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iauto8
iauto8, ASE Certified Technician
Category: Car
Satisfied Customers: 137
Experience:  15 year ASE Auto Technician. European import specialist.
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I have a 1992 Volvo 940 GL non-turbo. I have a unstable/rough

Customer Question

I have a 1992 Volvo 940 GL non-turbo. I have a unstable/rough idle that I am trying to diagnose. The idle bounces from 7-900 at about two cycles per second, it doesnt really run rough but the cycling idle makes it feel like a miss. The car drives excellent off idle under all loads. The car even runs ok if I hold the throttle open to keep the rpms around 1000. I have code 221 set. I also had code 311, but just fixed my malfunctioning speedometer last week so I believe that 311 code was related to that previous failure. My resources just say this is a lean reading on the oxygen sensor, but I dont know what is common for this car or if it would have anything to do with my idling issue. I have looked carefully for vacuum leaks by listening and spraying carb cleaner around the intake, replaced and cleaned the throttle body and fuel injectors (on and off the car), replaced fuel injector orings, and cleaned the mass air flow sensor. Any help or diagnostic advise would be appreciated. Thanks!
Submitted: 2 years ago.
Category: Car
Expert:  iauto8 replied 2 years ago.
Here are test for the Rex/Regina type fuel system. Chris....

1. Check for any intake or vacuum leaks.

2. Check output of the O2 sensor. If voltage is low, add fuel by blocking off the fuel return line or adding an external fuel source. If voltage does not rise, it has a dead O2 sensor.

3. Check Manifold Absolute Pressure (MAP) sensor signal output at the Blue/Red wire. At idle, voltage should be 1.4V. Voltage rises with engine load.

4. Check the vacuum source to the MAP sensor.

5. Check fuel pressure. Specification is 37-44 PSI. Check operation of the pressure regulator.
Tech Tips:
Rex/Regina type fuel systems will have a 2 wire intake air temperature sensor where the Mass Airflow (MAF) would typically sit in a Bosch system.
Customer: replied 2 years ago.

thanks for the reply. however, i have this list from mitchell on demand and other google searches. Specifically, I was wondering with my exact symptoms what you think the most likely failure would be. I did do a smoke test today on the intake and found no vacuum leaks in the intake that made a difference, only a very small leak I repaired at a vacuum tee. I also am guessing that I do not have the regina fuel system since I do have a MAF in the intake tubing. Looks like that will matter if I end up needing an oxygen sensor, which I have not tested yet.

I find this idling situation to be quite unique in that I have never seen such a quick oscillation in idle speed. Ive seen many vacuum leaks and i dont usually get a fluxuating idle for more than just a second or two, just higher than normal. also seen dead oxygen sensors that effect idle speed, but it happens much more slowly. Im not a Volvo tech specifically though, and work on all models, so I thought I'd ask somebody who sees these cars every day.

Expert:  iauto8 replied 2 years ago.
Ok on the bosch system you may run with the MAF unplugged to eliminate that from being the problem. If the MAF is running the car lean when the o2 signal tells the ecm to richen the mixture up you will get idle fluctuation that will follow the waveform of your o2 signal. Also make sure the o2 is working. Monitor signal and pull vacuum hose off and make sure it goes lean or add propane and make sure it goes rich. The other thing that can affect idle this way is the TPS. Make sure when when you slightly open the throttle then close you hear a click. The TPS has an idle switch in it. I will find TPS tests and post for you. Chris.....
Expert:  iauto8 replied 2 years ago.
graphicgraphic
iauto8, ASE Certified Technician
Category: Car
Satisfied Customers: 137
Experience: 15 year ASE Auto Technician. European import specialist.
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Customer: replied 1 year ago.
hey just wanted to give you an update to my condition on the car. I tested the o2 sensor, found it dead, and replaced it. The idle is 90% better, still has some audible oscillation to the idle speed, but you cant really feel it and it is barely noticible on the tachometer. I also cleaned the idle control valve, throttle body, and maf, and tested the tps as suggested. anything else i could do to further improve smoothness would be appreciated, but thanks again either way! ** I also realize i never accepted apparently, sorry for the delay
Expert:  iauto8 replied 1 year ago.
Do you have a motorvac fuel injection cleaning machine or access to one? Being a 92 Volvo I would assume this beast has some mileage on it. I have found that running a motorvac service on them can really smooth out that idle and also help with starting. The only other thing to check would be the pcv system. Make sure its clean and working properly. Chris....

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