850 surge/lag under load

shegarty · Post by **shegarty** » 26 Sep 2011, 07:29

hi everyone - i've been the proud owner of my 94 850 T5 wagon (206k km) now for over a year and have been chasing a persistent surge/lag in engine rpm since putting her on the road - under load rpm would drop approx 2-300 rpm (load dependent, higher load more drop) on a regular cycle (maybe every 15-20 secs) - much less noticeable at highway speed but still evident (definitely not related to A/C compressor, first thing i checked!)

things i have done that have resulted in marked improvement but have not relieved completely:
- reinstalled clamp holding exhaust downpipe to turbo housing (haning off with bolt missing!)
- replaced torn intake hose with silicone and others that i could get to (silicone kit from groton was tiiiiight)
- thoroughly cleaned throttle body (filthy!!!) and IAC
- replaced vacuum tree and all connected vacuum lines (still need to replace turbo waste gate actuator line)
- new air & fuel filters

surge/lag is still present but much less (maybe around 100- 150rpm) and is almost non-existent at highway speed - when driving home yesterday i unplugged the AMM just for fun and while she ran a little lumpy and smelled rich the surge/lag was not readily evident - i am thinking about removing the AMM and cleaning it with one of the specialized cleaners but have heard anecdotal evidence that this can cause failure of the part as they are quite sensitive ... any suggestions??? is there any way that i can diagnose the AMM before attempting to clean it???

Post by **jblackburn** » 26 Sep 2011, 07:32

If it ran better without the MAF plugged in, by all means clean it. Get yourself a can of the CRC mass airflow cleaner spray at an auto parts store. Disconnect the sensor tube from the airbox, spray the guts of it, and then let it dry for about an hour before reconnecting it.

Have you changed spark plugs and re-gapped them (Bosch Super Copper; .028" gap are preferred by these cars)

Once you do both of those, disconnect your positive battery cable, drain the charge by tapping the brake pedal, and reconnect it. This will let the ECU re-learn its fuel trim table with the sensors properly functioning.

shegarty · Post by **shegarty** » 03 Oct 2011, 06:45

had some time over the weekend to do a bit more work - removed and cleaned the MAF, also replaced a few vacuum lines to turbo while i had the access - turns out they were in bad need of replacement as one was cracked and the other had begun to crumble (two lines coming out of vacuum regulator attached to air box) - massive improvement in performance but still have noticeable surge/lag which is worst when cold - plugs should be fine as they are relatively new (within last 10k km) - any other ideas???

Post by **jblackburn** » 03 Oct 2011, 09:37

It could very well be time for a new mass airflow sensor. That it seems to run fine without the sensor plugged in suggests an air/fuel mixture metering issue that will happen when the sensor isn't reading right. They seem to have a lifespan of about 10 years give or take.

simonbyard · Post by **simonbyard** » 03 Oct 2011, 11:11

just seen this post, and wondering if I have the same problem with my S60. When starting after partially cooled, the engine surges on idle violently, but settles a little after a few "lusty" revs. Driving is jerky when reversing slowly or going forward, but clears when planting the throttle. thought it might be coolant sensor, but gauge is reading ok, so maybe a sensor?

simonbyard · Post by **simonbyard** » 03 Oct 2011, 11:12

just seen this post, and wondering if I have the same problem with my S60. When starting after partially cooled, the engine surges on idle violently, but settles a little after a few "lusty" revs. Driving is jerky when reversing slowly or going forward, but clears when planting the throttle. thought it might be coolant sensor, but gauge is reading ok, so maybe a sensor?

Post by **jblackburn** » 03 Oct 2011, 11:12

The S60 is likely an electronic throttle module issue, but it could possibly be your mass airflow sensor from your description.

Check out "Don's ETM Room" for more information/scenarios related to that to see if your issues sound similar.

simonbyard · Post by **simonbyard** » 03 Oct 2011, 16:46

Silly question - looks like my exhaust may be blowing. Could this also give rise to the surging mentioned above?

Post by **jblackburn** » 03 Oct 2011, 16:52

You hadn't mentioned that. Yes, an exhaust leak before the catalytic converter can fool the o2 sensor and cause the same thing to happen. Is it at the flex pipe or joint into the cat? Those are the usual weak points in the exhaust system.

simonbyard · Post by **simonbyard** » 03 Oct 2011, 17:18

Only had a quick look at it earlier but the light was fading - I think its the flex pipe, but I'll get under it properly tomorrow. Hopefully not too expensive, but then I hope I'll win the lottery, and I hope for world peace, and that I suddenly become more attractive to women, but none of that ever comes true either!