Volvo T5: problem caused by change in fuel octane?

Question

97 Volvo S70 T5, 102kI'm about to waffle somewhat but please bear with me, it's all relevant (honest) ;-)Took the car to a track day organised by the IAM at the weekend. During the day I did about 35 miles of flat-out track driving and later drove home from Bedford autodrome (125 mile journey) with no problems at all. During all of the above the car was running on Shell V-Power petrol. It gives better performance and Volvo recommend a 98RON fuel for this engine -    not compulsory but advised. For these reasons I filled up with it before I set off for the track day.Today, I was out of fuel and not near a Shell station or other super unleaded seller, so I put in some normal 95 unleaded. Most of the time, I tend to swap between both because V-power is better but it's also dearer and I often can't be arsed to drive across town to the Shell garage. About 15 miles from putting in the 95, the car started to misfire and struggle to maintain a steady idle. In fact, the engine actually stalled once on the way home and came close several times.Also, the lambda light came on (the MIL is a lambda symbol on this car). Fortunately, I have some OBDII reading equipment so I plugged in the laptop and have discovered the following:1) The fault code is P0171 - Fuel trim too lean2) Reading the fuel trim off the ECU, short term is 1%, but long term is -25%Given the above circumstances, I feel that the ECU has leaned the mixture considerably because I have been using a large amount of throttle with a high octane fuel, and now that I have returned to standard fuel the mixture is so lean that the engine is incapable of running properly. Am I thinking along the right lines? Is -25% a very lean setting? Is it likely that I can cure the problem by simply pulling the ECU supply, or is it a coincidence and more likely to be a component failure?Would appreciate your thoughts.Stu

Tunku · Accepted Answer

Stu  wrote innews:1163447812     My old 740 ecu used to like high octane fuel, but I always had problems when going back down to normal unleaded. I used to unplug its ecu to wipe whatever was in there, and that always cured it. Never had a problem going from 95ron to 98ron, but the car always had trouble going back down the way. Basically it didn't want to. Stuttered, heated up, and generally threw its mechanical dummy out of the pram. Putting the ecu to sleep seemed to be the only way.BTW, the car was always driven to the rev limiter, which seemed to magnify the problem ;-)

Duncan Wood · Answer

& it does seem a fairly common issue on the ovlov

Stu · Answer

Tunku wrote in news:Xns987ACD70E841Fstuartggraydslpipexc@216.196.109.145:

Thanks for that. It looks like I have the same problem. The reason it's never happened before must be because I've never driven it so hard so the ECU hasn't adjusted the fuelling all that much. I'll reset the ECU and see how it goes.

Cheers,

Stu

Tim.. · Answer

Unlikely.98 and 95 still require pretty much 14.7 parts of air to burn at stoic.What is much more likely is that the vacuum take off on the end of the manifold behind the power steering pump has split giving you a vacuum leak, or you have a split in the intake plumbing *after* the AMM.Tim..

Tunku · Answer

"Tim.."  wrote innews:     Your Volvo must be very well trained. :-)

Stu · Answer

"Tim.."  wrote innews:     Tim, you were absolutely spot on! The small vacuum pipe is indeed split, right on the elbow where it joins the inlet manifold. Appears to pass under the back of the manifold, where another pipe joins it within a single lagging, and both terminate to a sensor in the main inlet duct, just before the turbo. Looks fiddly but hopefully I can get a replacement in without any drastic dismantling. Any hints? 'Cos I get the impression you've been here before? ;-)Sincere thanks for that - I found it hard to spot the pipe even with your advice so god knows how I'd have managed without it!Thanks again,Stu

Tim.. · Answer

No problems. I have vast experience of the 4,5,6 cylinder 'white blocks' and their foibles...

You may have to pull the manifold to feed through a new pipe, but it can

*just about be done with it inplace if you R&R the PAS pump, but its abit of a bitch.

You local dealer will have the elbow on the shelf. (or should do if its anygood.)

Tim..

Stu · Answer

"Tim.."  wrote innews:     So I thought.....such accurate information isn't usually the product of a wild guess ;-)Unfortunately, I don't have a local dealer. Nearest one is about a 30 minute journey :-(. Methinks a temporary bodge will be required, probably involving silicone sealant ;-)I see from the VADIS diagram that the elbow fitting appears to be separate from the main pipe. This being the case, could I just replace the elbow by cutting off the old one and attaching the replacement with a suitable glue?Stu

Tunku · Answer

"Tim.."  wrote innews:     Nice one Tim, I only have experience of the 4 pot ones, usually red block.I'll remember that when I can afford to upgrade. :-)

Volvo T5: problem caused by change in fuel octane?

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