In the continuing quest to get to the bottom of my 200Vi's poor running, I have invested in a Vacuum Gauge.
History of problem;
Misfire at idle/down on power/jerking on part throttle.
Session on Rover Testbook diagnostic system revealed no faults.
Compression test revealed no serious problems (all within 10% of each other)
Recent service and replacement of HT Leads & Plugs.
Having connected the Vacuum gauge to the Plenum chamber I got the following results;
At idle - steady reading of 16, 2500rpm - steady reading of 22, blipping the throttle caused a swing from 0 to 22 and then back to 16.
The only test that caused concern was when I revved the engine hard and sharply closed the throttle... according to my documentation, the reading should return swiftly to that at idle but instead it hovered at around 22 for a second or two before returning to normal. I understand that this could be a sign of excess backpressure in the exhaust system (i.e. a blockage). Would you agree with this conclusion?
P.S. apologies for the long-winded post but I thought the extra detail might be usefull.
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Richy
The units of measurement you are using are inches of mercury I expect.If so the reading of 16 at idle is too low.You should have a steady 19 to 22 for an engine with no faults.I don't think you have an exhaust backpressure problem as this causes the return to idle to be extremely slow not just a second or two.You would also have reduced top end power and very sluggish acceleration.Your 16 reading at idle could be a number of things such as retarded or advanced cam timing,inlet manifold air leak,valves sticking at idle,hydraulic lifter problems or even retarded ignition timing. A vacuum test will tell you if something is wrong but won't tell you exactly what.
May I suggest a detailed check of camshaft timing to start with as the electronic and compression tests you have had don't really address this area.
Please keep us posted on your progress.
David Davies (Tune-Up Raglan)
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Also have a look at the small diameter vacuum pipe that goes from the inlet manifold to the engine ECU. This feeds the MAP (Manifold Absolute Pressure) sensor which the ecu uses to guage load. It may have some gunge in it,in which case clean it out with spray carb cleaner. Check the manifold end first, then the ecu end. If the ecu end has gunge in it then some may have got into the sensor. Roll cotton wool into strands, push into the sensor, twist then remove. Repeat this till all gunge gone - do not use any solvent in the sensor.
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Another common problem that can lead to low manifold vacuum is that the earth (gray wire) on O2 sensor fails causing the engine to run rich all the time. With a digital voltmeter measure the voltage on the earth wire - should be 0.025v max, if it is hovering round 1.4v bare a little of the wire and solder a wire to it which you can run to a known good earth. (had one this very morning!)
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I've been off work with the flu and have not felt up to getting my head under the bonnet.
The problem seemed to occur around the time I had the cam belts changed and I am not absolutely sure it was done correctly (see forums.mg-rover.org/viewtopic.php?TopicID=2092 for more info).
I’ll have a go at checking the MAP sensor and Oxygen sensor A.S.A.P. but am waiting for a response to my post on the MG rover forum before I take the car to the dealership.
Cheers guys for the all the suggestions, hope I'm able to contribute to the forum in the future. Will keep you informed of my progress
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