loss of power
 

hi there, just bought an old xj6 2.9 1988 last week and I'm wondering if anyone out there could help. When I got the car it was fine, it pulled and changed gears well, though I thought the fuel consumption was juicy for a 2.9. the digital dashboard was indicating 7-8 mpg. The previous owner said it was an anomaly because he hadn't driven it for a while. However taking it out on the third day after purchase there's now a
very noticeable and acute loss of power . Dashboard is now reading 6-1 mpg. Struggling to make even 10mph unless its moving downhill.
I'm a bit of an idiot in terms of car mechanics so I'm wondering if theres something iam doing wrong Or was a I sold a dud. If so why did it go so well before. Btw I have been filling it with unleaded fuel. In the meantime I have been told to get an additive to replace the lead in the fuel. Could this be the reason or is the lovely xj6 doomed. Any help would be much appreciated

Is your check engine light on? If he hadn't driven it in a while there's a chance that a few things could have happened. My first thought is maybe that fuel pressure regulator is bad (diaphragm rotted through), or your oxygen sensor is bad. It could be other stuff but I'd start there. When i get frustrated I like to go back to the basics. Generally, its only one of three things: Fuel, Air, Spark. if you can test each of those then you can start to narrow it down. Obviously there's things like timing, compression, etc. that will affect it, but the fact that it does run will help. I bought my 89 VDP while it was only running on 5 cylinders, and took my time diagnosing it, only to find crap had made its way into the fuel injectors and was plugging one completely, and two others about half way. Whole different car after that. I know nothing about the 2.9, but the principles are the same. Just take your time and don't get discouraged... On a side note, I drive a '65 Ford truck (I live in the states) and have rarely given the fuel a lead additive (haven't in many years), and don't have any strange symptoms. Maybe my valve seats are shot, but it still runs and drives great. Maybe someone else has better advice, but I hope this helps!

Lead additive shouldn't be necessary. Check your ignition as suggested. Gat the car to a dark area and look for sparks off the plug wires. Change your fuel filter- underneath, in front of left rear wheel.
How much fuel have you run through it? more than a tank? if so, it shouldnt be bad fuel.... Also, check your vacuum lines, they may be cracked causing leaks. I use a carb cleaner spray to check this. Good luck.

Many many thanks to Civicalized and Jerry-hoback for your responses. Its all greatly appreciated. The engine check light isn't on, but I'll be checking the fuel pressure regulator this coming weekend, however I do know that the sparks are virtually new. I'm not too despondent now about its condition so I'll investigate further and report any further developments here. Thanks