406 RFR engine jerks between 1850-2300 RPM

[DanielBukovics]

Hello guys,

I have a 2000 406 Coupe with an RFR (2.0 gasoline) engine (Euro II). My engine had been recently serviced (timing chain, clutch, all gaskets except head). Now my engine jerks between 1850-2300 RPM when it’s under very little load (eg. uphill, 1cm of throttle). It was present from 3rd to 5th gear, but now it’s present only in 4th and 5th gear. I had felt very minor jerking before the service, not noticable to non-car guys.

When my speedometer stops working for 30 seconds (very rarely) the jerking is not present, so I’m assuming that the timing was correctly set and it’s not a mechanical fault. The mechanic told me that the O2 sensor is not connected to the ECU.

Other symptoms: idle oscillates 50-100 RPM, very very rarely battery checklight comes on for 0.5 second (happened only 3 times), very rarely the engine stops for no reason, on one occasion the engine almost stalled when throttle was added while idling, when holding eg. 1500 RPM while stationary the RPM drops 50 RPM and goes back every 2-3 seconds, small pops in the exhaust between 1600-1800 RPM while using engine braking.

The engine runs nice at RPMs above 2300 and has power, MPG is exactly equal to factory data, no smoke, no oil burn, compression was tested and all cylinders have ~13 bars.

So the question is what causes the mixture/ignition problems? Could this be the O2 sensor which is not connected?

[steve_earwig]

Hello. Umm, I think I'm going to go for the o2 sensor, if it's getting no feedback the ecu falls back on a fixed map which won't take into account stuff like engine loads, like going up a hill or whatever. I'm surprised it's not throwing a MIL, but if it is disconnected you won't know if this is the cause or there are further problems.