Stumble at 2,000 RPM in 1994 Volvo 940 non-turbo

Maintenance/Repairs
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1

I have a 1994 Volvo 940 non-turbo that has a stumble that is most pronounced at 2,000 RPM and under load. The stumble is less noticeable at idle or under acceleration. It would be very noticeable with cruise control on at 55 mph on a flat stretch of highway, but not noticeable accelerating up a hill at 55 mph.



The onboard diagnostics insists there is no problem. I think the exhaust smells as if it is burning rich. Plugs give no indication of any problem.



I have made the following repairs or replacements: Air filter, cap, rotor, wires, plugs, fuel filter, Cleaned throttle body, air temp sensor, vacuum lines, fuel pressure regulator, fuel injector cleaned, replaced both coolant temp sensors, replaced intake manifold gasket, O2 sensor checked, replaced crankshaft position sensor.



I have checked the timing, which is at 10 degrees (manual says 12 degrees, but 10 degrees is the last mark).



One final note; the car seems to run better after a repair. Maybe for an hour, maybe for a day.



Any suggestions would be appreciated. I have replaced just about everything.



Martin



2

You have replaced about all the suspects.

I think your car has an EGR. I would try disabling it and see if that makes any difference.

You have, of course, carefully checked for air leaks between the mass airflow sensor and the throttle body?

Have you checked for leaks that could let air in to the exhaust ahead of the oxygen sensor. The fact that this happens at low load makes me suspect an exhaust leak.

OBD 1 is not very sensitive to how well the oxygen sensor works, but I am skeptical of oxygen sensors that have been ‘tested’. Unless it was put on an oscilloscope and cycled through a complete diagnostic, the test that most shops do on oxygen sensors does not tell you much.

3

Although other versions of the 940 may have an EGR, this car with the Regina Rex fuel system does NOT have an EGR. The EGR was on my list of possible things to repair until I figured out why I couldn’t find it.

Also, this car does not have a Mass Airflow Sensor. It uses the IAT sensor and another sensor to calculate the air mass. But good point, I will check for air leaks.

Another repair I forgot to mention was that I cleaned and tested the Idle Air Control Valve.

I do believe I may have an exhaust leak, but that is behind the 02 sensor, between the Catalytic Converter and the muffler. Could this cause the stumble?

Thanks,

Martin

4

I have now repaired the o-rings around the fuel injectors, as well as repaired the leak post catalytic converter and replaced the exhaust manifold and collector gaskets. Stumble has not disappeared.

5

Problem Solved. The cam was one tooth off. Timing appeared correct at the crank, and the crank and distributor were correctly timed; but the cam was one tooth off. Wow.