flat spot on RPM curve on 2 850's

Post by **JimBee** » 17 Jun 2012, 14:08

Both of these '96 850 wagons have the same flat spot and they are consistent.

Between about 1700 and 2500 rpm, they both flatten out a bit. They don't really misfire, just lag for a second or two. Then they get regular again and go smoothly up the curve. One's a 5 speed manual, the other automatic. The one with a more erratic issue (automatic) when starting cold (but hot it starts right up) has a check engine light. I'll be taking that for a scan tomorrow.

The other one doesn't have any dash lights on while running, but all the lights work while ignition is on.

Ideas on the flat spots welcome!

9394volvo850s · Post by **9394volvo850s** » 17 Jun 2012, 17:25

my 93 5 speed does the same thing but it does it for only like 100 revs between 1400 and 1500 then its fine. no CEL either. mine has an ecu tune, modded cam timing, 960 TB, and a 3" aluminum intake with cone filter, and a 2.25" exhaust w/ 320k

Post by **JimBee** » 17 Jun 2012, 18:08

I also have a '93 5 speed sedan. It's great. 165k. That's my trusty all seasons car for local driving. I bought the 5 speed wagon last summer for a trip to Toronto and it was fantastic. Best road car I've owned. The red wagon was presumed dead when I bought it but I got it running and have progressively solved some issues. The red wagon still doesn't run well when cold, so still need to work on that.
My '93 surprised me a couple of weeks ago in an onramp dash to dust off a Blazer that I think the driver thought was going to blow by me. When let the 2.4 have full reign and got a wicked yelp when I wacked second gear even I was surprised. I guess the air temp and humidity were just right or something. It really flew. The Blazer faded quickly.

9394volvo850s · Post by **9394volvo850s** » 17 Jun 2012, 18:11

yea they can be zippy with the 5speed and minor mods

Post by **JimBee** » 17 Jun 2012, 18:27

Mine's a GLC. I haven't done anything to it. Does that model have any extra factory juice?

9394volvo850s · Post by **9394volvo850s** » 17 Jun 2012, 18:31

GLT? all 93s were labled GLT. and only came with the 168hp 2.4L

Post by **jblackburn** » 17 Jun 2012, 19:42

Hmmmmmmmmmmmmmmmmmmm.

I wonder.

Disconnect the highlighted line I showed in this picture. Cap off the end sucking into the intake manifold with something, and then flip around the white one-way valve in the end of the line.

Take it for a drive and see if it still happens. If so, your EVAP purge valve at the bottom left corner of that picture (yours may be square like mine, but I think most 850's have a round valve) is likely sticking open and should be replaced. It's flooding the intake manifold with fuel vapors at low RPMs and causing a stumble.

And the 5-speeds are a lot of fun. It's amazing how much power the automatic steals from them.

Post by **Ben850** » 17 Jun 2012, 20:59

My '93 has a completely different configuration than that photo. All '93s were N/A. My evap looks like this.(attached) I would imagine others in '93 would be similar.
I also seem to notice a flat spot around 2,ooo to 2,500. I have an automatic and am in the process of replacing vacuum lines if they need it or not. I've noticed several improvements in different areas already.

Edit: It occurred to me that the '93s had a Variable Velocity intake manifold which has two different velocity runners actuated by a vacuum valve under the manifold. That is most likely what is going on during the actuation of the valve.
Look at this write-up for a modification. I sent a PM to this guy, but I don't think he comes to this site anymore.
https://www.matthewsvolvosite.com/forums ... =5&t=24681
So there is my Ah-HA moment.

Post by **jblackburn** » 17 Jun 2012, 21:12

Cheap gas (87) could do it too. Both my Cruze and S70 lag like crazy at low RPM in hot weather unless I run 89 or 93 octane - but they are turbo engines.

That's the effect of the knock sensor pulling timing to keep the engine from knocking/pinging when load is high and RPM is low (as in accelerating from a stop).

Post by **Ben850** » 17 Jun 2012, 21:58

Ok, I found a comparison shot of our '93 VVIS intake. It is obviously the one on top. Evidently the guy I llinked in the last post came up with an electronic alternative to give control of the actuating valve. I want one.
I'm fairly sure that is what causes our flat spot.
Hve Fun.