Engine surging

[Davidqhog]

Morning All,

I'm experiencing, well the car is, some surging at constant throttle opening. It occurs around 1500rpm and fluctuates by 50 - 100 rpm. I can see it on the rev counter and feel it. Think I can just about hear it too.

Paul Titheradge at Wiltshire Jaguar noted a lean mixture code when checking the cats (2 second hand ones coming in soon). Would this be the cause or should I be looking elsewhere?

Regards,

David Q

[J44EAG]

Lean mixture is usually caused by air leaks within the induction system. On supercharged cars, there are two small, cheap pipes to deal with. 1/8" ID rubber. A length of the pipe is available from Jaguar or indeed Halfords sell packs of it which in my opinion is of better quality.

One pipe comes off the cowhorn manifold above the supercharger. The pipe falls to bits given time and mileage. This is usually itemised as P0171 and P0174 on a code reader. That pipe sends pressured air to one side of the by-pass actuator. The other pipe is a vacuum pipe from the inlet elbow below the throttle body to, again, the other side of the by-pass actuator. So you have suck and blow which controls the by-pass actuator and flap. Any leaks on those pipes can upset mixture control...hence the lean codes. See my "Nineteen day glitch" article late 2015/early 2016 magazine pages.

The pipe fitting is not so easy due to lack of space at the back of the engine around the throttle body. Removing the throttle (Four Torx bolts and a new gasket gets more space and access). Any work at the back of the engine on SC models can be a nightmare of frustration.

Check out those pipes before buying new cats or Lambda sensors.

Mike K

[Davidqhog]

Thanks Mike. I'll check those pipes out. I haven't done anything to the engine yet - still trying to find time to fit the K&N air filter and change the SC oil. But I can see that working on anything here is a nightmare unless you have incredibly thin hands with extra long fingers and a 360 degree wrist.

Regards,

David Q

[iRS]

David,
I has something similar before I changed the gearbox oil. I seem to remember at 40mph and about 1600 rpm, the revs would fluctuate by around 50 -100 or so. Its amazing the difference a gearbox service makes.
Cheers, Roger

[Davidqhog]

Hi Roger,

Good point. But, when I bought the car, first thing I had done was the gearbox service! I'd read too many posts about the necessity of getting this done. Having had my XJ8 gearbox collapse on me once, I didn't need much reminding.

Once the engine is up to temperature, it seems to go away - I think.

Regards,

David Q