exhaust installed, new universal Bosch 3 wire O2 sensor crimped back, along with a few other items.


Starts great cold (though a little slow - 4-5 cranks), but once warm, the car has a surging idle... from 1500 to 2000 RPM, cycling over and over again. Here's what I've looked at...

IAC - seems not to be doing anything. I plugged the IAC 'port' in the intake pipe, and observed no marked difference in idle RPM. Maybe a tiny bit lower, but still surging from 1500 to 2k every 2 seconds or thereabouts.

Reset TPS. No change. Swapped with known good TPS. No change.

Checked Throttle body - seems set correctly, with plate just barely open 1/4 turn on the screw adjuster after letting it bottom out.

Checked engine coolant temperature sensor via ECU pins 13. ~3.3k Ohms around 50F, and ~290 Ohms once up to temperature. Seems to be functioning correctly.

Checked AMM with known good AMM from my daily driver. No change. That said, when unplugging AMM, idle drops down to around 1100pm, but sounds like ****, and still bounces +-100RPM.

Checked EGR system by plugging the port on the intake. Was curious if perhaps the valve was stuck wide open, resulting in a huge ass vacuum leak to allow surging up to 2k. No difference.

Swapped in an entirely different ECU. No change.

Checked airbox to ensure thermostat wasn't busted, and we weren't sucking in super hot air.

Threw on a known good powerstage for the hell of it, though it should have little to no relevance here. No change.

Noticed a big crack in the upper PCV box hose.. Replaced that with OEM, along with a new flame trap hose back to the air intake bellow hose. No change.

Unplugged the O2 sensor just to make sure I didn't somehow screw that up during the whole crimping process. Nope - still surging.

Speedo works fine, so I'm assuming that the speed sensor wiring is OK. From what I could tell, ECU/EZK had never been pulled or screwed with.


I'm stumped. When I bought the car, it didn't seem to have this high idle problem. To 'restore' the car back to some semblance of stage zero, I fitted an entire exhaust system from the header back. New O2 sensor, spliced into the old harness. Changed the water pump as it was leaking. Threw in a new air filter, and reattached the giant accordion bellows intake pipe, which had been quite literally hanging off the throttle body. The throttle body was also terribly misadjusted... The speedo cable was pulled tight, not allowing the throttle to be closed to the stop. The transmission kickdown cable was also thus pulled into the wrong idle position, resulting in this thing never wanting to shift until very high RPMs.

Outside of the weird shifting transmission, the car actually idled and started just fine. Even ran decently too, which was surprising given the total lack of an O2 sensor.

What am I missing here? What sort of size vacuum leak could actually result in such a strong idle? If the IAC line was plugged off, and the throttle plate was slammed shut to the stop... how could this thing surge from 1.5k to 2k? Where is this air coming from? Sprayed some carb cleaner around the intake gasket, but didn't notice a change. I have one on hand, and will be changing it anyway while swapping out the PCV box for a new OEM unit.

Outside of that, I'm all ears for ideas. I'm puzzled why the car seemed to idle smoothly before sans O2 sensor and with screwed up throttle body positioning, but now once correctly adjusted, does not.

Last edited by Kestrel; 02-06-2019 at 09:43 PM..
Kestrel is offline   Reply With Quote
Old 02-07-2019, 12:30 PM   #2
Jesse B
Fine Modern Gentleman
Jesse B's Avatar
Join Date: Jun 2009
Location: Somerville, Ma

Not sure about the high idle, but I'd imagine changing the intake manifold gasket will help with the lumpiness. I just changed out mine, and found that the #1 intake portion had become elliptical in shape, therefore creating a vacuum leak near the bottom of the intake (which probably wouldn't have been detected by spraying carb cleaner near the top. It idled like a diesel until I replaced it. Nice and smooth now.

Have you let the car run for a while? I cleaned my IAC because of a high idle (with both carb cleaner and a little Kroil), and after I put it back in, the car idled high and lumpy for a few minutes (like 1.5k+), but then eventually calmed down. I chalked it up the ECU getting its bearings.

Did you check that the kickdown cable isn't binding?
'73 1800ES, now converted to m41!
'95 944
'95 945
'96 855T, Autumn Gold, now converted to m56!
'11 XC70, T6
Jesse B is offline   Reply With Quote
Old 02-08-2019, 03:29 PM   #3
Board Member
wyatt8740's Avatar
Join Date: Mar 2017
Location: Fort Wayne, Indiana

Check the kickdown cable and clean the throttle body if it wasn't cleaned when repositioned.
Also check the ECU and the MAF. Looks like some non-volvo people (shouldn't matter as it's likely bosch anyway) have had bad ECU's cause these symptoms.

And of course make sure the throttle cable plastic bits (the tether point thing) aren't snapped and holding the cable out. That happened to me once.
1991 244 (manual 5spd)
1993 245 (auto 4spd [for now])
And the Dalai Lama smiled and said, 'Well my son, life is like a beanstalk, isn't it?'

Last edited by wyatt8740; 02-08-2019 at 03:44 PM..
wyatt8740 is offline   Reply With Quote
Old 02-09-2019, 09:40 AM   #4
Board Member
Kestrel's Avatar
Join Date: Sep 2010
Location: Albuquerque, NM

Mystery has been solved.

I compared a known well adjusted throttle body / TPS to the one on this car. It appears the previous owner(s) at some point had screwed with it in a way (not to mention shearing off the adjuster screw) that resulted in the throttle bottle prematurely bottoming out. That is to say... "Fully closed" wasn't really fully closed. It's something I should have caught, but didn't. After comparison with a throttle body off my DD, the difference was observed. Metal was cleaned up, and the butterfly finally drops into a true closed position.

Lesson learned - even the tiniest sliver of air is enough to significantly raise your idle RPM.
Kestrel is offline   Reply With Quote
Old 02-10-2019, 08:27 PM   #5
Reformed SAABaholic
Join Date: Nov 2009
Location: NW Georgia

"Confirm the setup"..... always step 1 when trouble shooting. I know both the Bentley manual, Greenbooks, and sages on the 7/9 Maint. Pages all detail the TB setup. Glad you found the real problem.
Project "cheap thrills" build thread: http://forums.turbobricks.com/showth...67#post4211467

Feedback thread: http://forums.turbobricks.com/showthread.php?t=198746
DET17 is offline   Reply With Quote

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On

Forum Jump

All times are GMT -4. The time now is 08:43 PM.

Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2021, vBulletin Solutions Inc.