I have been searching for information about "hesitation on acceleration" in an attempt to diagnose a problem with my 940.
Well maintained, primarily commuter highway driven 940 Estate developed engine bogging in "stop and go" city traffic.
The symptom only occurs after and during prolonged city stop and go traffic. When driven short distances, this symptom does not occur at all.
The symptom is; When at full stop or rolling stop/running at idle when gas pedal is depressed and acceleration is called for, it seems to be accelerating/starting in 3rd gear instead of 1st or 2nd depending on speed. After about 10 seconds even if pedal is "floored" it seems to engage the correct gear or fuel is now at proper delivery, and normal acceleration and shifting resumes at speed.
There is no rough shifting or transmission noise when displaying this symptom or when not, it is more like the clutch is slipping. The tachometer seems to be displaying the correct RPM's when in its relative gears at all times. Does not stall
It is important to note that this hesitation on acceleration does not happen at higher speeds or during highway driving and the "passing gear" engages normally. There has been no improvement or worsening of this problem over 9 months of driving and with all the repairs listed below. Also, this bogging only occurs [u]after[/u]and during driving in stop and go "hot" traffic for an extended period of time. Once car has been parked and "cooled down" this symptom is not present again until above driving conditions are encountered. Again if car is started up and operated at high speed , or even in very hot weather on the highway, this symptom is not present. So my (and several mechanics) attention has been on fuel and air intake system's. This has not fixed this problem. The injectors and throttle seem to be clean and operating normally.
Only a few small things left to check but now I am starting to think about the transmission, clutch and throttle pedal linkage unless someone has any other ideas..
No check engine light , and no codes, all clear so far...
Current notes :
Cold start is a bit touchy sometimes if the car has been sitting a while(days), warm start is instant. Idle is a touch rough in park but not in drive or neutral since I cleaned idle air control valve , still above 500rpm and doesn't stall out, cleaning seemed to improve power a bit.
Thanks in advance for any ideas and sorry for the long posting but I thought the more information the better since this has been a very mysterious problem. Devoted Volvo owner (previous 1988 740 R.I.P cracked head gasket , 290,000 miles) and my 1984 240 which odometer stopped at 320,000.. Hope my 940, can pass those gems.
1993 940 Estate with B230F "[u]Regina[/u]" engine
149,000 miles (primarily highway miles)
Work performed all in the last year (none of which has affected/solved my problem so far):
Replaced "in tank" fuel pump with new unit. (original failed at about 120,000)
Replaced in line fuel filter.
Replaced all spark plugs.
Replaced 4 ignition wires (set), minus the coil wire as it was still in good shape.
Replaced 3 drive belts , minus camshaft drive belt (no reason to assume in need of replacement?)
New catalytic converter.
New o2 sensor (not sure if 3 wire correct voltage or aftermarket splice in, regardless , no effect on problem before or after)
Cleaned throttle body.
Replaced fuel pressure regulator (Bosh).
Replaced air box thermostat.
Replaced air box filter.
Cleaned and confirmed proper function of idle air control valve.
Cleaned flame trap hoses and plactic flame trap.
Checked inlet air temperature sensor ([u]Regina[/u]) appears clean.
Checked all above electric connections for corrosion , all seem good.
Checked fuel pump relay module (drilled holes in cover for heat venting) works properly.
Checked all air intake hoses for vacuum leaks and cracking , all seem in good shape not leaks.
Added STP complete fuel treatment to full tank, ran through.
All pumps and cooling system working properly, engine operates at normal temp even in hot traffic or highway with a/c.
Not checked yet :
Knock sensor (although electrical connection is clean)
Pressure sensor/ "MAP "(not MAF) for [u]Regina[/u] , to be honest , don't know what it looks like or where it is. someone please post a photo ?
Throttle position switch (need adjustment ?)
Cold start injector. (don't know if it could be related)
Carbon canister (all hoses seem ok)
Heated oxygen sensor.
Transmission and/or Clutch (fluid clean and not leaking)
Throttle pedal linkage and cables (seem ok though)
Camshaft belt, pulley's, timing, etc.
Ignition HT coil (no reason to think it is related ?)