Hello,
I know that I just joined and this isn't the most polite way to start out in a forum, but I'm stumped and the car has become very exciting to drive with the current behavior.

So, this is what has occurred. Last week, when driving to work I noticed that the idle had risen to just under 1500 rpms per the tach in the car. This was odd since in the 5 years we've had the volvo the idle has been pretty stable. I started troubleshooting the usual culprits - vacuum leaks, iac, fuel pressure regulator, TPS. Didn't find any vacuum leaks testing with carb cleaner around the engine. I'm not able to check the IAC per the various videos and writeups because this car does not have the blue connector. Nor does it have the black **** on the throttle body for the idle air bypass to set a base idle. So, I did what I could. I pulled the IAC (a 2 pin model) and checked the resistance between the pins (10-11ohms). From what I could find online, this is a pretty low resistance reading for this IAC, although the information could be wrong. I then tested it by applying 12v to the pins and the IAC actuated (not sure if it actuated correctly, but it actuated). I also double checked that the "click" in the TPS is present when the throttle butterfly starts to just move off of idle.

After reinstalling the IAC and double checking all of the connectors, when cold the engine idles around 600-650 and stumbles off idle. Once the engine warms up, it begins to fluctuate at idle going from 1500-1750 rpms then dropping to 500 and stumbling, then back up, repeatedly like the ECU is trying to find idle but something is causing this to fail.

I'm a pretty good mechanic and know my way around electrical wiring, but I don't have enough information to continue testing. I'll check anything that I have good information for. My Haynes manual just doesn't have the details.

If anyone has seen this before and can make suggestions on what to check. Please share them.

Joe

 

I had a similar issue with our Volvo 850 a couple years back and it turned out to be the coolant temperature sensor was bad and would not send a signal to the ECU to lean out the fuel mixture when the engine warmed up.

 

Thanks for sharing your findings. I performed some resistance tests for both the ECU and the ICU side of the ECT sensor. At ambient, probably 70* to 75* in my garage and both sides were reading 2k Ohms, which based on the info I found is where it should be reading. I'll get the car warmed up and take the readings again later today. (Waiting on thermal paste to re-install the ignition power module that the previous owner had snapped one of the bolts on so it was over heating in the summer and causing the car to die).

Next check will be the voltage check at the TPS and checking the o2 sensor.

 

Measured the voltages at the TPS and found that behavior wasn't what had been demonstrated in videos of other people testing them.

Ignition on, engine off:
Pins 1 and 3 (based on what I found, could be wrong here.)
At closed, it reads battery voltage
Off idle to as open as it goes it reads a flat .62v

If I understand correctly, some (all?) TPS should read an increasing range from ~.5v to ~4.5v

The wiring diagram I have doesn't indicate any variable resistance in the TPS for a LH 2.4/EZK116 vehicle. It shows a idle switch that is closed at idle, open off idle and a WOT switch which is open unless at WOT.

Am I missing some documentation or am I testing the wrong pinouts?

Thanks in advance.

 

Further reading on appears to indicate that the TPS is behaving normally, at least that is what is indicated.

The MAF has been cleaned and checks good

I have gone through the engine checking for vacuum leaks using brake cleaner and nothing changes.

I did find a small fuel leak on the fuel return line where the rubber line connects to the fuel rail. In the past, it appears the previous owner had removed the clamp so the hose was just sealing on its own. I replaced the clamp and the leak has stopped.

I'm starting to wonder if I may have a stuck injector or a bad fuel pressure regulator. I understand that these sometimes fail allowing fuel to enter the vacuum side of the regulator. Mine is not doing this.

Anyone else have any suggestions for items to test? I have yet to find a good test procedure for the o2 sensor, but I'm still looking.

 

Finally got the o2 sensor measured with the engine running.
I measured the heating element of the o2 sensor cold and it was 3 ohms which the manual says is normal. Ran the motor for a few minutes to let it heat itself. I shut off the engine came into the house to post my findings and remembered I forgot to measure the resistance when the o2 sensor was hot. I went back out (about 2 minutes had past) and the resistance read 6 ohms and decreasing as it cooled.

With the o2 sensor output wire disconnected and hooked up to my multimeter. Red (+) lead in the spade connector, black (-) lead to ground, and the meter set on 2v, the meter started at .05-.06 volts and as the sensor warmed up it decreased to ~-0.04 (that's negative).

Interestingly the engine idled pretty steady at about 800rpms with a slight roughness but no fluctuation like had been occurring before with the sensor output wire disconnected.

Connected the o2 Sensor back to the harness and the fluctuating returned immediately.

I think I found the culprit for my fluctuation, a bad o2 sensor.

Anyone got an alternative theory based on the findings?